Abstract The Grizzly Creek shear zone is a newly discovered Proterozoic structure coincident with the southern margin of the Laramide White River uplift in west-central Colorado. The shear zone includes a 10-15 m basal mylonite oriented 255/44°NW that truncates foliation and magmatic fabrics in the footwall. The mylonite is overlain by >400 m of moderately north-dipping tectonites dominated by protomylonitic to mylonitic gneiss intercalated with mainly concordant pseudotachylyte veins and cmscale ultramylonite bands. At the upper boundary of the shear zone, the high-strain tectonic fabrics grade into relatively undeformed rocks of the hanging wall. The shear zone separates supracrustal gneisses and coarse-to-megacrystic granitoids in the footwall (south block) from fine-grained, foliated granite and supracrustal gneisses in the hanging wall (north block). Mutually overprinting brittle and plastic fault rocks, including cataclasites, pseudotachylyte fault veins, mylonitized pseudotachylyte, and ultramylonite bands, record cyclic deformation by both seismogenic rupture and plastic creep within the shear zone. Mineral lineations on mylonitic foliation surfaces plunge N-NE, and a variety of microscopic and mesoscopic shear-sense indicators indicate top-to-south reverse displacement. The Grizzly Creek shear zone is truncated by the Cambrian-Precambrian nonconformity, which is underlain by a 1-2-m-thick paleoregolith containing altered pseudotachylyte. We interpret the shear zone to record cyclic seismogenic faulting and aseismic plastic flow at the brittle-plastic transition during Proterozoic ~N-S shortening. The age, kinematics, and tectonic style of the Grizzly Creek shear zone are consistent with the 1.4 Ga Colorado mineral belt shear zone system; a NE-trending intracontinental deformation zone that formed under a regional strain field involving N-S shortening, E-W extension, and complex transpression on subvertical shear zones 55 km to the southeast. The Grizzly Creek shear zone was brittley reactivated as a Laramide, south-vergent reverse fault with a displacement of > 200 m. We suggest that the Proterozoic Grizzly Creek shear zone represents a persistent zone of weakness in the lithosphere that significantly impacted the Cenozoic structural evolution and present geomorphology of the region. Keywords: pseudotachylyte, ultramylonite, brittle-plastic deformation, tectonic inheritance, White River uplift, Glenwood Canyon.

The Eastern Klamath and Northern Sierra terranes of northern California consist of Devonian to Jurassic arc-related rocks that structurally and/or stratigraphically overlie Devonian(?) or older complexes that consist of quartzite, quartzofeldspathic sandstone, chert, and mafic and ultramafic rocks. These terranes lie within a regional belt of Paleozoic arc-related rocks that can be recognized from the Sierra Nevada to British Columbia. The Eastern Klamath and Northern Sierra terranes are geographically separate and have paleontologic linkage, but lack direct stratigraphic ties in strata of Late Devonian to Early Permian age. This chapter describes new stratigraphic correlations and structural interpretations of rocks that lie in the northernmost part of the Sierra Nevada. These rocks include the Butt Valley block of the Northern Sierra terrane, and rocks herein interpreted as dismembered Eastern Klamath terrane. The rocks of Eastern Klamath terrane affinity, Soda Ravine block, occur as a tectonic sliver that lies to the west of the Butt Valley block. The Soda Ravine block is about 2 to 4 km by 20 km and includes limestone lenses equivalent to zone A of the McCloud Limestone of the Eastern Klamath terrane, and upper Middle and Upper Triassic limestone, slate, siltstone, and pebble conglomerate. Similar Permian limestone with McCloud faunal affinities and slate-bearing slivers have been noted by previous workers in this area and to the south. The Butt Valley block had previously been interpreted as part of the west limb of a regional anticline, the Almanor anticline. New mapping suggests that the block is not contiguous with rocks that lie to the east. The Butt Valley block includes the Devonian and Mississippian Taylor Formation, which is unconformably overlain by an Upper Triassic (Carnian and Norian) basal conglomerate and sandstone, which in turn is overlain by Upper Triassic limestone. The Upper Triassic limestone is overlain by a sparsely fossiliferous, lithic-volcaniclastic sequence containing poorly preserved Triassic or Early Jurassic, and probable Early Jurassic, ammonites and clams. This sequence is similar to conglomerate, Triassic limestone, and volcaniclastic rocks of the Jurassic Sailor Canyon Formation, which overlie a regional Late Triassic unconformity west of Lake Tahoe, thus expanding the known distribution of the unconformity in the Sierra Nevada. The Triassic unconformity overlies Carboniferous and older rocks around the North Fork American River area to the south, and overlies Devonian and older rocks on the Butt Valley block, but overlies Permian rocks east and southeast of the Butt Valley block. These relations suggest that either the Butt Valley block was part of a highland or uplifted block along the western part of the northern Sierra terrane or that the Butt Valley block previously lay closer to the North Fork American River area and has been translated northward by right-slip displacement. Northern Sierra terrane east of the Butt Valley block consists of the Hough and Genesse blocks, which are separated by the Grizzly Mountain fault zone. The Grizzly Mountain fault zone is here interpreted to be a transpressional right-slip fault.

Abstract 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 256°/51°NW 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.