The Northern Highlands Terrane of Scotland hosts several thrust nappes that were deformed and metamorphosed during the Silurian Scandian orogeny. Quantitative petrological analysis of metamorphic assemblages indicates that the hinterland-positioned Naver nappe experienced decompression heating from 8–9 kbar and 600°C to 6–7 kbar and 700°C. Monazite–xenotime thermometry and geochronology delineate a detailed temperature–time history for the Naver nappe. Monazite often exhibits compositional zoning, which is used to establish multiple temperature–time points in several samples. These data indicate that the Naver nappe experienced relatively fast heating ( c. 50°C myr −1 ) and relatively slow cooling (15–20°C myr −1 ), with peak temperatures occurring at c. 425 Ma. This temperature–time evolution is compatible with the early Emsian (407–403 Ma) deposition of unmetamorphosed conglomerates that rest on high-grade metamorphic rocks in the Naver nappe, but requires an acceleration in the cooling rate to 40–50°C myr −1 at 420–410 Ma. Geochronological constraints from this study and previous work suggest that deformation and metamorphism in the hinterland of the Scandian orogen in northern mainland Scotland are younger than the c. 430 Ma deformation in the foreland-positioned Moine thrust zone. We postulate that heat from pervasive granitic intrusions in the Naver nappe weakened the crust, allowing deformation to retreat to the hinterland of the orogen. Supplementary material: A description of our analytical methods, all U–Pb-trace element data, additional figures explaining our petrological analysis and other relevant data are available at: https://doi.org/10.6084/m9.figshare.c.4458041

New 40Ar/39Ar thermochronological and deformation temperature analyses in the Scandian (c. 435–420 Ma) orogenic retrowedge of northern Scotland demonstrate accelerated cooling during late syn- to post-orogenic exhumation of the high-grade orogenic core. Initial cooling rates of 10–30°C myr−1 immediately following peak orogenesis transitioned to rapid rates of 45–90°C myr−1 during final exhumation of the Naver thrust sheet in the orogenic core. The flanking ductile thrust sheets exhibit a similar, albeit less pronounced, acceleration of cooling, with rates increasing by c. 150–300% following peak orogenesis. Closer to the foreland, the Moine thrust sheet did not experience increased cooling rates. Calculated unroofing rates of 3.75 mm a−1 in the high-grade Naver thrust sheet suggest increasing, rapid exhumation in the orogenic core during a presumed collapse phase of orogenesis. This is contrary to the expectation of decreasing erosional efficiency as topography is diminished and is interpreted to suggest that unroofing of the Scottish Caledonides may have been partially enhanced by upper crustal extensional deformation during ductile flow of the infrastructure of the orogenic core. Similar processes have been interpreted in the East Greenland Caledonides, which form the northern extension of the Scandian retrowedge.Supplementary material:40Ar/39Ar analytical data for muscovite (Supplementary Data Table 1), 40Ar/39Ar analytical data for amphibole (Supplementary Data Table 2), and electron microprobe analytical data for amphibole samples (Supplementary Data Table 3) is available at: https://doi.org/10.6084/m9.figshare.c.5087057

In the Caledonides of northern Scotland temperatures of metamorphism (Tm) and deformation (Td) progressively increase structurally up section in the Moine thrust sheet at the foreland edge of the Scandian (mid Silurian) orogenic wedge. However, the thermal history of the structurally overlying, more hinterland positioned thrust sheets is less well known. This study focuses on determining Td and Tm for both the central/upper part of the Moine thrust sheet and the lower part of the overlying Sgurr Beag thrust sheets in the middle of the Northern Highlands Terrane. Preserved microstructures and quartz c-axis fabric opening angles in the Moine and Sgurr Beag thrust sheets imply Td of 460 °C to 605 °C ± 50 °C. Thermobarometry and pseudosection-based P-T constraints, indicate Tm of ~550-680 °C at 4.8-7.2 kbar in the Moine thrust sheet and Tm of ~620 °C at 5.6-7.7 kbar in the Sgurr Beag thrust sheet. Together, Td and Tm in the Moine and Sgurr Beag thrust sheets indicate that deformation continued after peak metamorphic conditions in the Sgurr Beag thrust sheet. Monazite and xenotime petrochronology show that Tm, and possibly Td, record Precambrian metamorphism. Peak metamorphism is associated with the Knoydartian orogenic event (840-720 Ma), with possible reworking during Scandian thrusting (430-425 Ma).

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