Geology of the Okanogan Highlands 2012 - Stop 2

Stop 2: Volcanic rock in contact with granite

Here the roadcut exposes 50 million year old flow-banded volcanic lavas on the eastern half, and older granitic rock with aligned dark minerals on the western half.  Flow layers in the volcanic rock and alignment of dark minerals in the granite are sharply truncated at the contact (surface along which the different rock types touch), which indicates that the contact is a fault. This fault is a narrow fracture surface along which the rocks on either side have moved in opposite directions. The fault we see here contrasts with the broad shear zone of the Okanogan Fault, part of which we saw at Stop 1.

The Stop 2 fault is typical of the shallow crust, where rock is relatively cold and brittle, as opposed to the deeper crust where the rock has a higher temperature and is ductile.

This fault is part of a system of faults that form the boundary of the Toroda graben, a down-faulted block of rock. This fault appears to be a strike-slip fault, in which the two sides moved sideways (as opposed to up and down), based on the orientation of the grooved, shiny, slippery-feeling surfaces on the fault. Such grooves parallel the direction the rocks moved along the fault.

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