COEVAL PSEUDOTACHYLYTE AND MYLONITE IN THE GRIZZLY CREEK SHEAR ZONE, COLORADO: A RECORD OF LINKED COSEISMIC AND ASEISMIC PROCESSES IN A MID-CRUSTAL THRUST

The Proterozoic Grizzly Creek shear zone (GCSZ) at Glenwood Canyon, Colorado, presents an unusual opportunity to examine spatial, kinematic, and dynamic relationships between frictional and plastic deformation produced at coseismic and aseismic strain rates. Progressive uplift of mid- to lower-crustal fault zones can be expected to produce brittle faults overprinting plastic fabrics, such as frictionally melted, seismogenic pseudotachylyte (pst) hosted by older mylonite. This overprinting sequence can be interpreted to record continuing deformation during unroofing and cooling. More rarely, pst may be overprinted by mylonite or ultramylonite, as we have documented in the GCSZ. The sequence of brittle faulting followed by plastic deformation implies either burial and/or heating after seismic rupture or the interplay of brittle faulting and plastic creep within the complex brittle-plastic transition zone. We interpret the GCSZ as an example of the latter. The GCSZ separates supracrustal gneisses and megacrystic granitoids in the footwall (south block) from fine-grained, foliated granite in the hanging wall (north block) with top-to-S shear sense. The GCSZ is comprised of a 10–15-m-thick basal mylonite (foliation 255/44°NW) overlain by ~400 m of N-NW-dipping tectonites including protomylonitic to mylonitic gneiss (foliation 257/58°NW) and an upper fine-grained, tectonically foliated granite (foliation 257/49°NW). These rocks host pst fault veins (orientation 259/41°NW, n=281), mylonitized pst, and rare ultramylonite bands. Mylonitized pst and ultramylonite bands, some of which are clearly derived from pst as evidenced by truncated injection veins, are localized within the upper fine-grained granite and tend to form at dips ≤41°, the mean for all pst and approximate dip of the basal mylonite. We suggest that frictional melting produced cryptocrystalline and/or glassy, strain-softened layers that were preferentially susceptible to subsequent plastic flow. Coseismic ruptures recorded by pst throughout the shear zone were dynamically coupled to the aseismically creeping basal mylonite, and some suitably oriented pst veins in the homogeneous upper granite were overprinted by plastic flow as a result of coupled aseismic creep.