阿尔金断裂两侧构造单元的对比及岩石圈剪切机制

A Comparison between the Tectonic Units on the Two Sides of the Altun Sinistral Strike-slip Fault and the Mechanism of Lithospheric Shearing

中亚最大的阿尔金走滑断裂系构成了青藏高原的北部边界,阿尔金北缘断裂及阿尔金在行走滑断裂(阿尔金主断裂)所夹持的阿尔金地体自北往南由敦煌地块、北阿尔金加里东俯冲杂岩带、中阿尔金地块及南阿尔金加里东俯冲-碰撞杂岩带组成,研究表明这些单元分别可与阿尔金主断裂带东侧的阿拉善地块、北祁连俯冲杂岩带、中-南祁连地块及柴达木北缘加里东俯冲-碰撞杂岩带相对比。特别通过南阿尔金与柴达木北缘榴辉岩带的详细对比,发现它们具有相似的地质背景、产状、矿物组合、岩石地球化学、原岩特征、温度压力条件、退变质作用和围岩特征以及非常接近的峰期变质时代(500～503 Ma和495 Ma),可能构成了中国又一条被巨型走滑断裂所切割的高压-超高压变质带(?),并为阿尔金断裂带左行走滑400 km的确定提供了科学依据。天然地震探测剖面显示阿尔金主断裂由近直立低速低密度的物质组成,与低角度向南倾的阿尔金北缘断裂之间夹持由高速高密度组成的阿尔金地体。阿尔金北缘断裂与阿尔金主断裂在80km深度处交汇,并继续向南陡倾下插到150 km,反映出塔里木地块向南陆内俯冲于阿尔金山及柴达木盆地之下。代表上地幔物质剪切流动矢量的高强度各向异性,显示了剪切作用可抵达上地幔,阿尔金断裂的岩石圈剪切作用可能与塔里木地块往?

The Altun strike-slip fault is the largest strike-slip fault in Aisa and defines the northern boundary of the Qinghai-Tibet Plateau. The Altun terrane, bounded by the north Altun thrust fault and the Altun strike-slip fault on the south, consists of Dunhuang massif, northern Altun Caledonian subduction complex, central Altun massif and southern Altun subduction-collision complex from north to south. Our studies indicate that these tectonic units correspond to the Alxa block, northern Qilian subduction complex, central Qilian block and northern Qaidam subduction-collision zone respectively on the two sides of the Altun strike-slip fault. Especially, the eclogites in the southern Altun and northern Qaidam show strong similarities in geological setting, occurrence, mineral assemblage, geochemical and protolith feature, P-t condition of formation, retrograde metamorphism, associated country rock and almost consistent metamorphic age. This suggests the HP-UHP(?) metamorphic zone displaced by a large strike-slip fault be similar to the case of the Dabie - Sulu HP-UHP metamorphic zone which was truncated by the Tanlu sinistral strike-slip fault, and the offsets along the Altun sinistral strike-slip fault is about 400 km.A seismic tomographic section across the Altun Mountains , established by the natural earthquake experiment, indicates a steep low-velocity anomaly beneath and along the Altun strike-slip fault, and a high-velocity anomaly (implying dense material) beneath the Altun terrane located between the southern Altun strike-slip fault and the northern gently-dipping thrust fault. The two faults may meet at a depth of 80 km, then continue to underthrust steeply southward to 150 km, suggesting that Tarim block has been underthrusted southward beneath the Altun Mountains and the Qaidam block. The large magnitude of anisotropy represents the existence of shearing of upper mantle along Altun strike-slip fault , while the lithospheric shearing may be attributed to the southward subduction of the Tarim block.