ORIGINS AND RELATIONSHIPS OF UNITS IN AND ADJACENT TO THE ROSS LAKE FAULT ZONE, NORTH CASCADES, WASHINGTON: IMPLICATIONS FOR BURIAL OF META-SUPRACRUSTAL ROCKS INTO THE DEEP LEVELS OF ARCS

The Paleogene Ross Lake fault zone separates amphibolite-facies rocks of the North Cascades core, including deeply buried (~25-35 km) metasupracrustal rocks, from a panel of lower-grade rocks on the NE, and non-metamorphosed strata of the Jurassic-Cretaceous Methow basin farther east. Examination of the transition across the fault zone gives insights into transfer of sediments into deep levels of this arc. Detailed mapping of several areas was thus conducted across the fault zone. In two transects across the northern study area, medium-grade Little Jack metapelites appear to grade eastward into the Lower Cretaceous Jackass Mountain Group of the Methow basin, implying that the two units are in depositional contact. Initial detrital zircon studies, however, show that the Little Jack unit has similar signatures to the oceanic Napeequa unit (Twisp Valley schist) of the metamorphic core and is different than the Jackass Mountain Group. To the south, medium-grade meta-conglomerate, siltstone, and sandstone of the Easy Pass unit have protoliths similar to rocks in the Cretaceous Virginia Ridge Formation of the Methow basin. Farther south, the oceanic Twisp Valley schist of the Cascades core is overlain unconformably to the NE by the South Creek metaconglomerate and, in turn, by sandstone that is probably part of the Cretaceous Winthrop Formation of the Methow basin, as proposed by Dragovich et al. (1997). In the southernmost area, metamorphosed fine-grained turbitities sit on Twisp Valley metachert, and are overlain by greenschist-facies, metavolcaniclastic rocks. The latter rocks, which are mylonitic in the Ross Lake fault zone, may be correlative to volcanic rocks of the Midnight Peak Formation in the Methow basin. These various ties of Cascades core rocks to the Methow strata support the interpretation that rocks from the back-arc were underthrust into the mid- to deep crust of the North Cascades arc.