Climate and tectonism are both particularly intense in the Yarlung Zangbo (Tsangpo) Great Canyon in the eastern Himalayan syntaxis,which is characterized by the most rapid landscape evolution of anywhere in the world.Thus,the eastern Himalayan syntaxis is one of the best locations to study the interactions between climate and tectonics.This paper investigates the cooling ages of the Doxong La-Baibung profile using apatite fission track (AFT) dating on 11 bedrock samples at elevations ranging from 4210 to 710 m.There are topographic,climatic,metamorphic,and thermochronological gradients in the profile,providing good conditions to study interactions between climate and tectonics.AFT ages ranged from 4.6±0.6 Ma to 1.7±0.3 Ma,and the mean fission track lengths ranged from 11.0 to 12.4 μm.It was found that the cooling rates revealed by AFT ages increased with decreasing elevation.However,the tendency of the cooling rates revealed by the 40 Ar-39 Ar ages was different from that indicated by the AFT ages.Moreover,for most districts of the eastern Himalayan syntaxis,the compiled AFT age distribution correlates well with the annual average precipitation,indicating the coupling of the cooling and erosion rates of the near-surface rock and precipitation.The geothermal history modeling results indicate an obvious increase in the cooling and erosion rate between 1.0 and 0.5 Ma.This age is consistent with other research findings for this time,when the vapor channel of the Yarlung Zangbo Great Canyon began to take effect.These evidences suggest that climate,especially precipitation,has acting as a key factor influencing the rapid cooling and erosion in the Yarlung Zangbo Great Canyon since 1-0.5 Ma.
Compaction and silicon cementation are the dominant processes reducing porosity and permeability in quartzose sandstones during diagenesis.Despite the wealth of information about quartz cementation,there are still unanswered questions related to mechanisms of growth of quartz cement and the diagenesis processes.In this study we present an electron backscatter diffraction(EBSD)analysis,combined with optics and cathodoluminescence(CL)information,for the quartz sandstones from the Upper Triassic Xujiahe Formation of Sichuan Basin,in order to reveal the microstructural and crystallographic features of the silica cementation and detrital grain during the compaction.The EBSD is a crucial technique to provide essential crystallographic data on the quartz grain and its cement.Quartz cement is shown to be syntaxial to its host quartz grain.EBSD data-based orientation maps show dauphiné twinning and low angle boundary to be common in the host grains and quartz cement of the samples.The dauphiné twins occurred in grain-grain contacts and in cement-crystal boundaries,and commonly crossed grain cement boundaries.These features indicate that there may be two types of dauphiné twins,one inherited twins from the source area and the other developed by compaction-induced grain boundary deformation.These investigations suggest that strong mechanical compaction may occur after and/or during quartz cement growth in the later diagenesis of the Xujiahe sandstones.EBSD has a capability of revealing microstructural information and regarding mechanisms of diagenesis crystal growth in quartzose sandstones.
The uplift of the Ailao Shan-Diancang Shan(ASDS) along the Ailao Shan-Red River(ASRR) shear zone is an important geological event in the southeastern margin of Qinghai-Tibet Plateau tectonic domain in the Late Cenozoic,and it preserves important information on the structures,exhumational history and tectonic evolution of the ASRR shear zone.The uplift structural mode and uplift timing of the ASDS is currently an important scientific topic for understanding the ASDS formation and late stage movements and evolution of the ASRR shear zone.The formation of the ASDS has been widely considered to be the consequence of the strike-slip movements of the ASRR shear zone.However,the shaping of geomorphic units is generally direct results of the latest tectonic activities.In this study,we investigated the timing and uplift structural mechanism of the ASDS and provided the following lines of supportive evidence.Firstly,the primary tectonic foliation of the ASDS shows significant characteristic variations,with steeply dipping tectonic foliation developed on the east side of the ASDS and the relatively horizontal foliation on the west side.Secondly,from northeast to southwest direction,the deformation and metamorphism gradually weakened and this zone can be further divided into three different metamorphic degree belts.Thirdly,the contact relationship between the ASDS and the Chuxiong basin-Erhai lake is a normal fault contact which can be found on the east side of the ASDS.40Ar/39 Argeochronology suggests that the Diancang Shan had experienced a fast cooling event during 3–4 Ma.The apatite fission track testing method gives the age of 6.6–10.7 Ma in the Diancang Shan and 4.6–8.4 Ma in the Ailao Shan,respectively.Therefore the uplift of the ASDS can be explained by tilted block mode in which the east side was uplifted much higher than the west side,and it is not main reason of the shearing movements of the ASRR shear zone.The most recent uplift stages of the ASDS happened in the Pliocene(3–4 Ma) and Late Miocene(6–10 M
