Steep Faults, Narrow Basins, and High Topography in the "Dulan-Chaka Plateau": Observations and Implications for Growth in the Northern Tibetan Plateau

The importance of upper crustal shortening and its role in creating high topography in the Tibetan Plateau remains an important subject of debate concerning the dynamics of plateau evolution. Recently completed field work in the "Dulan-Chaka Plateau", the topographic highland east of the Qaidam basin and west of the Wenquan fault and Gonghe basin, yields several important constraints on upper crustal deformation along the northern Tibetan plateau margin. The Dulan-Chaka Plateau consists of several en echelon NW/SE trending mountain ranges that are bounded by steep, northeast-dipping reverse faults and separated by narrow Quaternary basins. These faults commonly place Triassic age arc-derived granitic, volcanic, and Paleozoic meta-sedimentary rocks over Neogene and Quaternary sediments. Patchy outcrops of Neogene mudstones and conglomerates are exposed throughout the Dulan-Chaka Plateau. Drainage patterns, conglomerate clast compositions, and structural geometry suggest that much of the preserved Neogene sediment is not syn- tectonic with range growth. The Neogene sediments, therefore, may represent remnants of a once continuous paleobasin that encompassed the modern Qaidam and Gonghe basins prior to dismemberment by the rise of the Dulan-Chaka Plateau. Lack of stacked thrust sheets, low angle faults, and flexural basins implies that the upper crust has not sustained significant horizontal shortening and crustal thickening; yet this region exists as a topographic highland with an average elevation of ~4500 m. Moreover, drainage pattern evolution and minimal range-basin relief support regional uplift over a lengthscale of several hundred kilometers as the main mechanism of topographic growth. One possible method of creating regionally high topography is by mid- lower crustal flow, a process evoked by other workers to explain both the eastern and southern Tibetan plateau boundaries. Alternatively, removal of a dense lithospheric root may explain the elevated landscape by buoyancy forces. The style of faulting and the topographic form of the Dulan-Chaka Plateau contrast with areas studied to the east. Thrust fault bounded ranges of the northeastern Tibetan Plateau create large, rhomb-shaped flexural basins. These basins contain several kilometers of sediment and often preserve growth strata as sediment was deposited during late Miocene and younger faulting episodes. Structural and depositional differences between the two regions are likely due to crustal anisotropies and show that the entire northern margin of the Tibetan plateau is not deforming in a uniform manner.