Triassic to Cenozoic multi-stage intra-plate deformation focused near the Bogd Fault system,Gobi Altai,Mongolia

The Gobi Altai region of southern Mongolia has been in the Eurasian plate interior since the midMesozoic,yet has experienced episodic phases of deformation since that time.In this paper,we document field evidence to characterize and date the intra-plate tectonic history of the Gobi Altai region from the Triassic to the present.To this end,we provide detailed mapping of the structure and stratigraphy of the eastern flanks of Mt.Ih Bogd that contains the widest variety of rock-time units in the area.We carry out geochronological analysis of basaltic lavas and basement granite in the area.We demonstrate that a crystalline basement with a 502 ± 8 Ma granitoid（U/Pb） underwent two phases of basin formation in the Mesozoic.which we date with new40Ar/39Ar lava ages of 218.5 ± 1.5,123.2 ± 0.7 and 124.8 ± 1.2 Ma,respectively.Both phases are linked to deposition of fluvio-lacustrine sediments and trap-like basaltic volcanics,with cumulative thicknesses of 1000-1500 m.Both basins were likely north-facing half-grabens that developed underN-S extension,but were subsequently overthrusted by Paleozoic and older crystalline basement during a less well constrained,but likely mid-Cretaceous phase of N-S shortening and basin inversion.Our results are consistent with recent seismic imaging of rift basins100 km to the NE of the study area where a similar history was reconstructed.The multiple phases of intra-plate deformation appear to have parallel structural trends,most likely due to reactivated Paleozoic basement structures created during the original terrane amalgamation of the Central Asian Orogenic Belt continental crust.This strong basement heterogeneity may predispose it to reactivation,and make it sensitive to changes in the overall stress field of the Eurasian plate driven by forces at its margins and base.Detailed study of Mongolia’s multi-stage tectonic history may thus provide a key proxy for the long-term dynamics of the Eurasian plate.In addition,the repeated reactivation of the Gobi Altai region during the last 200 My supports the contention that non-cratonized continental interior regions composed of Phanerozoic terrane collages are particularly susceptible to fault reactivation,much more than older cratonized continental crust.

Causes of Cretaceous subduction termination below South China and Borneo:Was the Proto-South China Sea underlain by an oceanic plateau?

The South China,Indochina,and Borneo margins surrounding the South China Sea contain long-lived arcs that became inactive at approximately 85 Ma,even though an embayment of oceanic crust(the‘Proto-South China Sea’)remained in the intervening region.This oceanic crust eventually subducted in the Cenozoic below Borneo and the Cagayan arc,while the modern South China Sea opened in its wake.To investigate the enigmatic cessation of Mesozoic subduction below South China and Borneo,we studied a fragment of oceanic crust and overlying trench-fill sediments that accreted to NW Borneo during the final stages of Paleo-Pacific subduction.Based on radiolarian biostratigraphy of cherts overlying the pillow basalts and detrital zircon geochronology of the trench-fill,we constrained the minimum age of the oceanic crust during accretion to 40 Ma.This shows that subduction cessation was not related to ridge subduction.Geochemical analysis of pillow basalts revealed an enriched mid-ocean ridge basalt signature comparable to oceanic plateaus.Using paleomagnetism,we show that this fragment of oceanic crust was not part of the Izanagi Plate but was part of a plate(the‘Pontus’Plate)separated from the Izanagi Plate by a subduction zone.Based on the minimum 40 Ma age of the oceanic crust and its geochemistry,we suggest that Mesozoic subduction below South China and Borneo stopped when an oceanic plateau entered the trench,while the eastern plate margin with the Izanagi Plate remained active.We show how our findings offer opportunities to restore plate configurations of the Panthalassa-Tethys junction region.

Triassic (Anisian and Rhaetian) palaeomagnetic poles from the Germanic Basin (Winterswijk, the Netherlands)

In this paper,we provide two new Triassic palaeomagnetic poles from Winterswijk,the Netherlands,in the stable interior of the Eurasian plate.They were respectively collected from the Anisian(~247–242 Ma)red marly limestones of the sedimentary transition of the Buntsandstein Formation to the dark grey limestones of the basal Muschelkalk Formation,and from the Rhaetian(~208–201 Ma)shallow marine claystones that unconformably overlie the Muschelkalk Formation.The magnetization is carried by hematite or magnetite in the Anisian limestones,and iron sulfides and magnetite in the Rhaetian sedimentary rocks,revealing for both a large normal polarity overprint with a recent(geocentric axial dipole field)direction at the present latitude of the locality.Alternating field and thermal demagnetization occasionally reveal a stable magnetization decaying towards the origin,interpreted as the Characteristic Remanent Magnetization.Where we find a pervasive(normal polarity)overprint,we can often still determine well-defined great-circle solutions.Our interpreted palaeomagnetic poles include the great-circle solutions.The Anisian magnetic pole has declination D±ΔDx=210.8±3.0°,inclination I±ΔIx=-26.7±4.9°,with a latitude,longitude of 45.0°,142.0°respectively,K=43.9,A95=2.9°,N=56.The Rhaetian magnetic pole has declination D±ΔDx=32.0±8.7°,inclination I±ΔIx=50.9±8.1°,with a latitude,longitude of 60.6°,123.9°respectively,K=19.3,A95=7.4°,N=21.The poles plot close to the predicted location of global apparent polar wander paths(GAPWa Ps)in Eurasian coordinates and are feasible for future apparent polar wander path construction.They confirm that the intracontinental,shallow-marine Germanic Basin,in which the Muschelkalk Formation was deposited,existed at a palaeolatitude of 14.1°[11.3,17.1]N,in a palaeo-environment reminding of the Persian Gulf today.In Rhaetian times,palaeolatitudes of 31.6°[24.8,39.8]N were reached,on its way to the modern latitude of 52°N.