DEFORMATION OF CONTINENTAL CRUST DURING UHP SUBDUCTION AND EXHUMATION

How continental crust deforms and transforms during its subduction and exhumation from ultrahigh-pressure depths dictates a panoply of tectonic processes, including large-scale mixing of crust and mantle, the rise or collapse of mountains belts, and local plate motions. Of central important are two main questions: i) How was deformation partitioned within the subducted and exhumed continental crust—were the UHP rocks subducted and exhumed as a coherent and intact sheet or were they strongly deformed? ii) How much of the material was transformed to UHP minerals during subduction and how much subsequently “back reacted” to lower pressure phases? Answers to these questions bear on the magnitude of stress [e.g., Stöckhert et al., 1997]. the applicability of geodynamic models [e.g., Gerya et al., 2002], and the body forces available to effect subduction or exhumation of the UHP rocks.

To address these questions we present outcrop to thin-section scale observations of structures and metamorphic minerals from the giant UHP terrane in the Western Gneiss Region of Norway. We find two principal indicators that internal strain within the UHP slab did not involve slab-scale overturning: i) only 2X thinning of the eclogite pressure gradient, and ii) no more than km-scale folding of overlying nappes into crystalline basement. Significant portions of the UHP terrane contain pre-UHP mineral parageneses with minimal conversion to high-pressure phases.