T96 Approaches for Extracting Shear Zone History from the Ductile Rock Record: Probing Their Initiation, Evolution, and Reactivation II

Tuesday, 27 October 2020
5:30 PM - 8:00 PM
Meeting Rooms

TEAMWORK OF SHEAR ZONES DURING EXHUMATION, THE ROAN WINDOW, NORWAY

Paper number: T96-13
Program: Technical Sessions
Time: 7:25 PM – 7:35 PM

Authors

Abstract

Regional studies of P-T conditions, including melt crystallization, structures, and localized shear zones inform the mechanisms of orogenic exhumation. The Roan peninsula, Norway contains HP granulites and migmatites of the northern Western Gneiss Region, the deepest structural level of this part of the Caledonian orogen. Roan is an elongate dome in which a series of shear zones were active, likely in concert. Two main high-strain zones, one sub-horizontal in the core of the dome, and the other sub-vertical along the NW margin, are separated by a domain of less deformed, crenulated gneiss with leucosome veins occupying the folds’ axial surfaces. U-Pb zircon ages from intensely deformed leucosomes fall between 410-404 Ma (Gordon et al., 2016; this study); axial plane leucosomes in crenulated gneiss and cross-cutting pegmatite dikes that are locally sheared record U-Pb ages between 400-396 Ma. This suggests that melt crystallized over 10-15 My and accompanied deformation, likely during exhumation and progressive cooling of the terrane, when strain localized within shear zones. A third shear zone, defined by a shallowly S-dipping mylonitic fabric, is exposed along the SW margin of the Roan peninsula. The plagioclase-hornblende mylonitic gneiss is well exposed along a 25 m road cut that displays m-scale mylonitic folds with hinges parallel to the stretching lineation; sense of shear is top to the S-SW. A U-Pb zircon crystallization age of ~407 Ma was obtained from a leucocratic mylonitic gneiss. The age of mylonitization is not known, but this shear zone is kinematically consistent with the sinistral shear along the NW-trending, steep bounding shear zone. We interpret the three shear zones to have worked “as a team”, with shallowly dipping shear zones allowing exhumation of HP granulites in the core of the dome and the sinistral shear zone bounding the dome to the NW accommodating exhumation in a pull-apart structure.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-358207

© Copyright 2020 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.