Controls on Late-Orogenic Exhumation of Subducted UHP Continental Basement

Ultra-high pressure (UHP) rocks from Norway's Western Gneiss Region (WGR) record late-orogenic (~420-400 Ma) subduction and exhumation of continental basement following protracted convergence. Recent models of UHP rock exhumation, in contrast, describe the exhumation dynamics of UHP continental margin rocks during the subduction-collision transition (Warren et al., 2008, G3, 9, Q04019, doi:10.1029/2007GC001839; Jamieson et al., this session). Here we present thermal-mechanical upper-mantle scale models focusing on exhumation mechanisms of UHP continental interior following substantial post-collision convergence. The initial model geometry includes a stationary retro-continent, a small ocean, and a converging pro-continent subdivided into a micro-continent, a rifted margin, and an “interior” (basement), with contrasting material properties. The models evolve through oceanic subduction, continental collision involving accretion of the micro-continent, and development of a thick orogenic wedge consisting of the stacked micro-continent and rifted margin. Subduction and subsequent exhumation of the continental interior beneath the orogenic wedge commence ~20 Ma after collision. Depending on the density and strength of the pro-continent system, one of three processes may result: (1) corner-flow exhumation following subduction channel closure; (2) buoyant exhumation in an effectively fixed-width subduction channel; or, (3) buoyant exhumation preceded by lithospheric delamination and rollback of the subducting slab. Weaker/lower-density interior crust facilitates early detachment from the subducting slab, favouring (1) over (2), while weaker/higher-density micro-continent and rifted margin crust facilitates early delamination, favouring (3). The potential for buoyant exhumation, expressed by the exhumation number E, depends on the subduction channel thickness, effective pressure gradient, effective viscosity, and subduction velocity. The model results offer possible explanations for late-orogenic exhumation of UHP continental basement, as observed in the WGR, and suggest that exhumation dynamics and timing may vary widely according to the density and strength of the subducting crust.