Miocene Volcaniclastic Sequence Within the Xiyu Formation from Source to Sink: Implications for Drainage Development and Tectonic Evolution in Eastern Pamir, NW Tibetan Plateau

Abstract

The formation of the Pamir salient and the Tashkorgan‐Yarkand River is highly debated with the ages ranging from pre‐Cenozoic to late Miocene. One approach to resolve these issues is to draw support from the sedimentary record in the surrounding basins. A volcaniclastic sequence, which divides into Lower and Upper Members, was identified in the southwestern Tarim Basin. The Lower Member was transported by dilute streamflows, which likely flowed during or soon after eruptions, while the Upper Member was formed by a syneruptive volcanic debris flow. Chronological, petrologic, and geochemical data consistently indicate that the sequence was derived from the Central Pamir at ~11 Ma. The ~11 Ma emplacement of the volcaniclastic sequence provides unique constraints for the evolution of the Tashkorgan‐Yarkand River and the Pamir salient. Provenance data indicate a multistage evolutionary history of the Tashkorgan‐Yarkand River. The paleo‐Tashkorgan River was initially formed in the piedmont of the Pamir marginal range before ~15 Ma. This river cut back into the Tashkorgan region at ~15 Ma, after which it has eroded the Central Pamir by ~11 Ma. The N‐S trending upper reaches of the Tashkorgan River and the Yarkand River were established after ~11 Ma. The emplacement of the volcanic debris flow, together with regional deformation evidence, indicates limited strike‐slip motion between Pamir and the Tarim at least since ~11 Ma, which refutes hundreds of kilometers offset between the Pamir and the Tarim after this time and supports an earlier indention of the Pamir salient.

Tectonic Evolution of the Pamir Recorded in the Western Tarim Basin (China): Sedimentologic and Magnetostratigraphic Analyses of the Aertashi Section

“…The composition of these gravels is similar to that observed in the clasts of the overlying conglomerates of unit F. Clasts of unit F are well rounded and up to 30 cm in diameter with clear imbrication structures. These conglomerates contain basement rocks and volcanic pebbles; they are clast supported and clasts are generally much larger and more rounded than those observed in the lower conglomerates of unit D. The unit is reported to extend upward another 2.4 km up to the 11‐Ma volcanic units (Wei et al, ).…”

“…Rates continue to increase toward the top of the section with a rate of 45 cm/kyr from 16.0 to 15.0 Ma before the onset of the proximal alluvial fan conglomerate deposition of unit F. The transition to unit F occurs within a few tens of meters stratigraphically (Figure ) but is gradual with increased occurrences of gray gravelly beds within continuing red clastic beds suggesting no large hiatus. For the overlying Xiyu conglomerates, accumulation rates of 60 cm/kyr can be further estimated based on the 11‐Ma deposition age provided by volcanic U/Pb zircons (Wei et al, ; Zheng et al, ) stratigraphically positioned circa 2.4 km above the base of the conglomerates. This suggests sustained or increasing accumulation rates associated with the conglomerate deposits despite potential differential compaction effects.…”

“…Between 15.0 and circa 11 Ma, dominantly conglomeratic deposits continue to be accumulated at high rates. The occurrence of fresh volcanic detritus (Blayney et al, ) at 11 Ma in the Aertashi record corresponds to the age of the 11‐Ma Dunkeldik volcanics providing a likely source from the Central Pamir terrane (Hacker et al, ; Kooijman et al, ; Wei et al, ). This age range also corresponds to the clockwise rotation (see above) and the suggested onset of QFTS (Wei et al, , Figure c).…”

“…Based on magnetostratigraphy, they further indicated that the start of the continental section lies at the base of C18r (circa 41 Ma) and dated through to C9.n (circa 27 Ma). More recently, the upper part of this section has been dated by Zheng et al () using magnetostratigraphy, combined with the basal constraints of Bosboom et al (), and an upper depositional age of 11‐Ma constraint provided by Ar‐Ar age control from a volcanic ash layer (Wei et al, ). In a previous paper, Blayney et al () provided provenance analysis of the Aertashi section based on detrital zircon U‐Pb ages coupled with zircon fission track, bulk rock Sm‐Nd, and petrography data.…”

“…In order to better constrain this evolution, we studied the Cenozoic Aertashi sedimentary section that constitutes the most complete and best‐dated sedimentary Cenozoic sequences from the western Tarim basin (e.g., Wei et al, ; Yin et al, ; Zheng et al, , , ). This work follows and complements previous dating and provenance results from this section (Blayney et al, ; Bosboom et al, , ) by providing (1) a sedimentological analysis of depositional environments, (2) magnetostratigraphic dating, (3) anisotropy of magnetic susceptibility analysis for stress orientation, (4) vertical‐axis rotation analysis, (5) backstripping analysis, and (6) additional provenance data in key intervals.…”