The role of deformation bands and stylolites in fault development in carbonate grainstones of Majella Mountain, Italy Emanuele Tondi (*), Marco Antonellini (**), Atilla Aydin (**), Leonardo Marchigiani (*), Giuseppe Cello (*) (*) Department of Earth Sciences, University of Camerino, Via Gentile III da Varano, 62032 Camerino MC, ITALY. e-mail: [email protected] (**) Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA (1995), Willemse et al. (1997), and Salvini et al. (1999). Abstract Graham et al. (2003) have recently documented a new faulting mechanism in platform carbonates of Majella In this paper we document deformation processes Mountain, in which multiple sets of stylolites and their in porous carbonate grainstones of Cretaceous age in subsequent shearing result in progressively evolving Majella Mountain in the central Apennines of Italy, by normal faulting. This mechanism produces zones of detailed mapping meso- and microstructural features as fragmentation, breccia, and fault rock (gouge) leading well as thin section observations and image analyses. to through-going mature normal faults structures. We distinguished three main deformation mechanisms: In clastic rocks, deformation bands are common (i) deformation bands developed by compaction and structures where localized strains accumulate within shear strain localization, (ii) stylolites formed by narrow bands (Engelder, 1974; Aydin, 1978; Aydin and pressure solution, and (iii) subsequent shearing of Johnson, 1978; Jamison and Stearns, 1982; Antonellini stylolites. The deformation bands occur in six sets: five et al., 1994). The micromechanics of this type of strain sets are compactive shear bands at high angles to localizations include pore collapse, grains fracturing bedding and one, which is the oldest, occurs parallel to and dissolution at grain to grain contacts (Aydin et al., bedding and is interpreted to be a compaction band. 2005). All these micromechanical processes give rise to Stylolites localize along and within all six sets of small but detectable displacement discontinuities, deformation bands and are commonly sheared, as typically from a few mm to a few cm across individual evidenced by striated surfaces and detectable offsets. bands, whereas larger displacement discontinuities are These sheared stylolites are often associated with one accommodated by wide zones of multiple, composite or two sets of subsidiary stylolites, oblique to the major deformation bands and slip surfaces (Aydin and set. The band-parallel sheared stylolites, together with Johnson, 1978; Antonellini et al., 1994; Mair et al., the associated oblique sets of stylolites, form a tabular 2000, Shipton and Cowie, 2001). zone of fine grained cataclastic material within the In this paper, we report the occurrence of several compactive shear bands and accommodate slip in the sets of deformation bands in carbonate rocks of Majella range of 5 to 75 cm. Mountain and discuss their significance. In addition, we The bed-parallel compaction bands and bed- recognized stylolites formed by pressure solution parallel stylolites, which are kinematically compatible, within the previously formed deformation bands. are interpreted to be pre-tilting structures developed in Through detailed mapping at various scales and response to the overburden, whereas the five sets of laboratory analyses of representative fault rock samples, compactive shear bands and the associated stylolites we document how shearing of these stylolites within the and sheared stylolites likely occurred during the syn- bands produce zones of intense deformation, leading to and post-tilting deformation phases recorded in the the development of cataclastic fault rock. We also Majella anticline. attempt to relate the observed change of the deformation processes from deformation banding to Introduction pressure solution to evolving mechanical behaviour of Previous studies have pointed out the significant the deformed rock and the syntilting and post-tilting role played by pressure solution in deformation of deformation phases of Majella Mountain. carbonate rocks (for review see Rutter, 1983 and Groshong, 1988). How pressure solution and the Geological setting subsequent shearing of solution surfaces (referred to as Majella Mountain is a thrust-related, asymmetric, stylolites in this paper) are responsible for fault box-shaped anticline (Figures 3a and b, Introduction to development in carbonate rocks has also been discussed the 2005 RFP workshop, volume 16B) composed of by several workers, including Alvarez et al. (1978), carbonates of Lower Cretaceous to Miocene age, Engelder and Marshak (1985), Peacock and Sanderson Stanford Rock Fracture Project Vol. 16B, 2005 D-1 covered by a silicoclastic sequence of Upper Miocene- rising to a maximum height of about 12 m. Two Middle Pliocene age. In map view, the anticline shows pavement levels are exposed at the basement of the an arcuate shape oriented along an axis curving from quarry; a larger, lower one in the northern part, and a the NW in the northern part and to NE in the southern smaller, somewhat higher one along the southern side. part. The fold is periclinal in form and plunges gently to The two pavements are connected by two short, vertical the northwest. walls with a narrow step in between. A map of the two The Cretaceous sequence is composed of lagoonal pavements (Figure 1) was constructed at 1:25 scale, platform limestones in the south, basinal carbonates in using the string line method and a photomosaic base the north, with platform margin sediments across the map. In addition, detailed maps at 1:1 scale were center of the anticline (Figure 3c, Introduction to the constructed by utilizing acetate sheets taped directly 2005 RFP workshop, volume 16B; Eberli et al., 1993; onto the outcrops. The locations of these detailed maps Lampert et al., 1997). In post-Cretaceous times, the are marked on Figure 1, but the actual maps and platform margin ceased to be active and a carbonate photographs are presented in separate figures as they ramp sequence developed. are introduced. The internal deformation of the Majella anticline In the Madonna della Mazza quarry, we have consists predominantly of normal faulting with some identified six sets of deformation bands. The oldest set minor strike-slip faulting (Marchegiani et al., 2005). of these occurs parallel to the bedding and is Evidence of small scale compressional features, such as consistently offset by other deformation bands. The small folds and thrusts, is rare. The timing of the bed-parallel deformation bands generally form within, faulting, in relation to development of the Majella box- the coarse-grained beds which are characterized by shaped anticline, has been placed in the Middle-Upper conglomerates with clasts ranging in size from 3 to 10 Pliocene (Ghisetti & Vezzani, 2002; Scisciani et al., mm (Figure 2). These bands have the same appearance 2002). as the other deformation bands, except that they show The location for the present study is a small quarry no evidence of shearing. They appear as light-colored named Madonna della Mazza, which is situated on the bands with respect to the parent rock and their thickness inner part of the forelimb of the Majella anticline varies from 1 mm to 5 mm. The bed-parallel (Figure 3a, Introduction to the 2005 RFP workshop, deformation bands aren’t developed extensively, and volume 16B). There, the basinal sediments are are made up of short, discontinuous segments. represented by calcareous turbidites of the Orfento Four of the younger sets of deformation bands Formation (Figure 3c, Introduction to the 2005 RFP show normal dip-slip components of motion and have workshop, volume 16B). The Orfento Formation is mutually cross-cutting relationships, suggesting that Campanian to Maastrichtian in age, is composed of they developed contemporaneously (Figures 1, 2 and 3). eroded subangular rudist fragments ranging in size from These four groups form two pairs of conjugate sets; silt to rudite, and includes some intercalated one striking N-NW and the other W-NW (Figure 4). megabreccias (Mutti, 1995). The Formation covers the Among the N-NW striking sets, the E-dipping bands underlying platform margin and is a ramp succession, show a steeper dip angle (70° - 80°), whereas the W- thickening to the north. dipping bands have a lower dip angle (50° - 60°) Bedding surfaces are represented by (Figure 2). The W-NW-striking sets dip to the NE and unconformities between calcarenites beds and lenticular to the SW at similar angles (60°-70°). Another set of carbonate conglomerate levels (Figure 1, inset I; deformation bands trends E-W with an almost vertical Gillespie et al., 2005). These surfaces, which are often dip angle and shows right-lateral strike-slip kinematics striated, define tabular portions of rocks which typically (Figures 1, 4a and 5a). This set is the youngest, since it range between 5 cm to 50 cm in thickness. They gently consistently displaces all the other structures described dip towards the NE with a mean orientation of above. 020°/10°. The high porosity of the grainstones, ranging All five of these sets of deformation bands include between 15% and 30%, is attributed to a lack of cement either single bands or zones of several individual bands during and after their deposition in a high energy (Figures 2, 3 and 5a). These features are reminiscent of marine environment (Mutti, 1995). those of deformation bands in sandstone documented by Aydin and Johnson (1978). Single deformation bands Field observations are characterized by a thickness value between 1mm and 2mm and an offset between 1mm and 2 mm. The The Madonna della Mazza quarry exposes a zones of deformation bands with a normal sense of slip pavement floor and three subvertical walls, oriented have a typical anastomosing pattern with interlinked north, south, and west (Figure 1, inset II). The eastern eye and ramp structures (Figure 3), as has been side of the quarry is open. The quarry is 80 m long described in sandstone (see Antonellini and Aydin, (east-west), and 50 m wide (north-south) with walls Stanford Rock Fracture Project Vol.