Relation between finite strain geometry and quartz petrofabrics in a folded conglomerate in the Norwegian Caledonides

Abstract

A relationship between crystallographic preferred orientation (CPO) and strain geometry (strain-ellipsoid shape) is often assumed, but not well documented. In this work, we use the EBSD method to study quartz CPO and microstructure in relation to strain geometry in a folded conglomerate in the SW Scandinavian Caledonides, where strain varies from strongly flattening (long limbs) to constrictional (hinges and inverted short limb). Our work shows that strain was accommodated by dynamic recrystallisation through subgrain rotation, bulging and Dauphine twinning. Differential stress during deformation is estimated at ∼125 MPa from recrystallised grain size. Deformed and recrystallised grains show similar patterns of crystallographic orientation, despite a significant difference in grain size. We find no correlation between strain geometry and CPO, which we explain by means of a flow perturbation fold model. In this model, the variation in strain geometry is a result of progressive layer rotation during shearing. A correlation may be expected for steady-state deformations with no folding of layers with strain markers. However, flow perturbation may be a common cause of complex 3D strain patterns in sheared rocks. Hence, estimating strain geometry from CPO alone is inherently difficult and should be avoided unless a simple and steady deformation history can be assumed.