HR: 17:45h
AN: GP12B-08    [PDF]
TI: Magnetotelluric constraints on the the nature of lower-crustal shear zones
AU: * Bedrosian, P A
EM: bedros@gfz-potsdam.de
AF: GeoForschungsZentrum - Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Unsworth, M J
EM: unsworth@ualberta.ca
AF: University of Alberta, Department of Physics, Edmonton, Al T6G 2J1 Canada
AB: While recent geophysical studies have shed light on the nature of upper-crustal (brittle) transform faults, little is known about the ductile roots of these shear zones. Models have been suggested encompassing varying degrees of strain localization, vertical coupling, and connectivity with upper-crustal faults. The lack of seismicity makes it especially difficult to discern the structure and geometry of such ductile shear zones. Magnetotellurics (MT), sensitive to zones of anomalous conductivity, is one of a handful of geophysical methods capable of imaging earth structure at such depths. Previous MT studies along the San Andreas (SAF) show strong indications that structural changes occur in the lower crust. Geo-electric strike, skew, and inductions vectors all suffer marked changes at a frequency of approximately 0.01 Hz, corresponding to depths of 10-15 km. A range of 3D and 2D anisotropic forward models have been examined in order to understand the changes in the observed MT data as well as the sensitivity of MT data to lower-crustal structure. The resistivity contrast across the fault provides the dominant MT signature and in some cases may mask the response of deeper structure. Assuming a conductive shear zone, the degree of localization is discernable from MT data. Furthermore, an anisotropic shear zone gives rise to a distinct response in the induction vectors, similar to that observed in data across the SAF.
DE: 0600 ELECTROMAGNETICS
DE: 8010 Fractures and faults
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting