The major suture zone of the Qinling orogenic belt

Abstract The Qinling mountains are a typical composite continental orogenic belt which underwent various stages of evolution characterized by different tectonic regimes. Its late Proterozoic to early Mesozoic evolution was dominated by plate subduction and collision processes. The Shangdan fault zone marks the major suture which resulted from the plate subduction and collision and represents the basic division in both the superfacial geology and the deep-seated crustal structure of the Qinling mountains. It is not a simple fault zone but an intricate geologic terrane, characterized by a prolonged and complicated history. It is referred to as Shangdan boundary fault terrane or Shangdan zone for short. The Shangdan zone consists chiefly of five parts: (1) fault systems as marked by various types of mylonite and cataclasite, (2) tectonic assemblages of Danfeng ophiolite blocks, (3) tectonic assemblages of obducted slices of a sedimentary prism, (4) granitic rocks related to orogenic processes of subduction and collision, and (5) basins controlled by late brittle faults. The evolution of the Shangdan zone can be traced through three major stages: the Caledonian period of subduction, the Indosinian period of collision and orogeny, and the Mesozoic-Cenozoic period of intra-plate deformation, among which the Indosinian period marks the most important stage in its development. In brief, the Shangdan zone is the major division between the ancient lithospheric plates in the Qinling belt and extends deep into the upper mantle.