TESTING TERRANE TRANSPORT ALONG TRANSPRESSIVE CONVERGENT MARGINS: HIGH-PRECISION U-PB GEOCHRONOLOGY, GEOLOGIC MAPPING, AND STRATIGRAPHIC ANALYSIS OF THE YAKUTAT AND SILETZIA TERRANES

The Northwestern Cordillera records a history of terrane accretion and regional strike-slip faulting since the Mesozoic. Currently, the Yakutat terrane is colliding with southeastern Alaska and sits outboard of the Queen Charlotte-Fairweather fault. One hypothesis for origin of this terrane is that it represents an oceanic plateau fragment that formed coeval with Siletzia offshore of Washington and Oregon. Such an origin requires that the Yakutat terrane was transported ~1500 km northward along strike-slip faults following initial accretion at 50 Ma.

Eocene—Oligocene sedimentary strata overlie both the Yakutat and Siletzia terranes with chronostratigraphies previously defined by limited geologic mapping, U-Pb zircon geochronology, and biostratigraphy. On the Yakutat terrane, the oldest stratigraphy is exposed in the Samovar Hills of the St. Elias Mountains. These units are the Hubbs Creek Volcanics, thought to represent the oceanic plateau basement, and overlying Oily Lake Siltstone (OLS) and Kulthieth Fm. New mapping of the OLS suggests that it includes two distinct lithofacies: a sequence of grey indurated marine siltstones and tuffs deposited in deep-water overlain by a tidal to deltaic sequence. A red paleosol at the top of this sequence is separated from overlying basal conglomerate of the Kulthieth Fm. by an angular unconformity.

Preliminary high-precision U-Pb zircon geochronology of the Hubbs Creek Volcanics indicate they are age-equivalent to the oldest part of Siletzia. The OLS represents pre-accretion deposits on the Yakutat plateau, similar to the Aldwell Fm., which overlies northernmost Siletzia. An upwards shallowing sequence and paleosol development indicates a period of subaerial exposure during deposition of the OLS. Collision with the continental margin resulted in angular unconformity development and subsequent deposition of the Kulthieth Fm., similar to the Aldwell and conglomeratic Lyre Fm. sequence on the Siletzia plateau. This data suggests that Siletzia and Yakutat may have a shared early history. Ongoing detrital zircon analyses (CA-LA-ICP-MS) in the Kulthieth Fm. will allow us to constrain the changing position of the Yakutat terrane in relation to exposed source terranes within the Coast Mountains Batholith, further testing translation models of the Yakutat terrane.