Abstract
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.
Highlights
Contractional fault systems that cut the crust and upper mantle are necessary consequences of plate convergence and continental collision [1]
Our calculated TitaniQ temperatures are generally lower than temperatures estimated by quartz recrystallization mechanism; recrystallization by bulge nucleation (BLG), subgrain rotation (SGR), and GBM are suggested to occur within temperature ranges of ~300-400°C, 400-525°C, and >525°C, respectively [55, 98]
This study presents a reconstruction of the geometry, internal structure, and rheology of a plate boundary-scale shear zone from ~10–20 km depth
Summary
Contractional fault systems that cut the crust and upper mantle are necessary consequences of plate convergence and continental collision [1]. In plate boundary-scale settings, these shear zones may be upwards of 20 km wide (depending on fault regime and geothermal gradient) for the quartz-rich crystalline continental crust [3,4,5,6,7,8] and even wider in the upper mantle [9]. A 1-2 km thick zone of mylonites, derived from midcrustal depths, has been described along the Alpine Fault in New Zealand, suggesting a mid-crustal shear zone of at least these dimensions [14]. These two systems exemplify the significant uncertainty concerning the deep structure and geometry of plate boundary-scale fault systems
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