EVIDENCE FOR RAPID DEPOSITION, BURIAL, PLUTON EMPLACMENT AND METAMORPHISM OF THE CHUGACH-PRINCE WILLIAM TERRANE TURBIDITES IN EASTERN PRINCE WILLIAM SOUND, ALASKA

A full appreciation of rapid exhumation (mm/yr) of deep crustal rocks during mountain building is one of Lincoln Hollister's seminal contributions. The rapid rate of some geologic processes is demonstrated by the geologic history of the Chugach-Prince William terrane (CPW) in southern Alaska. The CPW is one of the thickest accretionary wedge complexes in the world and is primarily composed of deep-water turbidites with abundant quartzofeldspathic and volcanic-lithic sandstones and basaltic rocks that were intruded by near-trench plutons of the Sanak-Baranof belt. Detrital zircon U/Pb dates from turbidites of the CPW show that these rocks were derived from a Late Cretaceous to Paleocene arc complex formed on an exhumed Jurassic to Cretaceous metaplutonic basement, and appear to match well with the crystallization ages found in the Coast Plutonic Complex. The vast majority of maximum depositional ages (MDA's) from the CPW are 73-54 Ma, and the total volume of sedimentary rocks deposited during this time interval is on the order of 3,083,000 km³. This implies a deposition rate of 162,263 km³/Myr; roughly equivalent to the entire area of the Coast Plutonic Complex being exhumed at a rate of ~1 mm/yr for 19 Myr.

The thickest part of the CPW turbidites occurs in Prince William Sound (PWS) and are part of the Valdez and Orca Groups. In eastern PWS, MDA's determined for the Valdez and Orca Group turbidites show that most of these rocks were deposited from 62-51 Ma. This includes two samples from the Valdez Group collected along the Richardson Hwy that are typically assumed to be Late Cretaceous, but have MDA's of 61.5 and 59.8 Ma. Plutons of the Sanak-Baranof belt intrude rocks of the Orca Group in this area. The Sheep Bay pluton has a crystallization age of 54.8 ± 0.7 Ma, and is indistinguishable from the MDA of the rocks it intrudes (55.9 ± 6.4 Ma). Most workers agree that the metamorphic rocks of the Chugach Metamorphic Complex that occur along strike and immediately east of PWS are the metamorphic equivalents of the Valdez and Orca Groups. Numerous studies show that these rocks reached peak metamorphic conditions at mid-crustal levels by ~54 Ma. These data combined with our new detrital zircon U/Pb dates show that the rocks of the CPW were deposited, buried, intruded by plutons, and metamorphosed in a geologically short period of time (< 1 Myr).