Provenance of upper Permian-lowermost Triassic sandstones, Wutonggou low-order cycle, Bogda Mountains, NW China:implications on the unroofing history of the Eastern North Tianshan Suture

This study investigates the provenance of sedimentary rocks in Bogda Mountains, NW China, and reconstructs the lithology and unroofing history of the Eastern North Tianshan Suture. Petrographic point counting data of sandstones and compositions of conglomerates of upper Permian-lowermost Triassic Wutonggou low-order cycle from Zhaobishan, North Tarlong, Taodonggou, and Dalongkou sections in the southern and northern foothills of Bogda Mountains were used to interpret the temporal and spatial variations of lithology of the Eastern North Tianshan Suture, which is the sediment source area. Three compositional trends were identified. A trend of upwardincreasing quartz content and granitic pebbles in Zhaobishan section suggests a change from the undissected volcanic arc, accretionary wedge and trench setting to predominantly transitional volcanic arc and subordinate accretionary wedge and trench, in the eastern part of the Eastern North Tianshan Suture. In North Tarlong and Taodonggou sections, however, the lithic content decreases and the contents of quartz and granitic pebbles increase up sections. These trends indicate that the western part of the Eastern North Tianshan Suture changed from an undissected volcanic arc to the transitional volcanic arc, accretionary wedge and trench. No clear trend in the lithic-rich sandstones of the Dalongkou section indicates that sediments were derived from the undissected volcanic arc in the Eastern North Tianshan Suture and local rift shoulders. Compositional variations of studied rocks suggest that the Eastern North Tianshan Suture was an amalgamated complex with great spatial and temporal heterogeneities in lithology and experienced persistent unroofing during late Permian-earliest Triassic. This study reconstructs a key element of the Chinese Tianshan Suture and serves as an example to understand the unroofing processes of ancient sutures.

Sedimentary characteristics and main controlling factors of the Middle-Upper Permian and Middle-Upper Triassic around Bogda Mountain of Xinjiang,NW China

Based on field geological survey,interpretation of seismic data and analysis of drilling and logging data,the evolution of geological structures,stratigraphic sedimentary filling sequence and sedimentary system around the Bogda Mountain were analyzed according to the idea of"structure controlling basin,basin controlling facies and facies controlling assemblages".The tectonic evolution of the basin around the Bogda Mountain can be divided into nine stages.The Middle-Late Permian–Middle-Late Triassic was the development stage of intracontinental rift,foreland basin and inland depression basin when lake,fan delta and braided river delta sedimentary facies developed.Early intracontinental rifting,late Permian tectonic uplift,and middle-late Triassic tectonic subsidence controlled the shape,type,subsidence rate and sedimentary system evolution of the basin.The Bogda Mountain area was the subsidence center and deposition center of the deep water lake basin in the Middle Permian with mainly deep-water deposition and local gravity flow deposition.This area had tectonic inversion in the Late Permian,when the Bogda Mountain uplifted to form a low bulge and a series of fan delta sand bodies.In the Middle-Late Triassic,subsidence occurred in the Bogda low uplift,characterized by extensive development of braided river delta deposits.

Mesozoic and Cenozoic uplift and exhumation of the Bogda Mountain,NW China: Evidence from apatite fission track analysis

Apatite fission track （AFT） analysis on samples collected from a Paleozoic series is used to constrain the cooling history of the Bogda Mountain, northwest China. AFT ages range from 136.2 to 85.6 Ma and are younger than rock depositional ages and the mean confined track lengths （11.0 13.2 μm） mostly showing unimodal distribution are shorten, indicating significant track-annealing. Thermal histories modeling based on the distribution of fission-track lengths combined with the regional geological data show that two rapid cooling phases occurred in the latest Jurassic-early Cretaceous and the Oligocene-Miocene. Those new data together with previous published data show that the AFT ages become younger from the southwest to northeast in the western Bogda Mountain and its adjacent areas. The fission-track ages of the southwest area are relatively older （〉100 Ma）, recording the earlier rapid uplift phase during the late Jurassic-Cretaceous, while the ages in the north pied- mont of the Bogda Mountain （namely the northeast part） are younger （〈60 Ma）, mainly reflecting the later rapid uplift phase in the Oligocene-Miocene. The trend of younger AFT ages towards the northeast might be explained by post-Cretaceous large-scale crustal tilting towards the southwest. In the thrust fault-dominated northern limbs of the Bogda Mountain, AFT ages reveal a discontinuous pattern with age-jumps across the major fault zones, showing a possible strata tilting across each thrust faults due to the thrust ramps during the Cenozoic. The two rapid uplift stages might be related to the accretion and collision in the southern margin of the Asian continent during the late Jurassic and late Cenozoic, respectively.

Shrinkage of Mt. Bogda Glaciers of Eastern Tian Shan in Central Asia during 1962–2006

Many small mountain glaciers have been reported undergoing strong shrinkage, and it is therefore important to understand how they respond to climate change. The availability of topographic maps from 1962, Landsat TM imagery from 1990 and ASTER（Advanced Spaceborne Thermal Emission and Radiometer） imagery from 2006 and field investigation of some glaciers allow a comprehensive analysis of glacier change based on glacier size and topography on Mt. Bogda. Results include：（1） an overall loss of a glacierized area by 31.18±0.31 km^2 or 21.6% from 1962 to 2006,（2） a marked dependence of glacier area shrinkage on initial size, with smaller glaciers experiencing higher shrinkage levels,（3） the disappearance of 12 small glaciers,（4） a striking difference in area loss between the southern and northern slopes of 25% and 17%, respectively. A subset of the investigated glaciers shows that the area 57.45±0.73 km2 in 1962 reduced to 54.79±0.561 km^2 in 1990 and 48.88±0.49 km^2 in 2006, with a relative area reduction of 4.6% during 1962-1990, and 10.8% during 1990-2006. The corresponding volume waste increased from 6.9% to 10.2%. Three reference glaciers were investigated in 1981 and revisited in 2009. Their terminus experienced a marked recession. Meteorological data from stations around Mt. Bogda reveals that glacier shrinkage is correlated with winter warming and an extension of the ablation period. Precipitation on the northwest side of the range shows a marked increase, with a slight increase on the southeast side.