Tectonometamorphic Evolution of Continental Crust During Ultrahigh-Pressure Metamorphism and Exhumation: Implications For Flow In Mature Collisional Orogens

Numerical models that account for the exhumation of high-pressure rocks in collisional orogens describe expulsion of buoyant (U)HP slices along the subduction interface; this process may also be linked to channel flow within higher levels of the orogenic wedge as the exhuming bolus enters the mid to lower crust. Both of these scenarios rely on a viscosity and/or negative density gradient between rocks in the channel and surrounding material to drive upward movement; such gradients are thought to result from syn-tectonic partial melting and metamorphic transformation. Strain and thermal weakening is also required to initiate detachment of the (U)HP crust from its downgoing substrate; upward flow is then typically accommodated between a coeval basal thrust and overlying normal-sense shear zone. Geologically, these processes should result in mixing and inversion of structural and metamorphic features, pervasive strain throughout exhumed (U)HP terranes, and perhaps extensive transformation to high-pressure assemblages. The metamorphic and deformation history of mid- to lower crustal sections that contain exhumed (U)HP rocks can yield constraints on the various defining characteristics of channel flow mechanisms. The Scandinavian Caledonides are an example of a continental collision where, in a deeply subducted part of western Norway known as the Western Gneiss Region (WGR), large (>30,000 sq. km), imbricate sheets containing scattered inclusions of eclogite-facies rocks have been exhumed between lower-pressure units. In the Nordfjord area of the southern WGR, early thrusting established a tectonostratigraphy that was subducted in entirety to UHP depths. Limited strain, or transformation to eclogite-facies assemblages, occurred during metamorphism at peak conditions. Exhumation was at least partially achieved by an overlying extensional detachment(s), but no evidence exists of a coeval basal thrust unless one lies buried beneath the current erosion surface. A coherent metamorphic transition from high-P amphibolite facies to coesite-eclogite facies that crosses tectonostratigraphy is retained, and bulk deformation during exhumation is consistent with extensional kinematics only. The overall extent of partial melting is also low. Apparently then, large tracts of ultrahigh-pressure crust can remain relatively untransformed and coherent during exhumation.