PROVENANCE AND THERMAL HISTORY OF THE PALEOGENE ORCA GROUP, EASTERN PRINCE WILLIAM SOUND, ALASKA, USING U-PB AND FISSION TRACK DATING OF DETRITAL ZIRCON

The Upper Cretaceous to Early Tertiary Chugach-Prince William terrane is exposed for approximately 2,200 km on the southern Alaskan continental margin, and constitutes one of the largest known subduction-related accretionary complexes in the world. In this study we present 12 new maximum depositional ages (MDA) from the flysch of the Orca Group in eastern Prince William Sound, Alaska. MDAs of the Orca Group range from 61 Ma to 51 Ma. Generally, the Orca Group samples young from inboard to outboard with the youngest ages on Hinchinbrook Island. One sample from the mapped Valdez Group collected along the Richardson Highway north of Thompson Pass yields an MDA of 62 Ma; this result is significant, because it implies much of the mapped Valdez group in eastern PWS might be Paleocene in age. All samples show three significant Phanerozoic grain-age populations including a dominant Late Cretaceous – Paleocene peak, a Jurassic peak, and a Devonian – Carboniferous peak indicating a source region composed of a Late Cretaceous to Paleocene arc built on top of a basement composed of Mesozoic and Paleozoic meta-plutonic rocks. Precambrian grains exhibit Archean and Paleoproterozoic populations, which indicate a predominately Northern Laurentian source for these grains. Some samples also have significant populations of Mesoproterozoic age.

Zircon fission track data show that most of the rocks in eastern PWS have cooling ages younger than the depositional age, and that heterogeneous annealing resulted in multiple populations of grain ages. Only the young population can be interpreted with confidence because partial resetting of older populations cannot be ruled out. There appears to be a significant thermal discontinuity across the Rude River fault where rocks to the east (or SE) have a greater proportion of young grain ages at ~30-32 Ma. These data indicate dip slip (east side up) motion that accompanied dextral strike slip.