Peeking into Continent-Building Processes Through the Bogda Window, Turpan-Junggar Basin, NW China

Yang, Wan; Crowley, James L.; Obrist, Jonathan; Feng, Qiao; Liu, Yiqun; Tabor, Neil; Luo, Xiaorong

The assembly of Central Asia Tectonic Belt into Eurasia was complex. Turpan-Junggar microplate, a part of the jigsaw-puzzle, may have been formed by amalgamation of volcanic arcs, microcontinents, and exotic terranes, followed by intracontinental rifting in late Paleozoic. A section in Tarlong-Taodonggou area, southern Bogda Mountains in the plate interior provides rare insights into the timing and nature of these processes. Pre-rift siliciclastic and volcanic strata are 571 m thick. Most volcanic rocks are theoleiites of pillow basalt flows 295 m thick. Thin felsic tuffs are intercalated with siliciclastics. A 10-m ignimbrite caps pre-rift deposits. The volcanics indicate bi-modal volcanism in a back-arc basin with nearby felsic sources. Felsic volcanism replaced mafic volcanism at the end of pre-rifting. Siliciclastics are sandstone, conglomerate, and shale. A 70-m interval in the lower section contains graded lithic arenite, wacke, siltstone, and shale, forming 10s of cm-thick laterally-persistent upward-fining successions with erosional bases. They are interpreted as coarse- and fine-grained turbidites comprising overall two upward-fining and thinning retrogradational and one upward-coarsening and thickening progradational submarine fan complexes. A 44-m interval in the uppermost part contains hummocky cross-stratified arenite, algal-laminated wacke with desiccation-cracked mud drapes, and clast-supported sheet conglomerate, containing plagioclase, K-feldspar, quartz, and basaltic and andesitic lithics. They are interpreted as shoreface to sand flat deposits. Massive matrix-supported conglomerates in the upper section are volcaniclastic debris flow deposits. The succession overall indicates shallowing from basin-slope to shoreline environments and mixed volcanic and continental sediment sources. A regional unconformity separates pre-rift deposits from overlying alluvial deposits in a half graben. Timing of arc-continent amalgamation, marine-terrestrial transition, and intracontinental rifting is constrained by U-Pb zircon TIMS dates. Back-arc theoleiitic and minor felsic volcanism occurred before 306.6 Ma and persisted to 301.1 Ma when crustal amalgamation completed and the thick ignimbrite accumulated. Submarine fans developed at 306.6 Ma, progressed at 305.5 Ma, and stopped at an extrapolated age of 303.9 Ma. After a regional uplift, rifting started before 301.3 Ma, indicating a rapid transition from amalgamation to rifting in an ~0.1-m.y. span.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013