Basin-and Mountain-Building Dynamic Model of “Ramping-Detachment-Compression” in the West Kunlun-Southern Tarim Basin Margin

Analysis of the deformation structures in the West Kunlun-Tarim basin-range junction belt indicates that sediments in the southwestern Tarim depression were mainly derived from the West Kunlun Mountains and that with time the region of sedimentation extended progressivdy toward the north. Three north-underthrusting （subducting）, steep-dipping, high-velocity zones （bodies） are recognized at depths, which correspond to the central West Kunlun junction belt （bounded by the Kiida-Kaxtax fault on the north and Bulungkol-Kangxiwar fault on the south）, Quanshuigou fault belt （whose eastward extension is the Jinshajiang fault belt） and Bangong Co-Nujiang fault belt. The geodynamic process of the basin-range junction belt generally proceeded as follows： centering around the magma source region （which largely corresponds with the Karatag terrane at the surface）, the deep-seated material flowed and extended from below upward and to all sides, resulting in strong deformation （mainly extension） in the overlying lithosphere and even the upper mantle, appearance of extensional stress perpendicular to the strike of the orogenic belt in the thermal uplift region or at the top of the mantle diapir and localized thickening of the sedimentary cover （thermal subsidence in the upper crust）. Three stages of the basin- and mountain-forming processes in the West Kunlun-southern Tarim basin margin may be summarized： （1） the stage of Late Jurassic-Early Cretaceous rampingrapid uplift and rapid subsidence, when north-directed thrust propagation and south-directed intracontinental subduction, was the dominant mechanism for basin- and mountain-building processes; （2） the stage of Late Cretaceous-Paleogene deep-level detachment-slow uplift and homogeneous subsidence, when the dominant mechanism for the basin- and mountain-forming processes was detachment （subhorizontal north-directed deep-level ductile shear） and its resulting lateral propagation of deep material; and （3） the stage of Neogene-present compression-rapid uplift and strong subsidence, when the basin- and mountain-forming processes were simultaneously controlled by north-vergent thrust propagation and compression. The authors summarize the processes as the “ramping-detachment-compression basin- and mountain-forming dynamic model”. The basin-range tectonics was initiated in the Late Jurassic, the Miocene-Pliocene were a major transition period for the basin- and mountain-forming mechanism and the terminal early Pleistocene tectonic movement in the main laid a foundation for the basin-and-mountain tectonic framework in the West Kunlun-southern Tarim basin margin.

Seismotectonics in the Pamir:An oblique transpressional shear and south-directed deep-subduction model

The 3-D geometry of the seismicity in Hindu Kush-Pamir--western China region has been defined by seismic records for 1975-1999 from the National Earthquake Information Center, the U.S. Geological Survey, and over 16,000 relocated earthquakes since 1975 recorded by the Xinjiang seismic network of China. The results show that most Ms≥ 5.0 hypocenters in the area are confined to a major intracontinental seismic shear zone （MSSZ）. The MSSZ, which dips southwards in Pamir has a north- dipping counterpart in the Hindu Kush to the west; the two tectonic realms are separated by the sinistral Chaman transform fault of the India-Asia collisional zone. We demonstrate that the MSSZ constitutes the upper boundary of a south-dipping, actively subducting Pamir continental plate. Three seismic concentrations are recognized just above the Pamir MSSZ at depths between 45-65 km, 95-120 km, and 180-220 km, suggesting different structural relationships where each occurs. Results from focal mechanism solutions in all three seismological concentrations show orientations of the principal maximum stress to be nearly horizontal in an NNW-SSE direction. The south-dipping Pamir subduction slab is wedge-shaped with a wide upper top and a narrow deeper bottom; the slab has a gentle angle of dip in the upper part and steeper dips in the lower part below an elbow depth of ca. 80--120 km. Most of the deformation related to the earthquakes occurs within the hanging wall of the subducting Pamir slab. Published geologic data and repeated GPS measurements in the Pamir document a broad supra-subduc- tion, upper crustal zone of evolving antithetic （i.e. north-dipping） back-thrusts that contribute to northsouth crustal shortening and are responsible for exhumation of some ultrahigh-pressure rocks formed during earlier Tethyan plate convergence. An alternating occurrence in activity of Pamir and Chaman seismic zones indicates that there is interaction between strike-slip movement of the Chaman transform fault system and deep-subduction of the Pamir earthquake zone. Pamir subdnction-related seismicity becomes shallower in depth with increasing distance east of the transform fault. Therefore, sinistral movement of the Chaman transform fault appears to be influencing continental deep-subduction in the Pamir region and may provide an explanation for the unusual south-dipping geometry of the intracontinental Pamir plate.

Seismogenic Structure of the April 20, 2013, Lushan Ms7 Earthquake in Sichuan

At 08：02 on April 20, 2013, a Ms7.0 earthquake occurred in Lushan, Ya＇an, in the Longmenshan fault zone, Sichuan. The epicenter was located between Taiping Town and Shuangshi Town, Lushan County and the maximum earthquake intensity at the epicenter reached class IX. Field investigations in the epicenter area found that, although buildings were seriously damaged, no obvious surface rupture structure was produced, only some ground fissures and sand blows and water ejection phenomena being seen. An integrated analysis of high-resolution remote sensing image interpretation, mainshock and aftershock distribution, and focal mechanism solutions indicated that this earthquake was an independent rupturing event in the southwestern segment of the Longmenshan fault zone, belonging to the thrust-type earthquake. Ruptures occurred along the south-central segment of the Shuangshi-Dachuan fault and the principal rupture plane dipped SW at 33-43% It is inferred that the Lushan earthquake might be related to the ramp activity of the basal detachment zone （13-19 kin） of the Longmenshan fault zone. Historically, there occurred at least two Ms6-6.5 earthquakes along the Shuangshi-Dachuan fault zone; thus it is thought that the Lushan earthquake, different from the Wenchuan earthquake, was a characteristic one in the southwestern segment of the Longmenshan fault zone. In-situ stress measurements indicated the Lushan earthquake was the result of stress release of the southwestern segment of the Longmenshan fault zone after the Wenchuan earthquake. This paper analyzes the tectonic setting of the seismogenic structure of this earthquake.