SHORTENING WITHIN THE NORTHERN LHASA BLOCK AT CA. 90OE–ASSESSING STRAIN CONTRIBUTIONS FROM THE INDIA-ASIA COLLISION

GeDepth / INDEPTH III surface geology investigations were conducted upon the tibetan plateau in the northern half of the Lhasa Block (30o30' to 31o15' N, and 89o45' to 90o15' E). Our work includes structural mapping, fault kinematic analyses, geochronology and palaeomagnetic studies, encompassing a section from immediately south of the ESE-trending en echelen dextral wrench faults of the Karakoram-Jiali system (the syntaxes' chord) to immediately north of the Zagya Tsangpo (adjacent to the Bangong suture trace). The prevalent stratigraphy comprises folded and thrust, mainly Cretaceous-age limestones with red and dark sand-, silt-, and mudstones interlayered with volcanics. In the northern portion of the traverse, structurally imbricated are volcanics, associated volcaniclastic sediments, and cherts of likely ophiolitic/island arc type affinity. In the southern portion, Permian and older Mesozoic rocks are thrust N-wards upon the Cretaceous. Locally scattered throughout the section are 100's m thick, S-ward thrust slices of serpentinised peridotite and other ophiolitic rocks; they are part of the Jurassic-emplaced Dongqiao obduction "nappe" that extends for >150 km south from the Bangong Suture (Girardeau et al. 1984). In the main Cretaceous section, folding is open to tight and, where thrust displacements are constrainable via stratigraphy, shortening amounts are 30-60 %, unevenly distributed along a bulk NNE axis. A range of K-Ar ages from interbedded volcanic rocks from Aptian to Campanian requires the shortening to be latest Cretaceous if it pre-dates the India-Asia collision (as suggested by Murphy et al. 1997 - for a similarly located section at ca. 85oE). In the northernmost portion of the traverse, a ca. 15 km section at Zagya Tsangpo comprises tightly folded red-beds complexly overthrust from the south by blocks (+melange?) of ophiolitic rocks. Palaeomagnetic results indicate an anti-clockwise rotation (left lateral sense) about the trace of Bangong suture. We infer N vergent reverse local fault reactivation of the suture. Based upon both the marine facies of the adjacent Aptian - Campanian rocks and nearby reports of late Cretaceous seaway rocks, this reactivation must be due to the India-Asia collision.