Yuqiu Zhao

B.Sc. (Honours) Thesis

Tiny Recorder of the Evolution of Major Crustal Faults in the Bhutan Himalaya: An Integrated Mineralogical, Morphological, and Geochemical Study of Clay Minerals from fault gouges

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The Himalayan Orogen was formed during ~ 50 million years of the on-going collision of India and Eurasia plates. The detachment at the base of the Himalayan orogenic wedge is the major active fault called the Main Himalayan Thrust (MHT). Its movement transferring gradually from ductile shear zone in the metamorphic orogenic core to brittle upper crust caused the formation of sequence of ductile shear zone and brittle thrust faults. From north to south, from older to younger, these mainly include the Main Central Thrust (MCT), the Main Boundary Thrust (MBT), and the Main Frontal Thrust (MFT). The Himalayan frontal faults, MBT and MFT, are the most important places for the study of how brittle fault formed and even understanding seismicity in the Himalayan orogen, due to its crucial role on accommodating deformation. This paper aims to characterize the mineralogy, morphology and geochemistry of clay minerals dominant in fault gouges, in order to understand the evolution of brittle fault and to reconstruct the conditions of faulting, as well as to further date faults for a larger project.

To identify clay and associated minerals, X-ray diffraction (XRD) analysis was primarily used, combining with petrographic microscope, scanning electron microscopy and energy dispersive spectroscopy (SEM/ EDS), and transmission electron microscopy (TEM). For the condition of clay formation, hydrogen (δD) and oxygen (δ18O) stable isotope analysis was also applied. The mineralogical and morphological investigations of 11fault gouge samples from the major crustal faults in the eastern Bhutan suggest that the fault gouges and protoliths are clay dominant with two types: (a) 2:1 layers, smectite, illite-smectite, and illite and (b) 1:1 layers, kaolinite. Both illite and kaolinite present in all samples, and illite and illite-smectite coexist only in Siwalik samples 182 while illite and smectite only in MFT samples 89. Such mineral aggregations suggest that no metamorphism happened but clay authigenesis occurred in fault gouges, combining with δ18O and δD data. For the three samples that have three grain size fractions (0.1μm, 0.4μm, &2μm), the δ18O and δD isotopic data show that there is no evident relation between grain size and isotopic composition. The isotope fractionation processes that occurred between fluids and clay minerals, establish the relation of the isotopic composition of clay minerals and the conditions of clay formation. Based on both smectite and kaolinite geothermometers, the equilibration temperatures of clay formation show two main results: 1) only the MFT sample 89C may have formed near surface at the lowest temperature, 35. 8 °C (± 1.6°C)sm and 20.6 °C (± 1.6°C)K; 2) other samples yield the same result that they have formed within the equilibration depth of ~ 2 km with ~ 35 °C − 46.3 °C (± 1.6°C)K of other samples. Using only known kaolinite fractionation factors, the isotopic composition of the meteoric waters is obtained with -10.6 − -9.0 ‰ δ18OK-W and -74.8 − -62.2‰ δDK-W. It suggests that the paleometeoric water was derived from elevations between 2000 and 2500 m, which are ~1500 – 2000 m higher than the outcrops of fault gouges. Therefore, the formation condition of fault gouges is reconstructed, and it is concluded that the fault gouges have formed within the equilibration depth of ~ 2 km and at ~ 3 km or longer horizontal distance from the site of the source of the paleo-meteoric waters. However, the deduction would not work for the MFT fault gouge, which was formed after the MBT and is still active, due to the fault barriers between the source site of paleo-meteoric water and the MFT. Therefore, the interpretation of the stable isotopic compositions of clays from the MFT fault gouge requires further research.

Keywords: Clay gouges, Authigenic illite, Meteoric water, Isotopic composition, XRD
Pages: 109
Supervisor: Djordje Grujic