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.
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?
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