THE TIMING OF ARC-CONTINENT COLLISION IN ALASKA FROM DETRITAL ZIRCON AGES IN THE CHUGACH ACCRETIONARY COMPLEX

Evidence for the timing of an arc-continent collision may be found in the accretionary complex typically associated with the arc. We determined U-Pb ages of detrital zircons from sandstones in the Mesozoic Chugach accretionary complex to better understand the tectonic history of the convergent margin. In our study area near Anchorage, accretion of the Potter Creek assemblage took place from 169 Ma to at least 156 Ma, based on maximum depositional ages from U-Pb dating of detrital zircons (these are older than reported radiolarian ages, e.g., Karl et al., 1979), and possibly until 125 Ma. Chert-argillite with metavolcanic rocks and sandstone in this unit represent deep-water oceanic sediment mixed with tuffaceous material derived from a Jurassic arc. This was followed by a period of tectonic erosion beginning at 125 Ma that was triggered by ridge subduction, and forearc trondhjemitic magmatism.

A major change at the margin occurred at ~100 Ma with an influx of massive graywacke and conglomerate. This switched the margin back into an accretionary mode. The conglomerate clasts include granitoid boulders with ages of 200 Ma and 180 Ma, consistent with the age of the Talkeetna arc (e.g., Rioux et al., 2010). 100 Ma is coincident in time with peak metamorphism during a major shortening event in the region (e.g., Rubin et al., 1990) and the resulting Coast orogen was likely the sedimentary source for this clastic influx. The source rocks are hundreds of km south of the depocenter, so either lateral sediment transport was prominent along the paleotrench, the forearc assemblage was transported laterally along younger strike-slip systems, or both. The presence of proximal-facies coarse clastic rocks, however, seemingly precludes long distance transport, favoring the strike-slip hypothesis.

The first evidence of North American continental sources in the accretionary complex comes in the form of Proterozoic zircons in the Late Cretaceous Valdez Group (max depo ages 90–72 Ma). The dominant population is at 1860 Ma. Sources with this age are found in the Yukon-Tanana composite terrane and their presence in the accretionary complex indicates the final closure of the ocean basin and breakdown of topographic barriers formed by the accreted arc terranes as rivers encroached into continental North America by the mid Late Cretaceous.