Shear zones and magma ascent: A model based on a review of the Tertiary magmatism in the Alps

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1 TECTONICS, VOL. 23,, doi: /2003tc001526, 2004 Shear zones and magma ascent: A model based on a review of the Tertiary magmatism in the Alps C. L. Rosenberg Institut für Geologie, Freie Universität Berlin, Berlin, Germany Received 25 March 2003; revised 15 October 2003; accepted 13 January 2004; published 8 May [1] The Alpine Oligocene plutons are spatially and temporally associated with the activity of the Periadriatic Fault System (PFS), an orogen-parallel, crustal-scale transpressive mylonitic belt. Excellent three-dimensional exposure, combined with a wealth of structural, seismic, petrological, geochronological, geochemical, and paleomagnetic data collected over the last decades help to constrain the relationships between deformation, ascent, and emplacement of the plutons. Magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the Periadriatic Fault System, which was used as ascent pathway to cover vertical lengths of 20 to 40 km. Therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth. Ascent of the melts is controlled by the mylonitic foliation of the PFS, which forms the only steep anisotropy, continuously traversing the entire Alpine crust. In contrast, the flow direction is not influenced by the specific kinematics of the faults. Final emplacement of the plutons occurred by extrusion from the Periadriatic Fault System into the adjacent country rocks. The transition from ascent to final emplacement is favored by partitioning of transpressive deformation. INDEX TERMS: 8010 Structural Geology: Fractures and faults; 8035 Structural Geology: Pluton emplacement; 8102 Tectonophysics: Continental contractional orogenic belts; 8110 Tectonophysics: Continental tectonics general (0905); 8434 Volcanology: Magma migration; KEYWORDS: plutons, emplacement, ascent, shear zones, Alpine magmatism, Alpine tectonics. Citation: Rosenberg, C. L. (2004), Shear zones and magma ascent: A model based on a review of the Tertiary magmatism in the Alps, Tectonics, 23,, doi: /2003tc Introduction [2] Tectonic controls on the ascent and emplacement of magmas have been debated by field geologists since the beginning of the past century. Cloos [1923] suggested that granitic magmas of Variscan age in the Bayerische Wald (Germany) were squeezed upward by regional shortening. This interpretation anticipated models of intrusion within Copyright 2004 by the American Geophysical Union /04/2003TC compressive settings postulated in the past decade [e.g., De Saint Blanquat et al., 1998]. With few exceptions [Paterson and Schmidt, 1999; Schmidt and Paterson, 2000], structural investigations concluded that faults are closely related to plutons in space and time [e.g., Pitcher, 1974; Pitcher and Bussell, 1977; Nicolas et al., 1977; Strong and Hanmer, 1981; Hutton, 1982; Hollister and Crawford, 1986] in any type of tectonic setting, either strike-slip [Hutton, 1982], extensional [Hutton, 1988] or compressive [Davidson et al., 1992; D Lemos et al., 1992; McCaffrey, 1992]. [3] Different causes were inferred to explain the above spatial and temporal coexistence. Interpretations focusing on final emplacement of plutons suggested that (1) the space needed for the plutons is made by displacement along regional, synmagmatic faults [Hutton, 1982]; (2) the site of emplacement is controlled by pre-existing faults [e.g., Pitcher and Bussell, 1977]; (3) shortening in the presence of magmatic bodies induces faults that accommodate further space for final emplacement [Benn et al., 1998]; (4) faults limit the lateral expansion of intrusions, leading to fault-bounded plutons [Roman-Berdiel et al., 1997]. Interpretations concerned with the ascent of magma suggested that ascent takes place (1) along pre-existing faults [e.g., Pitcher and Bussell, 1977]; or (2) along active fault planes or shear zones [e.g., Hollister and Crawford, 1986; D Lemos et al., 1992; Ingram and Hutton, 1994; Rosenberg et al., 1995; Handy et al., 2001; Galland et al., 2003]. A third group of interpretations discussed the spatial coincidence of plutons and faults in terms of melt generation along the fault planes, by either (1) rapid pressure reduction along the deeper parts of fault planes [Pitcher and Bussell, 1977; Pitcher, 1979]; (2) enhanced fluid migration along fault planes, hence lowering the solidus [Strong and Hanmer, 1981; Reavy, 1989]; or, (3) melt generation by shear heating along mylonitic zones [Strong and Hanmer, 1981; Nabelek et al., 2001]. Feedback loop processes involving the different mechanisms described above, were also proposed [Vauchez et al., 1997; Brown and Solar, 1998a; Kisters et al., 1998]. These models considered that strain localizes where melt is present, enhancing in turn further melt flow into the deforming region. [4] The present review shows that spatial, temporal and causal relationships between magmatism and deformation do exist in the Alps. This orogen is particularly suited for the study of such relationships because the excellent threedimensional (3-D) exposures of coeval and cogenetic plutons exhumed from different crustal levels, allow the construction of a synthetic cross section, showing the distribution of plutons and faults down to 30 km depth. This cross section and the numerous geochronological, structural, 1of21

2 Figure 1 2of21

3 Figure 1. petrological and seismic investigations of the past decades, form the basis for a discussion aimed at testing the applicability of current ascent and emplacement models to nature. 2. Periadriatic Fault System [5] The Periadriatic Fault System (PFS) is the most important Tertiary structure of the Alps, striking for more than 700 km along the entire length of the chain (Figure 1a). Overprinting relationships between plutons aligned along the PFS and mylonites formed within the PFS prove that deformation was active during and after magma crystallization, i.e., between 34 Ma and 28 Ma [Rosenberg et al., 1995]. Radiometric dating of synkinematically grown minerals of the PFS mylonites confirmed these ages of activity [Müller et al., 2000, 2001]. [6] The PFS consists of a set of kinematically linked large-scale faults, displaying different kinematics, but all (continued) together, accommodating dextral transpression [Schmid et al., 1989]. The intensity of north-side up movements (Figure 1a) varied along the strike of the chain, from a maximum of 20 km in the central Alps [Trümpy, 1980] to nearly zero in the easternmost part of the chain. Where deeper crustal levels are exposed, e.g., in the central Alps, the PFS forms a mylonitic belt of several kilometers in width (>5 km in the central Lepontine, including the Southern Steep Belt [Milnes, 1974]), whereas at higher crustal levels, e.g., in the Eastern Alps, the PFS is marked by cataclasites of a few tens of meters width [Mancktelow et al., 2001]. The extent of dextral displacements is still a matter of debate. A total offset of up to 300 km was estimated from paleogeographic reconstructions [Laubscher, 1971]. Schmid et al. [1996a] inferred 100 km of dextral displacement from the Oligocene to the present, while Müller et al. [2001] argued for a post-oligocene displacement of less than 30 km. Figure 1. Tectonic map and cross section of the Alps. (a) Simplified tectonic map of the Alps, with enlargements of some segments of the PFS showing the spatial distribution of the Oligo-Miocene dikes and of the Periadriatic plutons. Only the major faults of Tertiary age are shown. Redrawn after Bigi et al. [1990]. Faults are labeled as follows: CL, Canavese Line; DAV, DAV Line; GL, Giudicarie Line; GTL, Gailtal Line; IL, Insubric Line; LT, Lavantal Line; PL, Pustertal Line. Full circles indicate dikes. The size of the dikes is exaggerated by several orders of magnitude. Plutons are labeled as follows: A, Adamello; An, andesites of the Canavese Line; B, Bergell; Bi, Biella; K, Karawanken; L, Lesachtal body; M, Miagliano pluton; P, Pohorje; R, Rieserferner; Re, Rensen; T, Traversella; TL, tonalitic lamellae. Pink shading indicates areas with exposed Tertiary dikes. The x-x 0 is the trace of cross section of Figure 1b. Note that only parts of the dikes are dated. Most of them are inferred to be Oligocene on the basis of petrographic analogy with the dated ones and of crosscutting relationships with their country rocks. (b) Cross section of the central Alps, based on the interpretation of geologic and seismic data. Simplified after Schmid et al. [1996a]. Arrowheads mark the mylonitic belt of the PFS, which is the only steep structure crosscutting the entire crust. The northern boundary of the lower European crust is inferred to be aligned to the PFS at the time of intrusion of the Periadriatic plutons [Schmid et al., 1996a]. See color version of this figure at back of this issue. 3of21

4 [7] The PFS marks the boundary between the Southern Alpine indentor [Ratschbacher et al., 1991] and the Austroalpine Units (Figures 1a and 1b). In the central Alps, the PFS fits with the commonly observed steep zone of backthrusting, bounding the retrowedge of bivergent orogens [Beaumont et al., 1994] (Figure 1b). In contrast, in the Eastern Alps, no significant backthrust along the PFS is observed and its kinematic role is primarily to accommodate eastward extrusion of the orogenic wedge [Ratschbacher et al., 1991]. [8] The NNE-SSW oriented Giudicarie Line, east of the Adamello batholith (Figure 1a) constitutes a major truncation of the PFS. It is a NW dipping, transpressive sinistral fault [Werling, 1992; Prosser, 1998; Viola et al., 2001], whose age is inferred to be Miocene, or Oligo-Miocene [Müller et al., 2001; Viola et al., 2001]. The apparent leftlateral displacement of the Giudicarie Line in map view is 70 km. However, based on different estimates of shortening in the Southern Alpine thrust and fold belt, sinistral displacements of 87 km [Schönborn, 1992] or 30 km [Picotti et al., 1995] were inferred. 3. Tertiary Magmatism in the Alps Figure 2. Age of the Periadriatic plutons, modified and updated after von Blanckenburg and Davies [1995]. Data set based on Barth et al. [1989], Brügel et al. [2000], Dal Piaz et al. [1988], Del Moro et al. [1983b], Deutsch [1984], Elias [1998], Giger and Hurford [1989], Hansmann and Oberli [1991], Martin et al. [1993, 1996], Müller et al. [2000, 2001], Oberli et al. [2004], Romer et al. [1996], Romer and Siegesmund [2003], Stipp et al. [2004], Villa [1983], Villa and von Blanckenburg [1991], and von Blanckenburg [1992]. [9] All Tertiary plutons in the Alps are found in the vicinity of the PFS [Salomon, 1897; Bögel, 1975; Exner, 1976; Dal Piaz and Venturelli, 1983; Laubscher, 1983a] (Figure 1a), hence they were termed Periadriatic [Salomon, 1897]. Laubscher [1983a] suggested that the Periadriatic plutons were emplaced during a phase of orogenic extension. Structural investigations of several plutons [Mager, 1985; Martin et al., 1993; Rosenberg et al., 1995; Steenken et al., 2000; Wagner, 2004] showed, however, that syntectonic ascent and emplacement occurred in a transpressive setting. [10] Magmatism took place during continental collision and pluton ages vary between 42 Ma and 28 Ma (Figure 2). However, except for the southern part of the Adamello batholith all Periadriatic plutons fall in the short time interval Ma (Figure 2), which is coeval with backthrusting and dextral shearing along the PFS. [11] The Periadriatic plutons consist of calcalkaline I-type tonalites and to a minor extent granodiorites. Gabbros, diorites and granites constitute a small fraction of most plutons. Mafic enclaves and basic synplutonic dikes are widespread in most plutons. [12] The 143 Nd/ 144 Nd and 87 Sr/ 86 Sr of all investigated intrusions (Bergell, Adamello, Karawanken, Pohorje) show a typical mixing array between crustal and mantle isotopic values [von Blanckenburg and Davies, 1995; Pamic and Palinkas, 2000]. Mixing of basaltic partial melts with partially melted mafic lower crust, followed by fractional crystallization, gave rise to the dioritic, tonalitic, granodioritic and granitic magmas that formed the Periadriatic plutons [e.g., Thompson et al., 2002]. The geochemistry of ultrapotassic dikes shows an extreme enrichment in Cr, Ni, Sr, and Nd isotopes, attributed to be a primary feature of the mantle source [von Blanckenburg et al., 1998]. Such an enrichment is ascribed to the non-convecting mechanical boundary layer of the lithospheric mantle, since the convecting asthenosphere would dilute and homogenize any enrichment. Therefore melting took place in the lithospheric mantle, and not in the asthenosphere [von Blanckenburg and Davies, 1995]. The latter study inferred that two tectonic scenarios could explain melting restricted to the lithospheric mantle: (1) detachment of the thermal boundary layer [Houseman et al., 1981] or (2) slab breakoff. The latter model was favored, because it explains the alignment of the plutons in map view, inferred to reflect breakoff propagation along the axis of the Alpine chain. [13] A major implication of these geochemical data in terms of ascent and emplacement of the magmas is that the source region of the Periadriatic plutons is constrained to be at the base of the thickened continental crust, i.e., at a depth of 40 to 50 km. These values are consistent with geobarometric data constraining crystallization depths of the early crystallized phases of the Adamello suite [Ulmer et al., 1983]. Models of ascent and emplacement of the Periadriatic plutons must therefore consider melt transport through the entire continental crust. 4. Geometry and Distribution of the Periadriatic Plutons in Map View [14] All Periadriatic plutons, except the southern part of the Adamello batholith, have one margin located at a distance less than 4 km from a segment of the PFS and most of them 4of21

5 are directly in contact with the mylonites or cataclasites of the PFS (Figure 1a). The enlargements of Figure 1a show that there is a large number of small plutons that are located exactly along the PFS. None of the plutons is ever transected by the PFS, which encloses the roots and tails of the plutons, but not their main bodies (Figure 1a). Except for the Adamello batholith and the small Miagliano stock all plutons outcrop to the north (or northwest) of the PFS (Figure 1a). [15] The aspect ratios of the plutons in map view decrease with increasing distance to the PFS (Figure 3a). Post-intrusive shearing did increase these aspect ratios to some extent, as illustrated by the Bergell pluton, whose tonalitic tail was reduced in width by more than half during synintrusive to post-intrusive deformation [Berger et al., 1996]. These aspect ratios measured in map view are probably underestimated since the inferred intrusion direction of the magmas had a strong vertical component. Assuming that the latter underestimation counterbalances the overestimation due to post-tectonic shearing Figure 3 roughly reflects the primary intrusive geometries. These geometries indicate that all plutons with axial ratios larger than 4,5 are directly in contact and parallel to the PFS (Figure 3a), both in map view and cross section. These bodies are steep to subvertical (Figure 3b). In contrast, with few exceptions, plutons outcropping more distant from the PFS have aspect ratios 4 and are not parallel to the PFS in cross section. Although their long axis may strike parallel to the PFS, these magmatic bodies are gently dipping (Figures 3c and 3d), as observed in the roof of the Biella pluton, in the main bodies of the Bergell [Davidson et al., 1996], Rieserferner [Wagner, 2004] and Pohorje [Faninger, 1976; Exner, 1976] plutons. [16] The steep dip of plutons within the PFS and the more gentle dip further away from the PFS is not merely the result of post-intrusive overprint of the plutons by the PFS, but represents the primary intrusive geometry. This conclusion is supported by the preservation of subvertical magmatic foliations adjacent to the fault planes of the PFS [Martin et al., 1993; Rosenberg et al., 1995; Berger et al., 1996; Wagner, 2004], and of gently dipping magmatic foliations in the intrusive bodies that are more distant from the PFS [Davidson et al., 1996; Wagner, 2004]. Note that Figure 3a does not show a trend of progressively decreasing axial ratios away from the PFS. Rather, there are two distinct populations, one characterized by steep elongate sheets along the PFS, and a second one consisting of more circular and gently dipping bodies at distances of >1 km from the PFS. This observation is consistent with the rapid transition from the steeply dipping sheets along the PFS to the gently dipping circular bodies away from it, and it is presently exposed in the Bergell, Rieserferner and Karawanken plutons. 5. Geometry, Ascent, and Emplacement of the Largest Periadriatic Plutons [17] In the following the structures, geometries and emplacement mechanisms of the largest Periadriatic plutons are reviewed. The petrology and geochemistry of these magmas was reviewed by von Blanckenburg et al. [1998]. [18] For the sake of clarity, the term ascent, is used to describe magma flow from the source region to the crustal level of final crystallization, while final emplacement is used to describe the processes, acting at the depth of final crystallization, leading to the specific shape of the pluton [Clemens et al., 1997]. Whether these processes are physically distinct will be discussed in the concluding section of this contribution Biella Pluton [19] The Biella pluton (Figure 1a) is concentrically zoned [Cesana et al., 1976] and consists of a monzosyenite at its rim, grading into syenite, monzogranite and finally granite at its core [Bigioggero et al., 1994]. The pluton is located 1 km away from the Canavese Line (Figure 1a), the westernmost branch of the PFS. The intrusion level is poorly constrained, but inferred to be shallow, nearly subvolcanic [Bigioggero and Tunesi, 1988] on the base of the fine-grained and porphyritic texture formed at the pluton s rim, and of the low Al content of magmatic amphiboles [Bigioggero et al., 1994]. A crystallization depth of <5 km is likely. [20] The gentle outward dips of the northwestern and southwestern contacts suggest that parts of the roof are exposed. The concentric contacts between the different intrusive lithologies are often subparallel to the margin (roof), suggesting a 3-D oignon-like structure with slight updoming of a layered and gently inclined intrusive suite. By contrast, the eastern margin is steep and sub-parallel to the PFS. In this area, magnetic foliations, inferred to reflect magmatic foliations are also sub-parallel to the PFS [Hrouda and Lanza, 1989]). Evidence for stoping is rare and no ductile deformation of the country rocks is observed [Bigioggero et al., 1994] Bergell Pluton [21] The main body of the Bergell pluton is nearly concentrically zoned, with tonalite at the rim, granodiorite in the core, and a transitional zone in between [Moticska, 1970]. Post-intrusive tilting and erosion of the Bergell resulted in the present-day exposure of a 12 km deep crustal section between the eastern and western ends of the pluton (Figures 4a and 4b), as supported by the following observations: All structures plunge to the east [Cornelius, 1915; Staub, 1918; Trommsdorff and Nievergelt, 1983; Rosenberg et al., 1995; Schmid et al., 1996b]; petrologic data indicate a westward, post-intrusive pressure increase of 3 4 kbar [Reusser, 1987; Davidson et al., 1996] and a westward temperature increase [Rosenberg and Stünitz, 2003]; geochronologic data show progressively younger ages from east to west [Villa and von Blanckenburg, 1991], and finally, paleomagnetic data, suggest west-side-up tilting around a N-S striking axis [Rosenberg and Heller, 1997]. Moreover, 3 km of relief through the pluton, allow the reconstruction of the three-dimensional shape of the intrusive body (Figure 4b). [22] The floor of the Bergell s main body is exposed over more than 30 km along the western contact (Figure 4b), and within a tectonic window in the central part of the pluton. 5of21

6 Figure 3. Geometry and orientation of the Periadriatic plutons. (a) Aspect ratios of plutons versus distance to PFS. Open squares indicate steep sheets, subparallel to the PFS. Solid squares indicate gently inclined plutons and main bodies of plutons. Labels are as follows: Bi, Biella; Po, Pohorje pluton; Be, main body of the Bergell; Ri, main body of the Rieserferner. (b) Steeply, north dipping foliation of the tonalitic tail of the Bergell pluton. This tonalitic sheet and its foliation are parallel to the mylonites of the PFS. (c) Gently dipping floor of the Bergell pluton; Val Codera (Italy). Photo, C. Davidson. (d) Gently dipping roof of the Rieserferner pluton; Geltal (Italy). See color version of this figure at back of this issue. This contact is everywhere concordant with a high-temperature, synmigmatitic shear zone, which follows a previous nappe boundary [Diethelm, 1989; Davidson et al., 1996]. The three major lithological units of the pluton are parallel to its floor, suggesting that magmatic flow at the emplacement level was parallel to the gently inclined country rocks. The uppermost part of the exposed pluton (its side ) was intruded immediately below the base of the Austroalpine nappes, whose temperature was less than 300 C at the time of emplacement. As a result, the country rocks above the roof of the pluton behaved as a rigid lid [Laubscher, 1983b] while the migmatitic floor of the pluton was still highly ductile. [23] The southern margin of the pluton, commonly termed the tail, is a 40 km long, steep tabular body of tonalite, sub-parallel to the PFS. The tonalitic tail belongs to the Periadriatic mylonite zone. This tabular body is considered to represent the feeder of the pluton based on the following observations: (1) magmatic foliations and lineations are steep [Berger et al., 1996], in contrast to the rest of the pluton; (2) the orientation of the tonalitic tail itself is steep; (3) this tonalitic tail is the deepest portion of the intrusion, with crystallization depths ranging from 700 MPa in the east to 850 MPa in the west (Figure 4a), whereas the main body of the pluton crystallized at pressures of 450 to 650 Mpa [Davidson et al., 1996]; (4) all magmatic lithologies forming the Bergell body root steeply into the tail of the pluton (Figures 4a and 4b); and (5) the exposed base of the pluton and the overlying contacts separating the intrusive lithologies are never transected by steep magmatic 6of21

7 Figure 3. (continued) 7of21

8 Figure 4. Map, cross section, and structures of the Bergell pluton (modified after Rosenberg et al. [1995]). (a) Tectonic map of the Bergell pluton and its country rocks. Modified after Rosenberg et al. [1995]. (b) Cross section through the Bergell pluton. See Figure 4a for trace of the section in map view. (c) Small-scale magmatic folds at the base of the pluton. (d) Microphotograph of magmatic fold. The absence of crystal-plastic deformation of the grains indicates folding in the presence of melt. sheets, implying that the only existing steep conduit is the tonalitic tail. [24] After ascent along the transpressive PFS, final emplacement occurred in two stages. First, the magmas were emplaced to the north of the PFS, along an active and gently inclined nappe contact. During this phase a magmatic foliation formed, parallel to the foliation of the underlying country rocks [Davidson et al., 1996]. Second, the base of the pluton and its country rocks were folded, still in the presence of melt, as indicated by crosscutting relationships between folds and tonalitic veins [Rosenberg et al., 1995] and by microstructural observations of tight folds lacking any solid-state overprint [Davidson et al., 1996] (Figures 4c and 4d). The axial planes of these folds can be traced from the base of the pluton westward, into the underlying country rocks (Figure 4b) for several tens of 8of21

9 Figure 4. (continued) kilometers, indicating that the pluton was folded during large-scale regional N-S shortening. [25] At higher structural levels, now represented by the eastern contact, intense E-W horizontal shortening in the margin and metamorphic aureole [Conforto-Galli et al., 1988; Berger and Gieré, 1995] resulted in a steep, northsouth striking synmagmatic foliation [Berger and Gieré, 1995], which is perpendicular to the regional structures of Oligo-Miocene age in the area. This shortening event was therefore ascribed to final emplacement of the pluton by radial expansion or ballooning [Conforto-Galli et al., 1988; Berger and Gieré, 1995]. Synmagmatic folding at the base of the Bergell and ballooning at its top occurred simultaneously [Rosenberg et al., 1995]. [26] Finally, it is worth noting that the transition from the vertical feeder to the main body follows not only the general foliation trend of the enclosing rocks, but also a major rheological boundary within the Alpine crust, namely the boundary between the ductile Penninic Units and the rigid Austroalpine lid (Figures 4a and 4b) Adamello Batholith [27] The Adamello batholith is located at the junction between the Giudicarie and the Tonale lines (Figure 1). These two segments of the PFS border most of the northern and the eastern margins of the batholith (Figures 1 and 5a), which is composed of plutons ranging from 42 Ma to 28 Ma (Figure 5a) [Del Moro et al., 1983a; Villa, 1983; Hansmann and Oberli, 1991; Stipp et al., 2004]. A sequential emplacement history is indicated by the progressive younging of crystallization ages from south to north. This trend is confirmed by crosscutting relationships between the plutons [Callegari and Dal Piaz, 1973; Del Moro et al., 1983a] and by the progressive northward increase in 87 Sr/ 86 Sr [Dupuy et al., 1982; Macera et al., 1983] and 18 O/ 16 O[Del Moro et al., 1983b]. Contact metamorphic assemblages indicate pressures lower than 220 MPa [Riklin, 1983], consistent with clinopyroxene barometry on gabbroic rock [Nimis and Ulmer, 1998]. In contrast, Al-in-hornblende barometry of tonalites yields 350 MPa [Blundy et al., 1993]. On average these values are consistent with stratigraphic reconstructions, inferring the removal of 7 km of sedimentary cover [Assereto and Casati, 1965]. [28] The structural relationships between the intrusives and their enclosing rocks vary from one pluton to the next. Some intrusions are approximately concordant with the enclosing rocks, compositionally zoned and exhibit concentric foliation patterns [John and Blundy, 1993; Stipp et al., 2004] whereas others are strongly discordant and characterized by the occurrence of large, up to kilometer-scale stoped blocks [Salomon, 1908, 1910; Brack, 1983; Brack et al., 1983; Zattin et al., 1995] (Figure 5b). Space for these 9of21

10 Figure 5. Map and cross sections of the Adamello batholith. (a) Spatial distribution of Rb/Sr ages of biotites in the plutons of the Adamello batholith (modified after Del Moro et al. [1983b]). Thin solid lines separate different lithological intrusive units inferred to have intruded as separate pulses. (b) Fence sections through the southern Adamello. Open bar at the bottom of Figure 5a indicates the trace of cross sections. Redrawn from Brack et al. [1983]. plutons was primarily provided by stoping [Brack, 1983], whereas space for the former plutons mainly resulted from ballooning [Brack, 1983; John and Blundy, 1993; Stipp et al., 2004]. In the northern Adamello, magmatic foliations crosscut the contact between different intrusive bodies, suggesting that ballooning affected only the late stage of the emplacement process [Stipp et al., 2004]. Finally, other plutons, like the Val Fredda Complex, show subhorizontal bedding-parallel sheets [Blundy and Sparks, 1992]. These structures result in a ghost stratigraphy ascribed to a sill-like intrusion by fracturing along bedding planes. In summary, the mechanisms of final emplacement in the Adamello batholith differ from one batch of magma to another. None of these mechanisms can be directly attributed to the activity of the PFS. [29] The discordant contact between some parts of the Adamello batholith and the folded sedimentary rocks is considered as evidence for post-tectonic emplacement [Brack, 1981; Del Moro et al., 1983a]. However, structural and microstructural investigations of the northern Adamello plutons show that emplacement was syntectonic with respect to the PFS [Werling, 1992; Stipp et al., 2004]. Interestingly, the southern part of the batholith, which has no spatial relationship to any segment of the PFS, was intruded at 42 Ma (Figure 5a), i.e., prior to the activity of the PFS. In contrast, the northern and northeastern parts of the 10 of 21

11 batholith, which are adjacent to the PFS, yield intrusion ages (34 to 28 Ma) matching the inferred activity of the PFS (Figure 5a). [30] In conclusion, the magmas of the southern Adamello Batholith probably ascended independently of the PFS. Unlike the Bergell, the lithologies of the southern Adamello cannot be continuously traced northward into the PFS. It is therefore unlikely that magmas ascended along the PFS and subsequently migrated southward, along a gently inclined path at a depth of 7 km, over a distance of 50 km Rieserferner Pluton [31] The Rieserferner pluton is located along a sinistral, transpressive branch [Kleinschrodt, 1987] of the PFS (Figures 1a and 6a), namely, the Defereggen-Antholz-Vals Line (DAV). The pluton consists mainly of a coarsegrained, locally garnet-bearing, tonalite and a mediumgrained tonalite. Post-intrusive east-down tilting of the pluton [Borsi et al., 1978; Steenken et al., 2002] and 2000 m of relief provide an exceptional exposure of the roof (Figure 3d), sides and base of the intrusive body, which was emplaced at a depth of 12 to 15 km [Cesare, 1992]. The main body of the pluton is 2 km thick, relatively flat-lying and it abruptly becomes steeply dipping, as it gets closer to the PFS [Dal Piaz, 1934; Wagner, 2004] (Figure 6b). The contact between the two intrusive lithologies is relatively flat lying in the main body of the pluton (Figure 6b). [32] As shown by the foliation patterns (Figure 6a) and by the contour map of the plutons contact (Figure 6c), the roof of the pluton forms two domal structures separated by a synform (Figure 6c). The north-south striking axial plane of the synform, is perpendicular to all regional structures of the country rocks away from the pluton, suggesting that the domes result from buoyancy-driven uplift of this flat-lying magmatic body rather than from regional deformation [Wagner, 2004]. Although large parts of the roof are discordant to the foliation in the country rocks (Figure 6b), there is no evidence for stoping. Stoped blocks are lacking [Steenken et al., 2000] and rare xenoliths, generally <50 cm long, only occur in the very vicinity of the contact. Hence, instead of stoping, the discordant roof contacts are concluded to result from extensional fracturing along a gently inclined plane [Wagner, 2004]. [33] By contrast, the southern margin of the intrusion is steeply south dipping, parallel to the foliation of the country rocks and of the DAV shear zone (Figure 6b). Locally preserved magmatic fabrics oriented parallel to the solidstate mylonites of the DAV shear zone, call for a syntectonic intrusion of the pluton [Mager, 1985; Steenken et al., 2000; Wagner, 2004]. Within this steep zone, concentric foliation patterns with steeply plunging magmatic lineations are found locally, from the outcrop scale to that of several hundreds of meters (Figure 6a). Such foliation patterns do not occur in other parts of the pluton nor in the enclosing rocks. The spatial coincidence of these structures with the steeply oriented part of the pluton suggests that this zone acted as the feeder of the intrusion [Wagner, 2004]. Hence ascent of the tonalitic magmas occurred in the southern part of the pluton, adjacent to the DAV mylonites. As a result of tilting, the easternmost continuation of the steep zone (the eastern tail of the pluton in map view) represents a higher crustal level than the main body and is therefore inferred to represent the upper continuation of the Rieserferner feeder, which may have fed other plutons at higher crustal levels [Wagner, 2004] Karawanken Pluton [34] This 43-km-long tonalitic intrusion strikes parallel and immediately north of the PFS (Figure 1a). The rocks exhibit a pervasive foliation parallel to the PFS (Figure 7) that first formed in the magmatic state [Graber, 1897; Exner, 1976] and was then strongly overprinted by lower greenschist facies deformation [von Gosen, 1989]. Maximum temperatures of 300 C in the enclosing rocks [von Gosen, 1989] suggest an emplacement depth of less than 10 km, assuming a high thermal gradient. To the south of the PFS, deformation of Alpine age was only brittle, suggesting that the PFS in this area represents a brittleductile transition, like in the central Alps. [35] The pluton geometry consists of a subvertical, faultparallel tonalitic sheet, locally bending into a gently inclined tabular body further north [Exner, 1976; von Gosen, 1989] (Figure 7). Preserved parts of the roof suggest that the flatlying body of the intrusion forms a dome within the enclosing rocks of the Austroalpine [Exner, 1976]. The aspect ratios (X/Z) of magmatic enclaves rarely exceed a value of 3 [von Gosen, 1989], and as shown by other Periadriatic plutons that are less overprinted by solid-state deformation, such aspect ratios may have formed in the suprasolidus state. Therefore the present-day aspect ratio of the pluton (18:1) may roughly reflect the primary, magmatic geometry. In a Flinn diagram the enclaves plot in the field of apparent flattening, close to the line of plane strain [von Gosen, 1989], suggesting that deformation occurred under transpressive conditions [see also Polinski and Eisbacher, 1992] Tonalitic Lamellae [36] Elongate and extensively deformed [Exner, 1976] tonalitic bodies, hundreds of meters wide and several km long, termed lamellae [Dal Piaz, 1926] often occur along the PFS, from the Adamello Massif to the Slovenian Alps (Figure 1). Their original aspect ratios, varying between 5 and 1000 (Figure 3a) may have been significantly increased by solid-state deformation [e.g., Sassi and Zanferrari, 1973]. Petrologic investigations of some of the lamellae exposed along the Giudicarie Line [Martin et al., 1993] suggest that the emplacement depth, although poorly constrained, is 10 km (300 MPa). Locally preserved magmatic foliations oriented parallel to the Giudicarie fault plane call for synmagmatic shearing along this fault [Martin et al., 1993] or, alternatively, along the E-W segment of the PFS that was later rotated into the Giudicarie Line [Werling, 1992]. The geometric and spatial relationships of the lamellae with respect to the PFS are similar to the dike-like plutons exposed along the Patos shear zone of Brazil [Vauchez et al., 1997]. These plutons 11 of 21

12 Figure 6. Internal structure and geometry of Rieserferner pluton (modified from Wagner [2004]). (a) Foliation patterns in the pluton, modified from Wagner [2004]. (b) NNW-SSE cross section of the Rieserferner Pluton, modified from Wagner [2004]. Trace of cross section is shown in the inset map. (c) Contour lines of the contact of Rieserferner pluton. 12 of 21

13 [39] Considering that the Periadriatic plutons have a common source region, the spatial coincidence of coeval and cogenetic magmatic sheets with the PFS down to 30 km depth points to magma ascent of at least 30 km along the latter lineament. Note that the 30 km depth is constrained by geobarometric data from the westernmost Bergell tonalitic tail [Davidson et al., 1996], but field evidence suggests that the tonalite continues even further at depth. The vertical extent of the tonalitic tails of the Bergell and Rieserferner plutons may be even greater than their length in map view (30 40 km). In fact the longest axis of dikes and diapirs is often parallel to the propagation direction. Hence some of Figure 7. Cross section of the Karawanken pluton, redrawn and simplified after von Gosen [1989]. are elongate parallel to the shear zone, but their aspect ratio is very small compared to that of dikes (10 3 ) Pohorje Pluton [37] Although the Pohorje is one of the largest Periadriatic bodies, modern structural and geochronological investigations have not yet been carried out. The Pohorje intrusion (Figure 1a) is inferred to be a laccolith [Exner, 1976; Pamic and Palinkas, 2000], whose roof and possibly floor are exposed along the northern and southern contacts, respectively [Exner, 1976, and references therein]. Al-inhornblende barometry points to crystallization pressures of 680 MPa [Altherr et al., 1995]. The western end of the pluton is intruded by dacitic rocks, whose geochemical signature is similar to that of the Pohorje tonalites [Faninger, 1970; Altherr et al., 1995], suggesting a common magma source. Hence the same feeder zone may have been active from the time of emplacement at >20 km depth to that of exhumation close to the present-day surface level. 6. A Synthetic Cross Section [38] The spatial relationship between the PFS and the Periadriatic plutons whose geometry and emplacement depth are known is sketched in a synthetic cross section perpendicular to the PFS (Figure 8). Although biased by the fact that the plutons are not aligned normal to the PFS in map view, the cross sections, the depth and the distance of each pluton from the PFS are not schematic, but constructed on the base of geobarometric and field data. The following conclusions can be drawn from Figure 8: (1) Steep magmatic sheets continuously coat the PFS from the surface down to 30 km depth. (2) Within the entire orogen steep magmatic sheets only occur at distances smaller than 0.5 km from the PFS. Exposed pluton floors, such as the western margin of the Bergell confirm that the feeders are exclusively located along the PFS. (3) The main bodies of the plutons preferentially occur on the northern side of the PFS. (4) The main bodies of the plutons consist of gently dipping tabular bodies, whose thickness is much smaller than the inferred ascent distance. Figure 8. Synthetic cross section showing the spatial distribution of the Periadriatic plutons with respect to the PFS. Light shading marks the mylonitic belt of the PFS. Darker shading depict different magmatic lithologies within the plutons. Labels are as follows: A, Adamello batholith; B, Bergell pluton; Bi, Biella pluton; K, Karawanken pluton; P, Pohorje pluton; R, Rensen pluton; TL, tonalitic lamellae; Z, Zinsnock pluton. Emplacement pressures from Altherr et al. [1995], Biermeier et al. [1999], Bigioggero et al. [1994], Cesare [1992], Davidson et al. [1996], John and Blundy [1993], Martin et al. [1993], and Riklin [1983]. The depth of Karawanken Pluton is estimated on the basis of the temperature in the enclosing rocks [from von Gosen, 1989]. Cross sections from Nollau [1974], Rosenberg et al. [1995], von Gosen [1989], and Wagner [2004]. Cross sections through Adamello and Pohorje are schematic. 13 of 21

14 the Periadriatic plutons may be connected to their source region at the base of the crust. [40] The absence of any vertical magmatic sheets in the rest of the orogen suggests that the PFS is the only ascent pathway of the Periadriatic plutons in the Oligocene. The southern Adamello plutons are older than all the other intrusive bodies by several Ma (Figure 5a) probably predating the activity of the PFS. Hence these magmas are the only ones that were not channeled along the PFS. [41] Figure 8 and the above review allow to derive a simplified common characteristic shape for the Periadriatic plutons. Plutons are characterized by the transition from a steep sheet parallel to the PFS, to a more flat-lying tabular body further away from the PFS. Large, well-developed flatlying bodies are exposed in the Bergell (Figures 3c and 4b), in the Rieserferner Pluton (Figures 3d and 6b), and in the Pohorje Pluton. The upper part of such tabular bodies commonly rises into domal, or ballooning structures, as exposed along the side of the Bergell [Conforto-Galli et al., 1988; Berger and Gieré, 1995; Rosenberg et al., 1995], along the roof of the Rieserferner [Bianchi, 1934; Dal Piaz, 1934; Steenken et al., 2000; Wagner, 2004], in the northern part of the Karawanken pluton [Exner, 1976] and possibly in the eastern part of the Pohorje Pluton (P. Mioc, quoted by Exner [1976]). 7. Discussion 7.1. Linear Source or Linear Ascent Pathway? [42] The alignment of the Periadriatic plutons along the PFS may be attributed to (1) the subvertical, buoyancydriven ascent of magmas above a linear source region, inferred to result from the propagation of slab breakoff parallel to the strike of the chain [von Blanckenburg and Davies, 1995]; (2) an orogen-scale shear zone along which melts were formed, hence channeled [Pitcher and Bussell, 1977; Hutton and Reavy, 1992]; (3) localization of deformation into a partially melted zone, evolving into a meltbearing crustal-scale shear zone [Hollister and Crawford, 1986]; (4) channeling of melts into the shear zone [Brown and Solar, 1998b] during ascent, irrespective of the geometry of the source region. Structural and geophysical data, and the spatial distribution of plutons and faults, help to discriminate between these four possibilities Spatial Distributions of Dikes and Plutons [43] Dikes of similar age (Figure 2) and composition as the Periadriatic plutons are widespread in the Alpine chain [e.g., Pamic et al., 2002], but they are not particularly close to the PFS (Figure 1a). This observation is consistent with the inferred location of Oligocene volcanoes 25 km north of the PFS [Siegenthaler, 1974]. These dikes have no preferred orientation and are often discordant to the foliation in the country rock [Exner, 1976; Mancktelow et al., 2001]. [44] Since these dikes are inferred to have the same source as the plutons [von Blanckenburg et al., 1998], their spatial distribution may be used to constrain the lateral extent of the source region. Although the dikes show a linear arrangement on the scale of the orogen (Figure 1a), their spatial affinity to the PFS is much less pronounced compared to the plutons. Therefore it is suggested that the linear arrangement of the plutons in map view does not necessarily reflect a linear source region, but rather the channeling of magmas into the steep PFS mylonitic zone A Matter of Scale [45] Tomographic imaging of slab breakoff typically shows detachment gaps in the order of 10 2 km (e.g., Wortel and Spakman [2000] for the Appenninic chain). The width of the lithospheric mantle melted by upward flow of the asthenosphere and the resulting source region at the base of the crust should be similar to that of the gap in the downgoing slab. In contrast, the inferred width of the zone containing the feeders of the Periadriatic Plutons is <5 km wide (Figure 8). It is unlikely that the source region had a width of less than 5 km over a length of 800 km Is the Linear Arrangement of the Plutons Paleogeographically Controlled? [46] The Periadriatic plutons are located at the boundaries between Austroalpine and Southalpine (Figure 1a; Karawanken, Tonalitic Lamellae) and between Austroalpine and Penninic (Bergell), but also within the Austroalpine (Rensen, Rieserferner, Biella), and within the Southalpine (Adamello, Miagliano). Hence the alignment of plutons is not directly controlled by the older (Mesozoic) passive discontinuities, but rather by the synmagmatic activity of one fault system, namely the PFS Localization of Deformation Into Partially Melted Regions Versus Channeling of Melt Into Areas of Localized Deformation [47] Melt flow and deformation mutually interact, but the chicken and the egg question about which process controls the other is not solved [Vauchez et al., 1997; Rosenberg and Handy, 2000]. Faults may nucleate where melts have previously accumulated due to deformation partitioning into the weakest (melt) phase [Hollister and Crawford, 1986; Tommasi et al., 1994; Brown and Solar, 1998a] or melts may be channeled into previously existing faults. [48] In the Alps, several observations argue in favor of melt channeling into the already active PFS. The location, geometry, shear sense, and orientation of the PFS in the Alpine orogen are analogous to the commonly observed boundary of retrowedges of convergent orogens [Beaumont et al., 1994]. As shown by numerical models of the central Alps [Pfiffner et al., 2000] such crustal-scale backthrusts do not need any melts to nucleate. In addition, the areal percentage of the PFS presently covered by magmatic rocks is small (<30%). No deviations in the strike of the PFS occur between segments coated by magmatic rocks and others that do not contain any magmatic rocks. Hence a control of deformation on the site of melt transport is more likely than a control of melt on the sites of localized deformation Channeling and Ascent Mechanisms [49] Since the PFS-parallel tonalitic sheets are nearly everywhere concordant to mylonitic foliation of their country rocks, even at places where the primary intrusive contact is preserved [Berger et al., 1996] a mechanism of foliation- 14 of 21

15 Figure 9. Model for the transition from ascent to emplacement. (a) Schematic diagram showing the possible states of stress within the shear zone and in the rocks flanking the shear zone. Transpressive deformation is partitioned into a simple shear component within the shear zone and a pure shear component adjacent to the shear zone, leading to a swap of the intermediate and minor compressive stress axes. The vertical orientation of the minor compressive stress adjacent to the shear zone favors opening of horizontal fractures or dikes. (b) Horizontal dikes become shortened and hence folded and inflated. parallel fracturing is likely to account for magma ascent along the PFS. [50] Considering the strike-slip character of the PFS and the strong anisotropy of its mylonitic foliation, steeply dipping fractures are expected to form parallel to the foliation (Figure 9a). However, many segments of the PFS are inferred to be transpressive [e.g., Kleinschrodt, 1987; Schmid et al., 1989; Polinski and Eisbacher, 1992; Berger et al., 1996], suggesting that the angle between the fault plane and the maximum compressive stress may be well above 45. Experiments on anisotropic rocks showed that foliation-parallel fractures can form even if the angle between maximum compressive stress and schistosity is >60 [Donath, 1961]. If this angle reaches a value 70, foliation-parallel fracturing becomes difficult unless the melt pressure is sufficiently high and differential stresses are sufficiently small [Wickham, 1987]. Under these conditions fractures parallel to the mylonitic schistosity may open against the maximum compressive stress. This model was applied to explain ascent of the Bergell pluton along the mylonitic foliation of the PFS [Berger et al., 1996]. [51] The schistosity of rocks and lithological boundaries exert a strong control on melt flow, on the grain scale [Sawyer, 2001], outcrop scale [Weber et al., 1985; Brown et 15 of 21

16 al., 1999] and crustal scale [Brown and Solar, 1998b; Handy et al., 2001]. Generalization to the crustal scale is strengthened by the interpretation of field [Milnes, 1974; Milnes and Pfiffner, 1980] and seismic data of the central Alps [e.g., Schmid et al., 1996a], showing that the only steeply oriented anisotropy, continuously crosscutting the entire crust is represented by the mylonitic belt of the PFS (see arrowheads in Figure 1b). [52] An alternative and/or complementary process that may channel melt into a shear zone is the pressure gradient resulting from the strength contrasts between mylonites and the stronger, adjacent rocks. This contrast may induce melt flow toward the weaker rocks [Stevenson, 1989; Rutter, 1997; Rosenberg and Handy, 2000] of the shear zone, which support a smaller differential stress. If melt has already started to accumulate within the shear zone, the strength contrast between the melt-bearing shear zone and its host rocks may be significant (>1 order of magnitude [van der Molen and Paterson, 1979]), hence inducing a feedback process, which drives more melt into the shear zone Extrusion Mechanisms [53] Extrusion of melt from the PFS into the adjacent nappes leads to the formation of the main bodies of the Periadriatic plutons. This conclusion is supported by the observations outlined above, indicating that all vertical sheets lie along the PFS, while the gently dipping main bodies of the plutons are adjacent to the PFS (Figure 8). Several factors may account for a transition from subvertical flow to subhorizontal extrusion outside the shear zone. In particular, the rheology and anisotropy (foliation and/or lithological contacts) of the country rocks may control the site of final emplacement [Gilbert, 1877; Roman-Berdiel et al., 1995; Handy et al., 2001], in addition to the regional stress field. [54] The transpressive setting of the PFS suggests that regional stresses may have been different in the mylonites of the PFS and in the rocks flanking the PFS. Transpressional deformation may lead to partitioning of the simple shear component within the shear zone, while the pure shear component affects the adjacent block. This effect is enhanced by the presence of melt in the shear zone [Vigneresse and Tikoff, 1999]. Jones and Tanner [1995] briefly reviewed the occurrence of such shear zones, showing that the principal compressive stress in the rocks flanking the shear zone is oriented perpendicular to the shear plane. Under these conditions, melt-induced extensional cracks in the mylonites of the PFS and in the rocks flanking the PFS may have different orientations (Figure 9a). In the area flanking the shear zone, where the press component [Robin and Cruden, 1994] dominates, the least compressive stress is expected to be vertical, favoring the formation of sub-horizontal fractures (Figure 9a). In contrast, within the sub-vertical shear zone, where the strike-slip component dominates, the least compressive stress is expected to be sub-horizontal (Figure 9a) and hence extensional fractures are sub-vertical. [55] The latter change in orientation of extensional fractures may be facilitated by the change in orientation of the schistosity. As shown schematically in Figure 9, the transition from a strike-slip zone to one of folding with steep axial planes is marked by the rotation of the foliation from steep mylonites into gently dipping foliation planes within the folded region. This rotation favors steep fractures in the shear zone and gently dipping ones outside the shear zone. [56] An excellent example of the control exerted by the foliation of the country rocks on melt flow is given by the base of the Bergell intrusion, showing that the transition from a steep orientation along the PFS into a gently dipping orientation north of the PFS perfectly matches the orientation of the foliation in the country rocks (Figure 4b). The base of the pluton and its magmatic foliation are always concordant to the foliation of the underlying country rocks [Drescher-Kaden and Storz, 1926; Rosenberg et al., 1995; Davidson et al., 1996], and the transition from a steep root to a flat-lying main body corresponds to the transition from a steep zone of backthrusting to one of subhorizontal flow in the country rocks [e.g., Merle and Guillier, 1989]. [57] Except for the Adamello batholith, all main bodies of the plutons intruded from the PFS to the north or NW, i.e. into the crustal block that was preferentially uplifted during transpression [e.g., Schmid et al., 1989]. This differential uplift induced higher temperatures, hence lower strength in the North for a given crustal depth. It is therefore suggested that the lower strength of the northern block favored extrusion of the tonalitic magmas. The exception made by the Adamello (Figures 1a and 5a) may confirm the aforementioned rule. In fact, continuous intrusion of the Adamello plutons, from 42 to 29 Ma, leading to the construction of the Adamello batholith [Del Moro et al., 1983a], created a thermal anomaly, hence crustal weakness in the South Alpine block. This temperature increase was probably not balanced by differential uplift of the northern block, which was not significant in the Adamello region [Stipp et al., 2004] Space for Final Emplacement [58] The inferred emplacement mechanisms of the Periadriatic plutons range from cauldron subsidence (Biella pluton [Bigioggero et al., 1994]) to ballooning (several plutons of the Adamello [John and Blundy, 1993; Stipp et al., 2004]; eastern Bergell pluton [Conforto-Galli et al., 1988; Berger and Gieré, 1995]) to mode I fracturing, leading to the formation of a sill (Rieserferner pluton [Wagner, 2004]) to stoping (southern Adamello [Brack, 1983; Zattin et al., 1995]). [59] None of the above mechanisms can be directly related to specific displacements along the PFS and none of the plutons is inferred to be emplaced within a releasing bend, or a secondary, extensional structure of the PFS. Therefore the specific emplacement mechanisms of each pluton seem to be controlled by the local conditions at the site of emplacement, and not by the PFS. [60] A common feature of the main bodies of the Periadriatic plutons is the occurrence of discordant contacts along their sides and roofs, as shown by the Biella, by the northern and eastern sides of the Bergell, by several plutons of the Adamello, and by large parts of the roof of the 16 of 21

17 Rieserferner Pluton. Such contacts can only form by fracturing of the enclosing rocks, followed by flow of magma into the fractured region. These fractures may be thermally induced (stoping), or represent extensional cracks, resulting from regionally imposed differential stresses. Except for some of the Adamello plutons, the occurrence of xenoliths and stoped blocks is rare, suggesting that the discordant contacts result from mode I fracturing. Ballooning or doming of the roof is common (Bergell, Adamello, Rieserferner, Karawanken) during the last stages of final emplacement [Davidson et al., 1996; Stipp et al., 2004; Wagner, 2004], after the gently dipping main bodies have formed Dikes Versus Diapirs [61] The aspect ratios of the tonalitic lamellae and the exposed feeders of the Periadriatic plutons are generally small, varying from 2 to 80 (Figure 3a). Compared to typical aspect ratios of dikes [e.g., Rubin, 1993], these values are smaller by 2 3 orders of magnitude. Similar aspect ratios are described from shear-zone related plutons in Brazil [Vauchez et al., 1997] and mid-crustal vertical sheets in the Cascades (USA [Paterson and Miller, 1998]). The host rocks of the latter sheets indicate pluton-up shear senses, suggesting that the upward flow of the magmas occurred by ductile displacement of the host rocks [Paterson and Miller, 1998], i.e., by diapiric ascent. [62] The small aspect ratio of the Periadriatic feeders may be explained by lateral expansion during crystallization, when the viscosity ratio between magma and host rock decreases. Assuming a constant melt pressure, a reduction in the viscosity ratio can induce a switch to diapirism [Ramberg, 1981; Rubin, 1993]. Given that the main bodies had sufficient time and pressure to inflate after their emplacement, the same should be true for the feeders. Such a lateral expansion would reduce the aspect ratio of the tonalitic sheets. Alternatively, faulting along multiple planes can thicken the intrusion front [Rubin, 1993]. Examples of the latter process may be the western end of the Bergell tonalitic tail, which splits into different intrusion-parallel sheets in map view (e.g., 1:25,000 Swiss geological map Passo S. Jorio) and the top of the Karawanken tonalite, showing similar geometrical features in cross section [Exner, 1971]. [63] The different behavior of the Periadriatic plutons and their cogenetic dikes remains unresolved. Large volumes of magma used the anisotropy of the PFS, to form plutons at higher crustal levels. In contrast, small volumes of magma ascended often discordantly to the anisotropy of the enclosing rocks, forming the large number of Tertiary dykes (Figure 1a) that do not show a close spatial correlation to the PFS Role of Fault Kinematics [64] It has been suggested that the kinematics of faults may control the ascent of magmas, by enhancing ascent in case of vertical displacement and by inhibiting ascent in case of lateral, strike-slip displacement [Brown and Solar, 1998a]. Comparing segments of the PFS affected by different kinematics (Figure 1a) suggests that the presence or absence of magmas at a specific crustal level is totally independent of the fault kinematics at the time of emplacement. Tonalitic magmas from the same source ascended along the sinistral, transpressive DAV Line as well as along the dextral Insubric, Tonale and Gailtal Lines (Figure 1a). Also, the difference between dominating vertical displacement (i.e., along parts of the Insubric Line) and pure lateral displacement (e.g., Gailtal Line) does not appear to control the presence or absence of magmas (Figure 1a). Several Periadriatic plutons ascended up to shallow crustal levels along PFS segments characterized by lateral displacement and subhorizontal stretching lineations. Deformation rates along the PFS and shear zones in general are probably too slow to influence the much faster ascent rates of magmas Geometry of the Periadriatic Plutons and Statistical Analyses on the Distribution of Plutons and Faults [65] Image analysis has recently been used to characterize the spatial relationship between plutons and faults in map view, on the scale of orogens [Paterson and Schmidt, 1999; Schmidt and Paterson, 2000]. These investigations related the surface area of discrete portions of plutons to the nearest faults. In the case of faults bounding one side of a circular pluton, this type of image analysis yields a poor spatial correlation between fault and pluton [Schmidt and Paterson, 2000]. This result is in conflict with most structural investigations of syntectonic plutons, showing that the faults inferred to accommodate space for the intrusions always border, but never crosscut the core of the plutons. If this analysis was applied to the Periadriatic plutons, which only form thin sheets along the fault zone but cover large surface areas away from the fault, it would result in a poor spatial correlation between fault and plutons. In contrast, structural and petrologic investigations suggest that the entire volume of magma ascended along the fault plane, before it extruded northward. Therefore the poor spatial correlation indicated by the aforementioned analysis results from an assumption, that is not valid for shear-zone fed intrusions. [66] Geologic maps and cross sections of the Periadriatic Plutons show that all the feeders are located along the PFS. Therefore, except for the earlier intrusions of the Southern Adamello, all of the Oligocene magmas ascended along the PFS, even though the largest area of intrusive rocks in map view is located adjacent to, and not within, the mylonitic belt in map view Extrapolation to Other Orogens [67] The type of spatial and temporal relationship between Periadriatic plutons and PFS is not uncommon in other orogens, characterized by large-scale, synmagmatic shear zones. In the Variscan South Armorican Shear Zone the main bodies of the plutons are located adjacent to the mylonitic zone, whereas thin sheets of the same plutons lie along the shear zone itself [Jegouzo, 1980]. The same appears to be true for the Neoproterozoic West Pernambuco Shear Zone of Brazil [Vauchez and Egydio da Silva, 1992] and the Central Maine Belt of the United States [Brown and Solar, 1998b]. 17 of 21

18 [68] In spite of the small number of studies which document cross sections of plutons, several examples from outside the Alps illustrate the transition from a zone of ascent within a steep shear zone to flat-lying plutons adjacent to, but outside of, the shear zone [Witkind, 1973; Castro, 1986; Reavy, 1989; Cruden and Aaro, 1992; Aranguren et al., 1997; Archanjo et al., 1999; Gilder and McNulty, 1999; Kalakay et al., 2001]. In contrast, gravimetric studies of syntectonic plutons indicate that the deepest parts of the intrusive bodies are not located within the fault zone itself, suggesting that the feeders are systematically offset with respect to the fault zone [Vigneresse, 1995]. Note, however, that magmatic bodies must have a width of 500 m in order to be detected by gravimetric investigations [Vigneresse, 1990]. The example of the Periadriatic plutons shows that many of the potential feeders (the tonalitic lamellae) do not reach this width, and thus they would not be detected by gravimetric studies. 8. Conclusions [69] The Periadriatic plutons are both temporally and spatially related to the PFS. This relationship is a causative one, determined by the ascent of magmas along a steeplyoriented crustal-scale shear zone. The continuous occurrence of coeval and cogenetic magmatic rocks along one and the same shear zone down to 30 km depth and the absence of any other feeder channels of Oligocene age in the entire orogen strongly suggests that crustal-scale shear zones are effective agents of melt transport. [70] The linear distribution of plutons in map view is the result of melt channeling from the source region into the PFS. The source region may have been elongate parallel to the axis of the orogen, but its width must have been larger than that inferred for the ascent channel of the Periadriatic plutons (<5 km) [71] The occurrence of steeply-inclined feeders within or adjacent to the PFS mylonites and of flat-lying plutons outside of the mylonitic belt suggests that ascent and emplacement are different processes. Ascent is clearly controlled by the anisotropy and deformation of the PFS mylonites, whereas emplacement is largely unaffected by the PFS. [72] Ascending magmas are not affected by the kinematics and orientation of finite stretching in the PFS. Fault kinematics vary from one segment to the other of the PFS, but all segments are coated by magmatic bodies (Figure 1a) that ascended coevally. Ascent rates are probably faster than deformation rates in shear zones. Therefore deformation does not directly influence the flow direction of melt by imposing its direction of finite elongation. It is rather the anisotropy and the weakening resulting from deformation that create a preferential pathway for melt flow. This conclusion is supported by the fact that all intrusive bodies are situated along the only steeply oriented structure that continuously crosscuts the entire Alpine crust (Figure 1b). [73] Partitioning of transpressional deformation into strike-slip and pure shear deformation along and adjacent to the PFS respectively, favors the formation of steep foliation-parallel fractures along the PFS and gently dipping ones north of the PFS, and hence the transition from ascent to emplacement. The gently dipping fractures may evolve into sills, which finally inflate by uplifting the country rocks or, alternatively, by radial expansion as balloons. [74] The PFS formed independently of the Periadriatic magmas. Hence melting in the lower crust is not the cause of large-scale localization of deformation, while localization of deformation along the PFS is the cause of channeled of magma ascent through the crust. [75] Acknowledgments. The journal reviewers J.-L. Bouchez, N. Mancktelow, S. Schmid, and the Associate Editor A. Pfiffner are greatly acknowledged for their constructive and thorough reviews. Funding by the Deutsche Forschungsgemeinschaft (Ha 2403/3-2) is acknowledged. Alfons Berger, Peter Brack, Bernardo Cesare, Cam Davidson, Mark Handy, Silvana Martin, Stefan Schmid and Ralph Wagner shared helpful discussions on the Periadriatic plutons. Michael Stipp kindly provided preprints of his work on the Adamello batholith and a review of the present paper. References Altherr, R., B. Lugovic, H. P. Meyer, and V. Majer (1995), Early Miocene post-collisional calcalkaline magmatism along the easternmost segment of the Periadriatic fault system (Slovenia and Croatia), Mineral. 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21 Fabrics, edited by J.-L. Bouchez, D. H. W. Hutton, and W. E. Stephens, pp , Kluwer Acad., Norwell, Mass. Vigneresse, J. L. (1990), Use and misuse of geophysical data to determine the shape at depth of granitic intrusions, Geol. J., 25, Vigneresse, J. L. (1995), Control of granite emplacement by regional deformation, Tectonophysics, 249, Vigneresse, J. L., and B. Tikoff (1999), Strain partitioning during partial melting and crystallizing felsic magmas, Tectonophysics, 312, Villa, I. M. (1983), 40 Ar/ 39 Ar chronology of the Adamello gabbros, Southern Alps, Mem. Soc. Geol. Ital., 26, Villa, I. M., and F. von Blanckenburg (1991), A hornblende 39 Ar- 40 Ar age traverse of the Bregaglia tonalite (southeast central Alps), Schweiz. Mineral. Petrogr. Mitt., 71, Viola, G., N. S. Mancktelow, and D. Seward (2001), Late Oligocene-Neogene evolution of Europe-Adria collision: New structural and geochronological evidence from the Giudicarie fault system (Italian Eastern Alps), Tectonics, 20, von Blanckenburg, F. (1992), Combined high-precision chronometry and geochemical tracing using accessory minerals: Applied to the Central Alpine Bergell intrusion (central Europe), Chem. Geol., 100, von Blanckenburg, F., and J. H. Davies (1995), Slab breakoff: A model for syncollisional magmatism and tectonics in the Alps, Tectonics, 14, von Blanckenburg, F., H. Kagami, A. Deutsch, F. Oberli, M. Meier, M. Wiedenbeck, S. Barth, and H. Fischer (1998), The origin of Alpine plutons along the Periadriatic Lineament, Schweiz. Mineral. Petrogr. Mitt., 78, von Gosen, W. (1989), Fabric developments and the evolution of the Periadriatic Lineament in southeast Austria, Geol. Mag., 126, Wagner, R. (2004), Tectonics and magmatism along the TRANSALP seismic section between the Tauern Window and the Periadriatic Line (Eastern Alps), Ph.D. dissertation, pp , Freie Univ. Berlin, Germany. Weber, C., P. Barbey, M. Cuney, and H. Martin (1985), Trace element behavior and crustal anatexis during migmatization: Evidence for a complex melt-residuum-fluid interaction in the St. Malo migmatitic dome (France), Contrib. Mineral. Petrol., 90, Werling, E. (1992), Tonale-, Pejo- und Judicarien-Linie: Kinematik, Mikrostrukturen und Metamorphose von Tektoniten aus räumlich interferierenden aber verschiedenaltrigen Verwerfungszonen, Ph.D. thesis, Eidgen. Tech. Hochsch. (ETH), Zurich. Wickham, S. M. (1987), The segregation and emplacement of granitic magmas, J. Geol. Soc. London, 144, Witkind, I. J. (1973), Igneous rocks and related mineral deposits of the Barker Quadrangle, Little Belt Mountains, Montana, Prof. Pap. U.S. Geol. Surv., 752, Wortel, M. J. R., and W. Spakman (2000), Subduction and slab detachment in the Mediterranean- Carpathian region, Science, 290, Zattin, M., F. L. Bazzolo, S. Giorio, S. Martin, and V. Tornielli (1995), Intrusioni multiple nell area del Corno Alto, massiccio dell Adamello, Atti Ticinensi Sci. Terra (Ser. Spec.), 3, C. L. Rosenberg, Institut für Geologie, Freie Universität Berlin, Malteserstrasse, , D Berlin, Germany. (cla@zedat.fu-berlin.de) 21 of 21

22 Figure 1 2of21

23 Figure 1. Tectonic map and cross section of the Alps. (a) Simplified tectonic map of the Alps, with enlargements of some segments of the PFS showing the spatial distribution of the Oligo-Miocene dikes and of the Periadriatic plutons. Only the major faults of Tertiary age are shown. Redrawn after Bigi et al. [1990]. Faults are labeled as follows: CL, Canavese Line; DAV, DAV Line; GL, Giudicarie Line; GTL, Gailtal Line; IL, Insubric Line; LT, Lavantal Line; PL, Pustertal Line. Full circles indicate dikes. The size of the dikes is exaggerated by several orders of magnitude. Plutons are labeled as follows: A, Adamello; An, andesites of the Canavese Line; B, Bergell; Bi, Biella; K, Karawanken; L, Lesachtal body; M, Miagliano pluton; P, Pohorje; R, Rieserferner; Re, Rensen; T, Traversella; TL, tonalitic lamellae. Pink shading indicates areas with exposed Tertiary dikes. The x-x 0 is the trace of cross section of Figure 1b. Note that only parts of the dikes are dated. Most of them are inferred to be Oligocene on the basis of petrographic analogy with the dated ones and of crosscutting relationships with their country rocks. (b) Cross section of the central Alps, based on the interpretation of geologic and seismic data. Simplified after Schmid et al. [1996a]. Arrowheads mark the mylonitic belt of the PFS, which is the only steep structure crosscutting the entire crust. The northern boundary of the lower European crust is inferred to be aligned to the PFS at the time of intrusion of the Periadriatic plutons [Schmid et al., 1996a]. 3of21

24 Figure 3. Geometry and orientation of the Periadriatic plutons. (a) Aspect ratios of plutons versus distance to PFS. Open squares indicate steep sheets, subparallel to the PFS. Solid squares indicate gently inclined plutons and main bodies of plutons. Labels are as follows: Bi, Biella; Po, Pohorje pluton; Be, main body of the Bergell; Ri, main body of the Rieserferner. (b) Steeply, north dipping foliation of the tonalitic tail of the Bergell pluton. This tonalitic sheet and its foliation are parallel to the mylonites of the PFS. (c) Gently dipping floor of the Bergell pluton; Val Codera (Italy). Photo, C. Davidson. (d) Gently dipping roof of the Rieserferner pluton; Geltal (Italy). 6of21

25 Figure 3 7of21