CONSTRAINTS ON LATERAL OFFSETS ALONG THE GOBI-ATLAY FAULT SYSTEM, SOUTHWESTERN MONGOLIA

Nested between the Qilian Shan and the Baikal rift zone, the E-W Gobi-Altay fault system (GAFS) in southwestern Mongolia has been previously recognized as an important component of the Central Asia intra-continental strike-slip fault system associated with the Indo-Eurasian collision. Yet, although the ~250 km surface rupture of the M=8.3 earthquake of 1957 along the eastern segment of the GAFS has been investigated, as well as the Quaternary history of faulting along this part of the fault system with published lateral slip rates ranging from 1.5±0.4 to ~20 mm/yr, the Quaternary evolution of faulting along the central segment of the GAFS, east of the city of Altay, remains less studied. The focus of the present study is to add new constraints to Quaternary lateral offsets and slip-rates along the GAFS. As a preliminary stage, remotely sensed visible to thermal infrared (0.4-12.0 μm) multispectral data were used to map the composition of Quaternary depositional surfaces along the GAFS, match them up with their bedrock sources and thus identify potential markers for cumulative lateral offsets. Remote estimates of surface roughness and desert varnish development were used as proxies for relative age of the surfaces. Our analyses are consistent with the sinistral motion previously identified along parts of the GAFS, and have revealed apparent lateral offsets ranging from a few hundred meters to 5 km. Strain along the central part of the GAFS appears to be complexly partitioned among sub-parallel faults over a >50 km zone north of the Gobi-Tien Shan fault. A field campaign is planned during the summer of 2004 for validation of the preliminary interpretations and sampling for numerical dating. Age constraints on offset surfaces along transects of the sub-parallel strands of the GAFS are expected to provide us with new insights on Quaternary strain partitioning in this region located between the northern edge of the Indo-Eurasian collision deformation zone and the Baikal rift extensional zone.