Christophe Brunet - publications et communications

I. Publications

Migration of compression and extension in the Tyrrhenian Sea, insights from 40Ar/ 39Ar ages on micas along a transect from Corsica to Tuscany In press Tectonophysics C. Brunet , P. Monié (2), L. Jolivet (1) & J.-P. Cadet (1)

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Département de Géotectonique, ESA CNRS 7072, Université Pierre et Marie Curie, Tour 26-00 E1, CC 129, 4 place Jussieu, 75252 Paris Cedex 05, France. ([email protected], [email protected], [email protected]) (2) Laboratoire de Géophysique, Tectonique et Sédimentologie, UMR CNRS 5573, Université Montpellier II, CC 58, place Eugene Bataillon, 34095 Montpellier Cedex, France. ([email protected])

Abstract : The opening of the Tyrrhenian Sea is the consequence of the eastward retreat of the CalabriaApennines subduction from the Oligocene to the Present. Structural and petrological studies suggest a migration of extension from the Gulf of Lion to Alpine Corsica and to the Present-day Apennines from the Late Oligocene to the Present. During the same period the thrust front of the Apennines migrated eastward to its present position. Oceanic crust was formed during the migration first in the Liguro-Provençal Basin, then in the Southern Tyrrhenian Sea, while only thinning of continental crust occurred in the Northern Tyrrhenian Sea. Syn-rift basins deposits show a clear eastward migration, as well as frontal thrusts. Metamorphic rocks were exhumed during the migration process from Alpine Corsica to the Tuscan archipelago and the western coast of Tuscany. High pressure and low temperature parageneses are found along the whole transect and published stratigraphic and radiometric dates also suggest an eastward migration. We conducted a series of 40 Ar/39 Ar age determinations on metamorphic micas along this transect on grain populations and/or single grains. The results clearly show (1) the Late Oligocene - Early Miocene age of the top-to-the-east sense of shear in Alpine Corsica, (2) a transition from compression to extension around 32 Ma, (3) the eastward migration of the HP-LT event related to compression from 45 to 17 Ma, and of the LP event, related to extension from 32 Ma to recent. These results show a faster rate of migration of extension than compression. In the Quaternary, extension has almost caught up compression and the whole onshore northern Apennines are extending. This faster migration of extension is interpreted as the consequence of a progressive verticalization of the slab. The slab pull component was thus predominant in the process of slab retreat and extension.

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Christophe Brunet - publications et communications

Structural evolution of the Northern Tyrrhenian Sea from new seismic data Marine and Petroleum Geology, Vol. 16, p. 381-407, 1999 A. Mauffret, I. Contrucci & C. Brunet Département de Géotectonique, ESA CNRS 7072, Université Pierre et Marie Curie Tour 26-00 E1, CC 129, 4 place Jussieu, 75252 Paris Cedex 05, France. ([email protected], [email protected], [email protected])

Abstract : We present the results of the deep penetrating LISA seismic survey supplemented by several surveys of industrial oil surveys. The LISA survey shows the deep part of the crust and Moho reflections, which depth were previously precised by refraction data, are seen at 7.5-8 seconds TWTT on several seismic line on the Tuscan (eastern) margin of the North Tyrrhenian Sea. Two kinds of dipping reflectors are identified in the crust. The western dipping events are interpreted as traces of thrusts with an Apenninic vergence, the eastern dipping events are probably extensional detachments faults. These faults are seen deep in the crust and may be shear zone in the dutile crust. The extensional faults and the thrusts have been previously indentified on the industrial lines but these lines are processed at a depth of 4 seconds TWTT and consequently the deep reflections are not imaged. The industrial lines and the correlations with the exploratory wells helps to extends the results of the LISA cruise in the entire region. The compilation of all the seismic data allow us to present the first depth to basement map of this region. The major feature of the study area is the Corsica basin that is more than 8.5 km deep. This huge basin is probably filled by Oligocene-early Miocene deposited in an extensional regime. However this extension persisted during the Miocene. The steep faults that bounds the basin toward the west are imaged and we present a model in ramp and flat system that explains the particular geometry of the Corsica margin. The eastern flank of the Corsica basin which is bounded by the Pianosa ridge, is completely different. This region has been intruded by granitic plugs during the Messinian and the early pliocene and these intrusions are coeval with horst and graben formation on the Tuscan margin. Before this extension Pianosa ridge and Elba island were deformed by a compression related to the formation of the Apennines during the Miocene. Our observations on the north Tyrrhenian Sea are replaced in the tectonic framework of the Western Mediterranean. A first extension occurred during the Oligocene-early Miocene in the Provencal basin, west of Corsica and Sardinia. The western basins are bounded by strike-slip faults and are located on the top of deformed tectonic prism. The compressional deformation is related to the middle Miocene Apennines mountain buiding. Then compression migrated to the east and the inner Apennines belt collapsed during the late Miocene and the early Pliocene.This late extension is well illustrated by the LISA survey in the north Tyrrhenian Sea.

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Christophe Brunet - publications et communications

Blueschist to greenschist transition from the Gorgona metamorphic complex (Northern Tyrrhenian Sea, Italy): P-T-t paths, fluid inclusion analysis and geodynamic implications In press Ofioliti F. Rossetti (1), C. Faccenna (1), L. Jolivet (2), B. Goffé (3), R. Funiciello (1), C. Brunet (2), F. Tecce (4) & P. Monié (5) (1)

Dipartimento di Scienze Geologiche, Università di Roma Tre, Largo San L. Murialdo 1, 00154-Roma, Italy.([email protected], [email protected], [email protected]) (2) Département de Géotectonique, URA 1759, Université P. et M. Curie, Tour 26-00 E1, Case 129, 4 place Jussieu, 75252 Paris Cedex 05, France. ([email protected], [email protected]) (3) Département de géologie, URA 1316, Ecole Normale Supérieure, 24 rue Lhomond, 75232 Paris Cedex 05, France. ([email protected]) (4) Centro di Studio per il Quaternario e l'Evoluzione Ambientale, C.N.R., Dipartimento di Scienze della Terra, Università la Sapienza, Roma, Italy. (5) Laboratoire de Géochimie et de Géochronologie, UMR 55-67, USTL-Université de Montpellier II, place Eugéne Bataillon, 34095 Montpellier, France. ([email protected])

Abstract : We analyzed the metamorphic and structural evolution of the retrogressed blueschist Schistes Lustrés sequence exposed on the Gorgona island, located in the Northern Tyrrhenian Sea. P-T metamorphic conditions have been reconstructed by using combined petrologic and fluid inclusion techniques, in order to constrain the exhumation path of the blueschist sequence. Presence of Fecarpholite in metapelites in association with glaucophane and lawsonite in metabasites define the HP/LT (0.8-1.5 GPa and 300-350°C) stage. 39 Ar-40 Ar geochronological determinations constrain this blueschist event to Late Oligocene. The decompression part of the reconstructed P-T path, as also attested by preservation of carpholite-bearing associations, show no indication of any temperature rise during greenschist re-equilibration. Eastward asymmetric ductile shear, synkinematic to the retrograde greenschist stage, accompanied exhumation of the Gorgona HP complex, controlling with a normal-sense metamorphic break contact with the upper low-grade up to greenschist facies ophiolitic unit, which occurs along a NW-SE trending milonitic shear zone. We refer this nearly isothermal retrogressive evolution to a syn-convergence extensional unroofing mechanism, connected with the dynamics of the Northern Apennine-Alpine Corsica orogenic wedge.

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Christophe Brunet - publications et communications

Syn-versus post-orogenic extension: the case study of Giglio island (Northern-Tyrrhenian sea, Italy) Tectonophysics, Vol. 304, p. 71-93, 1999 F. Rossetti (1), C. Faccenna (1), L. Jolivet (2), R. Funiciello (1), F. Tecce (3) & C. Brunet (2) (1)

Dipartimento di Scienze Geologiche, Università di Roma Tre, Largo San L. Murialdo 1, 00154-Roma, Italy.([email protected], [email protected], [email protected]) (2) Département de Géotectonique, URA 1759, Université P. et M. Curie, Tour 26-00 E1, Case 129, 4 place Jussieu, 75252 Paris Cedex 05, France. ([email protected], [email protected]) (3) Centro di Studio per il Quaternario e l'Evoluzione Ambientale, C.N.R., Dipartimento di Scienze della Terra, Università la Sapienza, Roma, Italy.

Abstract : Giglio Island, located 20 km off the southern Tuscany coast in the Northern Tyrrhenian Sea, is constituted by Pliocenic monzogranitic bodies and, subordinately, by metamorphic and sedimentary Mesozoic rocks. Structural analysis, coupled with metamorphic petrology and fluid inclusion analysis pointed out a polyphased deformation, characterized by different deformation and P-T metamorphic regimes. Data from this study indicate a nearly isothermal exhumation of the Giglio HP/LT (Fe-Mg carpholite bearing) metamorphic complex, which occurred during the EarlyMiddle Miocene as a consequence of a top-to-the east syn-orogenic extensional shear. Subsequent extensional processes, during the Upper Miocene-Early Pliocene, also showed an eastward asymmetry in a HT post-orogenic back-arc environment. Post-orogenic extension caused uplift of isotherms, partial melting of previously exhumed deep crustal levels and emplacement of the Giglio pluton. Finally, probably during the Late Pliocene-Early Pleistocene, late normal faulting accompanied the solid-state ascent of the Giglio intrusives thus determining the present-day setting of Giglio Island. Similar post-thickening/syn-orogenic style of extension has recently been recognized in the surrounding peri-Tyrrhenian areas, suggesting a unique evolutionary tectonic context for the entire Northern Tyrrhenian Sea area.

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Christophe Brunet - publications et communications

Mid-crustal shear zones in post-orogenic extension: the northern Tyrrhenian Sea case Journal of Geophysical Research, Vol. 103, p. 12123-12160, 1998 L. Jolivet (1), C. Faccenna (2), B. Goffé (3), M. Mattei (2), F. Rosseti (2), C. Brunet (1), F . Storti (2), R. Funiciello (2), J.-P. Cadet (1), N. d'Agostino (2) & T. Parra (3). (1)

Département de Géotectonique, URA 1759, Université Pierre et Marie Curie, Tour 26-00 E1, Case 129, 4 place Jussieu, 75252 Paris Cedex 05, France. ([email protected], [email protected],[email protected]) (2) Dipartimento di Scienze Geologiche, Università di Roma Tre, Largo San L. Murialdo 1, 00154-Roma, Italy. ([email protected], [email protected], [email protected], [email protected], [email protected], [email protected]) (3) Département de géologie, URA 1316, Ecole Normale Supérieure, 24 rue Lhomond, 75232 Paris Cedex 05, France. ([email protected])

Abstract : Metamorphic core complexes of the Aegean region have revealed midcrustal, shallowdipping extensional shear zones. These shear zones display constant kinematic indicators over large regions (100-200 km). We analyze the example of the northern Tyrrhenian Sea and then compare it to the Aegean region. We first summarize our observations on ductile extension and metamorphic evolution in the northern Tyrrhenian Sea from Alpine Corsica to Tuscany. (1) Extension migrated from west to east from the early Miocene in Corsica to the Recent in the Apennines; (2) Extension is accommodated by shallow east dipping extensional shear zones at the depth of the brittle-ductile transition, from the early Miocene to the Pliocene. (3) West dipping normal faults accommodate extension on the eastern side of the volcanic arc. (4) Extension is preceded along the convergence front by the formation of a thrust wedge, where high-pressure and low-temperature conditions are recorded; maximum PT conditions decrease toward the east, and PT paths are systematically very cold, suggesting that a large part of the exhumation occurred during synorogenic extension. We discuss the possible mechanisms that account for constant shear sense over large domains. The model involves retreat of the slab and migration of the volcanic arc. Partially molten lower crust acts as a low strength zone where extensional strain is localized. Eastward motion of the upper mantle as a consequence of the migration of the slab induced a component of shear toward the volcanic arc at the base of the stronger upper crust. In the weak upper mantle and lower crust, to the west of the volcanic arc, extensional stresses are not transmitted; this produces a top-to-the-east sense of shear at the base of the upper crust that migrates eastward, following arc migration.

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Christophe Brunet - publications et communications

Ferro-magnesiocarpholite from the Monte Argentario (Italy): first evidence for high-pressure metamorphism of the metasedimentary Verrucano sequence, and significance for P-T path reconstruction European Journal of Mineralogy, Vol. 9, p. 859-873, 1997 T. Theye (1), J. Reinhardt (2), B. Goffé (3), L. Jolivet (3,4) & C. Brunet (4) (1)

Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenberstr, 18, D-70774 Stuttgart, Germany. ([email protected]) (2) Institut für Mineralogie, Technische Hochschule Darmstadt, Schnittspahnstr. 9 D-64287 Darmstadt, Germany (3) Département de géologie, URA 1316, Ecole Normale Supérieure, 24 rue Lhomond, 75232 Paris Cedex 05, France. ([email protected]) (4) Département de Géotectonique, URA 1759, Université P. et M. Curie, Tour 26-00 E1, Case 129, 4 place Jussieu, 75252 Paris Cedex 05, France. ([email protected]; [email protected])

Abstract : Ferro-and magnesiocarpholite [Mg/(Mg+Mn+Fe2+ )=0.36-0.70, Mn/(Mg+Mn+Fe2+ )