Abstract

The Gonjo basin consists of a long and narrow belt of lower Tertiary sedimentary rocks that record tectonic and environmental conditions during the early stages of India-Asia collision. The basin fill is dominated by continental sediments reflecting predominantly alluvial fan, fan-delta, floodplain, and lacustrine deposition. Growth stratal relations indicate that accommodation was created along with the onset of compressional deformation and the generation of structural relief associated with fault-propagation folds. Sediments were derived from proximal locations and deposited into a synclinal trough between bounding antiformal culminations. Deformation outlasted sedimentation, and the Gonjo basin sediments are now part of footwall synclines to bounding reverse faults that broke through the original bounding folds. The preservation of basin-margin facies suggests that the present exposure of Tertiary sediments broadly corresponds with the original extent of the basin. Shortening of the basin is limited, but the throw on the bounding faults increases from south to north, with perhaps as much as 4 km of throw on bounding reverse faults in the northern part of the basin. Beyond the basin, at least some, if not most, of the regional shortening occurred in Mesozoic time and thus likely predated India-Asia collision. Estimates of shortening unequivocally related to continental collision are hampered by the difficulty of distinguishing late Mesozoic from early Cenozoic deformation: only locally are the two deformations not coaxial. Strike-slip faulting postdates sedimentation, and is not associated with the creation of accommodation space in this basin. Strike-slip deformation consisted of distributed left-lateral faults where the uppermost rocks of the basin are shallowly detached, as well as a few prominent left- lateral faults, including one on strike with the active left-lateral Litang fault. Limited early Cenozoic shortening in eastern Tibet suggests that upper crustal deformation made only a minor contribution to the almost doubling of crustal thicknesses in this region.

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