MAJOR CHANGES IN EARLY EOCENE FAULTING AND BASINS IN CENTRAL WASHINGTON

An excellent record of Paleogene sedimentation, magmatism, and deformation is preserved in the central and North Cascades of Washington. The region is cut by four major high-angle faults that strike NW (Ross Lake, Entiat, Leavenworth faults) and north (Straight Creek, Leavenworth faults). Ongoing U-Pb zircon geochronology has improved our understanding of the timing of tectonic events such that we can interpret major changes in faulting and basin evolution at the submillion year scale. New U-Pb dates from tuffs and lava flows in basins suggest that strike-slip faulting initiated or accelerated at ca 50 Ma, followed by a reorganization of faulting at ca 47-45 Ma. Here we consider one model to explain these events. At 50 Ma, the NW segments of the Leavenworth fault were active and the Entiat fault may have accelerated its slip to form the early stage of the Chumstick pull apart basin characterized by rapid sediment accumulation rates of ~5 km/my. Strike-slip faulting and the formation of a pull apart basin was synchronous with bimodal volcanism and a mafic dike swarm as part of the Teanaway Formation. The orientation and location of the dike swarm is compatible with NW dextral shear west of the southern Leavenworth fault. The initiation of the pull apart basin is also synchronous with the start of a major episode of magmatism and ductile deformation in the core of the North Cascades. At ca 47 – 45 Ma, fold and fault patterns in the basin suggest the initiation of N-S oriented dextral-slip faults, and continued faulting on the NW faults: the Straight Creek fault initiated; north-striking Leavenworth fault segments localized strike-slip while NW segments became transpressive zones; the Entiat and Eagle Creek faults formed a narrow subbasin of the upper Chumstick Formation and the main western subbasin inverted and was folded. The Straight Creek – Leavenworth faulting caused subsidence that accommodated the Roslyn Formation on top of the Teanaway lavas and created folds compatible with N-S dextral slip. The 47 – 45 Ma fault reorganization was accompanied by the final magmatism and ductile deformation in the core of the North Cascades. We speculate that a newly formed spreading ridge – trench interaction at 50 Ma off southern Vancouver Island initiated the major NW strike-slip faulting episode and the change to N-S directed faulting at 47-45 Ma.