BEDROCK EXPOSURES OF THE TINTINA FAULT ZONE AT EAGLE, ALASKA: EXOTIC JUXTAPOSITIONS, CARBON-RICH FAULT ROCKS, AND EVIDENCE FOR TRANSPRESSIONAL AND TRANSTENSIONAL DAMAGE

Rare bedrock exposures of the Tintina fault zone and associated fault rocks at Eagle Bluff tell a complex story of entrainment of disparate lithologies, fluid flow, and brittle transpressional and transtensional deformation. Complex juxtapositions of Proterozoic metagreenstones, lower Paleozoic clastic and carbonate rocks, and Cretaceous-Tertiary clastic rocks were mapped in detail. There is at least one principal slip zone that is a few 10s of meters wide surrounded by an asymmetric zone of possibly lithologically controlled, distributed damage at least 200 m wide. In this zone, the greenstones show evidence of macroscopically ductile deformation overprinted by brittle deformation and pervasive chloritization. Intercalated carbonate infused rocks with shows of Cr mica indicative of listwanitization show pervasive macro- to micro-veining indicative of extensive fluid infiltration. This rock package is juxtaposed against Devonian-Silurian carbonaceous shales, sandstones, and conglomerates. Although a single principal slip surface is not exposed, limestone pseudomylonites are present within a few meters of the primary juxtaposition. Associated metallic mineralization consists of chalcopyrite veins and clots, malachite stains on a variety of surfaces, and extensive brittle slip surface coatings of hematite and quartz.

Rock fabric and kinematic data were collected from bedrock outside and within the fault zone. Country rock layering strikes in multiple directions and dips moderately, but foliations within the fault zone generally strike WNW-ESE and dip steeply. Striated brittle slip surfaces occur as complex arrays with predominantly NW-SE strikes and steep to moderate dips. Although kinematically complex, reverse, dextral and sinistral strike-slip surfaces are reactivated and overprinted by extensional-slip surfaces. Preliminary kinematic analyses of the relatively small dataset show SW-NE, subhorizontal shortening for each mode of slip, consistent with transpression followed by transtension, based on the overprinting slip vector relations. Interestingly, this kinematic configuration is independently consistent with evidence for late transtension along the Tintina fault ~80 km NW of Eagle reported in previous research.