Field Geology in the Time of Earthscope: Integrating Detailed Geologic Analysis with Earthscope-Scale Questions

Many structural and petrologic studies involve analysis of small-scale phenomena that seem incompatible with the scale and resolution of Earthscope images. The Athabasca granulite terrain (AGT), Saskatchewan, one of Earth's largest exposures of lower continental crust, is an essential resource for Earthscope Science. The AGT is a 20,000 km2 exposure of high-P granulite (1.0-1.2 GPa) that constituted the lower crust of the Churchill Province from 2.6-1.9 Ga. This exposure of lower crust is extremely heterogeneous. Lithologies range from mafic to felsic, massive to layered, regular to contorted, highly-strained to relatively undeformed, porphyroblast-rich to porphyroblast-poor, and relatively wet to dry. This heterogeneity, if characteristic of deep crust, informs a variety of geophysical observations. The mineralogy of foliated and lineated tectonites, with varying amounts of mica and/or amphibole can significantly affect seismic anisotropy. Metamorphic reactions involving garnet growth (i.e. opx + pl = grt) provide a mechanism for large-scale densification of the deep crust. Deep crustal orthogneisses develop large amounts of garnet even long after tectonism, decreasing the buoyancy of crustal roots. Recent interpretations of weak crustal layers and channel flow seem to contradict geophysical interpretations that crust and mantle are coupled. The AGT offers a view of a melt-weakened laterally flowing crust. Mapping of migmatite domains and flow fabrics documents temporal and spatial heterogeneity of melt-weakened domains, and provides a model for partial coupling of crustal layers. Finally, the distribution of mafic igneous rocks documents the heterogeneous transfer of mass and heat form mantle to crust. Dike swarms, underplates, and intraplates have signatures and consequences for heating, weakening, and coupling of crust and mantle. Regions such as the AGT can play an important role in building the Earthscope 4D image of North America. They serve as high-resolution laboratories that illuminate tectonic processes and provide a temporal context for Earthscope images.