Academic journal article
By Allen, Mark B; Vincent, Stephen J
Journal of the Geological Society , Vol. 154
MARK B. ALLEN & STEPHEN J. VINCENT
Cambridge Arctic Shelf Programme, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB:! 3EQ, UK
Abstract: Basement structures exposed at the margins of the Junggar Basin were created during the Altaid orogeny in the Late Palaeozoic. The most prominent structures are backstops to subduction accretion complexes (North Tien Shan Fault), or major thrusts within these complexes (Dalabute and Kelameili faults). Both types of basement structure are far more common in accretionary, Turkic-type, orogens such as the Altaids than true sutures. Probably the only suture sensu stricto in the Junggar area is cryptic, and lies under the Junggar Basin's thick Mesozoic-Cenozoic cover. The exposed Palaeozoic fault zones have been reactivated by Mesozoic-Cenozoic compressional events, which are the long-distance expression of orogenies at the southern margin of Asia. Latest Palaeozoic and Mesozoic events reactivated a larger number of fault zones than have been affected by the Cenozoic India-Asia collision, possibly because 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 orogenic belts around the Junggar Basin, and numerous thrusts and transpressive faults in regions marginal to and within the basin itself. Most major strike-slip faults are reactivated structures, and occupy narrower zones than their Palaeozoic precursors. Thrust zones follow the Palaeozoic basement grain, but active faults have propagated into the Mesozoic-Cenozoic cover.
Keywords: China, Altai Mountains, faults, basement, reactivation.
Central Asia is an excellent region in which to study basement reactivation within continental crust, because recurrent Mesozoic and Cenozoic compressional deformation has operated in an area constructed during Late Palaeozoic orogenies. The Tien Shan divides the Junggar and Tarim basins in northern Xinjiang (Fig. 1). This mountain range originated as an orogenic belt during the Late Palaeozoic, when it was created by two collisions (Allen et al. 1993a). In the Late Devonian and Early Carboniferous, the Tarim block collided with an elongate sliver of continental crust, known variously as the Yili microcontinent, the Central Tien Shan or Chu-Terskey (Allen et al. 1993a; $eng6r et al. 1993), which lay at the southern margin of the evolving Altaid orogenic collage. The Altaids began to form in the Vendian-Cambrian, when an ensialic arc, the Kipchak Arc, rifted from the Siberian and East European cratons. A subduction-accretion complex formed at the margin of the arc, and arc magmatism migrated out into this complex through the remainder of the Palaeozoic. Simultaneously, the arc deformed by a combination of oroclinal bending and strike-slip faulting, leading to the repetition of slices of pre-Altaid continental crust and newly-created accretionary complex (Sengor et al. 1993).
Although ,Seng6r et al. (1993) proposed a chronology in which Tarim collided with this collage in the Permian, the abundant evidence for compressional deformation within northern and central Tarim at the Devonian-Carboniferous boundary makes it more likely that Tarim collided with the Central Tien Shan at this time (e.g. Cai et al. 1995). This collision would have provided a driving force for the final oroclinal deformation of the Altaids in the early Late Carboniferous. As the subduction front tightened, the last-remaining oceanic crust in the region was eliminated, and sectors of what originated as one subduction complex faced each other across the area now occupied by the Junggar Basin. Thrust slices of volcanic and flysch material, similar to the exposed basement of the Northern Tien Shan, Western Junggar and Altai, most likely form the basement to the Junggar Basin itself. The actual suture is cryptic, and overlain by the thick Lower Permian-Cenozoic sediments of the basin. …