FAULT REACTIVATION IN THE JUNGGAR REGION, NORTHWEST CHINA - THE ROLE OF BASEMENT STRUCTURES DURING MESOZOIC-CENOZOIC COMPRESSION

Basement structures exposed at the margins of the Junggar Basin were c reated during the Altaid orogeny in the Late Palaeozoic. The most prom inent structures are backstops to subduction-accretion complexes (Nort h Tien Shan Fault), or major thrusts within these complexes (Dalabute and Kelameili faults). Both types of basement structure are far more c ommon in accretionary, Turkic-type, orogens such as the Altaids than t rue sutures. Probably the only suture sensu stricto in the Junggar are a is cryptic, and lies under the Junggar Basin's thick Mesozoic-Cenozo ic cover. The exposed Palaeozoic fault zones have been reactivated by Mesozoic-Cenozoic compressional events, which are the long-distance ex pression of orogenies at the southern margin of Asia. Latest Palaeozoi c and Mesozoic events reactivated a larger number of fault zones than have bean affected by the Cenozoic India-Asia collision, possibly beca use of an increase in the strength of the Junggar basement over time, following Late Permian rifting. Cenozoic strain is partitioned between strike-slip motion on basement structures within the Palaeozoic oroge nic belts around the Junggar Basin, and numerous thrusts and transpres sive faults in regions marginal to and within the basin itself. Most m ajor strike-slip faults are reactivated structures, and occupy narrowe r zones than their Palaeozoic precursors. Thrust zones follow the Pela eozoic basement grain, but active faults have propagated into the Meso zoic-Cenozoic cover.