Abstract

Previous studies in the Yakataga fold-thrust belt of the St. Elias orogen in southern Alaska have demonstrated high exhumation rates associated with alpine glaciation; however, these studies were conducted with only a rudimentary treatment of the actual structures responsible for the deformation that produced long-term uplift. We present results of detailed geologic mapping in two corridors across the onshore fold-thrust system: the Duktoth River transect just west of Cape Yakataga and the Icy Bay transect in the Mount St. Elias region. In the Duktoth transect, we recognize older, approximately east-west–trending structures that are overprinted by open, northwest-trending fold systems, which we correlate to a system of northeast-trending, out-of-sequence, probably active thrusts. These younger structures overprint a fold-thrust stack that is characterized by variable structural complexity related to detachment folding along coal-bearing horizons and duplexing within Eocene strata. In the Icy Bay transect, we recognize a similar structural style, but a different kinematic history that is constrained by an angular unconformity at the base of the syntectonic Yakataga Formation. At high structural levels, near the suture, structures show a consistent northwest trend, but fold-thrust systems rotate to east-west to northeast trends in successively younger structures within the Yakataga Formation. We present balanced cross sections for each of these transects where we project the top of basement from offshore seismic data and assume a subsurface structure with duplex systems similar to, but simplified from, structures observed in the onshore transects. These sections can account for 150–200 km of shortening within the fold-thrust system, which is <33% of the likely convergence based on the subsurface geometry of the subducted Yakutat terrane lithosphere. This mismatch with known convergence is the result of loss of the earliest thrust belt structures by erosion and recycling into the orogen, sediment subduction, and three-dimensional (3D) motions that move mass through the cross section. Based on order of magnitude estimates and regional geophysical studies, we suggest that sediment subduction has been significant and probably accounts for previously recognized low Vp/Vs (compressional to shear wave velocity) ratios in the mantle wedge above subducting Yakutat lithosphere.

Our section restorations also provide a simple explanation for the observed elongate bullseye pattern of low-temperature cooling ages in the thrust belt as a consequence of exhumation above the growing duplex and/or antiformal stack. Comparison with analog model studies suggests that structural feedbacks between erosion and development of décollement horizons in coal-bearing strata led to this structural style. Although previous studies based on thermochronology suggested an active backthrust at the northern edge of the thrust belt, section restorations indicate that a backthrust is allowable but not required by available data.

The Yakataga fold-thrust belt has been treated as a dominantly 2D system, yet our work indicates that 3D processes are prominent. In the Duktoth transect, we interpret a group of northeast-trending thrusts as younger, out-of-sequence structures formed in response to the rapid destruction of the orogenic wedge by glacial erosion and deposition immediately offshore. We infer that these northeast-trending thrusts transfer slip downdip into a duplex system that forms the antiformal stack modeled in cross-section restorations, and we infer that these structures represent thrusting stepping back from the active thrust front attempting to rebuild an orogenic wedge that is being destroyed as rapidly as, or more rapidly than, it is being rebuilt. In the Icy Bay transect, we use the relative chronology provided by an angular unconformity beneath the syntectonic Yakataga Formation to infer that early, northwest-trending fold-thrust systems were formed along the Fairweather transform as transpressional structures. Continued strike slip carried these structures into the tectonic corner between the Fairweather and Yakataga segments of the orogen, producing a counterclockwise rotation of the shortening axis until the rocks reached their present position.

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