ATHABASCA ANALOG FOR TIBETAN LOWER CRUSTAL FLOW AND STRAIN PARTITIONING

Micah Jessup has made significant field-based contributions towards our understanding of strain partitioning, strain localization, and middle crustal flow in the Himalayan-Tibetan orogen. The deep crustal record of this orogen beneath the Tibetan Plateau, however, has been mainly gleaned remotely via active and passive source seismology, magnetotelluric profiles, igneous geochemistry, and xenoliths. Here we highlight examples and recent results from the >20,000 km² Athabasca granulite terrane in the Canadian Shield that arguably represents spectacular field-based analog for lower crustal flow and strain partitioning inferred for the southeastern Tibetan Plateau. This region of Tibet is underlain by deep crustal zones of low shear wave speed and low resistivity, attributed to channelized flow of melt-weakened lower crust. In the Athabasca terrane, hundreds of km² regions are underlain by penetrative sub-horizontal L>S and L-tectonite ribbon mylonite fabrics. These tectonites exhibit uniform kinematics that developed in feldspar porphyroclast-rich orthogneisses and garnet-rich metasedimentary migmatites at >800°C and >0.7-1.4 GPa (>25-50 km-depths). In situ monazite petrochronology demonstrates that melt-weakening and ductile lower crustal flow in some domains occurred simultaneously with juxtaposition along moderately to steeply dipping transpressive shear zones adjacent to strong isobarically cooled domains. The Athabasca analog is remarkably similar to the rheological behavior hypothesized for the southeastern Tibetan Plateau, i.e., channelized weak lower continental crust presently flows out of central Tibet and is diverted around the cratonic lithosphere beneath the Sichuan basin. This hypothesis requires the presence of a rheological dichotomy in the lower crust that is well-documented in the deeply exhumed record of the Athabasca granulite terrane at scales spanning several orders of magnitude.