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Oblique Divergence Zones |
1 Cambridge Arctic Shelf Programme, Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, UK
2 Chinese Academy of Geological Sciences, Beijing, People’s Republic of China
Extensional basins with an element of strike-slip deformation can form because of a perturbation in a strike-slip fault zone (pull-apart and fault wedge basins), or where extension is oblique to the margins of the deforming zone (transtensional basins). Transtensional basins are characterized by en echelon arrays of normal faults which are individually oblique to the basin margins. The Bohai Basin, northern China, has previously been modelled as either (1) a giant pull-apart between NNE-SSW trending dextral strike-slip faults, or (2) a rift basin caused by WNW-ESE extension, without significant strike-slip deformation. We present a model for the Bohai Basin’s rift history in which the basin formed as a result of dextral transtension. The Bohai Basin is one of a family of early Tertiary extensional basins present within eastern Asia from northeastern Russia to southeast China. The structural grain in this basin was inherited from a phase of late Mesozoic sinistral transpression. Tertiary extension began in the Paleocene. Most half-grabens in the eastern and western regions of the Bohai Basin have master faults with a NE-SW or NNE-SSW orientation. Secondary normal faults strike oblique to the main structures, in en echelon arrays which indicate a component of dextral transtension. The central part of the basin, the Bozhong Depression, became a significant depocentre for the first time in the middle Eocene. It formed when activity on transtensional zones to its east and west created an extensional overlap between them. Thus the basin as a whole resembles a giant pull-apart basin, with the Bozhong Depression as its central depocentre, but dextral transtension rather than simple strike-slip controlled the deformation. The component of dextral deformation in the Bohai Basin is shared by other early Tertiary east Asian extensional basins, and is consistent with the sense of shear implied by the oblique convergence of the Pacific and Asian plates: an east-west convergence vector applied to a NE-SW trending plate boundary. The consistency of this dextral shear along the Asian margin, and the fact that several of these basins pre-date the India-Asia collision, supports an origin by subduction roll-back of the oceanic Pacific plate from Asia. The extrusion model for east Asian basin formation, whereby extension was caused by lateral transport of lithospheric blocks out of India’s northward path following the India-Asia collision, is not applicable to major basins east and northeast of the Red River Fault.
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