NEW GEOLOGIC MAPPING AND U-PB ZIRCON GEOCHRONOLOGY AT THE SOUTHERN END OF THE ROSS LAKE FAULT ZONE IN THE NORTH CASCADES OF WASHINGTON, USA

The >200 km long Ross Lake fault zone (RLFZ) marks the eastern boundary of exhumed mid-crustal rocks in the North Cascades crystalline core and juxtaposes these rocks with the Mesozoic sedimentary strata of the Methow basin. The RLFZ is steeply dipping and, in most locations, it preserves sub-horizonal lineation. For this reason, it has long been considered to represent a strike-slip fault with significant >100 km dextral displacement during the Mesozoic and Paleogene. At the southern end of the RLFZ a wide zone of mylonite is intruded and sealed by the Eocene Cooper Mountain batholith. U-Pb zircon and K-Ar hornblende dates from the mylonite zone suggest that deformation occurred as late as 55-50 Ma (Miller and Bowring, 1990). Existing U-Pb zircon geochronology from the Cooper Mountain batholith indicates that it was constructed between 49 and 47 Ma. Thus, this area offers a unique opportunity to tightly constrain the timing of last motion on the southern RLFZ and to better understand how magmatism interacts with major shear zones. To address these issues we present new geologic mapping and U-Pb zircon geochronology from the area where the RLFZ is intruded by the Cooper Mountain batholith and discuss how this data has a bearing on the tectonic evolution of the North Cascades and our understanding of the conditions necessary for magma to seal a structure rather than facilitate its motion.