The Longmenshan foreland basin is one of typical foreland basins along the eastern margin of the Tibetan Plateau. The Late Triassic—Quaternary strata there, more than 10000 m thick, can be divided into six tectonic sequences by unconformities. Two depositional patterns of tectonic sequences have been distinguished: one is wedge-shaped and the other is tabular-shaped. The wedge-shaped tectonic sequences include Late Triassic, Late Jurassic and Late Cretaceous—Eogene tectonic sequences. The tabular-shaped tectonic sequences include Early to Middle Jurassic, Middle Cretaceous and Neogene to Quaternary tectonic sequences. The results indicate that the wedge-shaped tectonic sequences are sedimentary response of active thrusting and the tabular-shaped tectonic sequences are sedimentary response of strike slipping. Taking the Late Triassic the wedge-shaped foreland basin as an example of the typical wedge-shaped foreland basin, we used a one-dimensional analytical model to simulate the subsidence in the wedge-shaped foreland basin due to the emplacement of tectonic loads in the Longmenshan thrust belt on an elastic plate. Taking the Neogene to Quaternary tabular-shaped foreland basin as an example of the typical tabular-shaped foreland basin, we used a one-dimensional analytical model to simulate the subsidence and uplift in tabular-shaped foreland basin due to the erosional unloading in Longmenshan on an elastic plate. The results indicate that the migration rate of the orogenic wedge in the early wedge-shaped foreland basin history (c. Late Triassic) was rapid (maximum of 15 mm/a) compared to the slow migration rate (maximum of 6.7 mm/a) during the later stage of the wedge-shaped foreland basin evolution (c. Late Jurassic and Late Cretaceous—Eogene). We infer that the episodic thrusting activity of Longmenshan was driven by the accretion of the Cimmerian continents and the India—Eurasia collision. The Late Triassic wedge-shaped tectonic sequence was a sedimentary response to the Qiangtang—Eurasian collision, the Late Jurassic wedge-shaped tectonic sequence was a sedimentary response to the Lhasa—Eurasian collision, and the Late Cretaceous—Eogene wedge-shaped tectonic sequence was a sedimentary response to the Kohistan—Eurasian collision and the India—Eurasian collision.
Chengdu basin developed ahead of Longmen Shan along the eastern margin of Tibetan Plateau, lying between the Longmen Shan and Longquan Shan, and extends from southwest to northeast with the long axis of the basin parallel to Longmen Shan. The filling in the basin with a maximum thickness of 541m include Dayi conglomerate, Yaan gravel layer and Late Pleistocene and Holocene gravel layer. In the transverse section of the basin(vertical to Longmen Shan), the wedge shaped sediments in the basin were derived from Longmen Shan and gradually northwestwards thickening, the depocenter is located in the northwestern part of the basin. As thrusting progressed southeastwards, the northwestern margin and the depocenter of the basin had been progressively migrated southeastwards. According to this evidence, it can be inferred that the direction of trusting and tectonic shorting was from NW-SE and vertical to Longmen Shan. In the longitudinal section (parallel to Longmen Shan), there are many minor echelon depressions and uplifts, as dextral strike-slipping progressed northeastwards, the minor depression (rhombocham) and fan had been progressively migrated northeastwards. According to these lines of evidence, we can infer that the direction of dextral strike-slipping was from SW-NE and parallel to Longmen Shan. So we draw a conclusion that the Chengdu basin is one of typical transpressional basin controlled by thrusting and strike-slipping along Longmen Shan since 3.6Ma.
Chengdu basin developed ahead of Longmen Shan along eastern margin of Tibetan Plateau,lying between the Longmen Shan and Longquan Shan.The filling in the basin include Dayi conglomerate,Yaan gravel layer and Late Pleistocene and Holocene gravel layer since 3.6Ma,and the sediments in the basin were derived from Longmen Shan with transverse drainage patterns.Based on results by the component analysis of gravel,heavy mineral and geochemistry of major elements in the sediments of Chengdu basin,we draw conclusion that there were two provenances and two paleo-rivers,one of them was paleo-Minjiang,which developed in the northern part of the basin,another was paleo-Qingyijiang,which developed in the southern part of the basin.According to the difference between Dayi conglomerate and modern fluvial deposit,we can infer Minjiang and Qinyijiang have changed their course after period of Dayi conglomerate deposition.
The last ten years have seen big progress and wide applications of a novel field,non-traditional stable isotope(NTSI)geochemistry,to high temperature geo-science studies.Invention of multi-collector-inductively coupled plasma-mass spectrometry(MC-ICP-MS)led to the big breakthrough of analytical methods for heavy stable isotopes.This contribution summarizes Li,Fe,and Mg isotope studies on igneous rocks and minerals,as representative of NTSI geochemistry.Li isotopes have been widely applied to the studies of mantle geochemistry,recycling of subducted materials,and metamorphism to constrain the source of magma and kinetic diffusion process.Fe isotope fractionation is related to partitioning of multi-valent Fe between Fe-bearing phases,which can occur in the course of mantle metasomatism,partial melting,and fractional crystallization.Mg isotopic compositions of igneous rocks most likely reflect the source signatures.Variation of Mg isotopic ratios of mantle peridotites is trivial and this provides a homogenous background for Mg isotope fractionation in low temperature processes.Furthermore,Cl,Si,Cu,Ca,and U isotopes are also promising in the future geochemical studies.Experimental studies and theoretical simulation for the mechanisms of isotope fractionation provide important guidances for understanding the NTIS data.Experimental studies show that light and heavy isotopes have different migration velocity at high temperature processes such as chemical diffusion,evaporation,and desublimation,which could produce significant kinetic isotope fractionation.Equilibrium isotopic fractionation could occur among mineral,melt,and fluid when chemical environment of the isotopes are different between the phases.Recent thermal diffusion and migration experiments on silicate material reveal a new mechanism of magma differentiation and isotope fractionation.Along a temperature gradient in silicate magma,large elemental variation and isotopic fractionation can occur,by which a wet andesite can even be differentiated to granite.This suggests that thermal migration could be important for continental crustal formation and evolution.If temperature gradient exists long enough during magma differentiation,thermal diffusion can produce significant stable isotope fractionation,which is contrast to the mechanism of traditional kinetic and equilibrium isotope fractionations.Such process can be fingerprinted by positive correlations among multi-stable isotopic systems.Due to thermal diffusion,concentration of material loaded or dissolved in the fluid is a function of Soret coefficient(ST).However,because ST is highly variable and sensitive to lots of factors,the basic physics of thermal diffusion is still poorly understood.As shown by Mg,Ca,and Fe isotope measurement of thermal diffusion experiments,isotope fractionation driven by temperature gradient is independent to the bulk composition and temperature of the system,suggesting that the difference of ST between two isotopes of the same element can be considered as a constant.This can simplify and help the studies on thermal diffusion and ST.
The large-scale morphology of the eastern margin of the Tibetan plateau can be divided into three zones, the Tibetan plateau, Longmen Mnt. and Sichuan basin. To understand the relationship between uplift rate and Minjiang incision rate in the region, we calculated the incision rate of the Minjiang River using terrace and its TL ages, and the results indicate that the Minjiang incision rate is 1.07~1.61 mm/a in the Tibetan Plateau, 1.81 mm/a in the Longmen Mnt., and 0.59 mm/a in the Sichuan Basin. Based on these key data and surface uplift rate of the Longmen Mnt.(0.3~0.4 mm/a, we established the late Cenozoic coupling relationship between incision rate and surface uplift rate, and incision rate is five times the surface uplift rate in the Longmen Mnt. According to the time span (3.48 Ma) of forming the maximum depth of dissection and the age (3.6 Ma) of oldest Minjiang Fan deposit in the Chengdu Basin, we inferred that the uplift of the Longmen Mnt. began from 3.6 Ma, and the mountain building models of the Longmen Mnt. had been constrained to both erosion uplift and tectonic uplift.
The denudation-accumulation system between upstream of Minjiang River drainage and Chengdu basin has been studied in the paper.Firstly, the remnant sedimentary flux has been calculated basing on Later Cenozoic isopach map of Chengdu basin which has been made from the borehole data in the basin by Sufer software.The potential sedimentary flux of the basin is 1665 km3 which is resultant of remnant sedimentary flux and eroded sedimentary flux.Secondly, the Later Cenozoic erosional rate, denudation thickness and denudation volume of upstream of Minjiang River drainage have been calculated basing on sediment discharge, incision rate,cosmogenic nuclides, digital elevation models and fission track.The denudation rate of upstream of Minjiang River drainage is 0.26～0.5mm/a,the denudation thickness of upstream of Minjiang River drainage is 0.94～1.44 km, and the denudation volume of upstream of Minjiang River drainage 21213.50 ～32636.16 km3.Based on the ratio of potential sedimentary flux to erosional volume of upstream of Minjiang River drainage which is 5.11%～7.85%, we inferred that it was not matching between potential sedimentary flux of Chengdu basin and denudation volume of upstream of Minjiang River drainage in the denudation-accumulation system system,and it was also not matching between Chengdu basin and Longmen Shan.
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