TECTONICS, VOL. 21, NO. 3, 1010, /2001TC900024, 2002

Transcription

1 TECTONICS, VOL. 21, NO. 3, 1010, /2001TC900024, 2002 Extension and compression in the Northern Apennines (Italy) hinterland: Evidence from the late Miocene-Pliocene Siena-Radicofani Basin and relations with basement structures Marco Bonini CNR, Istituto di Geoscienze e Georisorse, Sezione di Firenze, Firenze, Italy Federico Sani Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Firenze, Italy Received 7 June 2001; revised 10 September 2001; accepted 13 September 2001; published 9 May [1] The Northern Apennines (NA) hinterland area is characterized by a complex Neogene-Quaternary tectonics where both crustal extension, associated with the Tyrrhenian Basin opening, and crustal shortening in the onshore area took part in the deformation. Analysis of synorogenic deposits preserved in the NNW trending Siena-Radicofani Basin (SRB), extending along a large part of the NA hinterland, documents the evolution of deformation of this sector during the last 9 Ma. Information from subsurface geology (deep seismic lines, commercial seismic lines, and deep wells), surface geology (mapping and structural analysis), and from analogue modeling was integrated and used to infer the tectono-sedimentary history of the SRB and its relation with structures in the substratum, as well as the possible implications for the evolution of the NA hinterland. The results we obtain indicate that the SRB and the adjoining hinterland basins were bounded by thrust anticlines controlling basin development and deformation. Seismic lines across the SRB display various examples of compressional structures affecting the basin fill and the substratum, such as thrust anticlines and reverse faults. Notably, these basin-scale structures exhibit a good correlation with those observed in the field and are consistent with the kinematics of the outcrop-scale compressional structures. The thrust anticlines bounding the basins are often cored by Triassic evaporites (Burano Formation), suggesting that this weak layer decoupling the sedimentary cover from the underlying crystalline basement controlled their evolution. In this circumstance, three series of scaled brittle-ductile physical models have been used to investigate the development of a basement cover system, with a décollement ductile layer at the base of the sedimentary cover. These models simulate the evolution of the Northern Apennines hinterland, where the basement is involved in the thrusting, and the sedimentary cover is shortened above a basal ductile layer given by the Burano Formation. Longitudinal models cross sections display similar deformation patterns to those observed in the NA hinterland, such as characteristic wavelength of both basement and cover structures, as well as detachment and fault propagation folding in the cover. Extensional tectonics is instead found to control sedimentation in the southern part of the SRB (at 8 Ma) or representing recent deformation. The older extensional event is here related to the forelandward propagation of Tyrrhenian-related extension that could reactivate suitably oriented basement thrusts. Copyright 2002 by the American Geophysical Union /02/2001TC Transfer zones produced a differential propagation of extension, such that along-strike sectors of the hinterland were eventually deformed by different stress fields. Analysis of the SRB and of other adjoining basins reveals that the NA hinterland experienced alternated periods of forward and backward migration of the compression-extension transition. In our interpretation, the competition between extension and compression in the hinterland is related to the NNW directed Africa (Pelagian) indentation responsible for the lateral extrusion of the NA and the contemporaneous opening of the Tyrrhenian Basin in the inner zone. Increased marginal loading at the foreland promoting reactivation of hinterland thrusts may have influenced this process as well. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 8015 Structural Geology: Local crustal structure; 8094 Structural Geology: Instruments and techniques; 8025 Structural Geology: Mesoscopic fabrics; KEYWORDS: Northern Apennines, Hinterland basins, continental contractional orogenic belts, local crustal structure, mesoscopic fabrics, analogue modeling 1. Introduction and Aims of the Work [2] The dynamic relations between the NE vergent Apennine fold and thrust belt and Tyrrhenian Basin related crustal extension affecting its hinterland area has been representing a stimulating matter of research (Figure 1). In particular, crustal thinning, magmatic processes, and the current high heat flow, together with the numerous sedimentary basins outcropping in the Northern Apennines (NA) onshore hinterland area, commonly interpreted as grabens or half grabens [e.g., Martini and Sagri, 1993, and references therein], have been considered geological features related to the outward propagation of crustal extension associated with the Tyrrhenian Basin opening [e.g., Elter et al., 1975]. Tyrrhenian extension, which initiated in correspondence of the main NA suture zone, has been generally explained invoking a west dipping subducting (and eastward retreating) Adria plate producing crustal extension that has been referred to various geodynamic models considering back arc and/or postorogenic extension [Boccaletti and Guazzone, 1974; Reutter et al., 1980; Malinverno and Ryan, 1986; Royden et al., 1987; Carmignani and Kligfield, 1990; Carmignani et al., 1994; Jolivet et al., 1998; Jolivet and Faccenna, 2000]. Recently, continental extension has been associated with major east dipping normal faults, located both

2 1-2 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES o o o Figure 1. Tectonic sketch map of the Tyrrhenian-Apennines area, with indication of the main structural domains. The arcuated thrust fronts are interpreted as resulting from lateral extrusion of the Apennine fold and thrust belt (indicated by the large solid arrows) as a consequence of the roughly NNW directed shortening by the Pelagian Indenter; this process produced crustal opening and the development of the Tyrrhenian Basin (based on Boccaletti et al. [1982, 1984, 1985], Burrus [1984], Ben Avraham et al. [1990], Patacca et al. [1990], Mantovani et al. [1996], Faccenna et al. [1996], and Finetti et al. [1996]). in the northern Tyrrhenian area [Jolivet et al., 1998; Rossetti et al., 1999] and in the NA axial zone [e.g., Boncio et al., 2000]. [3] However, geological and structural studies have allowed the interpretation of most of the NA hinterland basins as developed in a compressional setting related to basement involvement in thrusting [i.e., Boccaletti and Sani, 1998]. Basement thrusting has been indeed documented by different geophysical approaches [Arisi Rota and Fichera, 1985; Sage et al., 1991; Ponziani et al., 1995; Cassano et al., 1998; Barchi et al., 1998; Finetti et al., 2001]. Particularly, the recognition of major contractional structures in the basin fill of numerous hinterland basins, together with their tectono-sedimentary evolution and sediments architecture showing strict relations to the activity of thrusts bounding the basins, strongly suggest that they evolved in a compressional rather than extensional regime [e.g., Boccaletti and Sani, 1998; Moratti and Bonini, 1998; Boccaletti et al., 1999; Bonini, 1998, 1999; Bonini et al., 1999]. Notably, piggyback basins situated in an approximately similar structural position to these in the NA have also been documented in the central [Cipollari and Cosentino, 1995] and Southern Apennines [Hippolyte et al., 1994a]. [4] Therefore the Neogene-Quaternary tectonic evolution of the NA hinterland area was dominated by two main contemporaneous processes: (1) the shortening of the basement and (2) the opening of the Tyrrhenian Basin. Both processes propagated toward the east, but the definition of their mutual time-space relations is still controversial, particularly the key point that concerns the position through time of the extension-compression boundary, which in our opinion was not coinciding with the hinterland basins but was significantly more internal. Obviously, the determination of these conditions will necessarily reflect on the assessment of any regional tectonic model. [5] An opportunity to achieve some insights for the comprehension of this geodynamic process is provided by the investigation of the Siena-Radicofani Basin that is suitably located and represents one of the most important hinterland basins of Tuscany (Figures 1 and 2). The availability of a grid of commercial seismic profiles, integrated with field geological-structural analyses and subsurface data from deep wells, has permitted the reconstruction in detail of the tectono-sedimentary evolution of this basin. This study has also allowed the refinement of the tectonic evolution and the timing of deformation of this area linking the structures controlling basin evolution to the major geodynamic stages. An analogue modeling approach has also been employed to investigate the complex relationships between thrusts in the metamorphic basement and those in the sedimentary cover and the development of compressive sedimentary basins. 2. Geological Setting [6] The NA constitute a NE to E vergent chain built up by ocean-derived tectonic units, namely Ligurian Units, tectonically overlying foreland Tuscan and Umbria-Marche units derived from the deformation of the Adria passive margin. The foreland units consist of 2 km thick Mesozoic carbonate succession resting above a 1 to 1.5 km thick Triassic basal evaporite layer (Burano formation); 2 3 km thick upper Oligocene-Miocene siliciclastic foredeep sandstones overlie this succession. [7] The NA orogenesis can be roughly subdivided into two main stages: (1) an earlier Late Cretaceous-early Eocene east dipping oceanic subduction phase during which the Ligurian Units were off-scraped, tectonically stacked, and accreted, and (2) a late Eocene to early(-middle?) Miocene stage dominated by the collision between the Corsica-Sardinia block and the Adria plate (Figure 1). The occurrence of an east dipping subduction during the older tectonic stage, contrasting with models proposing a continuous eastward slab retreating of the Apennines subduction, is based on the interpretation of the Crosta Profonda (CROP) deep Figure 2. (opposite) Schematic geological-structural map of the Siena-Radicofani Basin (SRB) and surrounding areas (location in Figure 1). Deposits indicated with a and b are local subunits. Dashed lines indicate inferred or buried structures. MTMR, mid-tuscany metamorphic ridge.

3 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-3

4 1-4 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES Figure 3. Geological cross sections across the study area (location in Figure 2). (a) Structure of the basement along profile AB is based on the interpretation of the deep seismic profile CROP03 [Finetti et al., 2001], whereas cover structures are based on the interpretation of commercial seismic profiles and surface geological-structural investigations. (b) In profile CD, the basement (and cover) structure is also integrated by deep wells data (Paglia 1, Radicofani 1, S. Filippo 1, BG 8, BG18, and wells in the Piancastagnaio area [Calamai et al., 1970; Elter and Pandeli, 1991; Liotta, 1996]; Figure 2) and the interpretation of the Cinigiano Basin structure is inspired to the analogue modeling results (see Figure 18). Symbols and patterns are as in Figure 2.

5 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-5 Figure 4. Stratigraphic columns of Siena Basin (SB) and Radicofani Basin (RB), reconstructed from outcrops, and comparison with deep wells Radicofani 1 and Paglia 1. Location of wells is shown in Figure 2. seismic profiles [Finetti et al., 2001; see also Boccaletti et al., 1971]. During the second tectonic stage, the Tuscan and Umbria- Marche Units continental margin sequences were shortened together with their underlying basement and progressively tectonically overlain by the Ligurian Units accretionary wedge. This process gave rise to an eastward migrating frontal foredeep basin that was progressively involved in the thrusting and accreted to the chain [Ricci Lucchi, 1986]. Successively, crustal extension affected the internal NA since the (middle?-)late Miocene, generating the Tyrrhenian Basin and dissecting the preexisting nappe pile [e.g., Sartori, 1990]. This process promoted crustal melting and the emplacement of several subalkaline magmatic bodies and volcanic rocks of the Tuscan Magmatic Province [Serri et al., 1993]. [8] In this context, the Tyrrhenian Basin development can be broadly considered as the boundary between the collisional and postcollisional phases in the NA evolution. Indeed, since the late Miocene onward the evolution of the whole Apennines was controlled by the roughly NNW indentation of the Africa Plate (i.e., Pelagian Block; Figure 1) [Ben Avraham et al., 1990; Boccaletti et al., 1990] inducing synchronous Tyrrhenian crustal opening and lateral extrusion in the Apennines fold and thrust belt [e.g., Tapponnier, 1977; Boccaletti et al., 1982; Faccenna et al., 1996; Finetti et al., 2001]. In this complex tectonic setting developed the NW to NNW trending continental and marine hinterland basins, whose tectonic evolution in relation to the NA fold and thrust belt is the main topic of this work. 3. Stratigraphic and Tectonic Setting of the Siena-Radicofani Basin [9] The Siena-Radicofani Basin (SRB) extends in a roughly NNW-SSE direction for a total length of 85 km (Figure 2). The SRB is composed of two subbasins: the Siena Basin (SB) to the north and the Radicofani Basin (RB) to the south. The boundary between the subbasins is commonly located along the Pienza structural high that has been interpreted as an extensional transfer zone [Liotta, 1991]. [10] The SB is bounded by ridges mostly uplifted by west to WSW dipping thrust faults: the Mid-Tuscany metamorphic ridge

6 1-6 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES W E 4 th unit 3 rd unit Figure 5. Angular unconformity (marked by the open circles) along the eastern SRB margin where subhorizontal fourth unit sands unconformably overlie steeply dipping third unit basal deposits composed of Pliocene conglomerates and sands (2.5 km NW of Monticchiello; Figure 2). This setting points to uplift of the eastern SRB margin above the back limb of the active Rapolano-Cetona thrust anticline. (MTMR) and the Mt. Labbro-Montalcino ridge to the west and the Chianti Mounts-Mt. Cetona ridge to the east. The latter also delimits the RB to the east, whereas the RB is bounded to the west by the Ripa d Orcia and Zoccolino-Castell Azzara ridges (Figures 1 and 2) [e.g., Losacco and Del Giudice, 1958; Losacco, 1959; Sani et al., 1998; Bonini et al., 1999, 2001]. The MTMR is an important arcuate west dipping thrust fault exposing Tuscan basement and metamorphic rocks in its hanging wall (Figure 2). The other more external ridges correspond to east verging asymmetric thrust anticlines generally exposing Mesozoic carbonate Tuscan rocks and occasionally small Triassic evaporites outcrops at their core (Chianciano area; Figure 2). Locally, these anticlines can be affected by normal faults [e.g. Losacco and Del Giudice, 1958; Losacco, 1959], structures that were used to interpret the SRB either as a graben or as a half graben in the frame of the classic extensional tectonic model for the Apennines hinterland (see section 2) [Costantini et al., 1982; Liotta, 1996]. [11] To provide a picture about the general structure of the study area, two transversal cross sections are illustrated in Figure 3. Construction of these sections integrates all the available data, such as surface geology, the deep profile CROP03 [Finetti et al., 2001], the stratigraphy of the deep wells, the results of the interpretation of the grid of commercial seismic lines across the SRB (see section 4), and the results of an analogue modeling investigation (see section 5) Stratigraphy of the SRB [12] The sedimentary succession outcropping within the SRB shows variable lithostratigraphic characteristics from the northern Siena Basin to the southern Radicofani Basin (Figure 4). The stratigraphic succession of the SRB has been subdivided into four main angular unconformities-bounded stratigraphic units (UBSUs) [Salvador, 1987], which can be correlated through the hinterland basins of the NA [Bernini et al., 1990; Moratti and Bonini, 1998; Boccaletti et al., 1999]. [13] The lowermost unit (first unit) is represented by the so-called Ponsano Sandstone [Giannini and Tongiorgi, 1959] which is exposed in small outcrops 10 km NW of Siena (Figure 2) and was encountered by the deep Radicofani 1 well in the RB (Figure 4). These deposits consist of shallow-water arenites of late Serravallianearly Tortonian age [Foresi et al., 1997]. [14] The following second unit consists of a late Tortonian- Messinian continental succession composed of lacustrine sandyclays with conglomerate levels [Signorini, 1966]. The deposits of this unit outcrop in the western part of the SB only, whereas in the RB they have been encountered by the deep wells for an approximate thickness of 960 m (Figure 4). Radiometric age determinations (K/Ar) carried out on a tuff layer embedded in the equivalent continental succession of the Radicondoli-Volterra Basin yielded an age of 8.07 ± 0.11 Ma, suggesting an age of 9 Ma for the base of the second unit [D Orazio et al., 1995]. Similar ages are obtained by dating of the lowermost fluvio-lacustrine cycle to the mammal zone MN11 in the Cinigiano Basin [Benvenuti et al., 2001]. [15] The third sedimentary unit is composed of a lower part consisting of late Messinian reddish continental conglomerates (the so-called Montebamboli Conglomerates ) and an upper part mainly composed of early middle Pliocene (G. margaritae to G. puncticulata zone) [Gandin, 1982; Liotta, 1996] marine clays, silty-clays, sands, and conglomerates. The apparent lack of the G. Sphaereodinellopsis zone might indicate the occurrence of a paraconformity between the Montebamboli Conglomerates and the overlying marine Pliocene sediments. The Montebamboli Conglomerates outcrop along the Asso River valley only, in the southern SB (10 km west of Pienza). Conversely, the Pliocene third unit deposits extensively outcrop in the RB, where they exhibit a gradual transition from basal polygenic conglomerates, interbedded with sandy layers, to gray-blue clays. Near the south-

7 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-7 Table 1. Mesoscopic Structural Field Data a Computed Site Number Kind of Mesostructures Number of Data s 1 AZ/Dip s 2 AZ/Dip s 3 AZ/Dip Estimated Mean Shortening Direction Unit or Formation Age of Deformed Rock Siena Radicofani Basin 1 faults 8 106/71 315/7 42/16 Burano Fm. Middle Triassic 1 folds 9 112/15 Burano Fm. Middle Triassic 2 faults 6 254/16 347/12 113/70 fourth unit late Pliocene 2 faults 3 178/55 293/17 33/29 fourth unit late Pliocene 3 stylolitic pits /15 third unit Messinian-middle Pliocene 4 folds 7 82/5 Diaspri Fm. Late Jurassic 5 faults 16 53/8 134/28 290/60 fourth unit late Pliocene 5 faults 6 158/3 249/12 34/77 fourth unit late Pliocene 5 faults 4 10/72 133/11 226/13 fourth unit late Pliocene 6 stylolitic pits /10 fourth unit middle-late Pliocene 7 folds 7 60/3 fourth unit middle-late Pliocene 7 faults 10 77/12 165/3 260/77 fourth unit middle-late Pliocene 8 faults /84 62/3 332/5 cherty limestone Middle Jurassic 9 stylolitic pits /3 third unit Messinian-middle Pliocene 10 faults 7 45/2 135/4 288/86 second unit late Tortonian-Messinian 11 stylolitic pits /10 third unit Messinian-middle Pliocene 12 stylolitic pits 54 11/2 third unit Messinian-middle Pliocene 12 stylolitic pits /1 third unit Messinian-middle Pliocene 13 stylolitic pits /35 third unit Messinian-middle Pliocene 14 folds 21 38/2 second unit late Tortonian-Messinian 15 faults 6 218/4 127/2 18/86 Ligurian Units Eocene 16 faults 10 13/11 106/15 246/71 Ligurian Units Eocene 17 faults 20 40/2 131/6 302/84 Macigno formation late Oligocene-early Miocene 18 faults 9 67/20 161/10 276/68 Macigno formation late Oligocene-early Miocene 19 faults 19 44/12 313/6 190/75 Macigno formation late Oligocene-early Miocene 20 stylolitic pits /8 fourth unit middle-late Pliocene 21 stylolitic pits 25 35/45 fourth unit middle-late Pliocene 22 stylolitic pits 93 65/55 fourth unit middle-late Pliocene 23 faults /68 296/17 56/8 Travertines late Pleistocene 24 faults /2 71/8 269/82 Travertines late Pleistocene 24 faults 4 279/67 184/2 94/23 Travertines late Pleistocene Velona Basin 25 faults 8 266/1 176/4 8/86 second unit Messinian 25 folds 6 80/3 second unit Messinian Cinigiano Basin 26 stylolitic pits 59 40/15 fourth unit middle-late Pliocene 27 stylolitic pits /12 second unit late Tortonian-Messinian 28 faults 13 53/8 139/13 335/72 second unit late Tortonian-Messinian 29 faults /5 144/2 37/85 second unit late Tortonian-Messinian a Palaeostresss orientation of fault slip data has been computed using the Carey [1979] inversion method, while a mean direction of shortening has been estimated for fold axes and stylolitic pits on calcareous pebbles surface. Location of sites is indicated in Figure 2. Sites 14 to 16 from Sordi [1998]; from Flaccomio [1998]; from Menichetti [1997]; 25 from Bonini et al. [1999]; from Landi et al. [1995]. AZ, azimuth; Fm, formation. eastern RB margin the clays grade upward to sands and conglomerates showing a marked regressive trend [Iaccarino et al., 1994] (Figure 4). The third unit deposits are also exposed in the NW sector of the SB (Figure 2). [16] The deposits of the fourth sedimentary unit mainly consist of middle to late Pliocene marine clays and sands (G. puncticulata- G. aemiliana zones) [Gandin, 1982; Gandin and Sandrelli, 1992; Bossio et al., 1993] and extensively outcrop in the SB (Figures 2 and 3). In the RB this unit crops out along the eastern margin where a high-angle angular unconformity separates subhorizontal fourth unit sands by underlying steeply dipping third unit deposits composed of Pliocene conglomerates and sands (Figure 5). These sands grade laterally to Amphistegina-bearing limestones (attributed to the late Pliocene, G. aemiliana zone) [Conti et al., 1983] that directly overstep the pre-neogene substratum structural highs bounding the RB to the east (Mt. Cetona area; Figure 2) Surface Structural Setting and Mesoscopic Data [17] In this study, particular attention has been devoted to the definition of the geometry of large structures, as well as to the collection of mesoscopic elements both in the basin fill and in the substratum adjacent to the basin. The results of this analysis are reported in Figure 2 and Table 1. The structure of the SRB can be schematically represented by a broad syncline basin bounded by two systems of NNW trending thrust anticlines (Figures 2 and 3). Along the eastern margin, the SRB is bordered by the roughly continuous major Cetona-Rapolano thrust anticline, which can be

8 1-8 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES Figure 6. Structural map of the Vagliagli anticline and the adjacent Casino Basin (modified and simplified from Lazzarotto and Sandrelli [1977], Flaccomio [1998], and Sordi [1998]). Note the progressive unconformities within the lower continental cycle indicating the synsedimentary uplift of the eastern Casino Basin margin in response to the activity of the Vagliagli anticline. traced further to the north of the investigated area. North of Siena the SB narrows and it is bounded to the east by the smaller Vagliagli anticline (Figures 2 and 6). Conversely, the western SRB margin is more complicated, being affected by a number of thrust anticlines (i.e., Grotti, S. Quirico, Montalcino, Zoccolino, Castell Azzara anticlines) as well as by the major MTMR basement thrust (Figure 2). These structures characteristically display an eastern vergence, as also demonstrated by the associated mesoscopic elements (folds and reverse faults; Figure 2). [18] Typically, such thrust anticlines exhibit an overturned eastern forelimb suggesting that they evolved according to a fault propagation folding model [Suppe and Medwedeff, 1984]. These structures involve both the Ligurian Units (LU) and the underlying Tuscan Unit (TU), which rests on a thick décollement layer composed of anhydrites and dolostones, with clay and marls that may be locally very important (i.e., the Burano formation) [Martinis and Pieri, 1964] (Figure 3). This ductile layer promoted the decoupling of the overlying TU from its basement, likely resulting in the development of detachment folds [Jamison, 1987]. This possibility is suggested by the thickening and the local outcropping of evaporites at the anticlines core (Cetona-Rapolano, Zoccolino, and Montisi anticlines; Figures 2 and 3b). For instance, the S. Filippo 1 well drilled the Zoccolino anticline stopping within the evaporite after having encountered 1300 m of anhydrite and dolostones [Losacco, 1959; Calamai et al., 1970]; obviously, this thickness represents only the minimum estimate for the ductile

9 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-9 thickening at the anticline core. This setting suggests that the thrust anticlines evolved from detachment folding to fault propagation folding (see Storti et al. [1997] for a review). In addition, during fold development, vertical ductile extrusion of the evaporites might have also taken place, being favored by the inversion at shallow depths of anhydrite to gypsum that is lighter than the surrounding rocks. This possibility is consistent with the radial extension cinematic obtained from the analysis of mesoscopic faults in the dolostones and by the geometry of pervasive folding observed in the gypsum layers of the Burano formation outcropping along the Cetona-Rapolano anticline (Chianciano area; site 1; Figure 2 and Table 1). Similar structural features characterize also the Montisi anticline, which is located a few kilometers in advance of the MTMR thrust front (Figure 2). [19] The recognition of the overthrusting of TU onto the LU at the front of the main thrust anticlines rather complicates the geometry of such structures. This geometry results in the inversion of the original stacking order and can be recognized at many places along the Cetona-Rapolano anticline [e.g., Passerini, 1964; Losacco and Del Giudice, 1958], at the Zoccolino anticline, as well as at the front of the Vagliagli anticline (Figure 6). This setting, which also characterizes the relationships between Tuscan basement and LU at the MTMR front [e.g., Moretti, 1991], is indicative of a thrust activity which lasted well beyond the emplacement of the LU onto the TU, which approximately occurred in the early Miocene [e.g., Merla, 1951; Abbate et al., 1970; Ricci Lucchi, 1986; Conti and Gelmini, 1994]. Indeed, polyphase thrusting has been hypothesized on the northern prolongation of the Cetona- Rapolano anticline, in the Chianti Mounts area [Bonini, 1999], as well as in other areas of the NA hinterland as a consequence of basement-induced cover thrusting [Boccaletti and Sani, 1998]. As outlined above, the development and the deformation of several hinterland basins is related to this polyphase thrust activity [Boccaletti et al., 1995, 1999]. [20] Field mapping and structural analysis of both the basin fill and the major structures bounding the SRB, as well as seismic profiles analysis (see section 4) indicate that the SRB formed and evolved mostly in a compressive tectonic setting. This interpretation is supported by the occurrence of open to gentle NNW trending folds, which affect the basin sediments and undulate the broad SRB syncline, as well as by the syndepositional activity of thrust anticlines at the basin margins and the mesoscopic compressional structural elements affecting the basin fill (Figure 7 and Table 1). [21] Notably, the oldest SRB sediments (i.e., the first and the second units) are exposed in correspondence of thrust anticlines: The first unit deposits ( Ponsano sandstones ) are lifted by the anticline delimiting to the SW the highly shortened Casino syncline basin [Lazzarotto and Sandrelli, 1977] (Figure 6), in the northernmost part of the investigated area (Figure 2). Similarly, the second unit fluvio-lacustrine deposits are exposed along the western SB margin and folded by the N-S trending Grotti anticline involving also the substratum (Figure 2). [22] Gentle folding affects the third unit deposits mainly in the RB, where NNW trending folds give rise to minor undulations across the basin [Calamai et al., 1970]. At the southwestern SB margin, the third and fourth unit deposits are involved in the east verging S. Quirico anticline, which also controlled the evolution of the Messinian Velona syncline basin that formed on the back limb of this structure (Figure 2) [Bonini et al., 1999]. The fourth unit deposits display clear evidence of folding in the center of the SB, where a NW-SE trending anticline has been mapped by long time [Servizio Geologico d Italia, 1968]. The local shortening direction obtained by the analysis of stylolitic pits on the surface of calcareous pebbles of the third and fourth units trends around E- W, in good agreement with the direction of the large folds affecting the basin fill (mostly in the RB; sites 12, 13, and 22; Figure 2 and Table 1). [23] Normal faults were rarely observed in the basin fill, while outcrop-scale reverse faults and thrust-related folds are locally well developed, such as at the emergence of thrust faults in front of the associated anticlines. For instance, the Pliocene third unit deposits are strongly shortened in front of the S. Quirico anticline, where the deformation induced by the main thrust propagates up to the surface (Figure 8). The identification of syntectonic deposition of fourth unit deposits (Figure 8) allows extending to the middle-late Pliocene of the timing of activity of this anticline. Analysis of mesoscopic folds and palaeostress orientation evaluated in this area (sites 7 and 25) indicate a roughly ENE trending shortening direction, consistently with the trend of the major S. Quirico anticline (Figures 2 and 8 and Table 1). [24] Similar structures can be also recognized in the Grotti anticline, where, at its front, mesoscopic reverse faults offset the boundary between the second and third units (site 10, Figures 2 and 7, and Table 1). Mesoscopic reverse faults pervasively affect the Amphistegina limestone (fourth unit) at the forelimb of the Cetona-Rapolano thrust anticline (sites 2 and 5; Figures 2 and 7, and Table 1). Activity of this structure during basin development is demonstrated by the architecture of the basin fill along the eastern margin, where strong bed tilting (Pliocene third unit beds locally show a vertical attitude adjacently to the margin) and progressive unconformities [e.g., Riba, 1976] are indication for the syndepositional uplifting of this anticline (see Figure 5). In this view, the compressive deformations detected in the Amphistegina limestone likely indicate a late Pliocene reactivation of this structure. [25] Syndepositional activity can be also inferred for the east verging Vagliagli thrust anticline that controlled the evolution of the Casino Basin during the late Miocene (Figures 2 and 6). This is indicated by the architecture of the basin fill, which characteristically displays high-angle angular unconformity and progressive unconformities to the back limb of the Vagliagli thrust anticline [e.g., Beer et al., 1990] (see cross section in Figure 6). [26] At places, crosscutting relationships observed between mesoscopic structures account for various deformative phases that acted in the study area. In particular, at sites 2, 5, and 24 different stress fields have been computed, and the clear superposition relations between structures allow the determination of a relative chronology of deformation (Figure 2 and Table 1). These data suggest the occurrence of an important stress field change. After the main ENE trending compression phases associated with the activity of thrust anticlines (sites 2, 4, 5, 6, 7, 10, 11, 13, 14, 15, 15, 17, 18, 19, 21, 22, 25, 26, 27, 28, and 29; Figure 2 and Table 1) a new stress field characterized by a roughly NW-SE direction of shortening was established (see site 5; Figure 7). Extensional deformations characterized by a NE-SW to E-W direction of extension followed the NW-SE shortening phase or were associated with it (see sites 2 and 24; Figure 2 and Table 1). This event

10 1-10 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES Second Unit (Buonconvento Basin) Site no. 14 Casino Basin Site no. 10 Radi Site no. 25 Velona Basin Third Unit Site no. 11 Site no. 12 Site no. 13 Fourth Unit Site no. 2 Sarteano Site no. 5 Mt.Cetona 1 2 Figure 7. Selected mesoscopic data collected in the sediments filling the SRB (location in Figure 2). Computed principal stress axes and number of data are reported in Table 1. Palaeostress orientation of fault slip data has been computed by the Carey [1979] inversion method, while a mean direction of shortening has been estimated for fold axes and stylolitic pits. Numbers 1 and 2 denote the sequence of faulting at site 5 inferred from crosscutting relations among mesoscopic structures.

11 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-11 a) b) W E c) c) d) Site 7 S. Quirico Figure 8. Example of outcrop-scale compressional structures affecting the Pliocene fourth unit deposits at the front of the S. Quirico anticline (quarry near to S. Quirico d Orcia; site 7 in Figure 2). The overall structural pattern is interpreted as an east verging triangle zone structure. (a) Photograph of the outcrop. (b) Line drawing of the main marker beds, folds, and faults. (c) Close-up showing the fault-related folds on the left-hand side of the outcrop and the prograding asymmetric wedge exhibited by the sediments in front of the thrust-related fold pointing to a synshortening deposition (F. Sani for scale). (d) Plot of structures collected at this site (site 7; Table 1); solid dots represent fold axes and solid lines indicate reverse faults (Schmidt net, lower hemisphere).

12 1-12 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES can be broadly referred to the middle-late Pleistocene, being the late Pleistocene travertines at site 24 deformed according to a roughly NW-SE shortening direction that was followed by a roughly E-W extension direction. To the same stress field can be referred the late deformations at sites 2 and 5, collected in the late Pliocene Amphistegina limestone (fourth unit), as well as the deformations recognized at sites 3, 9, 12, 20, and 23. It should be noted that to this stress field are also connected active deformations, consisting of roughly NW-SE trending normal faults, such as at Pienza [Sani et al., 2001] (Figure 2). Of different interpretation might be the normal faults collected at site 8, showing a NW- SE oriented extension direction that could be related to along-axis extension during the main emplacement phases of thrust anticlines bounding the SRB. 4. Stratigraphic and Structural Interpretation of the Seismic Profiles Across the SRB [27] The study of the dense grid of commercial seismic reflection profiles (for a total length of 400 km) across the Siena-Radicofani Basin allowed the illustration of some important stratigraphical and structural aspects of this basin. The overall quality of the seismic lines is rather good, and their interpretation has been tested in the field and integrated using the available well data. In order to illustrate the principal structural and stratigraphical features of the SRB, line drawings of selected seismic lines across the SB and the RB, as well as the longitudinal line throughout the whole SRB, are shown in Figures 9, 10, and 11, respectively (see Figure 2 for seismic profiles trace). The line drawings are integrated by details of the uninterpreted original seismic lines to provide a feeling for the data quality on which the line drawings are based. [28] Four main seismostratigraphic units, corresponding to those outcropping in the SRB, have been identified within the basin fill. Isopach maps of the top of the presedimentary substratum and of the four units have been reconstructed and reported in Figures 12 and 13, respectively. Analysis of seismic lines evidenced differences in age of sediments, stratigraphy, and tectonic evolution between the SB and the RB, whose description is treated separately in sections 4.1 and Siena Basin [29] Three of the four units identified in the whole SRB have been identified in the SB, particularly, the second, third, and fourth units (Figure 9). All of them crop out, thus allowing a good control of their age and lithological composition. The basal unconformity separating the basin sediments from the substratum is generally well recognizable, and its depth rarely exceeds 1 s two-way time (TWT) (Figure 9, line 2), being normally 0.8 s TWT or less (Figures 9, 10, and 12). This unconformity exhibits a gentle surface, suggesting widespread erosion, but mainly at the western part of the SB it is strongly displaced and folded by later thrust faults (Figure 12). [30] The overall structure of the SR is a broad syncline, mostly filled by the third and fourth units sediments. The architecture of the basin fill excludes the presence of significant normal faults controlling the sedimentation, while, although rare, only compressive structures have been observed to affect the SB sediments. In the axial zone, the SB is characterized by a NNW trending structural high in the substratum. This structural high has been interpreted as two partially overlapping thrust anticlines, one of which can be traced to the north up to the outcropping Vagliagli anticline, in the southern Chianti Mounts, whereas the other continues to the south into the S. Quirico anticline (Figures 2, 9, and 12). An interesting point concerns the recognition that these thrust anticlines delimited to the east a distinct basin, referred to here as Buonconvento Basin, that was also bounded by the Grotti and Montalcino thrust anticlines to the west (Figures 2 and 12). Second unit sediments filling the Buonconvento Basin are currently sealed by the third or fourth units deposits, as can be imaged in lines 1 to 4 (Figure 13b). These second unit deposits appear strongly tilted, and a secondorder unconformity can be recognized within them (Figure 9, lines 2 and 3). It should be noted that unconformities within late Tortonian-Messinian sediments (second unit) have also been recognized in the along-strike Casino [Lazzarotto and Sandrelli, 1977] and Velona [Bonini et al., 1999] basins, that most likely represent the remnants of the broader Buonconvento Basin (see also Figure 12). [31] Third and fourth units deposits constitute the most part of the SB fill and unconformably overlie the second unit sediments as well as the substratum (Figure 9, lines 2 and 3). Gentle folding affects the deposits of the third and fourth units in the syncline basin(s) between the axial thrust anticline at the southern end of the SB, such as at the Asso syncline in line 7 (most of the original seismic line 7 and the interpretative line drawing are given by Bonini et al. [2001]). Notably, the major structures in the SB fill inferred from the seismic interpretation (i.e., Grotti, S. Quirico, and Vagliagli thrust anticlines) show a good correlation to those observed in the field (compare Figure 2 with Figure 12). The isopach map reported in Figure 13c reveals that the third unit deposits apparently formed a basin isolated from the deeper RB, although a strong erosional event that could have removed a significant part of the sediments cannot be excluded. Similarly, the isopach map of Figure 13d shows that fourth unit sediments are currently confined between Siena and Pienza, that roughly correspond to the outcropping area of this unit (Figure 2). Sediments of the fourth unit show a characteristic progradation toward the NE, which most probably is marked on the seismic lines by the sandy levels widely outcropping in this SB sector (Figure 9, line 4) Pienza Threshold [32] The so-called Pienza threshold is imaged in seismic profiles as a structural high in the substratum separating the SB to the north from the RB to the south; only fourth unit sediments unconformably rest on this structure. This threshold conditioned the connection between the RB and the SB (or the former Bounconvento Basin) since the deposition of the first unit, which occurs in the RB but not in the SB (apart from the small outcrop in the Casino Basin, 10 km north of Siena; Figures 2, 10, and 13). Although it played an important role during sedimentation, its structural meaning is still not fully defined. Indeed, the seismic line 8 running along this structure is of poor quality and does not show significant structural features. In any case, it is likely that the Pienza threshold had a complex cinematic history. It presumably constituted a transfer zone delimiting to the north an

13 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-13 Figure 9. Line drawings of selected seismic profiles (location in Figure 2) through the Siena Basin. Heavy lines indicate faults, medium-heavy lines represent main unconformities, and thin lines are the picked reflectors. Boxes on lines 2 and 3 indicate details of the original seismic lines (seismic lines courtesy of FINA Italiana S.p.A.). The interpreted sedimentary units are indicated by the abbreviations 1st to 5th. The shaded area is the pre-srb substratum. important normal fault, well developed in lines 11 and 12, that controlled the sedimentation in the RB during the deposition of the second unit (Figures 11 and 13; see also section 6). Later, or partly contemporaneously with the activity of the above mentioned normal faults in the RB, this structure was reactivated in compression, as it is demonstrated by the reverse faults affecting the second and third units in correspondence of this threshold (Figures 9 and 13) Radicofani Basin [33] The general structure of the RB is an open synform, except along lines 11 and 12 where an important normal fault occurs at its

14 1-14 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES Figure 10. Line drawings composing the longitudinal seismic line 15 (location in Figure 2) extending across the whole Siena-Radicofani Basin. The box indicates a detail of the original seismic line (seismic line courtesy of FINA Italiana S.p.A.). Patterns and symbols as in Figure 9.

15 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-15 Figure 11. Line drawings of selected seismic profiles (location in Figure 2) through the Radicofani Basin. Boxes on lines 10 and 13 indicate details of the original seismic lines (seismic lines courtesy of FINA Italiana S.p.A.). Patterns and symbols as in Figure 9.

16 1-16 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES Casino Basin syncline Vagliagli anticline 15 N Siena km Trequanda 4 Buonconvento Cinigiano Basin 5 6 Montalcino S.Quirico 7 d Orcia8 Velona Basin 9 Pienza Montepulciano Normal fault Thrust fault Mt.Cetona 7 Radicofani 1 Syncline axis Anticline axis Seismic line Isopach line (sec. TWT) Well Mt.Amiata 12 Radicofani 1 13 Paglia 1 14 Castell Azzara Figure 12. Neogene-Quaternary isopach map of the Siena-Radicofani Basin showing the structures displacing the basin bottom (or substratum top). Contour lines indicate isopachs in two-way time (TWT) seconds. eastern margin (see below). This shape can also be observed in the longitudinal seismic line 15, showing a sudden deepening of the basin bottom southward of the Pienza threshold (Figures 10 and 13). The RB bottom reaches the maximum depth (1.8 s TWT) approximately in the central zone, whereas it gently rises up to 1.0 s TWT in correspondence of line 14 (Figure 12). The RB fill is composed of sediments belonging to the first, second, and third units; scattered outcrops of the fourth unit occur along the eastern margin of the basin, though they cannot be identified on seismic profiles because of their small thickness (Figure 2). On the other hand, sediments of the first and second units are obscured by the third unit deposits and have been drilled by the Radicofani 1 and Paglia 1 wells (Figures 2 and 4). The first unit sediments, correlated to the late Serravallianearly Tortonian Ponsano Sandstone, have been identified in the most part of the seismic lines crossing the RB. A thickness of 100 m has been encountered at the bottom of the Radicofani 1 well, while elsewhere the thickness commonly does not exceed s TWT (Figures 4 and 13). The reconstructed isopach map shows that the lateral extension of first unit sediments is considerably more restricted than the current outcropping area of the third unit sediments (Figure 13a). Reverse faults deform the first unit sediments in the axial zone and a roughly NNW-SSE trending normal fault displace them at the eastern margin, in the Mt. Cetona area (Figures 2 and 11; line 12). [34] The sediments of the second unit have a major extent, a greater thickness (0.8 s TWT) and show a complex structural setting. An important normal fault is clearly imaged in lines 11 and 12 at the eastern RB margin (Figures 11 and 12). The recognition of wedge geometry in the sediments architecture indicates that this structure controlled the sedimentation of the second unit. However, this extensional structure has only a local extent, not being identified either northward in line 10 or south-

17 0.6 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-17 a) Siena 15 1 st unit isopach map (thicknesses in sec. TWT) b) Siena 15 2 nd unit isopach map (thicknesses in sec. TWT) Trequanda 4 Buonconvento 5 Montalcino 6 S.Quirico Pienza Montepulciano 7 d Orcia Mt.Cetona Trequanda 4 Buonconvento 5 6 S.Quirico Pienza Montepulciano d Orcia Montalcino Buonconvento Basin Mt.Cetona N km Mt.Amiata Paglia 1 14 Castell Azzara Radicofani 1 15 N km Mt.Amiata Paglia 1 14 Castell Azzara 0.6 Radicofani 1 15 c) Siena 15 3 rd unit isopach map (thicknesses in sec. TWT) d) Siena 15 4 th unit isopach map (thicknesses in sec. TWT) Trequanda 3 4 Buonconvento Trequanda 5 Buonconvento 6 7 Montalcino 8 Pienza Montepulciano 5 6 Montalcino 7 S.Quirico Pienza d Orcia 8 Montepulciano Mt.Cetona Mt.Cetona N km Mt.Amiata Castell Azzara N km Mt.Amiata Paglia 1 14 Castell Azzara Radicofani 1 15 Figure 13. Isopach maps of (a) first (b) second, (c) third, and (d) fourth sedimentary units. Structures deforming the sediments are indicated in the corresponding map. Symbols are as in Figure 12. See text for details. ward in line 13 (Figures 11 and 13b). Indeed, in lines 10 and 13 the thickness of the second unit is higher in the RB axial zone and reduces toward both margins, where the deposits are affected by reverse faults (Figure 11). Successive shortening events affected the second units deposits, leading to the inversion of the normal fault and the folding of the associated sedimentary wedge, as well as to the development of a few reverse faults in line 12 (Figure 11).

18 1-18 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES Table 2. Analogue Modeling Parameters a Parameter Model (Bas-Cov.1) Nature Model/Nature Ratio BL density r b,kgm b 0.54 BL internal friction coefficient m DL density r d,kgm b 0 DL viscosity h, Pas b H s /H d 3 3 (H s + H d )/H b Strain rate e, s c Gravity acceleration g, ms Length l, m Stress s, Pa c Time t, s (1 Ma) c t* ¼ Hden vm Rate of displacement v, ms (2.7 mm yr 1 ) c v* ¼ l* t* ¼ l*s* h* a The model to nature dimensionless ratios are indicated in the text with an asterisk. Abbreviations are as follows: BL, brittle layers (sand); DL, ductile layer (silicone). The natural values refer to the sedimentary cover. b Parameters indicating average values. The mean anhydrites viscosity has been estimated for the characteristic décollement depth in the NA hinterland (Figure A1), while the models strain rate in the evaporite layer scale to natural strain rates varying between s 1 (Bas-Cov.1) and s 1 (Bas-Cov.3) [see also Bonini, 2001]. c Ratios calculated according to the other parameters. [35] In the axial part of the RB, a major open to gentle anticline can be imaged affecting the third unit sediments (and its basal unconformity) that show typical onlap relations to the folded underlying second unit sediments (Figure 11; lines 10 13). The core of this fold is locally deeply eroded and some small normal faults caused by crestal collapse can be imaged in the seismic lines (Figure 11, lines 12 and 13). In addition, some thrust-related folds locally complicate this major anticline, such as in line 13 where two oppositely verging reverse faults affect the basin fill lifting the center of the basin and producing synchronous crestal collapses (Figure 11). Notably, these folds accord well with the field observations showing roughly N-S trending open folds in the third unit sediments of the RB (compare structures in Figure 2 with seismic lines in Figure 11). Reverse faults are also imaged to affect the sediments of the third unit, although in general they appear less deformed than those of the older units (see Figure 11). [36] The possibility to observe a complete transect across both the RB and the Chiana Valley Basin (CVB) is offered by line 10, which cuts the Cetona-Rapolano anticline immediately south of the Burano Formation outcrop at Chianciano (Figures 2 and 11). Deposits filling the CVB show a thick and asymmetric wedge composed of fourth and third units sediments (maximum thickness 1.5 s TWT) generated by an important normal fault bounding the Cetona-Rapolano anticline to the east (Figure 11; line 10). This normal fault is imaged to die out in correspondence with a layer that we attribute to the Burano Formation, below the CVB bottom. In this hypothesis the normal fault could be interpreted either as accommodating the uplift of the Cetona-Rapolano anticline, or as due to the instability and sliding of this thrust anticline above the ductile evaporite Burano formation [e.g., Bonini et al., 2000] (see also Figure 3a). However, the hypothesis that this normal fault represents a second-order structure to the major Cetona-Rapolano anticline is consistent with the observation that the coeval third unit sediments are affected by reverse faults in the eastern RB margin, and the fourth unit sediments are deformed by mesoscopic reverse faults and stylolitic structures at the Mt. Cetona (sites 2, 5, 21, and 22; Figures 2, 3a, 7, 11, 13c, and 13d and Table 1). [37] The fourth unit deposits are unconformably overlain by the Pleistocene continental fifth unit sediments, which show a general dip to the NE and occur at a considerable distance from the normal fault without exhibiting any apparent relation with this structure (Figures 2 and 11, line 10). This observation raises the possibility that the forward shifting of the fifth unit sedimentation area was associated with the uplift of the Cetona-Rapolano anticline during the late Pliocene phase of thrust reactivation. 5. Deep Structure Across the SRB: A View From Analogue Modeling [38] Geophysical data indicate that the crystalline basement in the NA hinterland is shortened by west dipping thrust faults, as imaged in seismic refraction profiles [Ponziani et al., 1995] as well as by the interpretation of magnetic and Bouguer anomalies [Arisi Rota and Fichera, 1985; Cassano et al., 1998]. This interpretation has been confirmed and further improved by the reprocessing and analysis of deep seismic reflection profiles crossing the whole NA fold and thrust belt from the Tyrrhenian Sea to the Adriatic Sea (profiles M12A, CROP03, and M16A) [Finetti et al., 2001]. The imaged structures portray a complex structural pattern composed of basement thrusts and thrust sheets in the sedimentary cover, with the latter locally affected by polyphase thrusting. Tectonic evolution was strongly controlled by the m thick Burano formation [e.g., Calamita et al., 1994], representing a mechanically weak ductile layer interposed between the sedimentary cover and the underlying crystalline basement. This décollement layer allowed the decoupling of the sedimentary cover and transferred

19 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-19 deviatoric stress (σ1 - σ3) Pa Hs Hd 1.5 cm/h Hb 5 cm/h Hs=0.6 cm Hd= cm Plexiglas Squeeze Box Walls SEDIMENTARY COVER (Sand) 10 cm DECOLLEMENT (Silicone) BASEMENT (Sand) Hb=1.2 cm Moving Wall Piston 18 cm cm Figure 14. Geometric model parameters and initial model strength profile. Note that strength of the silicone layer depends upon the strain rate (i.e., v m ). forelandward the deformation produced by the thrusts in the basement [e.g., Bally et al., 1986] (see Figure 3). [39] Evolution of the SRB has been likely controlled by thrusting in the basement, being the SRB located between major basement thrusts: the outcropping MTMR and a buried thrust to the west, and the Cetona-Abetone basement thrust to the east, whose tip is located below the Mt. Cetona Rapolano ridge (see Figure 3). With the aim of investigating the tectonic evolution of this brittle-ductile system, particularly the propagation of deformation along the ductile décollement during compression and the comparison of the resulting structures with those in the NA hinterland, analogue experiments were performed at the former Analogue Modelling Laboratory of the CNR-Centro di Studio di Geologia dell Appennino e delle Catene Perimediterranee of Firenze, Italy Analogue Modeling: Materials and Strategy [40] Principles of analogue modeling have been efficaciously outlined in some recent review papers, such as Davy and Cobbold [1991] and Brun [1999]. The condition necessary for correctly comparing model results to nature is that models must be geometrically, rheologically, and dynamically similar to the natural prototype [Hubbert, 1937; Ramberg, 1981]. Geometrical similarity can be achieved if both model and natural prototype share the same angles and the same ratios between thickness and length of layers with homogeneous rheological behavior composing the natural system. In the study area the natural prototype can be schematically subdivided into three layers with different mechanical behavior. Models were designed to simulate this vertical rheological stratification given by a ductile décollement layer (the Burano formation) separating two layers with brittle behavior (the crystalline basement and the sedimentary cover). To this purpose, transparent SGM36 silicone putty simulating the décollement layer (with density r d = 965 kg m 3 and dynamic shear viscosity h = Pa s 1 )[Weijermars, 1986], was embedded within two layers of frictional material (dry quartz sand, with r b = 1350 kg m 3, m = 0.6, j 31, and cohesion c 40 Pa) representing the crystalline basement and the sedimentary cover (Table 2). Silicone SGM36 exhibits a Newtonian behavior for strain rates lower than s 1 [Weijermars, 1986], while the dry quartz sand obeys the Mohr- Coulomb criterion of failure, such that its strength is strain rate independent. Both materials represent convenient analog, providing the conditions for rheological similarity to be fulfilled [e.g., Weijermars et al., 1993] (for scaling, see also Appendix A). The models illustrated and discussed here had a sedimentary cover/ décollement density ratio of 1.4 (Table 2), which is greater than that of the natural prototype estimated to be around 1 (as anhydrites significantly contribute to enhance the décollement density). We deliberately introduced this model rheology to increase the diapiric potential for investigating the suspected diapiric component supposed for some thrust anticlines, characterized by the outcropping of (lighter) gypsum at their core (i.e., at the Rapolano-Cetona, Montisi, and Zoccolino anticlines). However, no significant diapirism was observed in the models, probably because it was not sustained by erosion. [41] The Newtonian behavior of the silicone can be expressed by t d ¼ h _g d ¼ h v m ; ð1þ where t d is the deviatoric shear stress on the viscous layer, h is the dynamic viscosity, and _g d is the engineering shear strain rate, which can be computed dividing the velocity of the piston v m by the thickness of the ductile layer H d [e.g., Weijermars, 1997]. As deformation in the décollement layer is nearly simple shear [e.g., Sans et al., 1996], strain rate in the models has been approximated to the engineering shear strain rate, such that _g d e m [e.g., Brun, 1999]. [42] Models were built in a Plexiglas squeeze box with internal dimensions of 24 cm 10 cm 10 cm. Models had a initial length of 18 cm and height of 2 cm: Height H s of the sedimentary cover was 0.6 cm, while height of the décollement H d and height of the basement H b were cm and 1.2 cm, respectively (Figure 14). These values were chosen to maintain the geometric ratios among the thicknesses (obtained from geophysical and stratigraphic data) of the three main layers composing the natural prototype. To investigate the effect of brittle/ductile coupling (ratio of brittle to ductile strength) [Allemand, 1988], three series of models were shortened at constant rates (v m )of5cmh 1 (series Bas-Cov.1), 0.75 cm h 1 (series Bas-Cov.2), and 1.5 cm h 1 (series Bas-Cov.3) by a piston driving a rigid wall into the model (Figure 14) (see also Appendix A). For simplicity, model strain rates have been computed at the onset of deformation. From (1), e m varied from 6.25 H d

20 1-20 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES Figure 15. Cartoon showing the progressive deformation of series Bas-Cov.3, shortened at the rate of 1.5 cm h 1. (a) Initial stage, (b) after 5% bulk shortening (BS), (c) after 21% BS, and (d) after 31% BS. The central part of the models was sectioned to expose the longitudinal cross sections illustrating the pattern of structures, notably the outward propagation of folding along the décollement layer accommodating the shortening of the basement. Most probably these folds evolved from detachment to fault-propagation folding mechanisms [see Storti et al., 1997], and the silicone at the folds core might have become partly diapiric if erosion were included in the model deformation. Numbers denote the fold and model basement thrusts (T) in order of appearance.

21 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-21 differential stress prior to T4 development (σ 1 - σ 3 ) Pa residual peak evaporite-cored mainly due to T4 anticline (4) (4) relative low peak λ 2 (3) evaporite-cored anticline (3) (2) (1) T1 0 T2 displaced roof thrust T4 T3 floor thrust Bas-Cov.3 5cm λ 1 Figure 16. Distance between thrusts in the basement sand (l 1 ) and between thrust-related folds in the sedimentary cover (l 2 ) of model Bas-Cov.3 at 31% BS. Differential stress (s 1 s 3 ) has been roughly estimated at the base of the basement sand prior to the development of basement fault T4, whose deformation has been restored in the basement sand but not in the complexly deformed sedimentary cover. Note the striking coincidence between the relative minor peak (between folds 3 and 4) in the differential stress distribution and the central part of fault T4, where brittle failure nucleate and then propagate both up and down. This may suggest that structures in the sedimentary cover might partly influence the position, or the trajectory, of a successive basement thrust, such as T s 1 to s 1 ; thus the material was in the range of Newtonian behavior Results of Modeling [43] Models were shortened up to 30%, bulk shortening (BS), which is very similar to the finite bulk shortening estimated for the basement in the NA hinterland [Finetti et al., 2001]. However, to investigate the progressive deformation and evolution of structures in the models, each series was composed of two other models shortened up to 10% BS and 20% BS, respectively. After deformation, each model was covered by dry white quartz sand to preserve the final surface morphology, before the model was soaked in water, frozen, and cut to expose longitudinal cross sections. All the three series of models showed a substantially equivalent evolution, such that results of modeling are illustrated below by describing the series Bas-Cov.3 (with v m = 1.5 cm h 1 ), which has been chosen as representative for the deformation style. [44] Model evolution is illustrated by the line drawing of sequential longitudinal cross sections reported in Figure 15. Shortening of the model resulted in the development of thrust faults in the basement sand, while the cover sand exhibited various patterns of folding and faulting associated with lateral and vertical displacements within the décollement layer. Thrust faults in the basement sand typically propagated into the décollement, where they turned into a shear zone. As a result, detachment fold structures [Jamison, 1987] developed in the foreland at a considerable distance from the corresponding thrust fault in the model basement. In models performed with lower brittle-ductile coupling (i.e., those deformed at lower strain rates; series Bas-Cov.1 and 2), this distance typically increased [see also Bonini, 2001]. Therefore the sedimentary cover was strongly detached from the underlying basement. This style suggests that deformation of the décollement layer can be roughly referred to a simple shear model, being the silicone sheared between a stationary base, given by the basement sand top, and a moving upper wall represented by the detached cover sand (see section 5.1). [45] The architecture of the basement thrust sheets array can be described as imbricate fan thrusts, being thrusts emanated from the floor thrust. Concerning the development of the detachment folds, with increasing shortening one limb was eventually sheared and faulted, such that deformation progressed from detachment folding to thrust tip folding, to thrust ramp folding [e.g., Storti et al., 1997] (see fold denoted by number 3 in Figure 15). Because fold and thrust belts developed above a décollement are typically characterized by no uniform vergence of structures [e.g., Davis and Engelder, 1985, 1987], folds and thrusts in the models displayed either foreland or hinterlandward vergence, though the latter appeared to be more developed in models shortened at low strain rates (i.e., series Bas-Cov.1 and 2). [46] Analysis of the sequential evolution of Figure 15 reveals a complex interference between structures in the cover and those originated in the basement. In particular, movement along the basement fault T3 transferred shearing along the décollement producing detachment folds 3 and 4. Successively, basement fault T4 developed forelandward of T3, and upward propagation of fault T4 interfered with horizontal shearing along the décollement generated by T3. This interference resulted in the concentration of deformation above the T4 thrust tip, such that the former fold 4 was squeezed laterally, and other detachment folds (cf. with anticlinal salt walls) developed ahead (Figures 15d and 15e). This evolutionary path, characterized by early detachment folds deformed by successive basement-related structures, may represent a possible way to generate superposed folding events in fold and

22 1-22 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES thrust belts developed above a décollement layer when the basement is also taking part to the deformation. [47] Distance between thrust faults in the basement (l 1 5 cm) is 2.4 times that between detachment folds in the sedimentary cover (l cm) (Figure 16). To this purpose, analysis of model results may offer additional clues about the relations between basement and cover structures, particularly how one may influence the development of the other. Observation of Figure 16 raises the possibility of a relation between the lateral differential stress at failure in the basement sand, estimated prior to the development of fault T4, and the position of this fault. Indeed, there is a striking coincidence between the relative minor peak in the differential stress distribution and the central part of fault T4 (i.e., the central part of the basement sand layer), which is the area where faulting nucleate and then propagate both up and down [e.g., Storti et al., 1997] (Figure 16). In this view, the detachment folds in the cover (and the amount of propagation along the décollement) might have partly controlled the position, or simply the trajectory, of the basement thrust T4. Obviously, the relationship between structures in the cover and those in the basement is certainly more complex in reality because other parameters should be involved in the problem, such as erosion and synshortening sedimentation or redistribution of sand, which would significantly affect the lateral distribution of differential stress. However, we draw attention to the fact that though the major control exerted by basement structures on those developing in the cover is indubitable, structures in the cover may also partly contribute to the localization of structures in the basement. [48] Another characteristic of these laboratory models is the development of potential sedimentary basins between thrust-related folds (see Figure 15). The current series of models did not incorporate synshortening erosion or sedimentation within these basins, though both features were partly simulated by the spontaneous hanging wall collapse that resulted in the deposition within the footwall (i.e., within the basins; Figure 15) Comparison of Model Results With Nature [49] Before comparing the results of modeling to nature, the main limitations of the method must be briefly outlined. Analogue modeling involves some important simplifications, such as temperature and rheology variation during deformation. Additionally, since our models were performed above a rigid base, approximately representing the brittle-ductile interface in the crust, we precluded both foreland flexure and isostatic compensation of the experimental fold and thrust belts. However, our models were designed to explore the deformation of a basement cover system, which is at upper crustal levels where the thermal gradient is low. Indeed, by comparing analogue models with numerical modeling, Storti et al. [2000] showed that at these structural levels the above limitations are negligible, and that analogue modeling has the advantage of offering a better resolution of the resulting brittle and ductile structures. [50] Analogue modeling techniques have been indeed widely and successfully applied to the study of the mechanical behavior and evolution of fold and thrust belts [e.g., Davis et al., 1983; Malavieille, 1984; Mulugeta and Koyi, 1987; Mulugeta, 1988; Colletta et al., 1991; Davy and Cobbold, 1991; Liu et al., 1992; Cobbold et al., 1993; Burg et al., 1994; Letouzey et al., 1995; Gutscher et al., 1998; Koyi et al., 2000; Storti et al., 2000], demonstrating the potentiality of this method in the investigation of these processes. In this study, analogue modeling was performed (1) to examine the relations and the interplay between thrusts affecting the basement and structures deforming a sedimentary cover resting on a décollement layer and (2) to compare the experimental results with the SRB area in the NA hinterland. [51] The current series of models can be compared with the structural style of the NA, which can be referred to as an evaporite-based fold and thrust belt. In particular, we compare the basement-cover and basin styles obtained in these laboratory models with those observed in geologic profiles across the study area. Although our models greatly simplify the complexity of the SRB area, the major structures observed in the models simulate remarkably closely those interpreted from seismic reflection data and field investigations (Figure 3). In particular, both distances l 1 and l 2 (see Figure 16) show a good agreement with wavelengths inferred for the sedimentary cover and the upper brittle crust in the NA hinterland. To facilitate comparison, a simplified version of the geologic profile AB across the study area is shown together with scaled model cross sections in Figure 17. Appropriate scaling to nature of distances l 1 and l 2 results in l 1 30 km and l km. Scaled distance l 1 is remarkably equivalent to that imaged in deep seismic profiles of the Northern Apennines [e.g., Finetti et al., 2001], while scaled distance l 2 shows a substantial similarity with thrust-related structures in the sedimentary cover (Figure 17; compare also Figure 16 with Figure 3a). [52] This similarity, together with careful analysis of cross sections of both models and nature, suggests that the SRB area evolved in a similar way to the evolutionary model of basement versus cover structures portrayed in the modeling results. At a first approximation, analogue modeling indicates that deformation of this brittle-ductile system resulted from superposition of two wavelengths: a longer (30 40 km) wavelength associated with deformation of the upper crust, and a shorter (10 km) wavelength associated with deformation of the sedimentary cover [see also Boccaletti and Sani, 1998]. These values are markedly similar to those reported for other areas [e.g., Nikishin et al., 1993; Cloetingh et al., 1999], as well as with those resulting from analogue and numerical models [Burov et al., 1993; Martinod and Davy, 1994; Cloetingh et al., 1999]. [53] Longitudinal models cross sections show a very similar structural style to that observed in the NA hinterland, particularly the decoupling of the sedimentary cover over the basal décollement (the evaporitic Burano Formation in nature), which is mostly caused by shortening of the basement. Indeed, the most striking observation from the models is the propagation of deformation along the décollement, such that models results support the hypothesis that thrusting in the basement is transferred horizontally along the basal décollement, giving rise to detachment folds and fault propagation folds which may control the evolution of hinterland basins that may eventually develop in between. Thrust-related folds observed in the models can be easily compared with the evaporite-cored thrust anticlines inferred in the study area on the basis of seismic profiles, field observations and drilling data (see Figure 3). [54] Analogue modeling also provides additional clues for the interpretation of other structural features characterizing the study

23 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-23 Figure 17. Comparison between the structures in the SRB area (derived from interpretation of deep and commercial seismic lines and surface geology) and the results of physical modeling, showing (a) a simplified version of the profile AB crossing the northern Apennines (NA) hinterland (simplified from Figure 3a), and (b) line drawing of (mirrored) model Bas-Cov.3 at 31% BS. In Figure 17b, a hypothetical, but realistic, erosion level has been drawn. Note the close similarity of structures between model and nature. area, such as the local outcropping of Burano formation rocks at the core of the Rapolano-Cetona anticline (SW to Chianciano; Figure 2). On the basis of model results it is suggested that this setting might be a consequence of detachment folding along the décollement and fault propagation folding generated by an underlying basement thrust (see Figure 17). This results in the accumulation of evaporites that could be also characterized by suspect diapiric uprise, as suggested by the unconformity relations exhibited by the Burano evaporites with respect to the country rocks composed of Ligurian Units (see Figures 2 and 3a). The elevated position of the evaporite walls in the correspondent structure of the model suggests that if erosion was involved during model deformation, it would have sustained diapirism and extrusion of the silicone to the models surface (see Figure 17b). Also, note that both in model and nature the basement thrust tip (i.e., the Abetone-Cetona thrust in nature and thrust T4 in the model) is characteristically located below this anticline (Figure 17). [55] These insights provided by the analogue modeling have been obviously taken into account in the construction of the presented geological cross sections (Figure 3). Indeed, results of modeling can also offer useful suggestions for extrapolating downward cover structures, whose geometry would be rather enigmatic at depth if based on the surface geology only. Model cross sections can be effectively compared to seismic profiles to interpret structures at depth, where the seismic record is generally of poor quality [e.g., Colletta et al., 1991]. Therefore analogue modeling may represent a useful tool for the construction and the balancing of geologic profiles [e.g., Koyi and Teixell, 1999]. In the study area these lessons may be applied to the Cinigiano and Velona syncline basins, which are peculiarly bounded by west verging (Cucco and Montalcino) and east verging (Labbro and S. Quirico) thrust anticlines on the western and eastern borders, respectively (see Figure 2). Modeling results give an effective solution for the interpretation of the deep structure of these basins that might be

24 1-24 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES not so obvious from observation of the surface geology only. In particular, the potential syncline basin exposed in model cross sections of series Bas.Cov.2 (and Bas.Cov.1) is characterized by a peculiar U-shaped thrust sheet geometry, which is likely associated with the strong decoupling of the sedimentary cover above the basal décollement layer in front of the imbricate basement fan thrusts (compare surface geology reported in Figure 2 with model Bas-Cov.2 in Figure 18; see also section CD in Figure 3 and sections of Bonini et al. [1999]). [56] In conclusion, analogue modeling results highlight (1) the efficiency of a ductile décollement in transferring forelandward the shortening generated by thrusts in the basement and (2) the complex superposition relations between thrusting in the basement and deformation in the cover. Modeling results exhibit a rather good matching of structural style to that of the NA hinterland, such that we consider the application of these results to the NA hinterland as realistic; thus we are confident that these results could also be applied to other natural cases. Finally, we remark on the validity of analogue modeling as a powerful tool for giving convincing interpretation of some types of structures, whose geometry at depth could not be simply solved by seismic profile interpretation or by the extrapolation of surface structures. Figure 18. (a) Photograph and (b) interpreted structures of cross section of model Bas.Cov.2 (deformed at a rate of 0.75 cm h 1 ). Note the potential syncline basin filled by debris collapsed during shortening and the U-shaped thrust sheet geometry. The pattern of structures portrayed by the model can be used to hypothesize the deep structures of the Cinigiano and Velona syncline basins. In Figure 18b, structures bounding the Cinigiano Basin are indicated above the correlated structures in the model (compare with Figure 2). 6. Discussion: Implications for the Evolution of the Tyrrhenian-Apennines System [57] The tectono-sedimentary evolution of the Siena-Radicofani Basin can be considered as indicative of the tectonics that affected the NA hinterland area, this basin being extended over a considerable part of this chain sector (see Figures 1 and 2). The SRB evolution accords with the hypothesis of a compressional origin for the NA hinterland basins (see sections 3 and 4) although it introduces more complexity to the tectonic scenario. In particular, the reconstructed SRB evolution shows that the eastward propagation of extension was not a continuous process but alternated with periods dominated by contraction (see Bernini et al. [1990] and Boccaletti et al. [1992]; see also Hippolyte et al. [1994a, 1994b] for the Southern Apennines). This involves the Tyrrhenian-related extension affecting most of the compressional basins (such as the RB), but in time it was again followed by compression. Additionally, analysis of the SRB evolution suggests that extension did not propagate eastward synchronously, but was controlled by transfer zones that eventually delimited along-strike sectors deformed by different stress fields. To illustrate this evolution, which is of basic relevance for understanding the complex tectonic history of the hinterland, we use the reconstructed SRB history and that documented for the other basins to schematically indicate the main structures active during six selected phases of the Tyrrhenian-Northern Apennines evolution (Figure 19). [58] However, the time-space distribution of structures in the NA hinterland, particularly the vicinity between compressional and extensional structures, requires the definition of a geodynamic model able to explain such a process. In our opinion, these observations fit well a regional model relating the evolution of the NA to a roughly NNW directed Africa (Pelagian) indentation producing the lateral extrusion of the chain and the contemporaneous opening of the Tyrrhenian Basin in the inner zone [e.g., Tapponnier, 1977; Boccaletti et al., 1982; Faccenna et al., 1996; Figure 19. (opposite) Schematic maps illustrating the main structures active during six selected periods of deformation. This evolutionary model shows the inferred position through time of the main thrust fronts as well as the expansion of the Tyrrhenian-related extension in the hinterland (ages of structures and magmatic rocks are based on Bigi et al. [1991], Vai [1987, 1989], Serri et al. [1993], Bendkik et al. [1994], Boccaletti et al. [1995, 1999], Bonini [1998, 1999], Bonini et al. [1999], Moratti and Bonini [1998], and Savelli [2000]). MTMR, mid- Tuscany metamorphic ridge. Basins are as follows: RV, Radicandoli-Volterra; CB, Cinigiano; E, Elsa; BC, Buonconvento; RD, Radicofani; SI, Siena; A, Albegna; F, Fine; M, Magra; MB, Montebamboli; CA, Camerino; AO, Aulla-Olivola; TB, Tiber; SP, Spoleto; FI, Firenze; MU, Mugello; CA, Casentino; GU, Gubbio; BO, Bologna; VM, Val Marecchia; C, Chiana; CO, Compiano; RI, Rieti; FU, Fucino; and G, Grosseto. Magmatic rocks are SS, Sisco; E, Elba; CA, Capraia; MC, Montecristo; ER, Etruschi Ridge; PA, Porto Azzurro; GI, Giglio; SV, San Vincenzo; LA, Laiatico; OR, Orciatico; RS, Roccastrada; R, Radicofani; and VU, Vulsini.

25 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-25

26 1-26 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES Figure 20. Schematic block diagrams portraying the kinematics of the NA hinterland in terms of competing Tyrrhenian extension and lateral extrusion processes for the phases indicated with A, B and D in Figure 19. Crustal profile is adapted from Finetti et al. [2001]. Surface structures are the same as those of Figures 19a, 19b, and 19d. See color version of this figure at back of this issue.

27 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-27 Mantovani et al., 1996] (see also Figure 1). Comparison of the hinterland tectonics reconstructed in Figure 19 with the interpreted deep seismic profiles crossing the whole NA fold and thrust belt [Finetti et al., 2001] allows the proposing of a model relating hinterland tectonics to competing Tyrrhenian extension and lateral extrusion (shortening) processes. In this model, which may also account for the alternated phases of compression and extension affecting the NA hinterland during the Neogene, the hinterland evolution is controlled by the activity of crustal structures whose kinematics reflects the prevailing mechanism of deformation (i.e., Tyrrhenian extension or lateral extrusion). [59] Three schematic block diagrams shown in Figure 20 illustrate the proposed evolutionary model of the system. The crustal profile shown in Figure 20c is a simplified version of the interpreted current crustal configuration of the NA hinterland reported by Finetti et al. [2001], while the lithospheric structure shown in Figures 20a and 20b is broadly retrodeformed. The deep structure of the NA hinterland is characterized by a marked uplift of the asthenosphere top in the Tyrrhenian area, approximately below Elba Island, whereas to the east the continental crust is dominantly affected by west dipping thrusts that may eventually expose basement rocks at the surface, like at the Mid- Tuscany metamorphic ridge (Figures 2 and 20). Further east, below the main Apennine watershed, first-order lithospheric thrust faults double the Moho and propagate far to the east up to the Adriatic foreland basin [Finetti et al., 2001]. These structures presumably correspond to the upper part of the west dipping subducting Adriatic lithosphere that has been hypothesized on the basis of the occurrence of relatively deep seismicity [Amato and Selvaggi, 1992; Doglioni et al., 1998]. Here the major east dipping normal fault related to active extension by Boncio et al. [2000] and also clearly imaged in the profile CROP03 (but not shown in Figure 20c because it developed in late Pliocene-Quaternary times) is instead interpreted as accommodating the uplift of a major thrust-induced basement culmination below the main NA watershed [Finetti et al., 2001]. [60] In the hinterland, the two most important thrust faults are notably rooted in correspondence of the asthenosphere uplift and cut through the whole lithosphere exhibiting a rather flat trajectory within the upper brittle crust (Figure 20). Several splay thrusts are also imaged to emanate from these major structures. The basement tip of the most external (and most important) thrust fault lies approximately beneath the Rapolano- Cetona Ridge (bounding the investigated SRB to the east), such that this structure corresponds to the Abetone-Cetona thrust of Boccaletti and Sani [1998] (Figure 20). Although these structures were generated by contraction of the lithosphere, some factors indicate that they also experienced a normal faulting reactivation during later periods of (lithospheric) extension. This occasion is indicated by a geometric relationship between the lithospheric layers displaced by the reactivated faults, as well as by the development of newly formed normal faults that appear to originate at depth along these structures. Extensional reactivation of the lithospheric thrust faults likely occurred to accommodate the uplift of the asthenosphere, which was in turn associated with the Tyrrhenian crustal opening. In this scenario, the east dipping normal faults reported by Jolivet et al. [1998] could represent antithetic faults to these extensionally reactivated west dipping crustal thrusts. [61] However, interpretation of the deep seismic profiles cannot provide a reliable resolution for constraining the timing of activity of crustal structures unless it is integrated by more detailed data. In this perspective, analysis of the seismic lines calibrated to investigate the SRB structure and field investigations can successfully be used to constrain the timing of activity of the crustal structures (imaged in the deep seismic profiles) that controlled basin evolution. [62] Analysis of the SRB evolution reveals that during the late Tortonian the RB was controlled by a normal fault along its eastern margin; however, this structure cannot be traced northward into the SB, which instead appears to be developed exclusively in a compressional setting (see section 4; Figures 12 and 13). The dissimilar evolution of these two subbasins, that apparently underwent contemporaneous different stress fields, was likely controlled by the Pienza line that separated them (Figure 20b). We speculate that at that time, this structure acted as a right lateral transfer fault accommodating the extensional reactivation of the Abetone-Cetona thrust induced by the asthenospheric uprising in the Tyrrhenian area (Figure 20b). Therefore extension propagated forelandward reutilizing (parts of) structures that had previously controlled the establishment of compressional basins in the hinterland, such as the Abetone-Cetona thrust for the SRB. [63] In this scenario, extensional reactivation of basement thrusts south of the Pienza line could be transferred to the sedimentary cover via the Burano Formation giving rise to low angle normal faults superposing the Ligurian Units above the evaporite layer (Figure 3b). Low-angle normal faulting has been previously invoked to explain this structural setting (i.e., LU above Burano formation) [e.g., Bertini et al., 1991; Carmignani et al., 1994] that is regionally know as Serie Ridotta, but in the mechanism we propose normal faulting is of more limited extent being alternated with compressional pulses. Indeed, Serie- Ridotta-like settings (or younger-over-older relations) can be also obtained by sole out-of-sequence thrusting [Morley, 1988; Roure et al., 1990, 1991; Finetti et al., 2001], a phenomenon that characterizes the study area [e,g., Boccaletti and Sani, 1998]. In any case, the Serie Ridotta (and the out-of-sequence thrust or the low-angle normal fault) appears to be folded or faulted by later compressional structures (see Figure 3b). These considerations raise the possibility that low-angle normal faulting may postdate and/or predate out-of-sequence thrusting, thus introducing even more complexity to the geometric relationship between tectonic units in the sedimentary cover of the NA hinterland. Indeed, after the extensional reactivation of parts of the Abetone-Cetona thrust (phase B in Figures 19 and 20), this structure was reactivated in compression during Messinian and Pliocene times (Figure 20c). Several compressional pulses can be ascribed to this phase that gave rise to out-of-sequence deformations and to new hinterland basins (such as the middle-late Pliocene Upper Valdarno Basin) [Bonini, 1999] at the leading edge of the reactivated structure. [64] The above described evolution is well represented in some seismic profiles of the RB, particularly the lines 11 and 12. In these lines, the deposits of the second unit exhibit a marked wedging against the border normal fault (Figure 21; see also section 4.3). In our interpretation this fault was generated by the horizontal transfer, via the evaporite layer, of the dip-slip

28 1-28 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES Figure 21. (a) Uninterpreted and (b) interpreted version of the migrated seismic line 11 (location in Figure 2) illustrating the structural style of the northern Radicofani Basin. The asymmetric wedge of second unit sediments associated with the late Tortonian half graben was successively deformed (together with the third unit sediments) by shortening events that folded and lifted the axial part of the basin (seismic line courtesy of FINA Italiana S.p.A.). movement along the extensionally reactivated segment of the Abetone-Cetona thrust (Figures 3b and 20b), in a similar fashion to the models described by Nalpas and Brun [1993]. The successive compressional reactivation resulted in the axial folding of the whole sedimentary wedge (Figure 21), as well as in the development of contractional structures both in the basin fill and in the organogenous carbonates (Amphistegina limestone) draping the Cetona-Rapolano anticline (Figures 2 and 7). Reactivation and syndepositional activity of the (blind) thrust delimiting the western RB margin during this strong phase of shortening is inferred by the consistent deformations (i.e., reverse faulting; Figure 13c) as well as by the large amount of olistostromes at this margin (Figure 2). [65] The so-illustrated NA evolution is not consistent with models hypothesizing a continuous and persistent rifting affecting the Tyrrhenian and the NA hinterland areas [e.g., Pascucci et al., 1999] but the proposed evolutionary model may reconcile into a unique scenario different observations that apparently may conflict, such as the crustal extension detected in the Tyrrhenian area [e.g., Jolivet et al., 1998; Rossetti et al., 1999] and the coeval basement shortening in the onshore NA hinterland [e.g., Boccaletti and Sani, 1998], both of which are imaged in the CROP profiles [Finetti et al., 2001]. Obviously, the actual evolution of the NA hinterland is more complex and other factors certainly contribute to this progress. A third factor controlling hinterland evolution may be represented by syntectonic foreland loading at the thrust belt front (marginal loading). Syntectonic loading at the front of the chains is indeed found to inhibit forward propagation of the thrust front and consequently to promote reactivation of hinterland thrusts, as documented both in natural cases (e.g., Pyrenees) [Coney et al., 1996] and in analogue models of thrust wedges [Storti et al., 2000]. Combining sedimentation rates and sediments thickness estimated for each stage along transects 1 3 of Figure 19 results that periods of thrust front retreating and renewed internal deformation with widespread synchronous thrusting follow periods dominated by higher sedimentations rates and sediment accumulation at the leading edge. Apparently, this model applies to the phase of internal thrust reactivation and uplifting that affected the axial part of the NA (including the hinterland) during the late Pliocene (phase E in Figures 19 and 22) [Boccaletti et al., 1992]. This phase was also characterized by a hinterlandward shifting of both the thrust front (along transect 1) [Vai, 1987] and the boundary extension-compression in the hinterland (Figure 22). [66] The close coincidence of fronts retreating (between phases D and E) with former marked increments in sedimentation rate and sediment thickness at the NA front (from phase C to D) suggests that both phenomena be intimately connected (Figures 19 and 22). In this specific case, the increased marginal loading may be associated with higher sediment supply resulting from an increased erosion of the growing Apennines in response to the major Messinian-early Pliocene phase of crustal shortening (see Figure 20c) [Finetti et al., 2001]. This trend is efficaciously illustrated in the diagram of Figure 22, where the peak of sediment accumulation (and sediment thickness) at D is a

29 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-29 Figure 22. Migration through time of both the northern Apennines (NA) thrust front and the boundary between extension and compression in the hinterland. The considered periods are the same as those of Figure 19. Length of the chain sectors affected by extension (dashed line) or compression (solid line) is measured along three distinct transversal transects (1 to 3) reported in Figure 19. Advancing or retreating of the thrust front and of the extensioncompression transition in the hinterland is compared with sedimentation rates and the absolute thickness of sediments at the foreland (marginal loading) for each considered phase (A to F). Sedimentation rates are obtained from Ricci Lucchi [1981, 1986], Dondi et al. [1982], Pieri and Groppi [1981], Bally et al. [1986]. Note the alternated period of compression and extension affecting the Siena-Radicofani Basin (SRB), whose position in the hinterland is indicated by the vertical dashed box. Grayed area indicates the along-strike variation of the extension-compression boundary along transects 2 and 3, which are those encountering the SRB. precursor for the major hinterlandward retreating of compression at E. In the SRB, this phase resulted in the reactivation of the Rapolano-Cetona thrust, which is emphasized by the deformation of the Amphistegina limestone, as well as by the deformation of the Upper Valdarno Basin (see above). [67] The above observations highlight the intimate linking between hinterland and foreland evolution in the development of a fold and thrust belt. This connection is also of basic relevance for the popular critical-wedge models that efficaciously describe the mechanics of fold and thrust belts [Davis et al., 1983; Dahlen et al., 1984; Davis and Engelder, 1985, 1987; Dahlen, 1990]. Particularly, critical-wedge models require the interior of the wedge be deformed as new thrust sheets are accreted at the thrust belt front, thus involving a continued superimposed deformation within the wedge. This requirement accords with the time-space distribution of active structures outlined here for the NA (Figure 19). Indeed, the outward progression of the NA front was closely matched by a persistent activity of the thrusts in the hinterland that locally controlled the basins evolution over a large time interval (Figure 19). This continued tectonic activity distributed in the whole internal area resulted in polyphase thrusting and superposed thrust reactivations events that were often characterized by an out-of-sequence geometry and by distinct stress fields [e.g., Pertusati et al., 1977; Bendkik et al., 1994; Fazzuoli et al., 1998; Boccaletti and Sani, 1998; Bonini, 1999]. 7. Conclusions [68] On the bases of different data sets, an evolutionary model focusing on the tectonic evolution of the Northern Apennines hinterland is here proposed. Seismic data integrating field mapping and structural analysis carried out on both the basin fill and structures in the sedimentary cover that controlled basin development allowed to obtain a better understanding of the tectono-sedimentary evolution of the Siena-Radicofani Basin (SRB) and adjoining areas. Similarly, the results of an analogue modeling study have provided helpful indications about the deep structure of the SRB, particularly the relations between deformation in the sedimentary cover and the thrusting in the basement, that is imaged in the deep seismic profile crossing the investigated area (CROP03) [Finetti et al., 2001]. These findings suggest the following main conclusions: 1. Both seismic data and mesoscopic structures affecting the late Miocene-Pliocene sediments support that the SRB evolved in a compressional setting dominated by the activity of thrust anticlines bounding the basin, though Tyrrhenian-related extensional structures are also found to locally control sedimentation (i.e., the second

30 1-30 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES l m /l n = (subscripts m and n refer to model and nature, respectively). This implies that our models simulate a 7.2 km thick basement, a 1.2 km thick dècollement, and a 3.6 km thick sedimentary cover, in good agreement with the values inferred for the natural prototype (i.e., H s /H d 3; see Figure 3a). The stress ratio is therefore s* =s m /s n = r*g*l* =9 10 7, assuming an average density of 2500 kg m 3 for rocks of the sedimentary cover and g* = 1, being models deformed in a normal gravitational field (Table 2). [70] To determine dynamic similarity, we use the procedure outlined by Sokoutis et al. [2000]. We start by assuming that both model and natural prototype share the same Ramberg number Rm exp, representing the ratio of gravitational to viscous forces acting on the viscous layer (Rm exp = r m gl 2 m/h m v m ) [Ramberg, 1981; Weijermars and Schmeling, 1986], we then estimate the natural bulk viscosity h n of the natural décollement layer using the experimental Ramberg number Rm exp [Sokoutis et al., 2000] e h n ¼ ngh 2 d v n Rm exp : ða1þ Figure A1. Temperature or depth dependence of viscous strength of selected rocks and minerals (at constant strain rate of s 1 and geothermal gradient of 30 C km 1 (redrawn after Davis and Engelder [1987], Dahlen [1990], and Weijermars [1997]). Anhydrite yields strength of 8 18 MPa at the average depth of the basal detachment in the NA hinterland (4 5 km), such that a mean effective viscosity for ductile deformation [e.g., Turcotte and Schubert, 1982] can be estimated in the order of Pa s 1. unit in the Radicofani Basin). Normal faults may also represent second-order structures accommodating the thrust activity or recent and active faults, like at Mt. Cetona-Pienza area (Figure 2) [Sani et al., 2001]. 2. On the basis of the present study and other data reported in the literature, a reconstruction of the main structures active in the Northern Apennines-Tyrrhenian system since the middle-late Tortonian has been attempted. In this reconstructed scenario, after the development in compression of the basins the hinterland area experienced alternate periods of compression and extension. 3. The results of the analogue modeling may have relevance on the relations between basement thrusting versus cover thrusting in the NA hinterland. These results highlight the importance of the basal evaporite décollement layer (Burano formation) in accommodating the shortening in the basement and offer clues for the extrapolation at depth of surface cover structures (i.e., the Cinigiano Basin). 4. Finally, we propose that the evolution of the NA hinterland, characterized by the backward and forward movement of the extension-compression boundary, resulted from the interplay among three main factors: (1) Tyrrhenian-induced extension, (2) lateral extrusion of the NA chain and of the Adriatic continental crust, and (3) marginal loading at the foreland, that might trigger important thrust reactivation in the axial zone. Appendix A: Scaling of Analogue Models [69] Models are conveniently scaled such that 1 cm in the model represents 6 km in nature (see Table 2), implying a length ratio l*= [71] A natural bulk viscosity of h n = Pa s 1 can be estimated applying equation (A1) to the series Bas-Cov.1 (similar values of h n can be also obtained for series Bas-Cov.2 and 3), taking v n = 2.7 mm yr 1 (the correspondent scaled displacement rate; see below), the correspondent Rm exp = 0.60, r n = 2400 kg m 3 (for the décollement) and H d = 1200 m. This h n estimate represents a somewhat high value for salt viscosity, which typically ranges between and Pa s 1 [e.g., Jackson and Talbot, 1986; Van Keken et al., 1993]. However, it can be considered fairly realistic for the Burano formation, being that this décollement is mainly composed of anhydrites yielding an effective viscosity [e.g., Turcotte and Schubert, 1982] in the order of Pa s 1 for characteristic décollement depths (4 5 km; Figure A1). Therefore we consider that the initial assumption of dynamic similarity between models and the natural prototype is justified by the very similar values of décollement viscosity obtained by equation (A1) and those estimated for the natural prototype in Figure A1 [e.g., Sokoutis et al., 2000; Bonini, 2001]. As sedimentary cover and crystalline basement in both model and nature show a similar brittle behavior, we extrapolate dynamic similarity to the whole brittle-ductile system. [72] Following Merle and Abidi [1995], the velocities in the models can be scaled down to natural conditions by v* ¼ v m v n ¼ e*l*; ða2þ implying that v n is very sensitive to the chosen length ratio l*. The scaled natural velocity v n can be estimated from e* =s*/h* =9 10 9, where s* = and h* =10 16 (h m = Pa s 1 and h n Pa s 1 ), obtaining v* = 14,400. From (A2) one can estimate that v n ranges between 9 mm yr 1 (v m =1.5 cm h 1 ) and 2.7 mm yr 1 (v m =5cmh 1 ), which compares well with values observed in natural thrust systems [e.g., Kukal, 1990; Calamita et al., 1994; Zapata and Allmendinger, 1996]. [73] Acknowledgments. Journal reviewers J. Dewey, L. Jolivet, and European Editor F. Roure are warmly thanked for the constructive

31 BONINI AND SANI: EXTENSION AND COMPRESSION IN THE APENNINES 1-31 comments that helped to clarify several points. FINA Italiana S.p.A. (currently TOTAL FINA ELF ITALIA S.p.A.) and R. Pasi, J. Staffurth, P. Dattilo, and C. Turrini are gratefully acknowledged for the kind permission to publish the original seismic lines illustrated in this work. A. W. Bally is gratefully acknowledged for the useful discussions during the early stages of seismic lines interpretation. Research funded by MURST (provided by P. Manetti, 1999, and by F. Sani, 2000), and CNR grants. References Abbate, E., V. Bortolotti, P. Passerini, and M. Sagri, Introduction to the geology of the Northern Apennines, in Development of the Northern Apennines Geosyncline, edited by G. Sestini, Sediment. Geol., 4, , Allemand, P., Approche experimentale de la mécanique du rifting continental, Mem. Doc. Cent. Armoricain Etud. Struct. Socles, Rennes, 38, 192 pp., Amato, A., and G. 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34 TECTONICS, VOL. 21, NO. 3, /2001TC900024, 2002 Figure 20. Schematic block diagrams portraying the kinematics of the NA hinterland in terms of competing Tyrrhenian extension and lateral extrusion processes for the phases indicated with A, B and D in Figure 19. Crustal profile is adapted from Finetti et al. [2001]. Surface structures are the same as those of Figures 19a, 19b, and 19d. 1-26