New insights on some structural geometries in Southern Apennines by multiscale analysis of potential fields

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New insights on some structural geometries in Southern Apennines by multiscale analysis of potential fields F. Cella ( 1 ), L. Ferranti ( 2 ), G. Florio ( 2 ) and L.

1 New insights on some structural geometries in Southern Apennines by multiscale analysis of potential fields F. Cella ( 1 ), L. Ferranti ( 2 ), G. Florio ( 2 ) and L. Maschio ( 2 ) ( 1 ) Dipartimento di Scienze della Terra, University of Calabria, Italy ( 2 ) Dipartimento di Scienze della Terra, University of Naples Federico II, Italy The Southern Apennines is one of the youngest orogens of the central Mediterranean belt and developed during the Neogene time collision between Europe and Adria, coupled to the rapid rollback of the Adriatic-Ionian slab (Malinverno and Ryan, 1986; Gueguen et al., 1998; Faccenna et al., 2001; Ferranti and Oldow, 2005). The orogen includes two vertically superposed contractional belts (Roure et al., 1991; Menardi Noguera and Rea, 2000; Mazzotti et al., 2000; Scrocca et al., 2005). The upper belt formed in response to Miocene-Early Pleistocene shortening, and underlies the central and western side of southern Italy (Patacca et al., 1990). Deformation of the belt progressed in a broad northeast direction toward the Apulian sector of the Adriatic foreland (Figure 1). Fig. 1 DEM of the Southern Apennines with GPS velocities of the PTGA network. The white dashed line indicates the boundary between extension to the west and transpression to the east. The black dashed lines indicate the lineament analyzed in this work (slightly modified from Ferranti et al., 2008). The lower thrust belt involves buried Mesozoic-Tertiary platform carbonates and perhaps slices of basement rocks, which represent the western extension of the Apulian rocks exposed to the east (Mostardini and Merlini, 1986; Roure et al., 1991; Menardi Noguera and Rea, 2000; Mazzotti et al.,

2 2000). Apulian rocks buried beneath the Apennines are the primary play for oil exploration, and thus efforts to decipher the subsurface structural arrangements are of timely importance. However, although a good knowledge of the tectonic evolution of the Southern Apennines has been spurred by surface geology studies, large uncertainties still surround a thorough characterization of the subsurface structural setting. In particular, the structures accommodating Quaternary deformation in the Apulian plate, when a major geodynamic change occurred (Hippolyte et al., 1994), are still poorly understood. Toward this end, we focused our joint geological and geophysical analysis on selected sectors of the orogen where recent tectonic studies suggest alternative interpretations. As a matter of fact, GPS geodetic velocities and morpho-structural studies document involvement in transpression of Apulian rocks in the whole eastern sector of southern Italy (Ferranti et al., 2008; 2009). It is well known that some sectors of the Apulian foreland, such as the Gargano promontory, have been experiencing mild shortening and strike-slip deformation since the Miocene (Bertotti et al., 2001; Ferranti & Oldow, 2005). This deformation is ongoing at Gargano, as documented by crustal earthquakes (Pondrelli et al., 2006). Little is known, however, of the structural setting south of Gargano along the border of the Murge plateau (Fig. 1). This border is traditionally envisaged as a staircase of normal faults which dip to the SW and downthrow the Apulian rocks beneath the foredeep basin (Sella et al., 1988). A second key area is represented by the frontal belt along the Ionian Sea coast of northern Calabria and Basilicata regions. Here, recent shortening behind the front of the thinskinned thrust belt is documented by studies on marine terraces and seismic reflection profiles (Ferranti et al., 2009). The young transpressional structures are related to deep-seated shear zones that involve the Apulian foreland plate underlying the thin-skinned accretionary wedge. Arrays of NW-SE striking strike-slip and thrust faults are traced from on-land (Catalano et al., 1993) to the offshore (Doglioni et al., 1999; Del Ben et al., 2007; Ferranti et al., 2009), and may be active today as indicated by GPS velocities (Figure 1). As for the faults involving the western side of Murge, it is possible that these faults were previous (Miocene-Pliocene) extensional faults which have been re-activated in transpression during Quaternary. For both the Murge border and the north-eastern Calabria coast, it would be desirable to have independent indications on the dip, dip direction and depth extent of the faults which have caused Quaternary transpression in the Apulia plate. Toward this end we interpret the Bouguer anomaly map of the Southern Italian region by using a multiscale full approach (Cella et al., 2009), involving the integrated use of Multiscale Derivative Analysis (MDA) and Depth from Extreme Points (DEXP) techniques (Fedi and Florio, 2001; Fedi, 2002; Fedi 2007). Methodology Our approach to the interpretation of selected profiles of the Gravity Bouguer anomalies in Southern Apennines is based on a two-step procedure. First, a high resolution boundary analysis is performed by using the MDA technique. The MDA is based on the Enhanced Horizontal Derivative function (Fedi and Florio 2001), defined as a summation of vertical derivatives of different order, weighted in such a way as to enhance contribution from source at different scales. Generally three maps are produced, displaying maxima of the EHD function, corresponding to boundaries of geologic bodies, with three different resolutions (large, medium and small scales) and related to structures of different dimension. This analysis applied to Southern Italy Bouguer gravity map gave detailed information on the shallow and deep structural setting (Fedi et al., 2005). The second step is the interpretation, by DEXP method, of the gravity field linked to a EHD lineament of interest. DEXP is a multiscale method based on the analysis of the gravity field in a vertical section, obtained by upward continuation. DEXP method implies a simple transformation of the original field, that is scaled according to a power law, where the exponent, connected with the source structural index, is determined by a specific analysis of the scale function along a ridge, defined as the ensemble of the points, at several altitudes, where the horizontal derivative of the gravity field is zero (Fedi and Florio, 2006; Florio et al., 2009). It was shown (Fedi, 2007) that maxima or minima of the scaled potential field will occur at an altitude corresponding to the depth to sources. Thus, by integrating Multiscale

3 Derivative Analysis with Depth from Extreme Points method we can retrieve a rather effective information about the sources, namely their horizontal boundaries, depth and structural index. Data Analysis We apply our scheme of interpretation to the Murge border and the north-eastern Calabria coast. It worth mention that although the results here presented are displayed along gravity profiles, all the computations (upward continuation and vertical derivations of the field) were correctly carried out on the whole map. Murge border The analysis of the MDA short-scale map shows that from the chain axis to the Adriatic Sea preferentially NW-SE trending structures alternate to a transversal (E-W) trends, especially localized within the Murge and Gargano sectors. More northwestwardly and around the Gargano Promontory, predominantly NE-SW and non rectilinear geometries highlight a peculiar activity which accompanies to the well known WNW-ESE fault systems localized inland and offshore. The western border of the Murge hills appears bounded by linear trends of maxima rather continuous from NW to SE. In such case of high-resolution analysis, we considered the fourth-order vertical derivative of the gravity field upward continued to altitudes up to 12 km. Such a high-order vertical derivative can be efficiently combined with upward continuation, allowing the resulting DEXP-transformed signal to be characterized by an acceptable signal-to-noise ratio. However, the combined filter allowed highlighting interesting small scale details of the gravity signal that give rise to linear ridges. The scale function analysis along these ridges yields a structural index of N=0 corresponding to sheet-like sources with depth positions at 2.25 km. The DEXP scaled field is shown in Figure 2. NE Calabria coast The analysis of the MDA short-scale map shows that in the onshore NE Calabria, NW-SE oriented arrays of EHD maxima coincide with sub-parallel left transpressional shear zones re-involved in shortening of the thick Apulia crust (Catalano et al., 1993; Monaco et al., 1998). A well highlighted transversal (NE-SW) EHD pattern is coincident with a NE- striking extensional fault system (Ferranti et al., 2009 and reference therein). As before, in such case of high-resolution analysis, we considered the fifth-order vertical derivative of the gravity field upward continued to altitudes up to 12 km. Such a high-order vertical derivative allowed highlighting interesting small scale details of the gravity signal that give rise to linear ridges. The scale function analysis along these ridges yields a structural index of N=1 corresponding to linear sources, with estimated depth position of about 4 km. The obtained DEXP section confirmed this depth. Discussion and Conclusions Murge border Our analysis suggests, although in a preliminary way, that an important tectonic lineament which borders the Murge plateau to the west is dipping at high-angle to the north-east, and not toward the southwest as traditionally depicted. This lineament can be traced at the morphological edge of the plateau, where it steeply falls down the Bradano foredeep, and the its top is buried between 2 to 4 km depth. Although the lineament may have acted as an extensional feature due to bending of the Apulian platform during Miocene-Pliocene, during Pliocene-Quaternary shortening it might have accommodated uplift of Murge (Ferranti and Oldow, 2005). Today, large geodetic strain might be accumulating along it (Figure 1; Ferranti et al., 2008).

NE Calabria coast The lineaments evidenced by our analysis might correspond to the high-angle transpressional faults documented by geological studies which caused the uplift and imbrications of the

Although our analysis is still in progress, it appears that these faults dip at high-angle toward the north-east and thus they represents back-thrusts in the regional reference frame.

4 NE Calabria coast The lineaments evidenced by our analysis might correspond to the high-angle transpressional faults documented by geological studies which caused the uplift and imbrications of the Apulian plate underlying the thin-skinned thrust belt. Although our analysis is still in progress, it appears that these faults dip at high-angle toward the north-east and thus they represents back-thrusts in the regional reference frame. Similar evidences have been found in the offshore by analysis of seismic reflection profiles (Doglioni et al., 1999; Del Ben et al., 2007; Ferranti et al., 2009). b) a) c) Figure 2. a) Gravity map of Southern Italy and analyzed profile. b) EHD function (blue), topography along the profile and location of the chosen ridge (red line). c) DEXP section computed for N = 0. Note the NW dip of the high density structure at x = 72 km and depth z 0 = 2.25 km. References Bertotti, G., Picotti, V., Chilovi, C., Fantoni, R., Merlini, S., Mosconi, A., Neogene to Quaternary sedimentary basins in the south Adriatic (Central Mediterranean): Foredeeps and lithospheric buckling: Tectonics 20, Catalano, S., Monaco, C., Tortorici, L., Tansi, C., Pleistocene strike-slip tectonics in the Lucanian Apennine (Southern Italy). Tectonics 12, Cella F., Fedi M. and Florio G., Toward a full multiscale approach to interpret potential fields. Geophysical Prospecting, 57, Del Ben, A., Barnaba, C., Toboga, A., Strike-slip systems as the main tectonic features in the Plio- Quaternary kinematics of the Calabrian Arc. Marine Geophysical Research, DOI /s Doglioni, C., Merlini, S., Cantarella, G., Foredeep geometries at the front of the Apennines in the Ionian Sea (central Mediterranean). Earth Planetary Science Letters, 168, Faccenna, C., Becker, T.W., Lucente, F.P., Jolivet, L., Rossetti, F., History of subduction and back-arc extension in the Central Mediterranean. Geophysical Journal International, 145, Fedi M. and Florio G., Detection of Potential fields source boundaries by Enhanced Horizontal Derivative method. Geophysical Prospecting, V. 49, n.1, Fedi M., Multiscale Derivative Analysis: a new tool to enhance gravity source boundaries at various scales. Geophysical Research Letters, 29, 16-1 to Fedi M., Cella F., Florio G., Rapolla A Multiscale derivative analysis of the gravity and magnetic fields of Southern Apennines (italy). In: Finetti I.R. (Ed.), CROP deep seismic exploration of the Mediterranean Region. Elsevier., Vol. I, Fedi M. and Florio G SCALFUN: 3D analysis of potential field scale function to determine independently or simultaneously structural index and depth to source. 76th Annual International Meeting, SEG, Expanded Abstracts, Fedi M DEXP: A fast method to determine the depth and the structural index of potential fields sources. Geophysics, 72, No. 1, Ferranti, L., Oldow, J.S., Latest Miocene to Quaternary horizontal and vertical displacement rates during simultaneous contraction and extension in the Southern Apennines orogen, Italy. Terra Nova 17,

5 Ferranti, L., Oldow, J.S., D Argenio, B., Catalano, R., Lewis, D., Marsella, E., Maschio, L., Pappone, G., Pepe, F., Sulli, A., Active deformation in Southern Italy, Sicily and southern Sardinia from GPS velocities of the Peri-Tyrrhenian Geodetic Array (PTGA). Boll. Soc. Geol. It., (Ital. J. Geosci.), 127/2, Ferranti L., Santoro E. Mazzella E., Monaco C. & Morelli D., Active transpression in the northern Calabria Apennines, Southern Italy. Tectonophysics, 476, doi: /j.tecto Florio G., Fedi M. e Rapolla A., Interpretation of regional aeromagnetic data by multiscale methods: the case of Southern Apennines (Italy). Geophysical Prospecting, 57, Gueguen, E., Doglioni, C., Fernandez, M., On the post-25 Ma geodynamic evolution of the western Mediterranean. Tectonophysics 298, Hippolyte, J. C., Angelier, J. Roure, F., A major geodynamic change revealed by Quaternary stress patterns in the Southern Apennines (Italy). Tectonophysics 230, Malinverno, A., Ryan, W.B.F., Extension in the Tyrrhenian Sea and shortening in the Apennines as a result of arc migration driven by sinking of the lithosphere. Tectonics, 5, Mazzotti, A., Stucchi, E., Fradelizio, G.L., Zanzi, L., Scandone, P., 2000, Seismic exploration in complex terrains: a processing experience in the Southern Apennines. Geophysics, v. 65 (5), p Menardi-Noguera, A., Rea, G., Deep structure of the Campanian-Lucanian Arc (Southern Apennine, Italy). Tectonophysics 324, Pondrelli, S., Salimbeni, S., Ekström, G., Morelli, A., Gasperini, P., Vannucci, G., The Italian CMT dataset from 1977 to the present. Phys. Earth Planet. In., 159, Roure, F., Casero, P., Vially, R., Growth process and mèlange formations in the Southern Apennines accretionary wedge. Planet. Sci. Lett., 102, Sella M., Turci C., and Riva A., Sintesi geopetrolifera della Fossa Bradanica (Avanfossa della catena appenninica meridionale). Mem. Soc. Geol. It., 41, Scrocca, D., Carminati, E., and Doglioni, C., 2005, Deep structure of the southern Apennines, Italy: Thinskinned or thick-skinned?: Tectonics, v. 24, TC3005, doi /2004TC001782

Ionian Sea and Margins: Recent Prospections and Interpretations

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