FAULT DEVELOPMENT IN PLATFORM LIMESTONES OF THE MAIELLA THRUST SHEET, ITALY Brita Graham, Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 e-mail: [email protected]

Abstract evolution of fault zones in sandstones (Aydin and The deformation mechanism associated with Johnson, 1978; Antonellini et al., 1994; Myers, evolution of normal faults in Cretaceous platform 1999), in granitic rocks (Segall and Pollard, 1983; interior carbonates of the Maiella thrust sheet, Itaiy Martel and Pollard, 1989), in dolomites (Mollema is documented in this paper. The earliest structural and Antonellini, 1999), and in limestone rocks features are bedding parallel stylolites that define (Willemse et al., 1997). Willemse et al. (1997) mechanical layers. The bedding parallel stylolites documented the nucleation and growth of strike slip were subjected to shearing probably in response to faults in the Jurassic limestones exposed at the Bristol bending of the thrust sheet with top to the West Channel, UK, where it was found that pressure motion, subparallel to the direction of transport. solution seams provide a weak plane for slip to Shearing along bedding parallel stylolites results in nucleate. In the study of the strike-slip fault stylolite clusters at a high angle to, and at the development in the Bristol Channel, slip occurs termination of, the sheared bedding parallel across solution seams, followed by the formation of stylolites. The high angle sylolite clusters are opening mode veins in an en-echelon stepping consequently sheared in a normal sense whereby pattern. Also tail cracks form in association with slip generating a third set of stylolites fragmenting the along the pressure solution seams. The difference rock. With increasing slip magnitude, further between the strike-slip fault development described fragmentation and weakening of the rock occurs by Willemse et al. (1997) and this study is with the within the adjacent mechanical packages. Linking of occurrence of opening (mode-I) features. In the the fragmentation zone in adjacent mechanical faulting processes described in this article no packages to form a larger fault occurs along the macroscopic vein formation (mode-I) occurs. The weakest packages aligned in the direction of the faults described here are formed in a thrust front normal faulting. At this stage the fragmented rock regime by a mechanism that includes formation of turns into breccia whose geometry and grain size stylolites and their sequential shearing. varies from one mechanical bed to the next based primarily on the spacing of the bedding parallel Regional Geology stylolites or the thickness of the mechanical layers. Maiella Mountain is located within the Central Eventually polished and striated surfaces form at the Apennine tectonic region, a fold and thrust belt edges of the breccia zones. Normal slip along hinging the Northern and Southern Apennines. clusters of stylolites accommodates extension Formation of the Apenninic chain is the result of typically provided by opening mode features such as Miocene African and European plate motion, where veins or joints in other settings. In this study, the continental blocks (commonly called Apulia) were stages of normal faulting in a thrust belt setting have caught up in subduction (Dewey et al. 1989; been documented from the incipient stage to just over Malinverno and Ryan 1986; Scandone 1979). The 2 meters slip magnitude. A model showing stages of units of the Maiella region were originally located growth in the development of normal faults by along the northern margin of the Apulian platform, hierarchical occurrence of pressure solution one of a series of platforms located on the Adria structures and their consecutive shearing is plate. The carbonate rocks of Maiella consist of presented. platform and basin sediments ranging in age from Lower Cretaceous to Miocene. Platform sediments to Introduction the south grading to deeper water sediments to the In an ongoing effort to understand the process of north are shown in the geologic map of Figure 1. fault formation and growth many studies have been Tectonic bending and uplift of Maiella began in the done to understand structural elements involved in early/lower Pliocene, evidenced by conglomerate faulting processes and how they interact and evolve to beds to the north of early/lower Pliocene age produce large scale faults. Understanding faulting (Centamore, 2000, personal communication). processes can provide a foundation for models of Thrusting and formation of the Maiella anticlinal fault zone growth and geometry as well as fault zone structure are related to slip along a basal decollement permeability. Many studies have documented beneath the anticline. Slip was accrued along the

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Triassic Burano Evaporites in late Pliocene time Stage 2 Structures: Bedding perpendicular stylolites (Ghisetti et al. 1993, Eberli et al 1993, Lavecchia et The wavy nature of stylolites is well known, and al. 1988, Lavecchia, 1988). Tilted gravel beds of has been documented in many studies including early Pleistocene age indicate the timing of the major Alvarez et al. (1976) and Alvarez et al. (1978). The thrusting (Eberli et al. 1993). The area investigated platform carbonates in this study are at the thrust in this study is highlighted in Figure 1. A front, and have experienced intense deformation. photomosaic of the thrust front in the study area is Stylolites are in fact the major component of displayed in Figure 2 along with outcrop locations. deformation, and macroscopic veins are almost absent in the study area. Among the many stylolite Methods orientations, two major sets of stylolites perpendicular to depositional bedding exist at 225- Maps documenting the fault evolution were 230 and 265-270. The origin of these sets of constructed using string maps and scan lines. Acetate stylolites is attributed to Miocene compression during was taped to outcrops for 1:1 mapping at key the Apennine orogeny, prior to accretion into the locations. On larger faults where the upper beds are thrust belt. inaccessible, structures were mapped on photomosaics. Stage 3 Structures: Slip along bedding parallel

stylolites and formation of a new stylolite set at high Fault Development angle to slipped bedding stylolites Normal fault development in platform carbonates The third stage occurs along discontinuous of a thrust belt is described here using stages of fault bedding parallel stylolites as they slip. Figure 4 growth as determined by detailed mapping in the documents slipped bedding parallel stylolites, with field. pressure solutions forming at a high angle to the slip patch. Slip along the bedding parallel stylolites is Stage 1 Structures: Bedding parallel stylolites evidenced by breccia pods located where Stylolites form parallel to depositional bedding, discontinuous stylolites meet and the tail pressure as indicated by algal laminates (stromatolites) in solution surfaces that form the next stage of section. The Cretaceous platform limestones contain structures. These stylolite clusters occur at an angle a high density of bedding parallel stylolites. Spacing that varies from 45L-75L to the slip patches. of the stylolites varies from approximately 0.5 cm to 20 cm for the area studied. These stylolites formed as Stage 4 Structures: Fragmentation of the rock a result of burial with a burial depth of ~0.7 km and a Stylolite clusters formed in previous stages slip maximum overburden pressure of ~17.5 MPa. in a normal sense, forming a new generation of Sequence stratigraphy detailed by Bernoulli et al. stylolites further fragmenting and weakening the rock. (1992) refers to the Lower Cretaceous platform Figure 4 part (c) and (d) show the formation of a limestones as having indistinct bedding with massive fragmented zone. All of the initial fragmentation packages representing cyclic deposition of ~20 m of occurs within individual mechanical packages. limestone. In the area studied the mechanical limestone packages typically range between 1 cm - 1 Stage 5 Structures: Linkage of fragmentation zone m thick, as not all stylolites become package and formation of continuous breccia with boundaries. For the purpose of this paper, the term throughgoing slip surface mechanical packages will be used to refer to layers The weakest layers are those with a high density defined by the slipped stylolite surfaces, as they of initial depositional bedding parallel stylolites - the represent a mechanical discontinuity rather than a subsequent shearing creates stylolite clusters, whose stratigraphic discontinuity. Figure 3 shows a shearing produces fragmented rock. These limestone section highlighting the bedding parallel mechanical packages link to form larger throughgoing stylolites formed due to a vertical stress and displays faults. The weak layers are also a locus for breccia their discontinuous nature. Frequency of the bedding development, where fragments can rotate and discrete parallel stylolites depends on the lithologic slip surfaces can form. Figure 5 documents a map composition of the carbonates, which in the platform view of sheared stylolites involved in normal faulting alternates with massive layers of mudstone, algal at a stage prior to development of a throughgoing laminates wackestone, and chalky bioclastic fault. Localized deformation in each of the five wackestone. The compositional controls on the mechanical packages highlights the containment of frequency of bedding parallel stylolites will not be structures within mechanical packages. Figure 6 addressed in this paper. documents the linkage of the mechanical packages in a throughgoing fault with 2.15 m offset. Note the

Stanford Rock Fracture Project Vol. 9, 1998 P-D-2 breccia forms in the package that has a high density Park, R.G. (eds) Alpine Tectonics. Geol. Soc. Lon. of bedding parallel stylolites. Sp. Pub. 45, 265-283.

Eberli, G. P., D. Bernoulli, et al. (1993). "From aggradation Model for normal fault development to progradation; the Maiella Platform, Abruzzi, Italy." A model showing stages of growth in the Cretaceous carbonate platforms 56: 213-232. development of normal faults by hierarchical occurrence of pressure solution structures and their Ghisetti, F., M. Barchi, S.W. Bally, I. Moretti and L. consecutive shearing is presented in Figure 7. Vezzani. (1993). "Conflicting balanced structural sections across the central Apennines (Italy); problems and implications." 3rd annual conference of the Discussion European Association of Petroleum Geologists and Faults in platform limestones near the front of the Engineers 3: 219-231. Maiella thrust sheet develop through the dissolution process where the deformation mechanism involved Lavecchia, G. (1988). "The Tyrrhenian-Apennines system; in fault growth is completely based of stylolite structural setting and seismotectonogenesis." formation. This deformation mechanism is Tectonophysics 147(3-4): 263-296. accommodated by a large amount of dissolution as a Lavecchia, G., G. Minelli, and G. Pialli. (1988). "The result of compressive stresses associated with the Umbria-Marche arcuate fold belt (Italy)." International thrust front. Reverse faults and associated features conference on The origin of arcs 146, no. 1-4: 125- have not been discussed. Understanding the 137. compositional controls on stylolite density is an area where further investigation is necessary. Malinverno, A. and W. B. F. Ryan (1986). "Extension in the Tyrrhenian Sea and shortening in the Apennines as References result of arc migration driven by sinking of the lithosphere." Tectonics 5(2): 227-245. Accarrie, H., Beaudoin, B., Cussey, R., Joseph, P. &

Triboulet, S., 1986. Dynamique Sedimentaire et Martel, S. J. and D. D. Pollard (1989). "Mechanics of slip Structurale au Passage Plate-Forme/Bassin. Les and fracture along small faults and simple strike-slip Facies Carbonates Cretaces du Massif de la Maiella fault zones in granitic rock." Journal of Geophysical (Abruzzes, Italie). Mem. Soc. Geol. It. 36, 217-231. Research, B, Solid Earth and Planets 94(7): 9417-

9428. Alvarez, W., T. Engelder and P. A. Geiser (1978).

"Classification of solution cleavage in pelagic Mollema, P.N. and M. Antonellini (1999). “Development limestones." Geology 6(5): 263-266. of strike-slip faults in the dolomites of the Sella

Group, northern Italy.” Journal of Structural Geology Alvarez, W., T. Engelder and W. Lowrie (1976). 21(3): 273-292. "Formation of spaced cleavage and folds in brittle

limestone by dissolution." Geology 4(11): 698-701. Myers, R. D. (1999). "Structure and Hydraulics of Brittle

Faults in Sandstone." Ph.D. Dissertation: Stanford Antonellini, M. A., A. Aydin and D. D. Pollard (1994). University. 176 p. "Microstructure of deformation bands in porous

sandstones at Arches National Park, Utah." Journal of Scandone, P. (1979). "Origin of the Tyrrhenian Sea and Structural Geology 16(7): 941-959. Calabrian Arc." Bollettino della Societa Geologica

Italiana 98(1): 27-34. Aydin, A. and A. M. Johnson (1978). "Development of

faults as zones of deformation bands and as slip Segall, P. and D. D. Pollard (1983). "Nucleation and surfaces in sandstone." Pure and Applied Geophysics growth of strike slip faults in granite." JGR. Journal of 116, no. 4-5: 931-942. Geophysical Research. B 88(1): 555-568.

Bernoulli, D., Eberli, G.P., Pignatti, J.S, Sanders, D., and Vezzani & Ghisetti, 1997. Carta Geologica Dell'Abruzzo. Vecsei, A. (1992). "Sequence stratigraphy of Scala 1:100,000. Montagna della Maiella." Roma: Libro-guida delle

escursioni Quinto Simposio di Ecologia e Willemse, E. J. M., D. C. P. Peacock, and A. Aydin (1997). Paleoecologia delle comunita bentoniche, "Nucleation and growth of strike-slip faults in Paleobenthos, Roma: P. 85-109. limestones from Somerset, U.K." Journal of Structural

Geology 19(12): 1461-1477. Centamore, E., personal communication, July 2000.

Dewey, J.F., Helman, M.L., Turco, E., Hutton, D.H.W., & Knott, S.D. 1989. Kinematics of the western Mediterranean. In: Coward, M.P., Deitrich, D., &

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