NEWLY RECOGNIZED STRIKE-SLIP FAULTS, SHEAR ZONES AND ASSOCIATED METALLIFEROUS DEPOSITS IN THE TALKEETNA MOUNTAINS, ALASKA

New geologic mapping, kinematic measurements, and geophysical interpretation in the Talkeetna Mountains C–4, C–3, and B–4 quadrangles highlight several previously unmapped or poorly understood structures that provide new insight into the complex structural history of this seismically active region.

The earliest deformation recognized is a series of northeast-striking, southeast-dipping thrust faults inferred from anomalous thickness and repetition of the Paleozoic volcanic and sedimentary section. These faults apparently cut and thus post-date the Upper Triassic Nikolai Greenstone.

Wrangellia stratigraphy is juxtaposed against Jurassic plutonic and metamorphic rocks along a northeast-striking, multi-kilometer-wide zone of distributed shear. Previous workers mapped a southeast-side-up thrust fault and vertical suture zone in Tsisi Creek. Our mapping suggests a broad zone of southeast-side-up, high-angle ductile deformation; this fabric is cut by undeformed Upper Jurassic plutons.

Our work failed to find a previously mapped Talkeetna Thrust; instead, geophysical data indicate a series of high-angle structures that separate Wrangellia from the Kahiltna Assemblage (Fog Lakes graben southern boundary) and dissect Wrangellia and the Paleogene volcanic fields. These structures bound domains of contrasting geomorphology, and delineate sharp transitions in metamorphic grade. Where the structures are exposed, they have normal-oblique kinematic indicators consistent with evolution in a complex strike-slip tectonic setting.

Geochemical analyses portray a diverse belt of structurally-controlled mineralization associated with a prominent newly-identified northeast-striking fault system that bisects the map area. The kinematics of the Central Raingellia Fault (CRF) appear to have varied through time and location, suggesting a complex ductile system overprinted by brittle deformation which may have received pulses of metal-rich fluids. The CRF appears to be the most recently active of the faults in the mapping area. It cuts north-northwest-striking faults, which offset earlier structures. The quartz ± sulfide veins and breccias associated with the CRF have widely varying metal associations and mineralization styles, including Cu-Ag, Cu-Au-Mo, Mo, Au, Au-Bi-W-Te, and As-Sb.