PALEOGENE BASIN EVOLUTION IN THE PACIFIC NORTHWEST: A WINDOW INTO RIDGE-TRENCH INTERACTION

Plate reconstructions require that a spreading ridge interacted with the North American margin during the Paleogene. However, the precise timing and location of this interaction remains uncertain, as the oceanic crust required to constrain the ridge’s position is now subducted. Based on anomalous forearc magmatism, some models place the ridge near present day Washington, thereby providing an attractive hypothesis from which to evaluate enigmatic features of the Paleogene North Cascades orogen; including the accretion of Siletzia, rapid exhumation of mid-crustal rocks, compositionally diverse magmatism in the arc and forearc, as well as abrupt changes in the regional stress field.

A record of Paleogene sediment accumulation, volcanism, and deformation is preserved in a series of sedimentary units in central and NW Washington that have been separated into discrete structural blocks by syn- to post early-Eocene motion on high-angle faults. The absence of diagnostic fossils and the use of relatively imprecise fission track and K-Ar geochronology have made stratigraphic correlations difficult, leading to contrasting correlation schemes and tectonic models. We present new U-Pb zircon CA-IDTIMS geochronology from tuffs and sandstones in the Teanaway structural block and the Chumstick basin that help clarify their relationship and their potential connection to ridge-trench interaction.

Detrital zircon data from the base of the Swauk Fm in the Teanaway structural block constrain the beginning of sediment accumulation to the latest Paleocene. Subsequent shortening resulted in an angular unconformity between the Swauk Fm and the overlying bimodal volcanic rocks of the Teanaway Fm and is bracketed to within 2 my. Shortening was immediately followed by extension, manifested as a voluminous dike swarm that fed basalt flows in the Teanaway Fm. Four new zircon dates from tuffs within the Chumstick basin demonstrate that sediment accumulation within this block began at approximately the same time as volcanism in the Teanaway Fm and was characterized by high rates (ca 5 m/kyr). These rapid changes in the regional strain (and presumably stress) field are consistent with ridge-trench interaction, and our new geochronology allows us to correlate them to events along the North American margin.