Interplay Between Crustal‐Scale Thrusting, High Metamorphic Heating Rates, and the Development of Inverted Thermal‐Metamorphic Gradients: Numerical Models and Examples From the Caledonides of Northern Scotland

Abstract

Understanding the processes that produce high prograde metamorphic heating rates and the development of inverted metamorphic sequences in collisional thrust belts remains a fundamental challenge for tectonics and metamorphic petrology. New 2D finite element models of crustal‐scale thrusts with variable slip rates (10, 20, 35, 50 km Myr−1) are used to examine how thrust sheet emplacement contributes to these processes. In the models, average prograde heating rates of 31–118 °C Myr−1 are observed in the footwall, with maximum transient heating rates of ∼167 °C Myr−1 occurring at the highest slip rate. Also, thrust sheet emplacement produces an inverted thermal gradient in the model footwall. At slip rates of 10 km Myr−1, heating magnitudes >200 °C are observed >7 km structurally beneath the thrust plane. At slip rates of 20–50 km Myr−1, models produce thermal penetration depths of 4–5 km for similar heating magnitudes. Petrologically determined heating rates for thrust footwalls in the Scandian orogenic wedge of northern Scotland mostly yield rates consistent with model results (10–230 °C Myr−1). The model‐derived magnitude of inverted thermal gradients is also similar to those indicated in texturally determined deformation temperature transects across the Scandian orogenic wedge (100 °C–180 °C). Combined, these results indicate that high heating rates can be produced by fault slip at typical plate velocities. Additionally, this implies that crustal‐scale thrusts are likely to produce inverted metamorphic sequences in most systems, provided that P‐T conditions during slip allow for metamorphic or recrystallization processes that would preserve evidence for such features.

Defining the Timing, Extent, and Conditions of Paleozoic Metamorphism in the Southern Appalachian Blue Ridge Terranes of Tennessee, North Carolina, and Northern Georgia

“…Peak granulite facies P‐T conditions in the CBR reached 775°C–850°C and 8.0–11.5 kbar (12NC03, this study; Moecher et al., 2004; Figures 9 and 10) at ∼ 458 Ma, essentially coeval with eclogite facies metamorphism occurring in the structurally adjacent EBR (Figure 12c). Although Phanerozoic subduction systems do not commonly preserve granulite‐facies metamorphism (e.g., Nasrabady et al., 2011), we attribute these lower P ‐higher T conditions to result from the bulk surface‐directed advection of isotherms in the growing accretionary complex simultaneous with mantle directed isotherm deflection in the subduction zone, similar to that observed in numerical models of collisional and subduction systems (Butler et al., 2011; Jamieson et al., 1998; Thigpen et al., 2021).…”

“…Because this schematic section is drawn to approximate the early Taconic collision at ∼460 Ma, WBR units of the Greenbrier (orange star) and Dunn Creek (green star) thrust sheets would not yet have been buried and heated to their peak Taconic P-T conditions until ∼450 to 440 Ma. The synorogenic geotherm of 29°C km −1 is derived from Thigpen et al (2021). both arc volcanism/plutonism and subduction of oceanic crust played a key role in Taconic orogenesis.…”

Thermal variation across collisional orogens: Insights from the hinterland–foreland transition zone of the Sardinian Variscan belt

“…Different factors control the tectonic architecture of orogenic wedges in collisional belts, such as the detachment zones, the efficiency of erosion or the temperature distribution (Agard, 2021; Avouac, 2003; Graveleau et al, 2012; Jamieson et al, 1996, 1998, 2002; Jamieson et al, 1998; Jamieson & Beaumont, 2013; Konstantinovskaia & Malavieille, 2005, 2011). Quantitative constraints about the thermal variation in space and time are necessary for understanding and modelling paleo‐wedges (Girault et al, 2020; Jaquet et al, 2018; Thigpen et al, 2021). However, except along a few favourable crustal sections (e.g., Beyssac et al, 2007; Grujic et al, 2020; Long et al, 2016), thermal estimations are often scattered and not continuously quantified across the chain, leaving large volumes of metasediments uninvestigated.…”

Implications of New Geological Mapping and U‐Pb Zircon Dating for the Barrovian Tectono‐Metamorphic Evolution of the Lepontine Dome (Central European Alps)

“…Heat is indeed advected and produced during nappe exhumation and overthrusting. Furthermore, heat can be transferred by diffusion to the colder units adjacent to the overthrusting nappe (e.g., England & Molnar, 1993;Huerta et al, 1998;Oxburgh & Turcotte, 1974;Thigpen et al, 2017), hence, it can generate metamorphic isograds that cross-cut nappe boundaries (e.g., Huerta et al, 1998;Thigpen et al, 2017Thigpen et al, , 2021. Therefore, the evidence of cross-cutting relationships between isograds and tectonic boundaries does not exclude a syn-tectonic origin of the Barrovian metamorphism.…”