Seismogenic structure of a crystalline thrust fault: fabric anisotropy and coeval pseudotachylyte–mylonitic pseudotachylyte in the Grizzly Creek Shear Zone, Colorado

Field and microstructural observations from the Proterozoic Grizzly Creek Shear Zone suggest that crustal-scale fabric anisotropy exerted a significant control on earthquake rupture propagation during deformation at mid-crustal depths. The shear zone developed in amphibolite-facies supracrustal gneisses and granitoids, and consists of a 0.4–0.7 km-wide zone of high-strain rocks with foliation transposed to 2568/518NW and top-to-the-south kinematics. The shear zone is overprinted by hundreds of veins of pseudotachylyte, mylonitic pseudotachylyte and ultramylonite. Field observations and whole-rock geochemical data suggest that pseudotachylyte fault veins formed as a result of first-generation rupture through intact rock. Pseudotachylytes are preferentially localized in as many as nine decametre-scale rupture zones dispersed across the width of the shear zone, concordant to foliation. We present a conceptual model for the asymmetric development of anisotropic fabric in a thrust-related fault zone in crystalline metamorphic rocks. Progressive tectonic exhumation of hanging wall rocks during thrusting results in the development of a crustal-scale anisotropic fabric that provides a preferentially weakened zone that could accommodate the propagation of earthquake ruptures from the seismogenic zone into the middle crust. Pre-existing, crustal-scale structures are known to guide tectonic inheritance on continental scales and serve as a focus for seismicity (e.g. Sykes 1978; Thomas 2006; Albaric et al. 2009). Within felsic continental crust, metamorphic fabrics in basement rocks that formed in the lower and middle crust appear to be a significant factor controlling brittle fault development when the rocks are exhumed and reactivated in the upper crust (Beacom et al. 2001; Watts et al. 2007; Butler et al. 2008). As fabric anisotropy in basement rocks can influence faulting, it follows that anisotropy may also serve as a focus for earthquake rupture propagation, either as a fault develops or during brittle reactivation. The continental fault-zone model of Sibson (1977) accounts for the development of pseudotachylyte by seismogenic frictional melting within and above the brittle–plastic transition (Fig. 1); in the former setting, it may be synkinematic with mylonite (Sibson 1980). In the plastic regime, mylonites result from crystal-plastic processes leading to viscous flow at much lower, aseismic strain rates (Sibson 1977). The presence of mutually overprinting pseudotachylyte and mylonite therefore indicates a deformational environment in which seismogenic rupture and aseismic creep occurred cyclically. This suggests strain rates must have fluctuated across at least 10 orders of magnitude (c. 10–10 s), dependent upon the thickness and rate of the aseismically creeping layer. Unequivocal preservation of this extreme contrast in strain rates is uncommon and has only been documented in a handful of locations around the world (see Sibson & Toy 2006, and more recent examples documented by Shaw & Allen 2007; Chattopadhyay et al. 2008; Moecher & Steltenpohl 2009). Sibson’s fault-zone model and subsequent modifications (Sibson 1986; Scholz 1988, 2002) predict that deformation zones should widen with depth. Seismically active faults may be rooted within broad mylonite–ultramylonite zones in the lower crust in order to accommodate high strain rates in the upper crust (Whitmeyer & Simpson 2003). Earthquake ruptures are interpreted from seismological data and modelling to occur below the seismogenic regime from the downward propagation of large ruptures (Das & Scholz 1983; Strehlau 1986), consistent with observations of coeval pseudotachylyte and mylonite in exhumed faults (e.g. Lin et al. 2005). Such occurrences offer an opportunity to examine the internal seismogenic structure of a fault zone and evaluate whether mid-crustal earthquakes are randomly dispersed or systematically distributed. Most studies of coeval mylonites and pseudotachylytes have focused on geochemical and From: Fagereng, A., Toy, V. G. & Rowland, J. V. (eds) Geology of the Earthquake Source: A Volume in Honour of Rick Sibson. Geological Society, London, Special Publications, 359, 135–151. DOI: 10.1144/SP359.8 0305-8719/11/$15.00 # The Geological Society of London 2011. microstructural relationships; comparatively fewer studies have aimed to document these fault-rock associations at macroscopic scales (e.g. Passchier 1982a; Camacho et al. 1995; Lin et al. 2005; Shaw & Allen 2007). In this paper, we examine the relationship between pseudotachylyte zones, mylonitic pseudotachylyte and the crustal-scale structure of the newly described Grizzly Creek shear zone, which is preserved in amphibolite-facies supracrustal rocks in the Palaeoproterozoic basement of the southern Rocky Mountains. We document the geometry and kinematics of the system, and discuss the significance of the shear zone to understanding tectonic fabric controls on seismicity in the middle crust.