Pre-Quaternary orogen-parallel extension in the Southern Apennine belt, Italy

ELSEVIER

Tectonophysics260 (I 996) 325-347

Pre-Quaternary orogen-parallel extension in the Southern Apennine belt, Italy L. Ferranti " J.S. O l d o w b M. S a c c h i c a Dipartimento di Scienze della Terra, Universit~ di Napoli "'Federico 11' ', Napoli, Italy J b Department of Geology and Geophysics, Rice University, Houston, Texas, USA 2 c Istituto di Ricerca Geomare Sud, C.N.R., Napoli, Italy

Received 30 June 1995; accepted 5 December 1995

Abstract

The Southern Apennine fold and thrust belt differs from other parts of the peri-Tyrrhenian orogen. In most of the peri-Tyrrhenian belt, hinterland extension is oriented at a high-angle to the orogen axis and appears to be related to rifting and formation of oceanic crust within the Tyrrhenian basin. The Southern Apennines share the late-stage development of normal faults related to the opening of the Tyrrhenian Sea, but also experienced an episode of extension parallel to the strike of the tectonic belt. The orogen-parallel extension was apparently formed in response to the increase in length of the deformed belt during arcuation. Arcuation ostensibly was related to asymmetrical rifting in the hinterland, which was greater in the Southern Tyrrhenian Sea than in areas to the north, and proportionately greater shortening in the frontal parts of the southern belt as compared to regions in the north. During arcuation, extension was spatially concentrated within structural domains and was accomplished by displacement on low-angle detachment faults cutting through a previously imbricated thrust stack. During the Miocene-Pliocene, NNW-SSE extension in the interior of the Southern Apennine belt formed coevally with ENE-WSW shortening in the foreland. Longitudinal extension ceased in the Pleistocene, when younger high-angle normal faults formed in response to the easterly migration of Tyrrhenian Sea rifting and NE-SW extension associated with lithospheric stretching.

1. I n t r o d u c t i o n Within the peri-Tyrrhenian orogen (Fig. 1), the Apennines form an arcuate belt that bounds the Tyrrhenian Sea to the east. Late Cenozoic contraction formed as Europe and Adria, a promontoryof North Africa, collided (Channell et al., 1979). Meso-

' now at the Istituto di Ricerea Geomare Sud, C.N.R., Napoli, Italy 2 now at the Department of Geology and Geological Engineering, University of Idaho at Moscow, Idaho, USA

zoic and Cenozoic rocks deposited on the western margin of Adria were detached from their basement and thrust from west to east onto the Apulian foreland of Italy (Fig. 1) (D'Argenio et al., 1975; Channell et al., 1979). Hundreds of kilometers of shortening were accommodated above a basal "decollem e n t " , which stretches from the foreland to depths o f 10-15 km beneath the inner parts of the orogen (D'Argenio et al., 1975; Bally et al., 1986; Mostardini and Merlini, 1986; Patacca and Scandone, 1989). In mid-late Miocene, severe stretching and subsidence affected the Africa-Europe collisional belt

0040-1951/96/$15.00 Copyright © 1996 XXXX. Published by Elsevier Science B.V. SSD! 0040-1951 (95)00209-X

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L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

and led to the formation of the Tyrrhenian Sea (D'Argenio et al., 1975; Malinverno and Ryan, 1986; Kastens et al., 1988; Sartori et al., 1989). Since then, the Apennine and Tyrrhenian basin acted as a linked tectonic system, characterized by foreland migration of the contractional front followed in space and time by orogen-normal extension related to rifting and formation of oceanic crust within the Tyrrhenian basin (Lavecchia, 1988; Patacca and Scandone, 1989). In the Northern Apennines and Corsica (Fig. 1), extensional structures locally reactivated thrusts and had displacements parallel to the regional shortening axis (Carmignani and Kligfield, 1990; Jolivet et al., 1990; Keller and Pialli, 1990). In the

Messinian, the earlier extensional detachments were cut by high-angle normal faults (HANFs) with displacements also oriented normal to the orogenic belt. In contrast, the late Miocene-Pliocene extension axis in the Southern Apennines was parallel to the strike of the tectonic belt (Oldow et al., 1993) and displacement was accommodated on low-angle normal faults (D'Argenio et al., 1987). In the Southern Apennines (Fig. 1), the low-angle detachments are cut by younger HANFs but the age of their initiation is younger than in areas to the north. The onset of late-stage extension on HANFs youngs southward along the axis of the Apennine chain from late Miocene in the north to early Pleistocene in the

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L Ferranti et al. / Tectonophysics 260 (1996) 325-347

south (Ciaranfi et al., 1983; Gars, 1983; Lavecchia, 1988; Carmignani and Kligfield, 1990), where they are probably still active as documented by faulting in recent deposits and fault-plane solutions of earthquakes (Cello et al., 1982; Gasparini et al., 1985).

2. Geologic setting of the Southern Apennines The Southern Apennines stretch from Abruzzi to Calabria and can be divided into external and internal structural domains, passing westward into the Tyrrhenian Sea (Fig. 2). The present-day foreland is bound to the west by imbricate thrusts composed of synorogenic rocks deposited in foredeep and piggyback basins (Mostardini and Merlini, 1986; Patacca and Scandone, 1989; Patacca et al., 1990). The age of synorogenic rocks progressively decreases from late Miocene to Pliocene-early Pleistocene toward the east. In external regions, these rocks are involved in contractional structures recording ENE directed tectonic transport (Patacca and Scandone, 1989). Within the internal sector of the orogen, from the Tyrrhenian margin to the axial part of the mountain belt (Fig. 2), allochthons derived from three groups of rocks are identified and differentiated (D'Argenio et al., 1993). For simplicity, the lithotectonic successions composing the allochthons are termed the CPBS (Carbonate-Platform and Basin System), Internal Basin and Foredoep assemblages. The CPBS assemblage is composed of thrust sheets derived from carbonate platform and intervening slope and basin domains. The rocks are mid-Triassic to late Miocene in age (D'Argenio et al., 1975, 1993) and form a number of tectonic elements that differ in thickness, areal extent and number of imbricate thrusts. The Internal Basin assemblage consists of thrust nappes derived from basin and slope domains formed on transitional and/or oceanic crust (D'Argenio et al., 1975, 1993; Channell et al., 1979; Marsella et al., 1995). They include, in ascending structural position, the Mesozoic to lower Miocene Lagonegro, Sicilid and Ligurid lithotectonic assemblages. To the southern border of the belt, Ligurid assemblage rocks and associated Mesozoic ophiolitic suites, along with pre-alpine and alpine crystalline rocks of Calabria and their Jurassic to Tertiary sedimentary cover,

327

structurally overlie the entire Southern Apennine edifice (Ogniben, 1969; D'Argenio et al., 1975; Amodio-Morelli et al., 1976). The Foredeep assemblage of the internal sector contains thrust sheets of Miocene clastic rocks related to ancient piggyback and foredeep systems (e.g., Cilento and Irpinian units). These sequences form relatively thin imbricates which lie both above the older deformed Apenninic units or in between them. The deformational history of the Southern Apennines was characterized by multiple episodes of non-coaxial tectonic transport. A schematic chronology of deformation across the Southern Apennines paleosedimentary domains is illustrated in Fig. 3. Contraction migrates toward the east from more internal to more external domains, and is accompanied by deposition in coeval foredeep and piggy-back basins (Patacca et al., 1990) and by different episodes of extension. The tectonic evolution is divided into two broad intervals separated by the late Tortonian onset of rifting in the Tyrrhenian basin (Channell et al., 1979; Patacca and Scandone, 1989). During pre-late Tortonian time, convergence between Africa and Europe and consumption of Tethyan oceanic crust led to imbrication of the Internal Basin nappes beneath the Corsica-Calabrian crystalline complex (Channell et al., 1979; Knott, 1988; Dewey et al., 1989). Ligurid and Lagonegro nappes were assembled in an accretionary prism by late Burdigalian-Langhian time (Fig. 3) (D'Argenio et al., 1993; Marsella et al., 1995) during westerly subduction of Adriatic lithosphere beneath Europe. Continued shortening caused the western portions of the CPBS domain to underthrust the accretionary prism in middle-late Miocene (D'Argenio et al., 1993) with E to NE transport on the order of a few hundred kilometers (D'Argenio et al., 1975; Patacca and Scandone, 1989). After Tortonian-Messinian time, as rifting was occurring in the Tyrrhenian basin, contraction moved toward the external part of the belt and accompanied orogenic arcuing. During the late Messinian to late Pliocene, more than 100 km of shortening took place in the internides on NE- to NNE-directed envelopment and/or out-of-sequence thrusts that broke the previously imbricated assemblages and their synoro-

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

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L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

genic cover (D'Argenio et al., 1993; Pappone and Ferranti, 1995). Late stage thrusting and migration of the contractional front to the external sectors of the orogen was accompanied by extension in the internides of the Apenninic carbonate belt (Fig. 3). During the late Miocene to early Pliocene, SSE-directed extension on low-angle normal faults (LANFs) dismembered the thrust pile (D'Argenio et al., 1987, 1993; Oldow et al., 1993). In the early Pleistocene, a non-coaxial extensional system cut across the already thinned Southern Apenninic belt in response to the southeast migration of the Tyrrhenian rift basin (Kastens et al., 1988; Sartori et al., 1989). WSW displacement on high-angle normal faults (HANFs) created differential uplift among various Apenninic blocks and the formation of NW-SE- and NE-SW-trending grabens on the coastal region of southern Italy (Fig. 2). The late stage extension was associated on the new continental margin (Figs. 1 and 2) with magmatism composed of high-K calcalkine, shoshonite, leucitebasanite, and leucite series magmas related to deep lithospheric rifting (Beccaluva et al., 1989).

3. L o w - a n g l e faults in the S o u t h e r n A p e n n i n e s

Low-angle faults are common structures found throughout the Southern Apennines. In most instances they are related to regional contraction, and they juxtapose sedimentary sequences in an olderon-younger configuration. In these areas, the fault surfaces are often folded and bedding on either side of the faults commonly parallels the fault zone. Little brittle deformation is associated with the faults, which typically have sharp contacts. The current attitude of the faults is not indicative of transport direction due to folding and possible rotation by later faults. These low-angle faults rarely have slip indicators on their surfaces but minor folds and locally developed tectonic foliations within the fault zones are consistent with mesoscopic and regional-scale folds of bedding and the allochthons themselves. In the Lagonegro region, fold orientation

Fig. 2. Generalizedstructuralmap of the SouthernApennines,showing regional tectonic features. Outlinedareas are shown in Fig. 5a (A), 6 (B), 8a (C), 9a (D), 10 (E).

L. Ferranti et al. / Tectonophysics 260 (1996) 3 2 5 - 3 4 7

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L Ferranti et al. / Tectonophysics 260 (1996) 325-347

of structural section yield older-on-younger configurations (e.g., Miocene sediments tectonically trapped between allochthons of Mesozoic rocks). These faults probably were earlier thrusts reactivated in extension (D'Argenio et al., 1987). Within all three structural domains, clear crosscutting relations are observed between older, contractional low-angle faults and younger structures attributed to extension. We describe the salient features of the structural domains below. 3.1. Matese domain

The Matese Mountains, located near the CentralSouthern Apennines boundary (Figs. 1 and 2) form one of the main axial culminations of the orogenic belt. Structures related to contraction and subsequent extension are found mostly along the southern border of the massif (Ferranti, 1995). The massif is composed of Upper Triassic to Miocene carbonate platform and margin rocks (D'Argenio et al., 1975) that were folded and imbricated during ENE and subsequent NNE to N directed shortening (Ferranti, 1995). Contractional faults show ramp-fiat geometries and frequently involve thin horizons of late Miocene terrigenous elastic cover rocks. Contractional shear zones are characterized by sharp slickensides and lithons aligned in the transport direction and the rocks on each side of the faults show negligible deformation. The carbonate imbricates form gentle anticlines and synclines with steep NW-SE axial planes, consistent with NNE-SSW shortening determined from the orientation of fault-related lithons and interlayer slip (Fig. 5b, stereonet a). The contractional structures are invariably cut by low-angle faults at the base and top of each imbricate (Fig. 5a, b). The younger faults exhibit a sheared cataclastic texture in zones a few tens of meters thick. Kinematic indicators (ploughing scratches, calcite fibers and steps) on low-angle surfaces indicate hanging-wall displacement down to the S (Fig. 5b, stereonet b). The younger low-angle faults form a regional detachment system that is kinematically linked to steeper (400-60 °) strike-slip faults oriented N-S in the hanging-wall block (Fig. 5a and stereonet b in Fig. 5b). The low-angle faults consistently place younger rocks on older, and where internal stratigraphic relations are well understood, omit up to 1

331

Ion of section. The detachment commonly juxtaposes Cretaceous limestone over subjacent Upper Triassic to Liassic carbonate rocks (Fig. 5a). Locally, the lower plate is itself divided into two imbricates by a lower fault branching from the main detachment surface (Fig. 5a). Imbricates in the lower plate are formed by Upper Triassic dolomite and overlying Upper Triassic-Liassic dolomitic limestone, respectively (Fig. 5b). The shallow-dipping fault surfaces outcrop as broad domes and are gently undulated at map-scale (M. Cila, Mass. Calcarone in the southern part of Fig. 5a). Corrugations define the long-axis of the domes and are parallel to the direction of upper plate transport. Both contractional and extensional low-angle fault systems are cut by WNW to SSE striking high-angle normal faults active during NE-SW extension (Fig. 5a, b). The younger normal faults reactivate at places older low-angle surfaces (forming the NE-SW slip lineations on shallow-dipping faults in Fig. 5b, stereonet b) and involve lower Pleistocene elastic rocks (Ferranti, 1995).

Fig. 4. Low-angle normal fault (traced by white line) near Giffoni Vallepianavillage,PiccntiniMountains(locatedin Fig. 8), which

juxtaposes Jurassic slope limestonesand marlsover Catalanshallow water dolomites of the CPBS assemblage. Rocks in the hanging-wall (overhangingin the upper part of the quarry) still retain bedding.Rocksin the footwallare pervasivelycrushedto a loose cataclasite. Dark glands includedin the footwallare composed of black dolomite,and represent tectonic slivers boudined duringextensionalshear(sense of shearin the hanging-wallblock is comingout of the quarry). Note undulationsof the faultsurface with axis parallel to transportdirection.Iron frameon the wall is about 3 m high.

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

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Fig. 5. Geologicalmap (a) and section(b) of SouthernMatese Mountains(locationin Fig. 2). Cumulativestereoplotsof thrusts, low-angle and high-anglenormal fault data collectedthroughoutthe area are inserted in b. Stereoplotsare lower hemisphere,equal angle nets in this and in the followingfigures; short arrows are in the sense of hanging-wall displacement, large white arrows outside the stereonetsshow kinematic axes. 3.2. Lattari-Picentini domain

The Picentini and Lattari Mountains are located in the central part of the Southern Apennines (Figs. 1 and 2) and are predominately composed of shallow water carbonate rocks of the CPBS assemblage with ages of Upper Triassic to Cretaceous (D'Argenio et al., 1987). Overlying and locally interleaved with the CPBS assemblage rocks are thrust sheets (Ietto, 1963; Seandone et el., 1967; Turco, 1976; Ferranti and Pappone, 1992) composed of the Internal Basin assemblage (Lagonegro and Sicilid units). Synorogenic rocks involved in contractional structures are Langhian-Serravalian(?) in age in the Lattari Mountains (De Blasio et el., 1981) and Serravallian-Tortonian(?) in the Picentini Mountains (Scandone et al.,

1967). Contractional structures young to the NE, where folds and thrusts involve rocks of late Pliocene age (Pappone and Ferranti, 1995). 3.3. Lattari Mountains

In the Lattari Mountains (Fig. 6) several klippen of younger carbonate rocks overlie older carbonates of the CPBS assemblage and form an E - W welt typified by domino-style faults and cataclastic shear zones. The age of rocks affected by and the magnitude of tectonic omission increases to the SE. In the west (section AA' in Fig. 6), Liassic to Lower Cretaceous rocks are structurally overlain by Lower to Upper Cretaceous carbonates (Cinque, 1980). In a few isolated areas, a thin horizon of

334

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

Langhian-Serravalian(?) synorogenic terrigenous deposits (De Blasio et al., 1981) is observed along the south-dipping fault that juxtaposes Upper Cretaceous above Dogger rocks. Rocks in the hanging-wall are folded and cataclasized and exhibit northwestward dips of 20 ° to 60 ° (Fig. 6). Rocks in the footwall are subhorizontal but are affected by minor low-angle faults that are synthetic to the overlying detachment. The base of the main fault zone is sharp and upper plate cataclasis gradationally decreases upsection over distances of few tens of metres. Secondary low-angle surfaces undulate on a 1 m wavelength

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about a N-S axis. Riedel shears and T-structures on the E - W striking low-angle faults and the consistent northward dip of upper plate bedding indicate top to the south displacement (stereonet in section AA', Fig. 6). In the eastern part of the Lattari Mountains, at Monte Avvocata (section B B ' in Fig. 6), klippen composed of Rhaetian-Liassic rocks of the CPBS are juxtaposed above Norian-Rhaetian(?) dolomite (Iannace, 1991). Upper plate rocks dip 25 ° to 70 ° to the north and in all cases have greater dips than rocks of the lower plate (10°-20°). The lower-plate

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Fig. 6. Generalized geological map and cross sections across the eastern Lattari Mountains (location in Fig. 2). Map redrawn after D'Argenio et al. (1987), sections AA' and BB' redrawn and modified after Cinque (1980) and lannace (1991), respectively. Stereoplots of slip vectors determined on low-angle normal faults are inserted. Extension direction on low-angle normal faults and variably oriented subsidiary faults is roughly N - S (large white arrows, 1). E - W lineations (large white arrows, 2, on lower stereonet) are referred to reactivation during later extension. Klippen numbered in map are: 1 = M. Pizzuilo; 2 = S. Maria a Castello; 3 = M. Faito; 4 = Colle La Serra; 5 = Piana di Agerola; 6 = M. Murillo; 7 = M. Lattaro-Cervigliano; 8 = M. Avvocata; 9 = M. Falerio; 10 = M. S. Liberatore; /1 Creste di Salerno; 12 = M. Stella.

L Ferranti et al. / Tectonophysics 260 (1996) 325-347

succession is internally dissected by bedding-parallel faults which record top to the south displacements (Fig. 6). At Monte San Liberatore (point no. 10 in Fig. 6) a klippe of Liassic carbonate is separated from underlying Upper Triassic dolomite and marls by a brittle shear zone more than one hundred metres thick. A complex pattern of faulting formed by the intersection of two low-angle extensional fault systems is responsible for the great tectonic omission found beneath the klippe. Throughout the Lattari Mountains, upper-plate rocks exhibit bedding-cutoff angles compatible with top to the south displacement. This transport direction is supported by structures measured on fault surfaces and is consistent with N - S extension proposed by D'Argenio et al. (1987). The low-angle, south-dipping detachments account for variable section omission (from few hundred metres up to 1 km) that increases to the southeast. The local occurrence of Langhian-Serravalian(?) deposits along the western exposures of the detachment (De Blasio et al., 1981) suggests an earlier history of sequence duplication (Cinque, 1980). Due to this occurrence, the upper plate rocks are related to a different thrust imbricate compared to the rocks residing in the lower plate of the present extensional detachment, indicating that omission estimates based on stratigraphic gaps greatly underestimate the structural removal of section. The present-day morphology of the Lattari Mountains is formed by high-angle normal faults that are superposed on all low-angle structures, and were active during E N E - W S W extension. Often, superposed lineations are observed on the earlier structures and are consistent with westerly extension related to the high-angle faults, indicating some degree of reactivation of the old features in the existing extensional regime (see stereonets in Fig. 6).

335

ESE belt of tectonic windows (numbers in Fig. 8a). The older-on-younger juxtaposition of lithotectonic assemblages derived from different paleogeographic domains, suggests a contractional origin for these faults (Ietto, 1963; Scandone et al., 1967). Rocks in both the hanging-wall and footwall successions are coherent and typically exhibit bedding orientations subparallel to the faults. Locally, the fault system is broadly folded around a N - S axis. No slip indicators have been observed on the faults, but analysis of folds within footwall units indicate northeastern tectonic transport (Pappone and Ferranti, 1995). Superposed on the earlier thrust faults is a pervasive system of shallowly south-dipping shear zones. Within the CPBS assemblage, which forms the upper allochthon in the Picentini Mountains, the younger shear zones consistently produce a younger-on-older configuration with large-developed bed-rotation (Fig. 8b). These low-angle faults define the base of several klippen (Fig. 8a) and are responsible for more than 2 km of stratigraphic omission within the CPBS (D'Argenio et al., 1987). A spectacularly developed cataclastic texture is associated with the younger faults (Figs. 4 and 8). A fault-rock succession is laid out in the footwall where coherent carbonates are transformed into brecciated carbonate, which passes into a very finegrained gouge zone. The breccia is internally organized by coalescing normal-slip faults arranged in

3.4. Picentini M o u n t a i n s

East of the Lattari Mountains, the Picentini form a mountain chain that lies between the Sele coastal plain of the Tyrrhenian sea to the southwest and the Irpinia region to the northeast (Fig. 8a). Low-angle faults that juxtapose CPBS allochthons above the Lagonegro unit of the Internal Basin assemblage (Ietto, 1963; Scandone et al., 1967) form a W N W -

Fig. 7. M. Salvatore low-angle normal fault idippe, Picentini Mountains (located in Fig. 8). The hanging-wall block is composed of Jurassic-Paleogene shallow water limestones dipping toward the North (toward the left-hand side of the photograph). The footwall block is composed of intensenly cataclasized Carnian dolomites. Note the well developeddomal configurationof the low-anglenormal fault surface(traced by white line).

336

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

lenticular or domino fashion. Fault displacements resulted in rigid-body rotations of fault-bound blocks. Immediately below the detachment, the fine-grained

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337

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

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the magnitude of tectonic omission and presumably upper-plate displacement (Ferranti, 1994). Cataclasite and bed rotation at places involve the older contractional faults that carry the CPBS over the Internal Basin units. In these areas, omission of substantial structural section is observed in both the upper and lower plates. Tracing individual faults laterally reveals significant differences in stratigraphic omission and occasionally produces younger-on-older configurations. These segments of the earlier thrusts are thought to have been reactivated during subsequent extension. Throughout the Picentini Mountains, the young low-angle faults show a gentle southerly dip and are planar or broadly undulate at the outcrop scale (Fig. 4). At the regional scale, the fault surfaces define

broad domes with wavelengths on the order of 1 km and flank dips varying from a few to up to 30 ° (Fig. 7). Some of the domes are elongated in the slip direction. Kinematic analysis of the brittle shear zones, using corrugations of fault surfaces, slickenlines, tension fractures and shear bands indicate normal-slip with the top to the SSE (stereonet in Fig. 8b). A strong asymmetry in the structures within and adjacent to the shear zones suggests a simple shear mechanism of deformation. Asymmetric intrafolial folds have a uniform shear sense to the SSE, consistent with the tilt direction o f upper-plate stratigraphic sequences outside of the shear zone. SSE-vergent drag folds are restricted to shear zones and rapidly attenuate away from the main detachment. The drag

338

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

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339

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

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folds typically are overprinted by brittle fractures and faults, suggesting a change from brittle-ductile to brittle conditions during progressive deformation. Slip directions determined on the young low-angle faults and variably oriented secondary structures in hanging-wall successions suggest that all of the structures were kinematically linked as part of a through-going displacement system. Detachment faults tend to branch and locally incorporate pre-existing thrust faults, such as the boundary between the CPBS and Internal Basin assemblages, and follow regionally expansive areas of subhorizontal bedding in the footwall. A single regionally extensive detachment is found in the northern part of the Picentini

Mountains, whereas two detachment systems are observed in the southern part of the range (left-hand side of section in Fig. 8b). Systematic cross-cutting relations indicate the superposition of the deeper on the shallower detachment, suggesting a stepping down of displacement during progressive deformation. NW-SE-trending high-angle normal faults cut the low-angle extensional structures (Fig. 8). Locally, the earlier structures were reactivated where favorably oriented. Reactivation is documented in several locations where earlier NNW-SSE lineations are overprinted by E - W to N E - S W lineations related to motion on the HANFs. The onset of high-angle

340

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

normal faulting is constrained as beginning in the early Pleistocene (D'Argenio et ai., 1987; Brancaccio et al., 1991).

3.5. Calabria-Lucania domain In the southeastern part of the Southern Apennines, from the Foraporta Mountain to Northern Calabria (Fig. 2), the fold and thrust belt is characterized by a complex structural architecture consisting of arcuated fault traces, thrust imbricates and envelopment and/or out-of-sequence thrusts (Scandone, 1972; Dietrich, 1976; Mazzoli, 1993, Marsella et al., 1995). In the south and west, the Calabrian crystalline rocks and subjacent Internal Basin nappes (Sicilids, Ligurids, and ophiolitic units) overlie the CPBS assemblage (Ogniben, 1969; D'Argenio et al., 1975; Amodio-Morelli et al., 1976). Eastward emplacement of the Internal Basin assemblage over the CPBS occurred in the early Miocene and contraction within the CPBS carbonate units continued through mid-late Miocene (Amodio-Morelli et al., 1976; Dietrich, 1976; Dewey et al., 1989).

cataclastic zone and the observation that fragments inside the microbreccia are themselves composed of tectonic breccia suggest that rock crushing was produced during progressive deformation. Kinematic analysis of selected low-angle faults exhibiting calcite fibers, tension gashes, and secondary shear planes indicates south to southeast upper-plate motion (Fig. 9b). The shallowly-dipping faults that imbricate the M. Foraporta sequences are soled into the thrust fault which carries the whole CPBS assemblage above the Lagonegro rocks, which is interpreted to have been reactivated during southward extension (Fig. 9b, section 2). Some uncertainty in transport direction arises from NE-SW striae observed on many low-angle detachments. SSE displacement was also determined on high-angle strike-slip and oblique-slip faults that sole into the low-angle detachments. Consequently, the NE-SW striae on the subjacent detachments are attributed to younger displacement associated with reactivation of the basal faults, which did not affect the inactive, overlaying structures.

3.7. Pollino region 3.6. Foraporta Mountain In the Foraporta Mountain region (Fig. 9), Upper Triassic-Dogger carbonates (De Alfieri et al., 1987) of the CPBS (Foraporta Unit) structurally overlie rocks of the Internal Basin assemblage (Lagonegro units), which were folded during ENE shortening (Mazzoli, 1993). Within the CPBS assemblage, numerous tectonic contacts are recognized and characterized by a consistent younger-on-older juxtaposition (Fig. 9a) and cataclasite zones hundreds of metres thick. Individual structural sheets rarely are more than several hundred metres thick and are strongly attenuated or missing along strike. At places, four to five structural sheets are found within one hundred metres of structural relief (Fig. 9b) and dolomites are intensely shattered and rendered to loose gouge by comminution and grain size reduction. Dark gray and white ultracataclastic shear bands, derived from stretching different stratigraphic horizons (dark intraplafform basinal and white peritidal dolomite of Upper Triassic age) are limited by hardened planar or undulating microbreccia ledges that exhibit polished surfaces. The sharp limit between

The Pollino massif is 2 km high and 40 km long and is one of the last outcrops of the CPBS assemblage north of the Calabrian arc (Fig. 2). In the Pollino massif, Mesozoic to Cenozoic carbonate platform sequences are unbroken (right-hand side of sections in Fig. 10), whereas to the west the same rocks are cut by low-angle faults characterized by structural discordance between beds in the hangingwall and footwall, cataclasite, and omission of stratigraphic section (center and left-hand side of Fig. 10). Thrust sheets composed of different paleotectonic elements of the CPBS are severely thinned on faults that cross thrusts at low-angles (Ietto and D'Argenio, 1990). Commonly, beds of the upper-plate successions have a moderate to steep northerly dip and are truncated against the underlying detachment faults (Fig. 10). Numerous synthetic and antithetic normal faults and tension fractures are found in the hanging-wall successions and consistently merge into or are truncated by the detachments. Though weakly dispersed, lineations on secondary low-angle faults in hanging-wall sequences are consistent with the sense of slip on master low-angle

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

341

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342

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

faults deduced from bedding cut-off criteria and indicate top to the south displacements (stereonet on section 1 in Fig. 10). Oblique-slip and strike-slip faults in the uppermost structural sheet appear to merge into the underlying detachment and are characterized by slip lineations recording SSE displacement (stereonet on section 2 in Fig. 10). All of the low-angle faults are cut by high-angle normal faults formed during SW displacement. Again, in many instances, the low-angle surfaces show evidence of later ENE-WSW reactivation, possibly related to Quaternary extension (stereonet on section 1 in Fig. 10).

4. Pre-Quaternary extensional architecture of the Southern Apennines As pointed out in the previous sections, kinematic analysis of structures and cut-off relations of bedding in hanging-wall assemblages indicate that transport on low-angle normal faults was subparallel to the axis of the orogenic belt and orthogonal to the main thrust direction. The structures related to longitudinal extension are concentrated in areally restricted zones, or domains of "hyper-extension", that parallel the axis of the tectonic belt from the Abruzzi region of Central Apennines to Calabria throughout the Southern Apennines (Fig. 2). The hyper-extension domains are developed across the west-central flank of the orogenic belt and expose rocks of the CPBS assemblage and the Internal Basin assemblage. Within each domain, greater extensional strain is recorded than is found in the belt as a whole and original thrust geometries are highly disrupted. Outside of the highly extended domains, earlier contractional structures are well preserved. Toward the foreland to the east, evidence of longitudinal extension disappears. Within the extension domains, estimates of the magnitude of stretching are imprecise and evenly scattered. Nevertheless, by using the contractional geometries preserved in adjacent regions, reasonable first-order reconstructions of the extensional history is possible. In the Matese domain, extension is relatively minor and by using stratal thinning as a guide, Ferranti (1995) has estimated extension at 25-30%. In the Pieentini region, three dimensional balanced

reconstructions of the extensional belt (Ferranti and Oldow, in prep.) point to values between 200 and 250%. In the Calabrian region, the profound thinning of thick carbonate platform assemblages suggests that extension may be in excess of 350%. Although the amount of extension recorded in two of the domains is very high, that of the tectonic belt as a whole is relatively low. Reconstruction of the Southern Apennines and restoration of the arcuate thrust front to a more linear N-S configuration requires no more than 50% extension for the belt as a whole (Oldow et al., 1993). The heterogeneous distribution of longitudinal extension within the internal Apennines and the abrupt difference in extension between the internal and external parts of the deformation belt requires explanation but is poorly understood. Within the internal sector of the Southern Apennines, we suspect that hyper-extension domains were linked by transcurrent fault systems like those depicted in Fig. 11. In the depiction, two spatially separated sectors experiencing longitudinal extension with the same vergence are linked by high-angle faults. Net NNW-SSE dipslip displacement on low-angle detachments dominates within each extensional sector (large open arrows in Fig. 11). The extensional sectors are kinematically linked by steeper NNW-SSE transfer faults which sole into underlying detachment systems and record strike-slip displacement (short black arrows in Fig. 11). In this simple model, motion is partitioned into pure dip-slip and strike-slip components. In a more complex and geologically significant system, displacement on the basal shear zone is resolved into various components of motion on faults in the overlying deforming mass. We speculate that the transition between the internal and external parts of the Southern Apennines is also accommodated by strike-slip fault systems that kinematically link hyper-extension domains and transfer the SSE displacement recorded on the lowangle detachments. Evidence for transcurrent motion on NW-SE faults through late Tortonian-Quaternary have been reported mainly in the southern part of the belt (Knott and Turco, 1991; Catalano et al., 1993). Transcurrent faults do not form through-going systems and appear to align in an en-echelon pattern from central Campania to the Calabrian arc. The lack of net slip determination on the proposed transcur-

L. Ferrantiet al. / Tectonophysics260 (1996)325-347 rent system and the strong overprint by later extensional faults, as yet, prevents a definitive solution to this problem. Nevertheless, we believe that coeval NNW-SSE strike-slip on the high-angle faults and dip-slip displacement on the low-angle detachments offer the best explanation to date.

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5. Discussion Extension in the Tyrrhenian basin and shortening in the Apennines show a dramatic increase in magnitude from N to S (D'Argenio et al., 1975; Bally et al., 1986; Malinverno and Ryan, 1986; Patacca and

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344

L. Ferranti et al. / Tectonophysics 260 (1996) 325-347

Scandone, 1989; Sartori et al., 1989). In the southern peri-Tyrrhenian, Miocene to Pliocene E - W extension in the hinterland basin was coeval with shortening in the foreland. Unlike the orogen-normal syn- to immediately post-collisional extension found in the Northern Apennines (Carmignani and Kligfield, 1990; Jolivet et al., 1990) the Southern Apennines experienced top down to the south/southeast syncollisional extension along an axis that parallels the tectonic belt (Fig. 1). Asymmetric opening of the Tyrrhenian basin, with larger magnitude stretching in the south and the general southeastern propagation of rifting account, at least in part, for the bending and arcuation of the Southern Apennines (Malinverno and Ryan, 1986; Lavecchia, 1988; Sartori et al., 1989; Patacca and Scandone, 1989). During arcuation, regions close to the thrust front were subject to foreland shortening and anticlockwise rotation. More internal sectors of the belt, incorporated earlier in the orogenesis characteristically had shorter longitudinal dimensions. During progressive arcuation, these internal units were extended parallel to the strike of the belt to accommodate lengthening of the orogen (Oldow et al., 1993). As a consequence, allochthons first involved in contraction and found in the highest structural positions of the tectonic belt underwent the greatest longitudinal

j'

displacement. Younging of longitudinal extension toward the southeast, in the direction of extensional transport, is consistent with late-stage displacement of crystalline and basinal allochthons over structural sheets composed of the more external CPBS domains (Dewey et al., 1989). Calabrian and Internal Basin nappes are found in the down-slip direction and lie above the lower part of the orogenic stack. Significantly, underlying Apenninic units are missing and the upper plate successions predictably were emplaced on low-angle faults such as the Sangineto line (Ietto et al., 1992; Oldow et al., 1993). Strain partitioning between hinterland extension and foreland shortening occurred as a continuum during progressive offscraping of sedimentary rocks from the westward sinking lithospheric slab, which is now underlying the Tyrrhenian basin (Gasparini et al., 1982; Anderson and Jackson, 1987). Arcuation processes were confined to the mobile belt (Malinverno and Ryan, 1986). Consequently, faults that accommodated along-strike extension within the ailochthonous succession do not necessarily penetrate the underlying decollement system (Oldow et al., 1993). During progressive deformation, changes in the regional strain pattern were expressed by structural superposition at different scales. The structural sce-

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Fig. 12. Cartoon showing the progressivedeformationin the peri-Tyrrhenianregion, the evolvinggeometryof faulting and the structure superpositionin the hinterlandand forelandof the mobilebelt. Tfl, 2, 3 = thrust front 1, 2, 3. Tfl, rot = thrust front 1, rotated.The outline of Italy in the back (dottedline) has been arbitrarilystretchedfrom A to D of about 30%. See text for discussion.

L Ferranti et al. / Tectonophysics 260 (1996) 325-347

nario is schematically depicted in the map-view forward model of Fig. 12. During early E - W contraction an accretionary complex was obducted eastward onto the foreland (large filled arrows in Fig. 12a). Where conditions of critical taper (Piatt, 1986) were exceeded, extensional collapse coaxial and opposite to thrusting ensued both in the northern ApenninesCorsica area (Carmignani and Kligfield, 1990; Jolivet et al., 1990) and in the Calabrian nappes (Platt and Compagnoni, 1990) (large open arrows in Fig. 12a). At the same time, continental rifting began in the back-arc region. Subsequent asymmetric propagation of rifting in the hinterland and related rotation of nappes and earlier contractional structures in the foreland occurred to the south (Fig. 12b). Northern and southern structural domains were separated by transfer faults in the rifted area (such as the N41°line in Figs. 1 and 12) and transverse lineaments in the contractional belt (such as the Ancona-Anzio and Maiella-Roccamonfine lines in Fig. 1). To the south, orogenic arcuation primed crustal thinning parallel to the strike of the belt (Fig. 12c). During NNW-SSE extension, rotated thrust fronts predictably were reactivated as transfer faults between earlier allochthons and domains being incorporated into the tectonic belt. Older transverse lineaments between segments of the contractional belt may have acted as breakaway faults for extensional allochthons, such as the Sangineto line in Figs. 1 and 2. Crustal extension in the interior of the belt was coeval and kinematically coordinated with crustal thickening in the external domains. Finally, during NE-SW lithospheric extension in the back-arc region, new systems of high-angle normal faults cut through the weakened belt (Fig. 12d), in response to rotation of stress patterns and SE migration of the locus of Tyrrhenian rifting. At this stage, earlier breakaway faults were characterized by dominant strike-slip displacement (such as the Maiella-Roccamonfina line in Fig. 1, where latestage motion seems predominantly strike-slip, Di Bucci and Tozzi, 1992), while transfer faults were reactivated in normal slip. To the foreland, new domains were affected by migration of the foredeep-mountain belt system. Orogen-parallel extension in the Southern Apennines can be considered only in minor part a product of gravitational instability within a critically tapered wedge (Platt, 1986), and is regionally driven by a

345

decrease in the lateral confining forces within the thrust allochthon as a consequence of the arcuation (Oldow et al., 1993). However, modes and kinematics of extension are strongly influenced by the complex history of imbrication involving late-stage non-coaxial overthickening. Sectors that were subject to higher-magnitude shortening relative to adjacent regions, were also possible sites of hyper-extension, where low-angle extensional detachments were localized in some instances along previous thrust faults. This helps to explain the spatial coincidence between envelopment thrusts and large-developed extensional structures in the axial-inner sector of the tectonic belt, as in the case of the Picentini Mountains (Pappone and Ferranti, 1995). The extensional transport parallel to the strike of the Southern Apennines brings an important constraint to the kinematic modelling of the belt. Longitudinal collapse of the orogen did not change the regional geometrical relationships between the main groups of thrust sheets, but it exercised a substantial control on the spatial arrangement of the allochthonous units. Consequently, any attempt of balancing ENE shortening should take into account volume loss in and out of the plane of the section and requires three-dimensional time-integrated kinematic analyses.

Acknowledgements This work was supported by the Ministero dell'Universith e Ricerca Scientifica through grants 40% 1991-1992 to B. D'Argenio. Wa are grateful to L. Ratschbacber, who provided many valuable critical comments on two versions of this manuscript, and to L. Carmignani and P. Scandone for helpful technical reviews on the regional geological setting.

References Anderson, H. and Jackson, J., 1987. The deep seismicity of the Tyrrhenian Sea. Geophys. J. R. Astron. Soc., 91: 613-637. Amodio-Morelfi, L., Bonardi, G., Coionna, V., Dietrich, D., Giunta, G., Ippofito, F., Liguori, V., Loranzoni, S., Pagfionico, A., Perrone, V., Piccaretta, G., Russo, M., Seandone, P., Zanettin-Lorenzoni, E. and Zuppetta, A., 1976. L'arco cal-

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