SLOW BURIAL OF CONTINENTAL CRUST IN COLLISIONAL OROGENS; INSIGHTS FROM SPATIALLY-RESOLVED LU-HF GARNET CHRONOLOGY IN THE WESTERN GNEISS COMPLEX, NORWAY

The burial of continental crust during collisional orogenesis is a first-order process in global geochemical cycles and continental tectonics. Decades of inter-disciplinary geoscience research into the exhumation of such deeply buried crust has led to the development of intricate models for continental subduction; however, we have yet to empirically constrain the tectonics and rates of burial. To address these knowledge gaps, we performed spatially resolved Lu-Hf garnet chronology on meta-mafic rocks in the Western Gneiss Complex (WGC) in Norway – the world’s largest and best-preserved continental ultrahigh pressure (UHP) terrane – along a 100-km vector oriented parallel to the paleo-subduction direction. Our calculated pressure and temperature estimates drop from the UHP domains at the coast to the HP regions in-land. In contrast, our Lu-Hf ages (419-400 Ma; ± 0.3-1 %RD) do not mimic such a trend. However, when comparing these chronologic data to Zr-in-rutile data for the same samples, it becomes apparent that our youngest ages (404-400 Ma) represent the cessation of recrystallization and the onset of exhumation, whereas the older ages (420-410 Ma) represent garnet relics that crystallized during burial. The discrepancies in the conditions and ages of garnet growth allow calculation of a vertical burial rate of ~5 mm/yr. The implications of this study are two-fold. First, our data provide detailed insight into the response of garnet to deep burial and eclogitization, and shows that its chemical and chronological records may uniquely represent peak burial, and the switch of orogenic systems towards extension and exhumation. Secondly, our data illustrate that during the transition from oceanic to continental subduction, unless the angle of the down-going plate decreases by a factor 4, this switch must result in a substantial slowing down of convergent plate motions. This prediction is reflected in observations from Earth’s main active collisional orogen, the India-Asia collision zone. The consistency of these data demonstrate that collisional orogenesis is a dominant factor in pushing converging systems into a regime where plate motions are controlled by the sluggish mechanics of subduction. This establishes continental collision as a prime mechanism for the reorganization of regional plate motions.