Description:

During the past 2.5 years numerous samples of the metamorphic rocks within the Grand Canyon have been obtained during Colorado River trips. We have nearly completed the argon data acquisition from hornblende, biotite, muscovite and K-feldspar. We have determined that a major thermochronological break occurs at the 96-mile shear zone where 1.4 Ga muscovite cooling ages are abruptly juxtaposed with 1.6 Ga muscovite ages. We are currently investigating the detailed nature of this shear zone (i.e. late normal movement vs. strike-slip juxtaposition) but we are confident that this is a major 1.4 Ga feature. Considering that there are no 1.4 Ga igneous rocks in the Grand Canyon, the thermochronological break represents significant 1.4 Ga tectonism at least 1000 km inboard of the collision zone.