Description:

_Journal of Structural Geology 32 (2010) 1291–1304_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg _Deformation along the leading edge of the Maiella thrust sheet in central Italy_ _Atilla Aydin a,*, Marco Antonellini a,b, Emanuele Tondi c, Fabrizio Agosta a,c_ _a Rock Fracture Project, Stanford University, California, USA b University of Bologna, Bologna, Italy c University of Camerino, Italy_ _article info_ _Article history: Received 5 December 2007 Received in revised form 1 October 2008 Accepted 8 October 2008 Available online 25 October 2008_ _Keywords: Thrust sheet Thrust-related anticline Maiella Mountain Platform carbonate deformation Apennines Fractured carbonates_ _abstract_ _The eastern forelimb of the Maiella anticline above the leading edge of the underlying thrust displays a complex system of fractures, faults and a series of kink bands in the Cretaceous platform carbonates. The kink bands have steep limbs, display top-to-the-east shear, parallel to the overall transport direction, and are brecciated and faulted. A system of pervasive normal faults, trending sub-parallel to the strike of the mechanical layers, accommodates local extension generated by flexural slip. Two sets of strike-slip faults exist: one is left-lateral at a high angle to the main Maiella thrust; the other is right-lateral, intersecting the first set at an acute angle. The normal and strike-slip faults were formed by shearing across bed-parallel, strike-, and dip-parallel pressure solution seams and associated splays; the thrust faults follow the tilted mechanical layers along the steeper limb of the kink bands. The three pervasive, mutually-orthogonal pressure solution seams are pre-tilting. One set of low-angle normal faults, the oldest set in the area, is also pre-tilting. All other fault fold structures appear to show signs of overlapping periods of activity accounting for the complex tri-shear-like deformation that developed as the front evolved during the Oligocene–Pliocene Apennine orogeny._ _© 2008 Published by Elsevier Ltd._ _1. Introduction_ _Understanding the processes of faulting and fracturing as it has occurred in existing structures provides a foundation for development of predictive models for faults and fractures, including their orientation, geometry, pattern, distribution, kinematics, and petrophysical properties. This knowledge is necessary for determining the structural framework of deformed rocks and regions and for evaluating fluid flow pathways in fractured reservoirs, especially those in low permeability carbonate rocks (Aydin, 2000; Graham Wall et al., 2006). Characterization of structural and hydraulic parameters is particularly challenging in carbonate rocks, which fail by opening mode fractures as well as by closing mode fractures. Furthermore, chemical dissolution of carbonate rocks at or near the surface further complicates direct observations by means of analog studies. Considering that vast amounts of resources, including water, oil, and gas are found in carbonate rock formations (Yose et al., 2001), understanding fractures and faults in this type of rock is all the more critical. However, there are relatively few publications on failure and faulting of carbonate rocks and their geometric and petrophysical properties in comparison to studies of similar features of silicaclastic detrital rocks._ _* Corresponding author. Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, CA 94305-2115, USA._ _E-mail address: aydin@pangea.stanford.edu (A. Aydin)._ _0191-8141 $ – see front matter © 2008 Published by Elsevier Ltd. doi:10.1016/j.jsg.2008.10.005_ _In some carbonate rock types, fault initiation and evolution are associated with opening mode fractures such as joints and veins (see, for example, Mollema and Antonellini, 1999 for dolomite; and Rawnsley et al., 1992; Kelly et al., 1998; Gross and Eyal, 2007 for limestone). The mechanisms described by these authors are similar to those in other brittle rocks (see Segall and Pollard, 1983; Martel, 1990 for granitic rocks; Myers and Aydin, 2004; Flodin and Aydin, 2004; Florez et al., 2005 and Gonzales and Aydin, 2008 for sandstone; Dholakia et al., 1998 for siliceous shale). However, because carbonate rocks are prone to dissolution under common geological loading conditions in the upper crust, deformation and failure of carbonate rocks usually involve pressure solution (Rutter, 1983; Groshong, 1988; Engelder and Marshak, 1985). Examples of carbonate rock deformation predominantly by pressure solution and the subsequent shearing of solution seams can be found in Alvarez et al. (1978), Salvini et al. (1999), Graham et al. (2003), and Billi et al. (2003). In some cases, however, both opening and closing failure modes simultaneously played equally important roles in carbonate rock failure in certain environments (Rispoli, 1981; Petit and Mattauer, 1995; Peacock and Sanderson, 1995; Willemse et al., 1997; Graham Wall et al., 2006; Agosta and Aydin, 2006; Antonellini et al., 2008). The conditions that lead to mixed failure modes are not well understood._ _Deformation of the carbonate rocks associated with the Apennine orogeny has attracted considerable interest (Alvarez et al., 1978; Marshak et al., 1982; Graham et al., 2003; Marchegiani et al.,_ _1292_ _A. Aydin et al. Journal of Structural Geology 32 (2010) 1291–1304_ _2006; Tondi et al., 2006; Agosta and Aydin, 2006; Agosta et al., 2007, 2008; Antonellini et al., 2008), as has its symmetric counterpart, the Albanian fold and thrust belt on the eastern side of the Adriatic Sea (Van Geet et al., 2002; Graham Wall et al., 2006). Early interest in these areas was, in part, due to the excellent examples of natural pressure solution features in carbonates in the region but the latest projects are more related to the growing need for analogs of carbonate reservoirs in fold-and-thrust belts for research related to resource location and extraction._ _Maiella (also spelled Majella) Mountain, in central Italy, offers a rich example of a marine carbonate setting, including the platform carbonates and the associated slope and basin deposits. These deposits were deformed during the Apennine orogeny and also perhaps during the postorogenic extension. Deformations of the basin and slope deposits (alternating turbiditic grainstones and micrites) and their spatial and temporal variations as a function of lithotype were the subjects of two recent studies (Tondi et al., 2006; Antonellini et al., 2008). In this paper, we document a number of structural assemblages, with an emphasis on the geometry, distribution, and formation mechanisms of the faults in the platform carbonates exposed in the hanging wall along the leading edge of the Maiella thrust sheet, immediately west of the town of Fara San Martino. This study follows two earlier investigations carried out in the same area which focused solely on the micromechanics of normal faulting in a contractional environment (Graham et al., 2003) and the geostatistical properties of the faults and fractures in the broader area (Marchegiani et al., 2006), and which were not concerned with the structural framework at the leading edge of the thrust sheet._ _2. Geological setting_ _Maiella Mountain is located about 200 km east of Rome (Fig. 1a), in the Abruzzo region, and is the easternmost of the major thrust sheets located within the Oligocene–Pliocene central Apennine fold and thrust belt (Fig. 1b, Mostardini and Merlini, 1986; Roure et al., 1991; Cello et al., 1997; Bigi et al., 1992; Ghisetti and Vezzani, 1997; Vezzani and Ghisetti, 1998; Scisciani et al., 2002; Tondi and Cello, 2003). The first-order structural feature of the Maiella thrust sheet is an elongated, kidney-shaped anticline with a steeply-dipping, curvilinear eastern forelimb and a basal thrust underlying the thrust sheet (Figs. 2a, c). Along the forelimb, the projected surface trace of the thrust fault separates the predominantly carbonate rocks of the Maiella series, which are of Lower Cretaceous to Middle Miocene age (Fig. 2b, Bernoulli et al., 1992; Eberli, 1993) from the argillaceous and silicaclastic foredeep deposits of the Messinian and Pliocene Psyc (Figs. 2b, c, Ori et al., 1986)._ _Aside from the underlying thrust, which accommodated several kilometers of eastward motion of the thrust sheet (Scisciani et al., 2002 and references therein), the internal structures of the Maiella anticline consist predominantly of normal faults (Figs. 2a, c) with about 4 km of total offset (Ghisetti and Vezzani, 1997; 2002; Scisciani et al., 2002). Among these, the Caramanico Fault marks the trailing western boundary of the thrust sheet. Many other smaller normal faults transect the anticline at various angles. These normal faults appear to be younger than the underlying basal thrust._ _The 2-km-thick carbonate succession of the Maiella series (Fig. 2b) records the conditions of Cretaceous to Miocene platform margins and the slope basin depositional environments of the Tethys. It is capped by silicaclastic sediments of Upper Miocene–Middle Pliocene age (Bernoulli et al., 1992; Eberli et al., 1993). The surface outcrops are dominated by platform carbonate rocks, in the south, and basinal rocks in the north (Fig. 2a). In the center of the anticline, platform margin sediments are exposed along several deep gorges such as the Tre Grotte Valley (Crescenti et al., 1969; Accarie, 1988; Eberli et al., 1993; Mutti, 1995; Lampert et al., 1997; Vecsei et al., 1998; Vecsei and Sanders, 1999; Ori et al., 1986). These earlier studies identified a carbonate ramp sequence developed in post-Cretaceous times, when the platform margin ceased to be active._ Ключевые слова: tectonophysics, apennines, hanging wall, underlying thrust, aydin flodin, thrust tectonics, folding, left-lateral fault, strike-slip faulting, parallel, set, thrust faulting, vezzani scisciani, author, della, eastern limb, bed, antonellini, elliot, aydin journal, splay psss, study area, vezzani, sheared, carbonate, wall, based, maiella mountain, psss, apennine orogeny, sequence stratigraphy, south showing, major, science, dal, eds, strike, showing, leading, upper, geological, ground photograph, surface, strike-slip fault, majella, slip, tavarnelli, angle, kink band, tondi, tectonics, steeper forelimb, reverse fault, aydin, platform, fracture, belt, italy, peacock, sheet, bed-parallel psss, geophysical, study, maiella anticline, pollard, figs, sheared psss, anderson, aapg, carbonate rock, maiella, normal fault, mechanical model, type, strike-slip, rock, structure, maiella series, suppe, bed-perpendicular psss, principal stress, structural, thrust, projected trace, oblique psss, mcclay, johnson, solution, leading edge, formation mechanism, normal, geology, central, bedding, secondary faulting, mechanism, structural framework, conceptual model, splay angle, mechanical, forelimb, occur, progressive deformation, structural geology, wide range, anticline, eastern forelimb, basin, deformation, wide variety, stress, sense antithetic, platform carbonate, limb, dip angle, exural slip, thrust fault, left-lateral, strike slip fault, area, structure type, apparent, graham, cutoff angle, subsequent shearing, basal thrust, edge, deformation mechanism, fold, thrust sheet, antiformal axis, bulletin, formation, location, map showing, splay, maiella thrust, limestone, antithetic fault, right-lateral, antithetic, pressure, zone, graham wall, layer, cello, roure, slip fault, strike slip, scale, left-lateral slip, rst set, projected location, central apennines, bed-perpendicular, seam, journal structural, conjugate, eastern, mutti, generally younger, maiella sheet, fracture pattern, ghisetti, mechanical layer, fold-and-thrust belt, normal faulting, mountain, christensen cooper, more-or-less consistent, kink fold, model, faulting, breccia zone, pattern, eld, petit, main thrust, formed, pressure solution, orientation, steeper limb, eastern slope, bed-parallel, special, petroleum, band, kink, steeper, view angle, society, agosta, thrustreverse fault, journal structural geology, pre, fault, journal, shear fracture, issue, lateral, extension, thrust belt, shearing, andersonian theory, geometry, solution seam, main, reverse, petrophysical property