The Deep Structure and Rheology of a Plate Boundary-Scale Shear Zone: Constraints from an Exhumed Caledonian Shear Zone, NW Scotland

Abstract

Below the seismogenic zone, faults are expressed as zones of distributed ductile strain in which minerals deform chiefly by crystal plastic and diffusional processes. We present a case study from the Caledonian frontal thrust system in northwest Scotland to better constrain the geometry, internal structure, and rheology of a major zone of reverse-sense shear below the brittle-to-ductile transition (BDT). Rocks now exposed at the surface preserve a range of shear zone conditions reflecting progressive exhumation of the shear zone during deformation. Field-based measurements of structural distance normal to the Moine Thrust Zone, which marks the approximate base of the shear zone, together with microstructural observations of active slip systems and the mechanisms of deformation and recrystallization in quartz, are paired with quantitative estimates of differential stress, deformation temperature, and pressure. These are used to reconstruct the internal structure and geometry of the Scandian shear zone from ~10 to 20 km depth. We document a shear zone that localizes upwards from a thickness of >2.5 km to <200 m with temperature ranging from ~450–350°C and differential stress from 15–225 MPa. We use estimates of deformation conditions in conjunction with independently calculated strain rates to compare between experimentally derived constitutive relationships and conditions observed in naturally-deformed rocks. Lastly, pressure and converted shear stress are used to construct a crustal strength profile through this contractional orogen. We calculate a peak shear stress of ~130 MPa in the shallowest rocks which were deformed at the BDT, decreasing to <10 MPa at depths of ~20 km. Our results are broadly consistent with previous studies which find that the BDT is the strongest region of the crust.

Natural and Experimental Constraints on a Flow Law for Dislocation‐Dominated Creep in Wet Quartz

“…They attributed the difference in the stress exponent to the activation of different rate‐limiting slip systems in response to changes in temperature and stress: experiments conducted at >900°C and low stresses fit the higher exponent, and those conducted at lower temperature and higher stresses fit the lower exponent. Data from Lusk and Platt (2020) used in this study, however, record c ‐axis CPOs indicative of slip on the basal <a>, ±rhomb <a>, and prism <a> systems, and a combination thereof, yet all samples plot closely along a line with a constant slope (Figure 6a). This suggests that the difference in n is not simply related to the active slip systems.…”

“…10.1029/2020JB021302 temperature and higher stresses fit the lower exponent. Data from Lusk and Platt (2020) used in this study, however, record c-axis CPOs indicative of slip on the basal <a>, ±rhomb <a>, and prism <a> systems, and a combination thereof, yet all samples plot closely along a line with a constant slope (Figure 6a). This suggests that the difference in n is not simply related to the active slip systems.…”

“…Figure 2. Sample photomicrographs representative of microstructures presented inLusk and Platt (2020) illustrating the range of dislocation creep recrystallization mechanisms and similarity to microstructures produced in experimentally deformed samples (e.g.,Gleason & Tullis, 1995, their Figures 4 and 7). (a) Recrystallization chiefly by bulge nucleation and minor subgrain rotation, roughly equivalent to Regimes 1 and 2 ofHirth and Tullis (1992).…”

Variations in the P‐T‐t of Deformation in a Crustal‐Scale Shear Zone in Metagranite

“…Exhumation was accommodated by a kilometer-scale ductile shear zone, in which progressive localization with decreasing P-T allowed the preservation of older, relict textures further into the footwall, as subsequent deformation at lower grades overprinted an increasingly narrow zone (Figure 12). This phenomenon has been well documented in both normal-and thrust-sense shear zones that experienced exhumation during deformation (e.g., Behr & Platt, 2011;Cooper et al, 2017;Haertel, 2012;Handy et al, 2007;Lusk & Platt, 2020).…”

“…These models are compared to the measured Si in phengite and Ti in quartz that, based on their microstructure, chemistry, and textural relations, appear to have been re‐equilibrated together during deformation. This approach has recently been used to estimate deformation conditions in metapsammites from the Moine Thrust in Scotland (Lusk & Platt, 2020). It benefits from using minerals typically present in rocks of granitic or similar compositions, and which are commonly involved in deformation over a wide P‐T range and thus have the potential to re‐equilibrate to reflect deformation conditions (Ashley et al., 2014; Bestmann & Pennacchioni, 2015; Grujic et al., 2011; Nachlas et al., 2018; Santamariá‐López et al., 2019).…”