Gross structure of a fault during its formation process in Westerly granite

We investigated the gross structure of a Fault in three dimensions during i ts formation process. The observed sample was Westerly granite recovered fr om compression after peak stress but before dynamic rupture. The sample was infused with a fluorescent resin, mechanically stripped, then observed wit h a stereo-microscope under UV light. The main fault plane was found to be macroscopically elliptical, and the fault trace on each polished surface wa s identical to that detected by X-ray CT scanning. Evidence suggests that t he fault plane widened its cracked zone as it grew. The gross structures of the fault system, such as a step-over structure and sub-faults parallel to the main fault, were also detected. We conclude that the step-over structu re was formed during the fault formation process. (C) 2000 Elsevier Science B.V. All rights reserved.