Mechanism on Moho offset induced by aseismic slip of deeply buried faults

Recent high-resolution deep seismic reflection profile across the Kunlun fault in northeastern Tibet shows clearly that the Moho is cut off by a complex thrust fault system. Moho offset is a general phenomenon, but little is known about the dynamic mechanism. In this study, contact models with Maxwell materials are used to simulate the mechanical process of Moho offset induced by the aseismic slip of deeply buried faults. Based on the seismic reflection data, we project a single fault model and a complex fault system model with two faults inter- secting. The deformations of the Moho, the aseismic slips, and contact stresses on faults in different models are discussed in detail. Results show that the Moho offset might be produced by aseismic slip of deeply buried faults, and the magnitude is influenced by the friction coefficient of faults and the viscosity of the lower crust. The maximum slip occurs near the Moho on the single fault or at the crossing point of two intersecting faults system. Stress concentrates mainly on the Moho, the deep end of faults, or the crossing point. This study will throw light on understanding the mechanism of Moho offset and aseismic slip of deeply buried faults. The results of complex fault system with two faults intersecting are also useful to understand the shallow intersecting faults that may cause earthquakes.

Moho offset beneath the central Bangong-Nujiang suture of Tibetan Plateau

By analyzing teleseismic waveforms recorded by 53 stations of Hi-Climb profile passing through the central Bangong-Nujiang suture(BNS) ,a total of 4764 high-quality receiver functions are obtained.The average crustal thickness and Poisson's ratio beneath each station are estimated using the travel time of Ps and PpPs of the Moho.The discontinuities such as the Moho,the 410and 660-km interfaces are also studied using the common converted points(CCP) time to depth migration of receiver functions. The main results are as follows:(1) The Moho of Lhasa terrane and that of Qiangtang terrane nearby BNS are overridden and offset by～10 km.The structural geometry shows a northward uplifting and the southward deepening for the Moho of Lhasa terrane and Qiangtang terrane,respectively,which is related to the reactivated structure beneath BNS since Cenozoic era.(2) The variation range of Poisson's ratios along the profile is between 0.237 and 0.280,indicating that the crust is mainly composed of felsic and intermediate rocks.The anti-correlation between the crustal thickness and Poisson's ratio suggests that thicker crust beneath the southern Qiangtang terrane may be related to the successive thrust of felsic and intermediate rock of Lhasa terrane.(3) The thickness of the mantle transition zone along the profile remains about 255 km,implying that the tectonic activities caused by the India-Asia collision are confined to the depths above 410 km.

深地震探测技术在四川盆地的应用

深地震探测技术是国际地学界公认的探测地球深部岩石圈精细结构、解决深部地质问题的最有效手段,目前国内外已广泛将其应用于研究板块内造山带的构造格局、岩石圈的精细结构以及刻画莫霍面的位置和走向。由于探测深度深,深地震探测技术要求偏移距大、药量大、记录长度长,因此与常规石油地震勘探存在较大差异。四川盆地开展的深地震探测技术,在观测系统设计、激发药量优化等方面探索和总结了适合深地震探测实验的采集技术方案和施工方法。

Remnants of the amalgamation of the east and west Cathaysia blocks revealed by a short-period dense nodal array

The Cathaysia block located at the southeast South China block(SCB)is considered formed by the amalgamation of the east and west Cathaysia blocks along the Gaoyao-Huilai and Zhenghe-Dapu deep faults(here referred as GHF and ZDF,respectively).Although the extension of the ZDF to the northeast,which represents the amalgamation of the two sub-blocks has been confirmed,the development of the GHF to the southwest remains to be verified.To better constrain the detailed deep structure beneath the southwest Cathaysia,which hold great significance for revealing the evolution of the SCB,a linear seismic array with 331 nodal geophones was deployed across the Sanshui basin(SSB).Combining with the regional 10 permanent stations(PA),we obtained two profiles with teleseismic P-wave receiver function stacking.The most obvious feature in our results is the ascending Moho towards the coastal area,which is consistent with the passive margin continental and extensional tectonic setting.The stacking profile from the dense nodal array(DNA)shows that the Moho is offset beneath the transition zone of the Nanling orogeny and SSB.We deduce that this offset may be casued by the deep extension of the GHF,which represents the remnants of the amalgamation of the Cathaysia block.From the other evidences,we infer that the widespread and early erupted felsic magmas in the SSB may have resulted from lithospheric materials that were squeezed out to the surface.The relative higher Bouguer gravity and heat flow support the consolidation of magmas and the residual warm state in the shallow crustal scale beneath the SSB.The sporadic basaltic magmas in the middle SSB may have a close relation to deep extension of the GHF,which serves as a channel for upwelling hot materials.