柴北缘骆驼泉剖面新生代地层磁组构特征及其构造意义

Magnetic fabric results of Cenozoic sediments from the Luotuoquan locality in the northern Qaidam Basin and their tectonic implications

青藏高原东北部新生代构造演化对理解高原隆升和变形模式具有重要意义,而目前对于该地区挤压应力方向转变过程仍存在很大争议.本文对柴北缘逆冲带北西部骆驼泉剖面新生代地层开展系统磁组构(本文特指磁化率各项异性)研究,以揭示该地区挤压应力方向的转变特征.系统岩石磁学结果表明,骆驼泉剖面新生代样品中主要磁性矿物是顺磁性组分和赤铁矿以及少量磁铁矿.通过对磁组构特征分析及其与古水流方向对比表明,骆驼泉剖面新生代地层磁组构主要为初始变形磁组构,可用于指示沉积成岩时期的挤压应力方向.磁组构结果揭示,骆驼泉地区挤压应力方向在上干柴沟组下部沉积时期为NNE-SSW向,而上干柴沟组上部和油砂山组沉积时期转变为NE-SW向.结合柴北缘逆冲带已有磁组构结果指出,该地区早期N-S向或NNE-SSW向挤压应力可能与印度—欧亚板块早新生代以来近N-S向碰撞挤压过程有关,指示印度—欧亚碰撞的挤压应力自下干柴沟组下部沉积时期就已传播至高原东北部地区;而后期NE-SW向挤压应力方向与该地区现今GPS揭示的上地壳运动方向一致,可能与该时段高原东北部巨型走滑断裂构造体系(尤其是阿尔金断裂)有关.此外,柴北缘逆冲带新生代挤压应力方向转变在其北西部起始于上干柴沟组下部沉积时期,而南东部起始于上油砂山组下部沉积时期,与地震反射剖面揭示的断裂活动等地质证据共同揭示柴北缘逆冲带新生代的构造活动自靠近阿尔金断裂的北西部向南东部传播和扩展.综合分析青藏高原东北部地区挤压应力方向转变和其他地质证据发现,挤压应力方向转变显示出自柴北缘逆冲带北西部向东、西和南向扩展特征,与阿尔金断裂在上干柴沟组下部-上油砂山组下部沉积时期剪切应力集中于断裂本身,而上油砂山组下部沉积以来开始散布于高原东北部内部地区的两阶段走滑活动相关.

The Cenozoic tectonic activities in the northeastern Tibetan Plateau are crucial for understanding uplift and tectonic deformation of the Tibetan Plateau.However,the transition of the compressional strain directions in this region is still controversial.Here,we conduct a detailed magnetic fabric(refer in particular to anisotropy of magnetic susceptibility,AMS) analysis on Cenozoic sediments from the Luotuoquan locality in the northwestern part of the northern Qaidam marginal thrust belt(NQMTB) to reveal the temporal variations of the compressional strain directions.Systematic rock magnetic experiments indicate that a paramagnetic component and hematite are the main magnetic contributors to the susceptibility,whereas magnetite plays a subordinate role.By analyzing magnetic fabrics and paleocurrent directions in this region,we propose that the Cenozoic magnetic fabrics are mostly embryonic tectonic fabrics and can be used to track the compressional strain directions that were induced during the deposition and lithification processes.The magnetic fabric results reveal that the compressional strain direction at the Luotuoquan locality was NNE-SSW during the deposition of the lower Shangganchaigou Formation,and changed to NE-SW during the deposition of the upper Shangganchaigou and Youshashan Formations.Together with the available magnetic fabric results along the NQMTB,we suggest that the former N-S or NNE-SSW direction was mostly related to the roughly N-S directed India-Eurasia collision,implying the compressional strain has already propagated from the collisional zone to the northeastern Tibetan Plateau during the deposition of the lower Xiaganchaigou Formation.The latter NE-SW direction is consistent with the upper crust movement from GPS observations and is likely related to the huge strike-slip faulting(especially the Altyn Tagh Fault(ATF)) after the deposition of the lower Shangyoushashan Formation.Moreover,the transition of the compressional strain directions occurred after the deposition of the lower Shangganchaigou Formation in its northwestern part,but after the lower Shangyoushashan Formation in its southeastern part.Combined with fault activities revealed by seismic reflection profiles,these results indicate that the tectonic activities may have initially commenced at the northwestern part of the NQMTB(adjacent to the ATF) and propagated to its southeastern part.Integrating this with the compressional strain directions and other geological evidence in the region,we propose that the transition of the compressional strain directions in the NQMTB has propagated from northwest to east,west and south.This process was probably related to the two-stage strike-slip faulting of the ATF,which is characterized by concentrated sinistral shear during the deposition of the lower Shangganchaigou to lower Shangyoushashan Formations,whereas by distributed sinistral shear after the deposition of the lower Shangyoushashan Formation.