Fault Zone Weakening in Low-Angle Normal Faults

Steven A.F. Smith

Department of Earth Sciences, University of Durham, United Kingdom, DH1 3LE

[email protected]

Traditional ‘Anderson-Byerlee’ frictional fault models predict that normal faults should initiate at dips of ~60°, rotate and lock up due to friction at angles of ~30-40°. This prediction seems inconsistent with the increasingly wide documentation of active Low-Angle Normal Faults from around the world. Recent models explain continued slip on LANF over long periods of time by invoking syntectonic changes in fault rock rheology causing ‘fault zone weakening’. This project will adopt a multidisciplinary approach to increasing our understanding of several important factors in the evolution of LANF, including fault zone geochemistry, fluid flow and alteration histories, and the controls on fault zone architecture.

The Zuccale LANF is very well exposed on the Island of Elba, western Italy. Significantly, recent investigations in central Italy have highlighted the existence of an active LANF – the Altotiberina Fault - which produces abundant microseismicity. Field observations of the exhumed Zuccale LANF can therefore be used to gain key insights into processes currently occurring at depth. The core of the Zuccale Fault is characterised by a distinct sequence of fault rocks, including a heterogeneous assemblage of foliated phyllites. Preliminary fieldwork has highlighted the importance of high-angle footwall structures, present throughout the history of the main fault, in controlling the internal distribution of fault rocks. Footwall extension was achieved along early flat-lying cataclastic shears, followed by sequential high-angle normal faulting and fault rotation. Future work will focus on quantifying conditions of deformation within the fault zone, the permeability structure of the fault core, and on understanding the role of fluid flow.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid