TECTONICS, VOL. 20, NO. 1, PAGES 112-133 FEBRUARY 2001 Alpine structural and metamorphic signature of the Sila PiccolaMassif nappe stack (Calabria, ltaly)' lnsightsfor the tectonic evolution of the Calabrian Arc Federico Rossetti, • Claudio Faccenna, • Bruno Goff6, 2 Patrick Moni6, 3 Alessio Argentieri, • Renato Funiciello, • and Massirno Mattei 4 Abstract. Combined structural and petrographical investigations, coupled with 4øAr/39Ar geochronology, were carried out in the Sila Piccola Massif of the Calabrian Arc in order to define the structural geometry and map out the major structural and metamorphic breaks within theexposed nappe sequence. On the basis of the contrasting Alpine pressure- temperature (P-T) and structural signatures the nappe stack can be divided intwo major tectonic complexes, bounded bya fiat- lyingductile to brittle extensional shear zone. The upper complex consists of a nappe-like structure, where a major top to the east compressional shear is recorded. The lower tectonic complex consists of an ophiolite-bearing sequence showing typicalhigh-P/low-T parageneses (Mg-carpholite and Na-amphibole). The 4øAff39Ar geochronology on phengites in equilibrium withblueschist minerals provided a minimum age estimatefor the blueschist event in the lower complex rocks at the Oligocene-Eocene boundary (around 35 Ma). Ductile to brittle top to the west extensional shear accompanied the nearly isothermal retrogression and exhumation of the lower complex rocks,reworking the previous nappe contacts with shear localization alongthe upper/lower tectonic complex discontinuity. The 4øAr/39Ar dating indicatesthat this postnappe stacking tectonic evolution took place from30 Ma onward. It is proposed that exhumation of the deep-seated rocks occurred below a top to the west extensional detachment active during convergence and orogenic complex formation (synorogenic extension). The age of this detachmentis bracketed between 30 Ma and the post-orogenic Neogene basin sedimentation (middle-upper Miocene). Therevised structural and metamorphic scenario is here integrated into a new tectonic evolutionary reconstruction, which involves an early high-P/low-T top to theeast crustal thickening episode during theconstruction of the Apennine orogenic wedge (Eocene-Oligocene), followed and overprinted by a top to the west extensional shear, probably active from the !ate Oligocene. •Dtpammento d• Scicnze Geologiche, Umversith "Roma Tre", Rome, Italy. 2 Laborato•re de Gdologm, Ecole Normale Superieure, Paris, France. 3 Laborato•re de Gdophysique, Tectonique et Sddimentolog•e, UMR 5573, UniversitdMontpellier, Montpellier, France. 4 Dipartimento Strutture, Geotecnica eGeologia Applicata, Universith degliStudidella Basilicata, Potenza, Italy. Copyright 2001 by the American Geophysical Union. Paper number 2000TC900027. 0278-7407/01/2000TC 900027512.00 1. Introduction The nappe-structured belt of the CalabrianArc is located in the center of the Mediterranean region, in between the Apennine-Maghrebian thrust belt (Figure 1). It constitutes a distinctive tectonostratigraphic domain within the largely sedimentary rocks exposed in this chain, as allochtonous ophiolitic and pre-Alpine basement rock nappes, affectedby Alpine metamorphism, are widely exposed [e.g., rimodio Morelli et al., 1976]. Its tectonic evolution is commonly referred to the plate convergence between Eurasia and Africa- Adria continental plates during the consumption of the Liguro-Piedmont and Ionian oceanic lithosphere (Thetyan realm), activesinceLate Cretaceous [e.g., ribbateet al., 1986; Dercourt et al., 1986], leadingto the formationof the Alps- Apennines orogenic belt [e.g., Dewey et al., 1989]. Sincethe middle Miocene, the inner sector of the chain has been affected by extensional tectonics which accompanied the breakup of the continental lithosphere and the formation of the Tyrrhenian Sea basin [Sartori, 1990, and references therein]. During the last phases of the Tyrrhenian rifting (late Miocene onward) the Calabrian Arc drifted apart fromthe Sardiniamargin and experienced a 20 ø clockwise rotation [Scheepers et al., 1994], extensional faulting, and uplift [e.g., Bordoni and Valensise, 1998].Presently, the Calabrian Arc constitutes theforearc of thesouthern Tyrrhenian subduction zone, well defined by seismicity [Isacks and Molnar,1972] and seismic tomographic images [Lucente et al., 1999].The Calabrian Arc thus gives the opportunity to investigate in a small areathe transition between the earlynappe stacking event and the subsequent back arc extension,which is a common feature of the wholeMediterranean region [e.g., Horvath and Berckhemer, 1982]. Despite this peculiarity, few studies have been addressed to the comprehension of the nappepile architecture, the metamorphic zonation, and the kinematics of themain nappe contacts. With minordifferences [see, e.g., Dietrich, 1988; Bonardi et al., 1992,1994; Knott, 1994; Thomson, 1994;Cello et al., 1996], thegeneral tectonic scheme accepted for theCalabrian Arc is based on thesynthesis of rimodio Morelli et al [1976]. This scheme is mainly focused on the paleogeographic pertinence of the different stacked units, which arearranged in threemain tectonic complexes, from top to bottom: (1) Calabrian units, constituted by pre-Alpine (Paleozoic in age) continental-derived metamorphic andigneous rocks andtheir Meso-Cenozoic sedimentary or weakly metamorphosed cover;(2) ophiolitic units (Liguride Complex of Ogniben [1969]), consisting of Cretaceous to lateOligocene ophiolite- bearingfiyschoidsequences [Knott, 1987; Bonardi eta!., 1988, 1994]with localhigh-pressure/low-temperature (high- 112