Valdez Creek shear zone

Composite lineation in the upper portions of the south-vergent Valdez Creek shear zone

The Clearwater Mountains in the central Alaska Range (Fig. 1) expose the tectonic boundary between the preexisting North American margin with the accreted Wrangellia Composite Terrane (WCT). The region hosts a large orogenic lode gold district in Alaska, which has been actively mined for more than a century. Despite the large placer endowment, little is known about the origin(s) or even existence of viable lode source(s), and even less is known about how the complex structural evolution relates to the orogenic gold lode system or the accretion of the WCT with ancestral North America. Establishing modern field constraints and interpretations will aid in solving numerous outstanding issues in the Geologic community by: 1) obtaining structural information pertaining to the collision of the WCT; 2) establishing timing/geochemical controls on both shear zone activity and the currently mined placer gold system; and 3) collecting and interpreting brittle fault and fracture data, which will pertain to the development of the Black Creek fault and ongoing deformation along the proximal and seismically active Denali Fault. The Clearwater Mountains are accessible by road and ATV trails and thus offers a unique opportunity to safely introduce a geology student(s) into modern geologic problems in a relatively understudied region of the northern Cordillera.

            The northwestern Clearwater Mountains is mostly composed of a single pelitic bulk-rock composition, but metamorphic grade varies from lower greenschist- to upper amphibolite-facies. The entire metamorphic gradient is exposed, and inverted across the Valdez Creek shear zone (VCsz), which preserves thrust-sense north-side up relative motion. Both hanging and footwalls are intruded by texturally diverse late Cretaceous plutonic rocks. However, despite the compositional homogeneity across the VCsz, there is a significant crustal break hidden within the mylonitized lithologies that define the suture between ancestral North America with Wrangellia within the Alaska Range suture zone (Ridgway et al., 2003). Paired with the interesting and dynamic geologic framework of the region, the Valdez Creek catchment is one of the most productive gold-placer systems Alaska, having been actively mined since 1903, adding an economic component to the project.

Discrete thrust faults at the base of the Valdez Creek shear zone, near the transition from phyllite to slate
Cobalt the geologizing dog investigating rigid Cretaceous plutons variably overprinted by the Valdez Creek shear zone