Thermal structure of a major crustal shear zone, the basal thrust in the Scandinavian Caledonides

Crustal-scale thrust zones accommodate most of the horizontal shortening at the front of orogenic wedges. Their thermal state is a key feature of collision zones, depending on critical parameters such as thrust rate or initial thermal properties of involved lithosphere units. We present here the first direct imaging of the thermal envelope of such a thrust zone: the Jotun Basal Thrust (JBT) in the Scandinavian Caledonides, through Raman Spectroscopy of Carbonaceous Material in the alum shales, an organic carbon-rich unit of Cambro-Ordovician age along which the basal decollement of the JBT developed. Maximum temperature mapping within this unit shows isotherms grading from ∼320 °C in the south–east to ∼500 °C in the north–west in the trailing end of the nappe stack. Based on bt + chl + grt + ph equilibrium, we estimate that the trailing end reached a temperature of 500 °C at 1.2 ± 0.1 GPa pressure. 2-D thermo-kinematic modelling constrained with these new natural data and timing considerations for the Scandian collision indicates that (1) peak temperature mainly reflects maximum burial stage, (2) thrust rate and dip angle must have been low for the JBT and (3) the Scandinavian Caledonides represent a relatively cold orogenic wedge compared to other orogens.