Abstract

Geochronological data on metamorphic minerals within and outside the Bancroft shear zone, a large-scale roterozoic shear zone, provide evidence for extensional displacement over a period of 150 m.y. This is the longest time interval yet demonstrated for any fault responding to one orogenic phase. Extensional displacement rates along the Bancroft shear zone, averaged over 150 m.y. from 1040 to 893 Ma, are 0.06 to 0.13 km/m.y. During extension, unroofing of the footwall was accompanied by rotation as 5 to 15 km of overlying crust was unloaded. Similar lower-plate rotation has been inferred for many of the Tertiary metamorphic core complexes exposed in the western United States. The protracted displacement and rotation history of the Proterozoic Bancroft shear zone contrasts with the much faster extensional unroofing of the younger metamorphic core complexes, suggesting inherent differences in the tectonic processes operating in these two geologic settings. Similarities, however, indicate that both settings may share common aspects of tectonic deformation. These findings underscore the geologic importance of large, deep-crustal faults and indicate that present-day zones of active extension, such as in the Himalayas, could remain active many millions on years into the future.

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