Early Jurassic extensional basin formation in the Daqing Shan segment of the Yinshan belt, northern North China Block, Inner Mongolia

Structural and stratigraphic studies of the western Daqing Shan segment of the Yinshan belt have recognized an Early Jurassic extensional episode supp orted by several lines of evidence. First, normal faults cut the lowermost Jurassic sequence and are overlapped by younger Lower Jurassic rocks. Secon d, Lower Jurassic rocks include growth strata in small-scale graben at the base of the Jurassic basin. Third, rapid lateral facies changes are mapped from boulder conglomerates along the basin-bounding faults to lacustrine an d meandering fluvial rocks in the basin center. Fourth, paleodrainage syste ms provided sediment input from three sides of the basin, two transverse an d one axial. Finally, there is a strongly asymmetric distribution of coarse proximal and fine distal facies within the basin. The Early Jurassic exten sional episode was responsible for formation of an east-trending half-grabe n basin in the western Daqing Shan in which at least 1800 m of syn-extensio nal nonmarine sediments accumulated and are preserved. Previous studies in the Triassic and Jurassic of other parts of northwest a nd north-central China have concluded that the early Mesozoic was a time of continental amalgamation and contractile deformation. The recognition of a n Early Jurassic extensional episode along the northern mar.-in of the Nort h China Block is problematic in this context, at least superficially. We pr opose two possible explanations for tile Early Jurassic extension: transten sion associated with strike-slip tectonics, or gravitational collapse of a pre-existing Late Paleozoic-Early Mesozoic contractile orogenic belt. The f irst possibility, transtensional deformation, is problematic because specif ic strike-slip faults have not been identified that could control extension in the western Daqing Shan. However, several lines of evidence allow the p ossibility of such a driving mechanism, including: documented Early Jurassi c transtensional systems in the southwestern North China Block, several can didate strike-slip faults along the China-Mongolia border region, structura l discontinuity between the Daqing Shan and southern Mongolia, and along st rike-changes in structural style within the Yinshan belt. The second possib ility, orogenic collapse, is similarly difficult to establish because of th e limited amount of data concerning the regional distribution and orientati on of pre-Jurassic contractile structures and Early Jurassic extensional st ructures. However, documented Late Paleozoic-Triassic contractile deformati on, as well as the close temporal and spatial association, and parallelism between Early Jurassic extensional structures and older contractile structu res in the western Daqing Shan requires consideration of gravitational coll apse as a driving mechanism for Early Jurassic extension in the Yinshan bel t. (C) 2001 Elsevier Science B.V. All rights reserved.