Competing Processes of Clastic Deposition and Compartmentalized Inversion in an Actively Evolving Transpressional Basin, Western Mongolia
Howard, J. P., Cunningham, W. D., Davies, S. J., Journal of the Geological Society
The Dariv Basin is an actively evolving intracontinental transpressional basin located on the eastern flank of the Mongolian Altai. The basin occupies a complex tectonic position between a restraining bend, a thrusted basement block, and two major conjugate strike-slip fault systems. Structures and sedimentary strata exposed within the Dariv Basin suggest a Mesozoic and Cenozoic two-stage evolution. Jurassic-Cretaceous strata fine upward and record alluvial fan, fluvial and lacustrine depositional environments. The distribution of Mesozoic sedimentary rocks and the presence of a suspected Jurassic normal fault array suggest that the Dariv Basin initially formed as an extensional basin. Following Palaeogene tectonic quiescence, Oligocene-Recent basin fill is dominated by alluvial fan sediments derived from basin-flanking ranges. The modern basin is deforming by thrusting, normal fault inversion and folding along discrete belts expressed as intrabasinal ridges and domes. These belts define a rhomboid of active deformation that compartmentalizes the basin. Sediments derived from these discrete deforming belts and from basin flanking ranges continue to accumulate in the basin centre. Thus, modern fans contain reworked older basin fill and competing processes of sedimentation, deformation, erosion and resedimentation can be observed. The Dariv Basin is an excellent example of a transpressional piggyback basin in the early stages of basin inversion and destruction.
In this paper, the structural and stratigraphie characteristics of the Dariv Basin, an actively evolving transpressional basin along the eastern margin of the Mongolian Altai (Fig. 1), are described. Transpressional basins are a major sedimentary basin type, which form in tectonic settings where strike-slip, oblique-slip and contractionul fault displacements dominate. Major transprcssional basins that are developed along transform plate boundaries, such as the Ventura Basin along the San Andreas Fault (Yeats et al. 1994), are commonly petroliferous and are consequently well studied with good seismic reflection data. Conversely, transpressional basins that form in an intraplate and intracontinental setting have received less attention and their development and architecture are poorly understood. Intracontinental transpressional basins are found on all major continents and are potential sites of valuable economic resources including hydrocarbons, mctallogenic commodities (i.e. placer gold, platinum), industrial minerals (e.g. salts, sands) and ground water.
Transprcssional basins in the eastern Altai region of Mongolia are linked to seismically active faults within a broad corridor of northwestward dextral strike-slip displacements and NE-SW shortening (Fig. 1). The basins occur in various stages of evolution from incipient to mature and represent depoccntres adjacent to, and between, actively uplifting crystalline basement blocks, which are typically thrust over basin fill and laterally displaced by regional-scale strike-slip faults. Consequently, most basins are internally faulted or folded as revealed by tilted successions along their margins and within interior zones. The basins provide a superb opportunity for analysing the complex dynamic interplay between competing processes of clastic deposition, faulting, inversion, erosion, and resedimentation in an evolving system. For these reasons, the eastern Altai region is perhaps the world's finest natural laboratory for studying active processes of intracontinental. intraplate transpressional basin evolution.
Regional geology of western Mongolia
Transpressional basins in western Mongolia are constructed on basement rocks that comprise Palaeozoic subduction accretion belts and arc-back-arc complexes that accreted between older cratonic blocks in Siberia, China and central Mongolia (Badurch et al. 2002: Chen & Jahn 2002; Windley et al. 2002). Progressive tcrrane accretion resulted in the development of regional foliation and fault trends that are typically NW striking and NE dipping. …