Sedimentary evolution of the Yanshan fold-thrust belt, northeast China.

摘要

The Yanshan fold-thrust belt is an east-west trending zone of intracontinental shortening that developed within the North China block in Mesozoic time. Sedimentary rocks within the thrust belt record a polyphase history of tectonism: (1) Permian-Triassic shortening due to the collision of North China with Paleozoic arc terranes of Mongolia (the “Altaid” arcs); (2) Late Jurassic shortening due to continental collisions along the southern, and possibly the northern, margin of the amalgamated North China plate; and (3) Early Cretaceous extension, which affected all of northeast China and eastern Mongolia.; The first phase of deformation in the Yanshan belt resulted from convergence between the North China block and arc terranes of Mongolia. Detrital zircon provenance data from Carboniferous-Permian nonmarine strata within the Yinshan segment of the Yanshan belt imply that the northern margin of the North China block constituted a continental margin arc prior to this collision (ca. 400–275 Ma) and that collision took place via south-directed subduction beneath North China.; The second phase of deformation in the Yanshan belt led to the formation of two sedimentary basins that developed in close association with contractional structures. The Gubeikou basin developed as a foredeep in the footwall of the south-vergent Gubeikou thrust and records syntectonic unroofing of the upper plate of that thrust in its basin fill. The Chengde basin developed atop the inactive Chengde allochthon in response to north-vergent thrust faulting along its southern margin. The leading edge of the Chengde thrust was subsequently reactivated, and now cuts Upper Jurassic strata within Chengde basin. These cross-cutting relationships lead to estimates for timing of emplacement of the Chengde allochthon that are too young and alternatively reflect reactivation of the allochthon rather than the age of initial emplacement.; The final phase of Mesozoic deformation in the Yanshan belt is represented by an extensive rift system that developed in northeast China and southern Mongolia during Late Jurassic-Early Cretaceous time. This rift system is superimposed on and interacts with a number of pre-extensional zones of intracontinental shortening, most notably the Yanshan belt. Throughout most of its length, the Yanshan belt is characterized by north and south-vergent structures which occupy a strongly linear, east-west trending belt of shortening. In its eastern segment, however, Jurassic fold and thrust structures are strongly southeast-vergent and the belt trends to the northeast.; The regional Cretaceous extension direction was oriented northwest-southeast. The interaction of this northwest-southeast extension with variably oriented pre-rift fabrics produced three styles of extensional structures and associated basins: (1) Where extension was imposed upon pre-rift fabrics trending orthogonal to the extension direction, normal faults that bound half-graben basins are localized along older southeast-vergent thrust structures and reactivate them in a normal sense. (2) Where pre-rift thrust belt fabrics lie oblique to extension direction, extension was accommodated by localized high-strain deformation in the form of metamorphic core complexes. (3) Regions external to the Yanshan fold-thrust belt are composed of heterolithic crystalline basement, and developed a number of northeast-oriented graben and half-graben with variable basin geometries. This tripartite subdivision of extension styles reflects the superposition of a regional extensional regime upon a complex crustal fabric.