Sedimentary evidence against large strike-slip translation on the Northern Altyn Tagh fault, NW China

Whether the oblique left-reverse Northern Altyn Tagh fault has large cumulative offset (>120 km) is one of the most controversial issues concerning the deformation mechanics for the northern Tibetan Plateau. In order to obtain material constraints on its total displacement, sedimentologic and bedrock provenance analysis of Oligocene-Pleistocene strata on the northern side of the fault was conducted. Lithofacies analysis and 970 paleocurrent measurements show that these strata were products of fluvial systems originating from south of the fault, making it possible to constrain the fault's slip history by establishing cross-fault sediment/source linkages. Identification of 5217 conglomerate clasts in the field indicates that major clast types are quartzite (30-50%), schist/gneiss (increasing upsection from 10% to 40-50%), and marble (decreasing upsection from 20-30% to <10%). Although granitic clasts can be observed throughout the entire section. they do not constitute a major clast type in any single bed. Amphibolite clasts first occur in the middle part of die Miocene strata, and reach up to 5% in the Pliocene and Pleistocene strata. Accordingly sandstone heavy mineral analysis indicates: (1) dominance of metamorphic over igneous minerals, (2) absence of contact metamorphic minerals, (3) dominance of greenschist facies; minerals lower in the section and coexistence of both greenschist and amphibolite facies minerals in the upper part of the section. These observations are consistent with unroofing of Precambrian basement immediately to the south of the studied Oligocene-Pleistocene section and argue against any left-lateral translation greater than 30 kin along the Northern Altyn Tagh fault. Therefore. our results preclude the possibility of large (>30 km) strike-slip translation on the Northern Altyn Tagh fault and support the model that strain induced by Indian indentation has been partitioned into left-slip on the Altyn Tagh fault and reverse dip-slip on the Northern Altyn Tagh fault. (C) 2004 Elsevier B.V. All rights reserved.