Publication Date

Spring 2022

Degree Type


Degree Name

Master of Science (MS)




Robert Miller

Subject Areas



The North Cascades of Washington preserves a wide range of crustal levels in andadjacent to a Cretaceous magmatic arc. Field, petrographic, and detrital zircon geochronological research focused on a discontinuous, 55-km-long belt of metasedimentary rocks in the NW-striking Ross Lake fault zone (RLFZ) within the North Cascades. In the NW, metapelitic rocks of the Little Jack unit have an Early Jurassic maximum depositional age (MDA) and strong ductile fabrics in contrast to clastic rocks on the E with an Early Cretaceous MDA and primary structures, confirming the presence of the dextral Hozameen fault. To the SE, the oldest unit (~110 Ma), the greenschist-facies South Creek metaconglomerate, lays unconformably on Triassic Twisp Valley Schist (TVS) quartzite. Along strike, ≤ 88 Ma (MDA), fine-grained TVS meta-turbidites (phyllites) structurally overlie the quartzite unit along an unconformity or fault. Structurally higher mylonitic, greenschist-facies metavolcanic rocks may, in turn, be younger than the phyllites. Other metaclastic rocks between the core and Methow basin include the Cretaceous (~99 Ma MDA) Easy Pass unit and amphibolite-facies biotite schist (MDA ~90 Ma) in the Foggy Dew segment of the RLFZ. The units with ~100–88 Ma MDAs contrast with rocks in the Cascades core arc and postdate almost all clastic rocks of the Methow basin. The phyllites and metaconglomerates are likely tied to the core rocks and imply incorporation of backarc rocks into the Cretaceous arc.