4 - Structures in UHPM rocks: A case study from the Alps

Summary

Abstract

Structures in UHP rocks may provide some insight into the rheology and deformation regime of continental rocks subducted at mantle depth, as well as on their exhumation tectonics. We focus on the Dora-Maira case study (Western Alps), with references to a few other areas of UHP or eventually HP-eclogite facies metamorphism. In southern Dora-Maira, UHP Eoalpine rocks are found as a coherent, 1000-m-thick lensoid unit within a pile of HP eclogite and blueschist-facies slices. The whole pile was affected by a common, Mesoalpine greenschist-facies overprint. From the local preservation of Pre-Alpine textures, and from the various UHP and HP relic structures (either irrotational or rotational), we infer a strong deformation partitioning of ductile flow in the subducting continental crust down to a 100-km depth. The blueschist, then greenschist-facies overprint developed during westward thrusting of the UHP-HP eclogitic slices onto more external zones, then during the collapse of the thickened orogenic wedge. The early exhumation of the UHP rocks up to the 30-km depth is controversial. If a continuous convergence between Apulia and Europa is accepted, then a forced-flow, or extrusion tectonics of imbricate slices in the subduction wedge would be hypothesized. If not, an extensional, metamorphic core complex stage could be invoked during the Eoalpine-Mesoalpine interval.

Introduction

While a number of occurrences of ultrahigh pressure metamorphic (UHPM) rocks have been described from various Phanerozoic belts in the last decade, detailed structural studies dedicated to these rocks are still lacking.