| 1 | The East Anatolia-Lesser Caucasus ophiolite:An exceptional case of large-scale obduction,synthesis of data and numerical modelling显示文摘We present new,geological,metamorphic,geochemical and geochronological data on the East Anatolian-Lesser Caucasus ophiolites.These data are used in combination with a synthesis of previous data and numerical modelling to unravel the tectonic emplacement of ophiolites in this region.All these data allow the reconstruction of a large obducted ophiolite nappe,thrusted for>100 km and up to 250 km on the Anatolian-Armenian block.The ophiolite petrology shows three distinct magmatic series,highlighted by new isotopic and trace element data:(1)The main Early Jurassic Tholeiites(ophiolite s.s.)bear LILEenriched,subduction-modified,MORB chemical composition.Geology and petrology of the Tholeiite series substantiates a slow-spreading oceanic environment in a time spanning from the Late Triassic to the Middle-Late Jurassic.Serpentinites,gabbros and plagiogranites were exhumed by normal faults,and covered by radiolarites,while minor volumes of pillow-lava flows infilled the rift grabens.Tendency towards a subduction-modified geochemical signature suggests emplacement in a marginal basin above a subduction zone.(2)Late Early Cretaceous alkaline lavas conformably emplaced on top of the ophiolite.They have an OIB affinity.These lavas are featured by large pillow lavas interbedded a carbonate matrix.They show evidence for a large-scale OIB plume activity,which occurred prior to ophiolite obduction.(3)Early-Late Cretaceous calc-alkaline lavas and dykes.These magmatic rocks are found on top of the obducted nappe,above the post-obduction erosion level.This series shows similar Sr-Nd isotopic features as the Alkaline series,though having a clear supra-subduction affinity.They are thus interpreted to be the remelting product of a mantle previously contaminated by the OIB plume.Correlation of data from the Lesser Caucasus to western Anatolia shows a progression from back-arc to arc and fore-arc,which highlight a dissymmetry in the obducted oceanic lithosphere from East to West.The metamorphic P-T-t paths of the obduction sole lithologies define a southward propagation of the ophiolite:(1)P-T-t data from the northern Sevan-Akera suture zone(Armenia)highlight the presence and exhumation of eclogites(1.85±0.02 GPa and 590±5℃)and blueschists below the ophiolite,which are dated at ca.94 Ma by Ar-Ar on phengite.(2)Neighbouring Amasia(Armenia)garnet amphibolites indicate metamorphic peak conditions of 0.65±0.05 GPa and 600±20 C with a U-Pb on rutile age of 90.2±5.2 Ma and Ar-Ar on amphibole and phengite ages of 90.8±3.0 Ma and 90.8±1.2 Ma,respectively.These data are consistent with palaeontological dating of sediment deposits directly under(Cenomanian,i.e.>93.9 Ma)or sealing(Coniacian-Santonian,i.e.,≤89.8 Ma),the obduction.(3)At Hinis(NE Turkey)PT-t conditions on amphibolites(0.66±0.06 GPa and 660±20℃,with a U-Pb titanite age of80.0±3.2 Ma)agree with previous P-T-t data on granulites,and highlight a rapid exhumation below a top-to-the-North detachment sealed by the Early Maastrichtian unconformity(ca.70.6 Ma).Amphibolites are cross-cut by monzonites dated by U-Pb on titanite at 78.3±3.7 Ma.We propose that the HT-MP metamorphism was coeval with the monzonites,about 10 Ma after the obduction,and was triggered by the onset of subduction South of the Anatolides and by reactivation or acceleration of the subduction below the Pontides-Eurasian margin.Numerical modelling accounts for the obduction of an'old'~80 Myr oceanic lithosphere due to a significant heating of oceanic lithosphere through mantle upwelling,which increased the oceanic lithosphere buoyancy.The long-distance transport of a currently thin section of ophiolites(<1 km)onto the Anatolian continental margin is ascribed to a combination of northward mantle extensional thinning of the obducted oceanic lithosphere by the Hinis detachment at ca.80 Ma,and southward gravitational propagation of the ophiolite nappe onto its foreland basin. | Y.Rolland M.Hässig D.Bosch O.Bruguier R.Melis G.Galoyan G.Topuz L.Sahakyan A.Avagyan M.Sosson | 2020 | Geoscience Frontiers2020，11，1： | 2 |
| 3 | Large earthquakes along slow converging plate margins:Calabrian Arc paleoseismicity based on the submarine turbidite record显示文摘The Calabrian Arc subduction-rollback system hosts seismogenic faults capable of generating earthquakes exceeding magnitude 7.Since earthquakes are the result of long-term geodynamic processes,documenting seismic activity during a sufficiently long time interval is of fundamental importance for hazard scenarios.Instrumental and historical data provide critical information on seismogenesis,but they cover time periods shorter than the recurrence times of large earthquakes,especially in areas with low deformation rates such as Calabria.If onshore paleoseismological studies are fundamental to compile earthquake catalogs,they are sometime affected by the relatively poor continuity of sedimentation in the subaerial environment.In this study we applied the paleoseismological approach to the submarine environment to reconstruct the record of high-energy sedimentary events triggered by seismic activity.We analyzed three gravity cores collected in disconnected sedimentary basins to reconstruct resedimentation processes during the Holocene,integrating inland information for a better assessment of tectonic activity and seismogenesis.Multiproxy analyses of the sedimentary record constrained by radiometric dating allowed reconstructing event stratigraphy and linking resedimented deposits to specific earthquakes.Onshore and offshore data allow to identify large-magnitude earthquakes in the central Calabrian Arc subduction system during the Holocene,with inferred epicenters located either along normal faults onshore and/or related to the slab dynamics.The turbidite record reveals 20 major events during the last 10 ka,with sources including crustal faults in Calabria(i.e.Lakes,Rossano and Cittanova faults).Analyses of sediment samples and high-resolution seismic reflection images allowed identification of different types of resedimented deposits during the last 30–50 ka.The basin-wide occurrence of three megaturbidites/homogenites suggests they are related to megatsunamis sourced by far field earthquakes along the Hellenic Arc.Megaturbidites with a more limited spatial extent are interpreted as subduction-type events in the Calabrian Arc,while thinner seismo-turbidites record the activity of crustal structures including faults onshore.Results suggest a recurrence time of 2–3 ka for major Calabrian Arc events that needs to be considered for a reliable hazard assessment in the Mediterranean region. | A.Polonia R.Melis P.Galli E.Colizza D.D.Insinga L.Gasperini | 2023 | Geoscience Frontiers2023，14，5： | 0 |
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