Abstract

Cross fiber chrysotile asbestos deposits within serpentinized alpine-type peridotites are syntectonic. The asbestos veins fill tension fractures induced by regional stress fields moderated by local structural heterogeneities. Inverting this process by measuring the orientation of the veins yields the regional stress regime at the time the veins were formed. Measurements at several asbestos mines (Cassiar mine, British Columbia; Thetford mines, Quebec, and Woodsreef mine, New South Wales) suggest that the asbestos develops during a change in the orientation of the regional stress which appears kinematically as a change from dip-slip to strike-slip fault motion. This interpretation is compatible with other independent sets of structural data.Several factors determine whether or not an asbestos deposit is developed. First, the serpentinite must have a nonfoliated texture; mineralogy does not appear to be important. Second, it must be situated near a fault that is active during a change in the orientation of the stress field. Third, the serpentinite must be in the stability field of chrysotile when the change in orientation occurs. Last, subsequent deformation must not continue, or temperature increase, such that the fiber is destroyed.

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