TEXTURAL DEVELOPMENT IN GRANITIC FAULT ROCKS OF THE RHINE GRABEN, WESTERN EUROPE: A XTG AND HRTEM STUDY

The Rhine Graben of western Europe provides an excellent area for studying the neotectonic and structural development of active, intracontinental rift systems. Despite numerous investigations of the areas history, there is relatively little known concerning the microstructural and textural evolution of alteration products in faulted granitic basement. In this study we examine two localities from different parts of the Rhine Graben: the Soultz-sous-Forets borehole site (sampled below 1417 m), located in the western part of the rift basin, and the E-W trending Schauenburg Fault, which crops out at the surface along the eastern graben shoulder. Fault rock samples (cataclasites and associated hydrothermal veins) were analyzed by X-ray diffraction, X-ray texture goniometry (XTG) and electron microscopy (SEM and HRTEM) in order to obtain textural, compositional and microstructural data. Significant differences are observed both in mineralogy and texture. The main alteration products of the Soultz samples are predominantly 2M polytypes of anhedral and fibrous muscovite with minor chlorite, which mainly occur as replacement products of feldspar and biotite. The white mica fabric is slightly better developed in the host granite (maximum intensity in m.r.d. 3.1-3.7), but also shows reasonably good oblate fabrics in the hydrothermal veins (m.r.d 2.7-3.0). In the Schauenburg Fault, 1Md illite/smectite polytypes dominate, along with significant quantities of kaolinite. XTG results reveal complex fabrics across the fault zone. Textural patterns often have multiple maxima attributable to the presence of relatively large vermiform stacks of illite and kaolinite. The best fabrics are developed in cataclasites lying along the fault plane (m.r.d >3.33). These results demonstrate that the formation of authigenic and hydrothermal phyllosilicates in these faulted granites play a critical role in influencing the permeability anisotropy, with enhanced fabric development along the shoulder of the Rhine Graben and poorer textures characterizing the younger, hydrothermally altered granite within the basin.