Paleocene–early Eocene post-subduction magmatism in Sikhote-Alin (Far East Russia): New constraints for the tectonic history of the Izanagi-Pacific ridge and the East Asian continental margin

New isotopic,geochemical and geochronological data justify the widespread occurrence of middle Paleocene to early Eocene(60–53 Ma)post-subduction felsic magmatism across the entire Sikhote-Alin territory(southeastern Russia),conform with previous observations in Northeast China,the southern Korean Peninsula,and the Inner Zone of Japan.This igneous activity in East Asia coincided with the reactivation(after tectonic quiescence between~93–60Ma)of left-lateral strike-slip displacements along the Tan-Lu and Central Sikhote-Alin faults and with the post-60 Ma cessation of subduction/accretion recorded in the Shimanto belt of SW Japan.The Sikhote-Alin post-subduction igneous A-type rocks present diverse mineralogical and geochemical features that suggest interactions of the subducting plate with anhydrous mantle upwelling through slab tears in the continental margin.The middle Paleocene–early Eocene magmatismis not related to subduction but is synchronous with strikeslip tectonics and the termination of accretionary prism development,suggesting a shift in tectonic regime from oceanic plate subduction at a convergent margin to parallel sliding and initiation of a transformcontinental margin.These new observations are inconsistent with the current tectonic model of 60–50 Ma Izanagi-Pacific ridge subduction beneath East Asian continental margin.

Tectonic implications of the subduction of the Kyushu-Palau Ridge beneath the Kyushu,southwest Japan

The Kyushu-Palau Ridge(KPR),a remnant arc on the Philippine Sea Plate(PSP),is subducting beneath the Kyushu,southwest Japan.Influenced by the subducting KPR,the Kyushu subduction zone corresponding to the KPR is significantly different from Shikoku subduction zone in terms of gravity anomalies,seismicity,the stress state,and the subducting slab morphology.Significant negative free-air and Bouguer gravity anomalies are observed in a prolonged area of KPR,southeast of the Miyazaki Plain,indicating that this is where KPR overlaps the overriding plate.The gravity anomaly in this area is much lower than that in other areas where the inferred KPR extends,suggesting that the subduction of the buoyant KPR may cause the lower mantle density to decrease.More earthquakes have occurred in Hyuga-nada region where the KPR subducts than in Shikoku forearc and other areas in the Kyushu forearc,indicating that the subduction of the KPR enhances the local coupling between the subducting and overriding plates.The centroid moment tensor(CMT)mechanism of earthquakes shows that stress is concentrated in the accumulated crust beneath the Kyushu forearc corresponding to the KPR,and the shallow thrusting events in the obducting plate are caused by the KPR subduction.The buoyant KPR,with a large volume of low-density sediments,was responsible for the differences of the subduction depth and dip angle of the subducting Philippine Sea(PS)slab between northern Kyushu and Shikoku.The seismic gaps and the sudden change of the dipping angle of the subducting PS slab indicate that slab tear may have occurred along the west side of the KPR beneath southwest Kyushu.A two-tear model was proposed,and the subduction of the buoyant KPR was believed to play an important role in the slab tear.

Generation and Disruption of Subducted Lithosphere in the Central-Western Mediterranean Region and Time-Space Distribution of Magmatic Activity Since the Late Miocene

The long migration of the Balearic Arc (Alpine-Apennine and Alpine-Maghrebian belts) in the Early-Middle Miocene caused the formation of a subducted lithospheric edifice in the western and central Mediterranean regions. Then, since the Late Miocene, this slab was almost completely disrupted, only maintaining a narrow and deformed remnant beneath the southernmost Tyrrhenian basin. This work describes a tentative reconstruction of the tectonic processes that caused the formation of major tears and breakoffs in the original slabs and the consequent disruption of the subducted lithosphere. In particular, it is suggested that this relatively fast process was produced by the collision between the Anatolian-Aegean system and the continental Adriatic domain, which triggered a number of extrusion processes. Possible connections between the proposed tectonic evolution and the spatio-temporal distribution and geochemical signatures of magmatic activity are then discussed. It is supposed that such activity has been mainly conditioned by the occurrence of transtensional tectonics in the wake of escaping orogenic wedges.

Late Jurassic oceanic plateau subduction in the Bangong-Nujiang Tethyan Ocean of northern Tibet

Oceanic plateau accretion and subsequent flat-slab subduction in modern convergent settings have profoundly influenced the nature of subduction and mantle dynamics.However,evaluating similar impacts in ancient convergent settings,where oceanic plateaus have been subducted but geological records are limited,remains challenging.In this study,we present geochronological and geochemical data for a suite of ore-associated plutonic rocks from the Gaobaoyue area of northern Tibet.These rocks have zircon U-Pb ages of 152-146 Ma,with high Sr contents and Sr/Y and La/Yb ratios,low MgO,Yb,and Y contents,and depleted Sr-Nd-Hf isotopic compositions,consistent with an adakitic affinity that was generated by the partial melting of subducting oceanic crust.We compare the Late Jurassic adakitic magmatism with the spatiotemporal evolution of magmatism in northern Tibet to infer oceanic plateau subduction and subsequent flat-slab subduction in the Bangong-Nujiang Tethyan Ocean.This tectonic model explains(ⅰ)slab-derived adakitic magmatism,(ⅱ)the observed lull in magmatic activity,(ⅲ)intraplate compression and uplift,and(ⅳ)subduction jump and initiation.We also propose that the subduction of heterogeneous oceanic crust(i.e.,buoyant oceanic plateau subduction)provided favorable conditions for tectonic exhumation,vertical slab tearing,and the formation of Cu-Au deposits.Our findings not only have implications for establishing the fundamental process of oceanic plateau accretion in ancient subduction zones but also provide an alternative explanation for Late Jurassic complex tectonomagmatic activity in north-ern Tibet.

Paleocene A-type igneous suites in the Sikhote-Alin(the East Asian continental margin):Petrological,geochronological,isotopic,and geodynamic constraints

Global geodynamic reconstruction models of the Pacific margin of Asia refer to materials collected throughout Japan,Korea and northeastern China,but they lack data on southern Sikhote-Alin.Therewith,the Sikhote-Alin orogenic belt(NE Asia)constituted a single eastern margin of the paleo-Asian continent with the abovementioned territories in the Paleocene.New isotopic,geochemical,and geochronological data show that Paleocene igneous activity(-61-55 Ma)is widely developed in south-ern Sikhote-Alin.Bulk rock compositions of the igneous rocks of this period yield ferroan,peraluminous,calc-alkaline to alkaline affinities,highly abundant LILE and HFSE(with pronounced Ba,Sr,Eu,and Ti neg-ative anomalies)and depleted HREE.The initial melts,displaying Zr+Y+Ce+Nb>350 ppm and 10,000×Ga/Al>2.6,derived from an OIB-like mantle source crystallised under fairly reducing conditions(below FMQ buffer),and yield high temperature of zircon saturation(>850℃),indicating typical A-type granite and related volcanic rock features.It is logical to associate variations in A-type rock geochemical composition with an enrichment of the upper part of the magma chamber with fluid-mobile components involving a reditribution of major and trace elements through fluid-magmatic differentiation.Strong depletion in Ca,Mg,Ba,Sr,Eu in the A-type rocks is caused by an outflow of these elements by an oxi-dizing,initially reduced,acidic fluid beyond the zone of magma generation.Whole-rock Sr-Nd isotope data argue for the generation of the A-type rocks by melting of dominantly a juvenile mantle component with a subordinate component of the ancient crust.Isotope variations of igneous rocks of the reference area:^(87)Sr/^(86)Sr(t)(0.7024-0.7118),ε_(Nd)(t)(-0.9 to-5.1)and T_(DM2)(934-1277 Ma),result from the mixing of the OIB-like mantle source with selective melts or from the metapelite contamination of the Samarka terrane accretionary prism and of the Zhuravlevka-Amur turbidite basin,later followed by fluid-magmatic differentiation that led to the formation of anatectic or hybrid melts.We further suggest that the origin of the A-type granites and related volcanic rocks is the result of the oblique interaction of ocea-nic and continental plates.This interaction accounts for the simultaneous formation of tears in the slab,enabling sub-slab asthenospheric upwelling,and strike-slip fault-related extensional structures in the overriding continental plate.