FOLLOWING THE CRUMBS: NEW FIELD OBSERVATIONS, FAULT ROCKS, AND STRUCTURAL GEOLOGIC ANALYSES OF DISMEMBERED OPHIOLITE KLIPPEN IN THE ALASKA YUKON-TANANA UPLAND

The nature, kinematics, and timing of ophiolite contraction in the Alaska Yukon-Tanana Terrane (YTT) are poorly understood. Northern klippen outcrop along a ~1,000 km-long by 5 to 50 km-wide arcuate belt of discontinuous ultramafic and mafic rocks (UM/M). UM/Ms, associated greenstones, and oceanic sedimentary rocks are late Permian to Early Triassic age, were obducted onto supracrustal Devonian-Mississippian YTT basement, and are regionally dismembered. Documenting the physiochemical continuity of the UM/Ms, from regional to outcrop scales, provides new opportunities to refine existing hypotheses for YTT crustal evolution.

Analyses of foliations from existing maps confirms the presence of concentric domains associated with previously documented, extensional gneiss domes in lower plate basement. Upper plate UM/M foliations reflect long-wavelength, low-amplitude open folds, consistent with belt-perpendicular NW-N-NE directed shortening. Northern outcrops of UM/Ms show variably serpentinized greenschist facies, flat-lying to open-folded klippen. Outcrops of minor klippen ~300 km south of northern klippen show near isoclinally folded amphibolite facies Mg/Ca silicate schists and gneisses. These upper plate rocks likely reflect complex thrust geometries and crustal thickening.

New field observations of basal shear zones (BSZ) that bound the upper and lower plates include the Salcha ophiolite to the NW with a serpentinized BSZ showing complex NW-directed, shortening and discrete carbonate-silica-Cr mica (listwanite) hydrothermal alteration. In the Healy area to the south, BSZs show talc-rich schists and gneisses with meter-long anthophyllite crystals, indicative of H₂O-rich reactions without listwanite. These BSZs show knife-sharp, extreme strain localized, contact with basement and SW to NNW directed shortening.

If UM/M klippen are petrogenetically, chemically, and geochronologically the same, and their BSZs form a regional datum, they indicate a large thrust sheet system ~300 km wide. Spatially dispersed southern klippen may alternatively reflect previously hypothesized regional-scale upper crustal extension. In either case, if the klippen are cogenetic, they may reflect very large, horizontal, fluid assisted translations of poorly resolved timing and kinematics.