DEFORMATION HISTORY OF THE FENGHUOSHAN, NORTHERN TIBETAN PLATEAU, QINGHAI PROVINCE, CHINA

The magnitude and distribution of Late Eocene to Oligocene deformation across Tibet has been the subject of considerable speculation and debate. This, in part, reflects the relative paucity of post-collisional, Eocene and Oligocene rocks across the plateau. The Fenghuoshan of the northern Tibetan Plateau is one exception, where some 4+ km of Eocene fluvial clastics and interbedded lacustrine limestones are well exposed near Erdaogou. Previous mapping, reported by Kidd et al. (1988) and Coward et al. (1988) has interpreted the structural history as a simple, southward vergent, piggy-back stack of thrust sheets. Recent mapping in the vicinity of Erdaogou, requires considerable modifications of existing map and structural interpretations of this area. Three major phases of shortening are present in this area, 2 of which pre-date 31 Ma. Phase 1 is characterized by fault-bend folding, along NW-dipping, SE-vergent thrusts that replicate at least the northern part of the Fenghuoshan basin. Phase 2 is marked by NE-vergent thrusting and fault-propagation folding of the footwall that produced northward overturning of a structural package of Eocene strata > 8 km thick. Structurally above these overturned and imbricated strata are openly folded and deformed strata of the hangingwall along a multi-km flat, with local duplexes. The structural overlap of the main phase 2 thrust may exceed 25 km if the northern thrusts of the Fenghuoshan are extensions of this sheet. NE-directed transport on structurally lower thrusts below the exposed phase 2 footwall is evidenced by the refolding of the overturned footwall sequence into a large, N-trending, S-plunging, downward-facing antiformal-syncline. We interpret this refolding to be the result of transport of both phase 2 footwall and hangingwall sequences over a NE-trending lateral ramp along a deeper level detachment. An open, phase 2 syncline at the northern edge of the Fenghuoshan is intruded by a post-kinematic hypabyssal biotite syenite from which a 31 Ma K-Ar age has been reported (Harris et al. 1988). This indicates deformational phases 1 and 2 occurred prior to the end of the Early Oligocene. Phase 3 involves S-vergent thrusting of Eocene over Plio-Pleistocene sediments along the S flank of the range.