HETEROGENEITY IN THE MIDDLE AND DEEP CONTINENTAL CRUST: INSIGHTS FROM ISOBARICALLY-COOLED TERRANES

Geologic studies of exposed isobarically cooled (IBC) terranes provide information that is difficult to glean from seismic data, xenoliths or other remote data sources about compositional relationships and structural and metamorphic processes at specific levels of the crust. The granulite facies East Athabasca area, Saskatchewan, is an example of an isobarically cooled terrane that resided in the deep crust (30-40 km) from ~2.5 Ga to at least 1.9 Ga. Proterozoic rocks in the southwestern USA may have been in the middle crust (10-25 km) from at least 1.65 Ga to 1.4 Ga. The single overwhelming characteristic of both regions is the extreme heterogeneity at all scales in lithology, structure, and tectonic history. At the largest scale, both regions have block architecture, with lithologically and structurally distinct shear-zone-bounded blocks or domains. Domains in the East Athabasca area are dominated by tonalite, charnockite/granite, felsic gneiss, or deformed migmatite. Middle-crustal rocks in the SW-USA are dominated by amphibolite facies sedimentary/volcanic rocks and granitoids. Both terranes have a similar structural signature, with an early, initially shallow- dipping fabric, overprinted by domains of intense upright fabric. In the middle-crustal rocks, the shallow fabric may have originated during thrusting and collisional processes, but was reactivated during later events, such that steep and shallow fabrics together characterize the overall architecture and rheologically strong character of the middle crust. In the deep crust, shallow fabrics are largely older, reflecting an early thermally weakened (extensional or channel flow) state that was overprinted by upright fabrics reflecting cooler and rheologically stronger conditions as rocks decompressed. Obviously a primary goal for Earthscope and other remote investigations is to recognize the signature of this type of heterogeneity on geophysical images. However, because apparently similar crustal geometry and heterogeneity may reflect different stages of tectonic evolution and different rheological behavior, it is also important to constrain the character and history of the heterogeneity. This will require better integration of direct studies of exposed crustal sections and both synthetic and natural geophysical images.