Deformation and temperature variation along thrust-sense shear zones in the hinterland-foreland transition zone of collisional settings: A case study from the Barbagia Thrust (Sardinia, Italy)

Abstract

In the Internal Zone of a continental collisional orogen, first-order contractional shear zones accommodate crustal shortening. Structural investigations at different scales, flow kinematics, and finite strain analyses are fundamental tools to determine how deformation is accommodated and partitioned. Spatial temperature variations can be responsible for the dynamic weakening and strain localization in the crust, therefore understanding the thermal conditions of shearing and deformation is critical. We integrate field observations, meso- and microstructural analyses, kinematic vorticity estimations, and finite strain data with a quantitative thermometric analysis by Raman spectroscopy on carbonaceous material along a ductile shear zone: the Barbagia Thrust (BT) in the hinterland-foreland transition zone of the Sardinian Variscan belt. These analyses, performed in two different parts of the shear zone, yield similar finite strain gradients, albeit with an increasing component of simple shear with increasing temperature, highlighting the feedback between temperature and vorticity. Our results best fits with a tectonic scenario with shear heating, where higher magnitude gradients correspond to higher vorticity and finite strain values, which indicate greater shear and heating values. The heating quantified along the BT (~50°C) is compared favorably to numerical and mechanical models. We demonstrate how the BT represents a major tectonic boundary separating the internal sector belonging to the metamorphic core of the belt from the external one involved in the orogenic wedge system.