NEW EVIDENCE FOR BACKARC BASIN INTERPRETATION FOR EOCENE COAST-RANGE TERRANE

The western margin of Oregon, Washington, and Vancouver Island are underlain by an 800 km-long Eocene ophiolite, called the Coast Range terrane. The terrane is often considered to be a collided seamount chain, but it lies inboard of a Late Jurassic igneous complex, called the Point of Arches, and is depositional underlain on its west side by continental-derived sediments, called the Blue Mountain unit. The seamount interpretation fails to account for these observations.

Surface exposures of the Coast Range terrane are mainly pillowed and sheeted basalts, with minor gabbros, but geophysical data indicate that the more eastward parts of the terrane are underlain by peridotite. Where exposed in the Olympic Mountains, the Blue Mountain unit consists of a stratally coherent sequence of sandstone and conglomerate, ranging from 0.5 to 2 km thick. In many places, we and many others have observed clear depositional contacts between the Blue Mountain and the overlying basalts of the Coast Range terrane. We collected 2 samples from the type locality at Blue Mountain and dated 167 detrital zircons by the U/Pb method using the UC Santa Cruz LA-ICP-MS. Both samples have well defined young peaks with ages of 50.0 and 54.7 Ma (six grains in each peak). Wells et al (2014) reported U/Pb ages for detrital zircons from another Blue Mountain sandstone (Buckhorn Mountain), which has a well-defined young peak of 47.9 Ma. The overlying basalts have 40Ar/39Ar ages that range from 56.0 to 45.4 Ma.

We envision that the basalts formed within an obliquely rifting backarc basin. The outboard forearc sliver was displaced northward to southern Alaska, creating the modern Cascadia subduction zone in its lee. Sediments in the Blue Mountain may have been derived from that outboard forearc sliver. The Point of Arches igneous complex could have been derived from the westernmost Klamaths, which we envision was also truncated during this same event.