CONSTRAINING THE ROLES OF LITHOSPHERIC FOUNDERING AND UNDERPLATING IN THE RISE AND FALL OF OROGENIC PLATEAUX

Orogenic plateaux are integral to many collisional orogens and the mechanism(s), rates, and timing of uplift and collapse are critical data for understanding collisional tectonics. Processes occurring in the lower continental crust are thought to play important roles in the evolution of orogenic plateaux, particularly those associated with crustal thickening and thinning. However, the lower continental crust is rarely exposed and processes are generally inferred from seismology or xenoliths. Hence, it has been challenging to develop time-resolved, quantitative records of change in the thickness of the lower crust. We utilize recent methodological developments integrating igneous geochemical crustal thickness proxies (chemical mohometry) with metamorphic P-T-t paths. This ‘full crustal’ approach permits quantitative estimation of spatial and temporal changes in the thickness of the lower crust. Application to Proterozoic and Paleozoic orogenic plateaux highlights dynamic lower crustal processes. The 1.47-1.37 Ga Picuris orogeny of the Southwest USA yields evidence for ~10 km of lower crustal thickening. This accords with widespread ferroan granitic magmatism and a lower crustal 7.x layer, suggesting basaltic underplating played an important role in the development of a 50-60 km thick Picuris orogenic plateau. Data from the 1.2-1.0 Ga Grenville orogeny indicate ~10 km of lower crustal thinning associated with ~1.15 Ga AMCG-suite magmatism, consistent with models invoking lithospheric delamination. They also require a component of lower crustal thickening (magmatic underplating?) during the 1.08-1.05 Ga Ottawan phase of the Grenville orogeny. Evidence from the Paleozoic Acadian and Variscan plateaux suggest lower crustal thinning during orogenic collapse, likely associated with ductile flow and lithospheric delamination, respectively. Combining chemical mohometry with P-T-t histories provides insights on the delicate balance of variables (including rheology, temperature, shortening) beneath the current level of exposure of metamorphic terranes through the lifetime of plateaux. Furthermore, quantifying the rates of lower crustal thickening and thinning provides new constraints on underplating and foundering, their role in altering crustal strength, and lower crustal growth and recycling.