NEW CONSTRAINTS ON LATE QUATERNARY SLIP RATES ALONG THE GOBI-ALTAY FAULT IN SW MONGOLIA

The Gobi-Altay fault system (GAFS) is an active 650-km-long strike-slip feature in southwestern Mongolia. Late Cenozoic activity in the GAFS, associated with the Indo-Eurasia collision, has resulted in one of the most prominent morphotectonic elements in Central Asia northeast of the Tibetan Plateau and also in one of the largest seismic events ever recorded in Central Asia — the 1957 M_w~8.3 Gobi-Altay earthquake. The 1.55 mm yr⁻¹ late Pleistocene–Holocene lateral slip rate previously constrained in one location along the 150-km-wide GAFS by Ritz et al (1995; 2006) has been used to classify the GAFS as a slow-moving fault zone and thus a fairly minor component in continental-scale deformation models of Central Asia. To test this hypothesis we mapped for the first time the distribution of Quaternary shear across the entire GAFS and applied single grain luminescence dating of laterally offset units to constrain the timing and rate of lateral deformation. Our results reveal asynchronous deformation that migrated north towards the northern zone of the GAFS where late Quaternary activity has been concentrated. Four new slip-rate measurements obtained along the northern GAFS reveal a constant, spatially invariable lateral slip rate of ~5–6 mm yr⁻¹ since the late Pleistocene and imply a significantly more important role than previously attributed to discrete lateral shear north of the Tibetan Plateau during this time period.