Thermal structure and tectonic evolution of the Scandian orogenic wedge, Scottish Caledonides: integrating geothermometry, deformation temperatures and conceptual kinematic-thermal models

In the Caledonides of northwest Scotland, two independent geothermometers (Fe-Mg exchange and quartz c-axis fabric opening angle) are used to characterize the thermal structure of the lower part of the Scandian (435–420 Ma) orogenic wedge within the Moine, Ben Hope and Naver-Sgurr Beag thrust sheets. Traced from west (foreland) to east (hinterland), Fe-Mg exchange thermometry yields peak or near-peak temperatures ranging from 484 ± 50 °C to 524 ± 50 °C in the immediate hangingwall of the Moine thrust to 601 ± 50 °C in the immediate hangingwall of the Ben Hope thrust, to 630 ± 50 °C in the Naver thrust sheet. Preserved metamorphic facies and textural relationships are consistent with thermometric estimates. Deformation temperatures calculated from quartz c-axis fabric opening angles across two similar orogen-perpendicular transects also yield systematic increases (Glen Golly – Ben Klibreck, 520–630 °C; Ullapool-Contin, 465–632 °C) traced towards the Naver and Sgurr Beag thrusts. In addition, deformation temperatures show a pronounced increase along the leading edge of the Moine thrust sheet moving south towards the Assynt window, which is interpreted to reflect deeper exhumation of the thrust plane above the Assynt footwall imbricate stack. Because temperatures calculated from metamorphic assemblages are within error of the quartz fabric-derived deformation temperatures that are of demonstrably Scandian age, the metamorphic sequence between the Moine and Naver-Sgurr Beag thrusts is interpreted to have developed during the Scandian orogeny. Integration of our results with previous 2D thermal-mechanical studies allows development of new conceptual thermal-kinematic models of Scandian orogenesis that may be broadly applicable to other collisional systems. Furthermore, it highlights the critical nature of coupling between orogen kinematic and thermal evolution.