SOME MUSING ON CORE COMPLEXES

 

For those of Nick’s followers who might be interested in what was avant-garde tectonic thinking in the era prior to terrane-speak and detailed knowledge of core complexes, read the paper below.  Ken did most of the writing and analysis; I merely supplied the paleomagnetics.  Note that he had the extensional stress regime dead on.  Here is the paper.

Paleomagnetic results for Eocene volcanic rocks from northeastern Washington and the Tertiary tectonics of the Pacific Northwest

KF Fox Jr, ME Beck Jr - Tectonics, 1985, vol 4, pp 323-341.

And now, a question:  The classical picture of a core complex has the upper crust sliding off an upwelling lower (gneissic) crustal layer.  This configuration need not necessarily imply extension; for instance, a thermal anomaly might cause a segment of lower crust to rise, and in doing so shed its “overburden” along normal (detachment) faults.  However, in this case one might expect to find compressional features in the detached sheets.  Finding extensional features instead must surely require crustal extension.  In the case of the Cordilleran core complexes the configuration seems to require that the western edge of the continent moved away from points fixed in the North American craton.  And, in passing, the significant clockwise rotation found in Eocene volcanic rocks from the Republic graben certainly implies a trans tensional, dextral, stress regime.

Or am I wrong?