CENOZOIC EXTENSION OF THE NORTH AMERICAN CORDILLERA: NEW INSIGHT ON METAMORPHIC CORE COMPLEXES (Invited Presentation)

Metamorphic core complexes and their bounding extensional detachments pepper the North American Cordillera from British Columbia to Mexico. The northern complexes exhumed migmatite domes in their cores (e.g. Thor-Odin, Valhalla-Passmore, Okanogan-Kettle of the Shuswap Complex). Most of these record significant Cordilleran crustal melting: zircon and monazite in migmatite yield Paleocene-early Eocene ages. The development of hot, thick, partially molten crust ended with rapid ascent of the deep crust driven by regional extension at ~50 Ma, as indicated by the similarity in ages yielded by high- and low-temperature thermochronometers in migmatite and other high-grade metamorphic rocks and associated granite.

Low-T thermochronology and paleoaltimetry studies have yielded several other important results about the late stages of orogeny in the core-complex belt: (1) migmatite dome rocks the developed in the mid-to-deep orogenic crust (> 30 km) reached the Eocene near-surface (<2 km) during a single exhumation event; (2) the ~45 Ma paleosurface was not far above the present-day peaks, for example in the Monashee Mountains of British Columbia, and not far from flat-lying detachment faults, for example in the Okanogan complex of Washington State; and (3) meteoric water precipitated at high-elevation and penetrated the detachment systems; Eocene elevation was 3000-4000 m: ~1000 m higher than it is today.

Insights from field-based studies of northern Cordilleran core complexes and from numerical modeling show that feedbacks between rapid, detachment-related extension and upward flow of deep crust have the effect of stabilizing orogens by the rapid cooling of deep crustal rocks exhumed to shallow levels. Models show that exhumation can occur at ‘tectonic rates’ of cm/year, consistent with low-T thermochronology data for migmatite and high-grade metamorphic rocks in northern Cordilleran core complexes.