This work deals with the preliminary relationship between strain path and strain partitioning pattern in a sinistral transpressional zone,Lancangjiang shear zone,located to the southeast of Tibet.Various ductile rocks provide an opportunity to investigate quantitative finite strain(Rs),kinematic vorticity values(Wm),and proportions of simple and pure shear components.The mean kinematic vorticity values(Wm) were evaluated based on three methods,such as Rs-θ,prophyroclast hyperbolic distribution method(PHD),and polar Mohr diagram construction(PMD).Wm for mylonitic metasediments of the low-grade schist belt ranged from 0.19 to 0.54.Protomylonites,mylonites,and ultramylonites recorded differential Wm values,0.19―0.73,0.11―0.8,and 0.84―0.95(even 0.99) respectively in the high-grade gneiss belt.Based on classical strain analysis and these kinematic vorticity evaluations,the shear zone is classified as a preliminarily bulk monoclinic symmetry.The pattern of vorticity path suggests that a previously undocumented transpressional setting and strain partitioning pattern in the southeast region of Tibet Plateau,characterized by steep-narrow domain of simple-shearing(Tertiary strike-slip shearing) deformation and wider domain of mostly pure-shearing(Tertiary contraction) to accommo-date intracontinental deformation.
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.
