History and tectonic implications of low-angle detachment faults and orogen parallel extension, Picentini Mountains, Southern Apennines fold and thrust belt, Italy
TECTONICS, VOL. 18, NO. 3, PAGES 498-526, JUNE 1999
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History and tectonic implications of low-angle detachment faults and orogen parallel extension,Picentini Mountains, Southern Apenninesfold and thrust belt, Italy Luigi Ferranti Departmentof GeologyandGeologicalEngineering,Universityof Idaho,Moscow GeomareSud, CNR, Napoli, Italy
John S. Oldow Departmentof GeologyandGeologicalEngineering,Universityof Idaho,Moscow
North Africa [Channell et al., 1979]. Rocks of different paleotectonicenvironmentswere thrust toward the east onto Apulia, part of the Adriatic foreland in Italy, and toward the southonto the Hyblean foreland in Sicily (Figure 1). Several the belt duringorogenparallelextensionas thrustimbrication hundred kilometers of shortening[e.g., Bally et al., 1986] continuedin the foreland. Extreme tectonic thinning defines coincided with a complex history of thrust imbrication, discretestructuraldomainsof hyperextension which are linked allochthonrotation, and orogenicarcuation [D'Argenio et al., by a complex systemof extensionaland transcurrentfaults. 1975; Charmell et al., 1979, 1990; Oldow et al., 1990; Patacca Some of the best examplesof hyperextension structuresin the and Scandone, 1989]. From the Miocene to Pleistocene,contractionmigratedfrom SouthernApenninesare exposedin the PicentiniMountains. In this area, detailed mapping, fault-kinematicanalysis, and internal (western and northern) sectorstoward external (eastern
Abstract. Late Miocene to Pliocene movementon low-angle extensional faults within the internal Southern Apennines orogenicbelt was superposed on an earlier, Mioceneimbricate thruststack. The low-anglefaultsformedwithin the interiorof
excellentstratigraphic controlcontributed to the construction of
and southern)regionsof the orogenand was followed in time and space by extensionassociatedwith rift propagationand With these constraints,it is possibleto documentextensional openingof the TyrrhenianSea [D'Argenio et al., 1975; Patacca displacement on shallowlydipping supercrustal faults whose and Scandone, 1989]. Extension associatedwith Tyrrhenian orientationis a primary featureand is not due to later tilting. rifting is active today and occurson high-anglenormal faults During movement,upper plate rocks were disarticulated by that predominantlydip towardthe oceanbasin(Figure 1). The listric and planar normal faults that soled into a ramp-flat axis of Tyrrhenian-relatedextensionchangesorientationalong detachmentsystem. The depth of the basal detachment the strike of the tectonicbelt but is essentiallyorthogonalto the increasedin the directionof upper-platemotionandrangedfrom axis of the curvedorogenthroughoutmainlandItaly and Sicily 5 to 10 km. Displacement on the low-angledetachment was [Cello et al., 1982]. Substantial variation exists in the structure of the periaccompanied by block tilting in the upper plate assemblage, incisementand excisementof the upper and lower plate rocksas Tyrrhenianbelt alongstrike. Geophysical datashowcontrasting fault trajectorieschangedthroughtime, and the progressive crustal and lithospheric structure between and within the cataclasis of hangingwall andfootwallassemblages. Preexisting TyrrhenianandApenninicdomainsof the orogen. The Adriatic thrustswereonly locallyreactivated duringextension, andfaults continentalmargin has a crustal thicknessof 35 km in the emanatingfrom the underlyingdecollementsystemscrossthe NorthernApenninesand increasesto about55 km beneaththe imbricated thrust sheets at moderate to high angles. SouthernApennines[Nicolich,1989]. In the norththe transition Longitudinalextensionresultedin thinningof the thruststackto betweenthe Apenninesand the thinnedcontinentalcrustof the less than half the original thicknessand had a cumulative northern Tyrrhenian Sea is gradational with Moho depths magnitude of between200 and250% (beta> 2). decreasingfrom 35 to 30 km from east to west acrossthe Tyrrheniancoast. In the souththe Apenninesand the southern Tyrrhenian Sea, which is composedof oceanicand stretched 1. Introduction continental crust, are characterizedby an abrupt boundary The SouthernApennines fold andthrustbelt wasconstructed [Lavecchia,1988]. The crustchangesfrom over 50 km thick in duringCenozoiccollisionof Europeand Africa as part of the the east to less than 15 km beneaththe southeasternTyrrhenian peri-Tyrrhenianorogen(Figure 1). The peri-Tyrrhenian belt Sea, which alongthe Calabrianmargin (Figure 1) is underlain containsimbricatesof Mesozoic and Cenozoic deep-basinand by a well-definedBenioff zone dippingsteeplyto the northwest. carbonate-platform rocksthat were deposited adjacentto andon The downgoingslab is imagedby seismictomographyand by thatprogressively deepento the northwestto depths the Adriatic continentalmargin, whichformeda promontoryof earthquakes restorable cross sections and forward
models of deformation.
of 400-500 km [Giardini and Velond, 1988; Suhadolc and Panza, 1989; Arnato et al., 1993; Cirnini and Arnato, 1993;
Copyright1999by theAmericanGeophysical Union
Selvaggi and Chiarabba, 1995]. Beneath the northern Tyrrhenian Sea a less distinctbut discerniblewest-southwest dippingslabhas beenimagedto depthsof 250 km by seismic
Paper number 1998TC900024 0278-7407/99/1998TC900024512.00
FERRANTI AND OLDOW: OROGEN PARALLEL EXTENSION IN SOUTHERN ITALY
,• Corsica Frontal thrust ofthe Alps, and Apennines •,, High-angle normal fault
• Tectonic line
Sense of upper plate
tectonic transport in extensional provinces
• _. _ _ .•18o
•.......... Forelsnd •nd foredeep deposits ofthe Apennines, Sicily, Southern Alps, •nd Corsic• ::!• Mildly deformed undeformed •ndpiggyback b•sin deposits ofto the Apenninesforedeep •nd Sicily '.-•• CalabrianArc • ;;I Apennine-Sicily belt :•/•Alpine belt
Areas floored with oceanic crust inthe Tyrrhenian Sea (MP--Late Miocene-Pliocene; PP=Pliocene-Pleistocene) Metamorphiccore complexesof the NorthernApennines (AA=AipiApune; El:Elba Island) and Corsica (T=Tenda)
Hyperextension domains ofSouthern Apennines (MM=Matese Mrs.; PM=Picentini Mrs.' F=Foraporta; P=Pollino)
Figure 1. Tectonicsketchmapof Italy, showingthe generalized compressional andextensional featuresof the Apennine-SicilybeltsandTyrrhenianbasin.
substantiallyto the north, where it becomes60-70 km in the
tomographyand to 90 km and possiblydown to 160 km by seismicity[Amato et al., 1993; Selvaggiand Amato, 1992;
centralnorthern Apennines andAdriaticSeabutagain,increases
Cimini andAmato, 1993].
Along the Adriatic continentalmargin, the lithospheric lithospheric thicknesspattern is interpreted to be a relict thickness[Calcagnile and Panza, 1981; Panza et al., 1982; structurerelatedto Mesozoicrifting ofßthe Adriatic continental Suhadolcand Panza, 1989] varies from southto north. In the margin during formation of the Tethyan Ocean [Lavecchia, south the thickness increases from 90 km below the mountain 1988;Ziegler, 1988;Doglioniet al., 1994]. -beltto 110km in Apulianforeland.Thethickness decreases The magnitudeof Tyrrhenianrifting also variesfrom north
FERRANTI AND OLDOW: OROGEN PARALLEL EXTENSION
IN SOUTHERN ITALY
Trace oflongitudinal extension breakaway fault ....[•Quaternary continental deposits
% Trace oflongitudinal extension transfer fault
Tlwast fault, teeth toward upper plate• ß• High-angle faultside balls towardnormal doamthrown • Axial trace of
Pliocene piggyback basindeposits
Middle-Upper Miocene Foredeep assemblage
regional syncline • Mesozoic-Cenozoic Basin assemblage Deposifional
Gulf of Salerno
Figure2. Generalized geologic mapof partof westernSouthern Apennines, showing regionaldistribution of majorlithotectonic assemblages, locationof physiographic elements,andinferredboundaries of thePicentini Mountainshyper-extensional domain.Tracesof restored cross-section of Plates2-4 areshownby dashed lines AA', BB', CC'. Boxedlettersare locationof wells(logsdescribed in Figure7): I, Irpinia1 Well; MF, Monte Forcuso1 Well; N, Nusco2 Well; C, Contursi1 Well. Low-anglenormalfaultsof thePicentiniMountainsare omittedfor clarity.
to south. The northern Tyrrhenian Sea, north and east of
Sardinia(FigUre 1), is underlainby stretchedcontinentalcrust with a thicknessof 20-25 km [Nicolich, 1989]. The ductilely stretchedpart of the continentalcrust is locally preservedand
exposedin the metamorphiccore complexesof Alpi Apuane, Elba Island, and northern Corsica [Zitellini et al.,
Carmignaniand KligfieM, 1990; Jolivetet al., 1990; Keller and Pialli, 1990; Bartole, 1995]. To the souththe Tyrrhenianbasin is underlainby oceaniccrust of 6-15 km thickness[Nicolich, 1989] which formed during rifting, that decreasesin age to the southeastfrom Miocene to Pliocenein the central Tyrrhenian Sea and from Plioceneto Pleistoceneage near the Calabrianarc [Kastenset al., 1988; Sartori et al., 1989]. Similarly, the lithospheric thickness decreasesfrom 50 km beneath the northernTyrrhenian Sea to 20 km below the central abyssal plain in the south[Suhadolcand Panza, 1989]. The history of contractionaldeformationvaries along the lengthof the tectonicbelt and becomesmore complexas the magnitudeof hinterland extensionand foreland thrust-belt
arcuation increases to the south. In northernItaly, Apenninic contraction is characterized by homoaxial deformation, typically directedtowardthe foreland[Lavecchiaet al., 1987;Boccaletti
et al., 1987;Bally et al., 1986]. In contrast, in southern Italy and Sicily deformationis clearly more complexand is characterized to varyingdegrees by changes in tectonic transport directionthroughtime, development of transpressional fault systems,and vertical-axisrotation of allochthonsduring progressivedeformation[Channellet al., 1990; Incoronatoand Nardi, 1989;Dewey et al., 1989; Oldowet al., 1990;Mazzoli, 1993]. Much of the foreland belt is still active and Pliocene to
Recentcontraction in the forelandportionof theApennines and Calabrianarc (Figure1) is documented by involvement of recent sedimentary units in structuresand by the focal mechanisms of earthquakes[Pieri, 1983; Ori et al., 1986; Patacca and
Scandone, 1989; Pataccaet al., 1990;Argnaniet al, 1991; Frepoli andAmato, 1997; Pieri et al. , 1997]. Within the Southern Apennines, tectonism'is further
complicated by the development of widespread low-anglefaults
FERRANTI AND OLDOW: OROGEN PARALLEL EXTENSION IN SOUTHERN ITALY that omit hundreds of meters to kilometers of structural section.
The low-anglenormalfaultsare distributed alonga northwestsoutheast trendingbelt lying within the internalzone of the orogen(Figure1). Theselow-angle faultswerefirstdescribed by Ietto  in the PicentiniMountains,locatedeast of
Thrust imbricatesderivedfrom the platformassemblage are composed of Upper Triassicto Mioceneshallowmarineto slope
carbonaterocks [D'Argenio et al., 1975, 1993; Channellet al., 1979; Marsella et al., 1995] which form two major thrustsheets withinthe PicentiniMountains[PapponeandFerranti, 1995]. In descendingstructuralorder they are the Terminio-Accellicaand Naples,alongthewestern flankof theorogenic belt(Figure2). For manyyearsthe faultswereinterpreted in a compressionalthe Southern Picentini sheets (C1 and C2, respectively, in contextasgravity-driven [Ietto,1965]or out-of-sequence [Roure Figure 3 and Plate 1). Differences in internal stratigraphy (Figure 3) allow recognition of individual stratal horizons et al., 1990]thrusts. Reinterpretation of the tectonicsettingof throughoutthe PicentiniMountains,evenwherethe stratigraphic the PicentiniMountainsled D 'Argenioet al.  to arguethat is disarticulatedby structuresand is only partially these faults formed in extension. Regional and structural succession investigations throughout the Southern Apennines belt, fromthe preserved. Matese Mountains in the northwest to the Pollino Mountains in Sheetsof the basin assemblagerocks include, in ascending the southeast (Figure1), document the existence of substantial structuralposition,the Lagonegroand Sicilid nappes(Figure 3),
andpreviously unrecognized extensional displacement parallelto which consist of Lower Triassic to Miocene and Cretaceous to Miocene successions, respectively[Ogniben, 1969; D'Argenio the axisof the orogen[D'Argenioet al., 1993; Oldowet al., 1993; Ferranti et al., 1995]. The areasof greatestextension et al., 1975, 1993]. Within the Picentini Mountains these rocks form discretedomainsboundby breakawayfaults, transcurrent composetwo major thrustsheets,the Lagonegro(B1) and Sicilid displacement transfersystems, andextensional allochthons with (B2) nappes,bothof whichare internallyimbricated(Plate 1). Relatively thin thrust imbricatescomposedof Miocene to a geometrysimilarto the extensional complexes of the North AmericanGreatBasin(e.g., Wernicke,1992). On thebasisof lower Quaternary clastic rocks of the so-called flysch
crosscutting relationsit is established that the low-anglefault systemsdevelopedduring orogen-parallelextensionthat precededinceptionof high-anglenormal faults related to Tyrrhenianrifting. Someof the bestexposures of the low-anglenormalfaultsin the SouthernApenninesare found in the PicentiniMountains, where detailed mapping, structural analysis, and a
successions are found above or imbricated between the deformed
basin and platform units. The clastic sequencesrepresent piggyback and foredeep basins whose ages decrease
systematically to theeast(Figure2). Deposition in theforedeep and piggyback basins spans the timing of contractional deformation[Pataccaand Scandone,1989]. In the more externalparts of the orogento the eastof the
were depositedin the comprehensive understanding of the Mesozoicstratigraphy of PicentiniMountains,flysch sequences middleto late Miocenein basinslying to the northeastof the deep-basinand carbonate-platform rocks allow examinationof thruststack[Coccoet al., 1974]. Thesesynorogenic the relationship betweenearliercontractional andlaterlow-angle advancing extensional structures.
Cross-section restoration and forward
modelingillustratethe displacement historyof extensional lowangle decollementsand related faults of the hanging wall assemblage. Geometriccontrol afforded by exhumationon youngerhigh-anglefaults duringTyrrhenianrifting documents the lack of basement tilt during extension on the basal decollementsystems. Displacementon the basal decollement occurredat present-dayattitudesand supportsthe conceptof supercrustal extensionon shallowlydippingfaults.
2. Structural Setting
rocks were involved, in turn, in the contractionaldeformation, and the easterlyto northeasterlymigrationof the thrustfront is recordedby the agesof rocksin the flyschsheets. Two thrust sheets(F1 and F2 in Plate 1) composed of thesesynorogenic rocksare recognized in the southern andnorthernportionsof the study area. One thrust sheet (F1) exposedin the southern
PicentiniMountainsis composed of a synorogenic sequence (Figure4) about500 m thick (Vallimalaflysch[Scandone et al., 1967]) and is reportedto be Langhian-Serravallian in age by Scandoneet al.  but may be as young as Tortonian
according to recentwork [Bigi et al., 1992]. In the Irpinia foothillsto the northeast(Figure2), anotherthrustsheet(F2)
of a thickwild flysch(Castelvetere flysch[Coccoet al., The SouthernApenninescontain a pile of thrust sheets consists consisting primarilyof Mesozoicto Cenozoicsedimentary rocks 1974]) depositedmostlyduringthe late Tortonianto Messinian detachedfrom their basementand imbricatedon the Apulian [Patacca and Scandone, 1989]. Farther to the northeastin the segmentof the Adriatic foreland(Figure 1) duringthe Neogene external parts of the orogenic belt, later (Pliocene-early Pleistocene) foredeepsequences are foundwith varyingdegrees [D'Argenio et al., 1975; Channell et al., 1979; Mostardini and of involvement in the contractional deformation[Hyppoliteet Merlini, 1988; Patacca and Scandone,1989]. In the westernor al., 1994].
internalsectorof the belt, threemain lithotectonic assemblages are recognized. Two are composedof Mesozoic to early Cenozoicrocksderivedfrom eithercarbonate-platform systems 3. Contractional Structures or from deep-basin environments.The third, whichis typically referredto as fiyschin the literature,is composed of Miocene The Picentini Mountains (Figure 2) form an axial and youngerclastic rocks depositedin foredeepor piggyback culmination in the internalportionof the Southern Apennines basins during regional contraction. The three lithotectonic orogen and expose a complex imbricate stack composed assemblages form the primary buildingblocksof the Southern predominantlyof interleavedslices of platform and basin Apennineorogenand have beenthe objectof intensestudyfor assemblage rocks [Scandoneet al., 1967; Turco, 1976; Ferranti decades(summarizedby Ogniben, , D'Argenio et al., and Pappone,1992]. Thrustimbricationand foldingoccurred [1975, 1993], Patacca and Scandone,, and Marsella et al. , ).
progressivelyfrom the middle Miocene to Recent and can be dividedinto multiplegenerations of contractional structures.At
FERRANTI AND OLDOW' OROGEN PARALLEL EXTENSION IN SOUTHERN ITALY