INTRODUCTION

In this paper, we describe the structural evolution of the Narni Anticline in the central Apennines (Lotti, 1926). This structure provides an opportunity to study at outcrop the final deformation pattern of a structural trap typical of the western and central parts of the Apennines. The structure developed as a result of sequential thrust-related folding, thrust breakthrough, extensional faulting and strike-slip faulting; due to this structural complexity, the term ???anticline??? is oversimplified. Particular emphasis is given to the role of the inherited fracture pattern in constraining the location and geometry of younger deformation structures, and to the progressive evolution of fracturing and cataclasis in the carbonate rocks of which the Narni Anticline is composed (Fig. 3).

Fig. 2. Geometric relationships between pressure-solution cleavage and fault surfaces in cross section (above) and map view (below) for (a) passive folding at fault bends, (b) extensional faulting and (c) strike-slip faulting. See text for details

REGIONAL TECTONIC FRAMEWORK

The Apennine-Tyrrhenian Sea region (Fig. 1) has evolved in response to Neogene oblique convergence between Africa and Eurasia (Laubscher, 1971). A particular feature of the region is the presence of eastward-migrating zones of compression and extension (Elter et al., 1975; Patacca et al.,1993; Jolivet et al., 1998). Compressional deformation is mostly located near the toe of the Apenninic thrust wedge, where the foredeep successions of the Adriatic foreland have been accreting since the Miocene in an overall piggyback sequence (Principi and Treves, 1984). Synchronous thrusting (Cipollari et al., 1995) and out-of-sequence thrusting (Morley, 1988) have further increased the structural complexity of the orogen (Bigi et al., 1989; Storti, 1995a). Extensional deformation in the northern Tyrrhenian Sea and on the western side of the Apennine thust belt is related to an eastward-verging/ dipping, asymmetric, low-angle detachment system (Kastens et al., 1988; Storti, 1995a; Jolivet et al., 1998) which separates the Sardinia-Corsica continental block (lower plate) from the overlying Apenninic belt (upper plate). Extension in the upper plate is accommodated by both east- and west-dipping, high-angle normal faults (Elter et al., 1975; Keller et al., 1993). Strike-slip faults overprinting this compressional and extensional framework have been described throughout the Apennines from the northern sector (Storti 1995a), via the central zone (Salvini, 1992), to the south (Monaco et al., 1998).

Fig. 3. The stratigraphy of the Umbria-Marche Succession, which is unconformably overlain by Pliocene-Pleistocene siliciclastic rocks (after Chiocchini et al., 1995). The large arrow indicates the main d̩collement at the base of the multilayer. Small arrows indicate the position of secondorder d̩collements.

GEOLOGICAL SETTING OF THE NARNI ANTICLINE

The Narni Anticline is located on the western margin of the central Apennine thrust belt which, to the east of the Tiber River Valley (Fig. 4), is made up of an east-verging array of unmetamorphosed carbonate thrust sheets and related terrigenous deposits. The rocks involved in the Narni Anticline belong to the Umbria-Marche Succession, a stratigraphic unit encompassing rocks of Triassic to Miocene ages (Fig. 3) (Chiocchini et al., 1995). The oldest rocks were deposited in a shallow-marine littoral setting (Burano Evaporites and Calcari a Rhaetavicula contorta Formations), which evolved into a carbonate platform (Calcare Massiccio Formation), then to a basinal environment (Corniola to Bisciaro Formations), passing up to the foredeep terrigenous clastics of the Marnoso Arenacea Formation. Pliocene-Pleistocene conglomerates and sandstones unconformably overlie the older, more deformed rocks (Parotto and Praturlon, 1975).

Fig. 5. SW-NE cross-section across the Narni Anticline (see Fig. 4 for location). Note the presence of an overturned succession involving the Umbria-Marche multilayer from the Corniola Formation to the Bisciaro Formation. Strike-slip re-activation of thrust ramps and extensional faults is evident. Ornaments as in Fig. 4.

The Burano Formation crops out in few places in the central Apennines; however, the oldest exposed rocks of the Umbria-Marche Succession generally belong to the Calcari a Rhaetavicula contorta Formation. Nevertheless, Burano Evaporites have been encountered in many boreholes (Martinis and Pieri, 1964), and this suggests that they may provide the basal d̩collement for the imbricate fan structure to the east of the Tiber River Valley (Bally et al., 1988). A number of marly formations within the stratigraphic succession may provide second-order d̩collements (Fig. 3).

In order to investigate the tectonic evolution of the Narni Anticline, a detailed research programme was carried out. This included field mapping, meso- and microstructural analysis of brittle deformational features, cross-section balancing, analogue modelling and geometric modelling (Storti 1995b). Illite crystallinity and clay mineralogy data support a shallow crustal level for the development of the Narni Anticline. In particular, illite crystallinity (K?bler index = 0.49?s ???2?) and mineral assemblages (calcite + quartz + illite + smectite + chlorite + illite/smectite ?hlorite/smectite mixed layers) strongly support early diagenetic conditions during deformation (Lezzerini et al., 1995). Geometric forward modelling including syntectonic erosion and sedimentation suggests that the structure evolved as a growth fold at the toe of the late Burdigalian Apenninic thrust wedge (Storti and Salvini, 1996).

A simplified structural map of the Narni Anticline is shown in Fig. 4. Most of the structural complexity developed during thrusting and folding. Four main thrust sheets are separated by three NW-SE trending thrust faults (Fig. 5). The tectonic transport direction is toward the NE. The lowermost thrust sheet crops out in the NE of the area, in the southern part of the Terni Basin (Fig. 4). At its surface are exposed the Marnoso Arenacea sandstones, marls and clays, whose overall attitude is generally flat-lying. This thrust sheet is overlain by a second one made up of pelagic carbonate rocks (Scaglia Rossa to Bisciaro Formations). The central and southern sectors of the roof fault of this thrust sheet were reactivated as a left-lateral strike-slip fault during the Pleistocene (Storti, 1995b). The third thrust sheet is made up of an overturned pelagic carbonate succession (Corniola to Bisciaro Formations). Second-order, bedding-parallel thrusts are located in the Marne a Fucoidi and Rosso Ammonitico. The presence of this overturned rock panel, which is more than 22km long and 4km wide (Fig. 4), is the most significant feature of the Narni Anticline. The overturned sector is in turn overlain by an upper thrust sheet which is also made up of a carbonate succession; this comprises the Calcare Massiccio which crops out widely in the northern and central sectors, up to the Scaglia Cinerea, exposed in the southern sector. A main lateral ramp occurs to the NW of Mt Cosce. Finally, remnants of the hangingwall ramp of a thrust-related anticline overlying the Narni Anticline are preserved along the western margin of the structure, and includes the Calcari a Rhaetavicula contorta, the Calcare Massiccio and the Corniola Formations.

Fig. 6. Forward-modelled stages in progressive rollover fault-propagation folding. (a) Simple-step progressive rollover fault-propagation folding (after Storti and Salvini, 1996). (b) d̩collement breakthrough. (c) synclinal breakthrough. (d) anticlinal breakthrough. (e) multiple breakthrough resulting from the superimposition of the earlier breakthrough geometries.

Development of the Narni Anticline can best be explained by progressive rollover fault-propagation folding (Storti and Salvini, 1996). In this model, the fault-fold kinematics of Suppe and Medwedeff (1990) have been modified by introducing additional bending in the frontal region of the fault-propagation anticline to account for the mechanical stability of the overturned forelimb (Fig. 6a). With continuing contraction, the simple-step anticline is translated foreland-ward, and is dissected by a multiple breakthrough array. This consists in sequence of d̩collement breakthrough, synclinal breakthrough and anticlinal breakthrough (Figs 6b, c and d). The superimposition of these breakthrough events ultimately produces the complex imbricate array modelled in Fig. 6e.

Fig. 7. Pitted stylolites perpendicular to bedding in the overturned forelimb of the Narni Anticline. In the stereographic projection, heavy lines are stylolites; broken line indicates the average attitude of bedding; and the dot indicates the average attitude of stylolitic pits. Schmidt net, lower hemisphere.

Thrust tectonics were overprinted by extensional and strike-slip faulting. A major NWSE trending, SW-dipping extensional fault system is located along the western margin of the Narni Anticline, and forms the eastern margin of the Tiber River graben (Bigi et al., 1989). These faults mark the boundary between the carbonate succession in the Narni Anticline and the Pliocene-Pleistocene clastic rocks infilling the Tiber River Valley; they were re-activated as left-lateral strike-slip faults during the Pleistocene (Storti, 1995b). Pleistocene NW-SE trending, left-lateral strike-slip faults also occur in the central region and along the eastern boundary of the Narni Anticline. A north-south trending, regional-scale, right-lateral brittle shear zone (the Sabina Fault of Alfonsi et al., 1991) delimits the Narni Anticline to the SE. To the NW, the anticline is separated from the Amelia Anticline by a NE-SW trending tear fault system, which was reworked during the Pleistocene by east-west trending, left-lateral strike slip faulting (Storti, 1995b).

FRACTURE DISTRIBUTION AND DEVELOPMENT IN THE NARNI ANTICLINE

The overprinting of three main deformation phases (thrusting, extensional faulting and strike-slip faulting) has resulted in a complex structural pattern ??? and a complex fracture distribution ??? in the study area. Fractures in the Narni Anticline can be grouped into three major patterns all of which overprint a pre-existing solution cleavage system related to layer-parallel shortening: (a) hinge-parallel and hinge ???perpendicular??? fractures (mostly pressure-solution cleavage) in the protolith rocks; (b) fault-parallel fractures in extensional damage zones; and (c) fault-oblique fractures in strike-slip damage zones.

Fracture panels

Rock panels between faults are much less fractured than damage zones. Three sectors can be identified in the Narni Anticline, based on the number, attitude and spacing of cleavage systems: (i) an outer sector, which includes the two eastern thrust sheets; (ii) an intermediate sector corresponding to the overturned thrust sheet, and (iii) an inner sector, which corresponds to the roof thrust sheet. The oldest structural elements that have been panel (Fig. 8d). The cleavage intensity in the overturned sector is high, and cleavage spacing varies from less than 1cm to about 20cm.

Fig. 8. Pressure-solution cleavage populations associated with folding and faulting in the Narni Anticline. (a) Relationship between cleavage (heavy lines) and bedding (broken lines) in the eastern thrust sheets. (b) Cleavage-bedding relationships in the overturned sector; note the presence of two cleavage populations trending parallel to bedding. (c) Cleavage-bedding relationships in the upper thrust sheet. (d) Typical fractured carbonate rocks cropping out in the overturned sector; note the presence of a fracture system oblique to bedding. (e) Beddingperpendicular, pressure-solution cleavage in the moderately fractured rocks in the upper thrust sheet. a, b, and c: Schmidt net, lower hemisphere.

Fig. 9. Former extensional fault in the Tiber River graben reactivated as a high angle reverse fault in a positive flower structure.

The inner sector consists largely of the limestones of the Calcare Massiccio Formation, which generally dip towards the SW, apart from minor, second-order gentle folds. The main pressure-solution cleavage system trends NW-SE, parallel to bedding, and dips steeply towards the NE (Figs. 8c, e). Another fracture system is more dispersed and strikes NESW. The spatial distribution across strike of the main fracture system is not uniform, and few, relatively narrow, intensely fractured rock panels are intercalated within the less deformed, wider ones.

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Fig. 10(a) Fault-cleavage relationships in extensional damage zones along the western boundary of the Narni Anticline. Pressure-solution cleavage surfaces (broken lines) trend parallel and dip at slightly shallower angles than the faults (heavy lines). (b) Fault-cleavage relationships in leftlateral strike-slip damage zones. Pressure-solution cleavage trends oblique to the faults. Schmidt net, lower hemisphere.

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Damage zones

Damage zones (Caine et al., 1996) in carbonate rocks are mainly exposed in the western and central sectors of the Narni Anticline, and are most clearly developed in the massive limestones of the Calcare Massiccio Formation. They have evolved in response to both extensional and strike-slip faulting. However, due to the footwall position of the Narni Anticline with respect to the Tiber River Valley boundary fault system, and to its strike-slip re-activation (Fig. 9), first-order extensional damage zones have not been preserved. Second-order extensional damage zones consist of an array of pressure solution cleavage surfaces trending almost parallel to the fault surfaces, which overprint the fracture fabric of the protolith rocks. The dip of the extension-related, pressure-solution cleavage system is slightly shallower than that of the corresponding faults (Fig. 10a), and the spacing of the cleavage surfaces ranges from less than 2cm to about 10cm. Fault cores are generally not well developed, probably due to the small displacement along these fault zones.

The best-developed damage zones relate to strike-slip faulting. A particularly well exposed zone crops out along a NW-SE trending belt in the central sector of the Narni Anticline (Fig. 4). Its width ranges from few metres along splay fault strands in anastomosed arrays, to about 50m where the fault zone is localised. Strike-slip damage zones are typically accompanied by a pressure-solution cleavage system which makes an acute angle with the related fault surfaces (and faces the fault-slip direction) (Fig. 10b) (???fault-propagation cleavage??? of Salvini et al., 1999). This angle varies from about 30?o 55?ith an average value of about 40? very similar to that characterising the left-lateral, strike-slip Mattinata Fault in the Gargano Promontory (Salvini et al., 1999; Billi and Salvini, 2001). Pressure-solution cleavage in strike-slip damage zones in the Narni Anticline overprints both the fracture pattern of the protolith rocks, and also the extension-related, pressure-solution cleavage system where extensional damage zones have been re-activated. The spacing of the cleavage in the strike-slip damage zones varies from less than 1cm to about 16cm (Fig. 11a).

Fig. 11. Field photographs of fault rocks associated with strike-slip faulting in the Narni Anticline. (a) Strongly fractured Calcare Massiccio limestones in a damage zone. (b) Poorly cohesive cataclasite in the Calcare Massiccio Formation. (c) Vertical foliation in the Scaglia Cinerea Formation, here involved in a positive flower structure.

Fig. 12. Forward-modelled fracture panels related to progressive rollover fault-propagation folding (see Fig. 6). The increasing intensity of shading indicates increasing fracture intensity. (a) Simple-step fault-propagation folding. Note the high fracture intensity predicted in the forelimb, and the virtually undeformed crest. (b) d̩collement breakthrough. (c) synclinal breakthrough. (d) anticlinal breakthrough. (e) multiple breakthrough resulting from the superimposition of breakthrough geometries in b, c and d. Forward models of fracture panels have been generated following Salvini and Storti (1997).

Strike-slip damage zones surround cataclastic and brecciated bands constituting the fault cores (Caine et al., 1996) (Fig. 11b). Fault cores are lens-shaped and their along-strike width varies from less than 1m to more than 10m. Fault-core rocks are poorly cohesive to non-cohesive and their grain size varies from less than 0.1mm to approximately 50mm. Localisation of fault motion has produced centimetric to decimetric bands of fault gouge within the fault cores. The distribution of fracturing is more complex where strike-slip faults have re-activated previously extensional damage zones. The high-angle superimposition of the two fault-related fracture arrays favoured rock fragmentation, comminution and the development of wider fault cores.

A very different fabric characterises the fault zones exposed in the eastern side of the Narni Anticline. Fault zones mostly occur in the marls and marly limestones of the Scaglia Cinerea and Bisciaro Formations. The weak rheology of these rocks favoured re-orientation of the pre-existing planar fabric (bedding and thrust-related pressure-solution cleavages) within highly-localised, elongate belts of near-vertical to vertical foliation (Fig. 11c).

DEVELOPMENT OF FRACTURES AND THE LOCALIZATION OF FAULTS

Fracture systems developed throughout the evolution of the Narni Anticline, from layer-parallel shortening predating folding to much later strike-slip faulting. The mechanical anisotropy produced by the pre-existing fracture patterns influenced the location and geometry of younger structural elements. To model the interaction between fracturing and faulting over time, we assumed that the discontinuous fracture distribution developed by faulting and folding overprinted an earlier homogeneous one resulting from parallel shortening.

Fig. 13a. Forward-modelled multiple breakthrough structure (Fig. 6e) overlain by a passivelyrefolded, thrust-related anticline. Notice the steeply-dipping ramp (46? at the rear of the structure. (b) Extensional re-activation of the former thrust ramp at the rear of the structure. (c) strike-slip re-activation of the former extensional fault at the rear of the structure and of thrust ramps in the frontal part of the structure. The heavily-fractured, overturned sector favours strike-slip fault location in the central sector of the structure.

Storti and Salvini (1996) predicted the distribution of longitudinal deformation panels (i.e. panels of rock in which the deformational features are expected to be homogeneous, henceforth referred to as ???fracture panels???) during simple-step progressive rollover faultpropagation folding (Fig. 12a). We used the same semi-quantitative approach to predict fracture panel distribution for breakthrough configurations. Fig. 12a shows a heavilyfractured panel in the overturned core and recumbent forelimb, sandwiched between two less-deformed sectors. However, the crest of the anticline is virtually undeformed. The fracture pattern which is developed during progressive rollover fault-propagation folding favours fault breakthrough during folding (Suppe and Medwedeff, 1990) and, consequently, the increased complexity of the fracture panels. Figs. 12b to 12d model the distribution of fracture panels for d̩collement, synclinal and anticlinal breakthrough, respectively. These breakthrough arrays occurred sequentially during development of the Narni Anticline, and superimposition of the corresponding fracture panels has produced a complex final pattern (Fig. 12e). It is worth noting that the predicted attitude of hinge-parallel fractures is generally not perpendicular to bedding. Even in a complex breakthrough array like that in Fig. 12e, the upper thrust sheet preserves wide panels whose fracture intensity can be predicted to be moderate, due to the gentle angles by which the rocks have been folded. Moreover, fractures and bedding are nearly perpendicular. This deformation pattern closely matches that which we observed in the Narni Anticline, where the Calcare Massiccio limestones in the roof thrust sheet are generally only moderately fractured, with a few localised panels in which the fracture intensity is higher.

When the thrust-related anticline at the rear of the Narni Anticline is included in the forward-modelled structure (Fig. 13a), the passively backward-tilted footwall ramp constitutes a weak zone suitable for extensional re-activation. In the study area, it is likely that this pre-existing zone of weakness localized extensional faulting to the west of the Narni Anticline, while the bulk of the fold was preserved unaffected by significant extension. This model implies that only a major thrust-related damage zone was re-activated during the onset of the extensional tectonics and the development of the Tiber River Valley graben.

The model in Fig. 13b shows two main weak sectors at the end of extension: the highly-localized extensional fault zone on the western side; and the overturned panel, which is heavily fractured and bounded by thrust faults, and in which bedding dips steeply towards the SW. The overturned rock panel thus provides a number of possibilities for fault localization: the boundary thrusts, the hinge zones separating adjacent fracture panels, and weaker layers within the steeply-dipping rock multilayer. In the Narni Anticline, all these zones of weakness in the overturned limb, as well as the pre-existing extensional fault zone along the western boundary, have been re-activated by NW-SE trending, left-lateral strike-slip faulting. The near-vertical attitude of strike-slip faults does not prevent re-activation of thrust-ramps at depth in a staircase trajectory, as proposed in Fig. 13c.

The overprinting of deformation associated with strike-slip damage zones onto a preexisting fracture pattern favoured rock fragmentation and comminution, and the development of cataclastic rocks. The spatial variability of folding-related fractures may have favoured the three-dimensional discontinuous occurrence of cataclastic rocks.