MICROSTRUCTURAL ANALYSIS OF FAULTING IN QUARTZITE, ASSYNT, NW SCOTLAND - IMPLICATIONS FOR FAULT ZONE EVOLUTION

Macroscopic fracture arrays, microstructures and interpreted deformati on mechanisms are used to assess the development of a minor reverse fa ult (backthrust) in quartzite from the Moine Thrust Zone, Assynt, NW S cotland. Fracturing dominates the faulting via the progression: intrag ranular extension microcracks; transgranular, cataclasite absent exten sion fractures; through-going, cataclasite filled shear microfaults, w ithin which fracturing and particulate flow operate. However, both dif fusive mass transfer (DMT) and intracrystalline plasticity (low temper ature plasticity, LTP) processes also contribute to the fault zone def ormation and lead to distinct associations of deformation mechanisms ( e.g., DMT-fracture and LTP-fracture or low-temperature ductile fractur e, LTDF). Over a large range of scales the fault zone consists of bloc ks of relatively intact rock separated by narrow zones of intense defo rmation where fracture processes dominate. The populations of fragment s/blocks of different sizes in the fault zone have a power-law relatio nship which is related to the dimension of the fault zone. These obser vations are used to develop a general model for fault zone evolution b ased on the distribution of deformation features as a function of eith er time or space. A systematic variation in the deformation rate : tim e histories is recognised, associated with different positions within the fault zone. Thus, the fault zone preserves elements of the ''birth , life and death'' sequences associated with the displacement history and strain accommodation.