Kinematic Analysis of Ductile Features within the Northern Wenatchee Block, North Cascades, Washington

The Cascade crystalline core of Washington State is a block of high grade metamorphic and plutonic rocks at the southeast end of the Cretaceous-Tertiary Coast Plutonic Complex. Previous workers have debated whether northwest translation or southwest-vergent thrusting was responsible for regional metamorphism and deformation. The present study finds structural evidence supporting both orogen-normal and orogen-parallel displacements, thus indicating that the regional deformation can be broadly characterized as transpressional. Units of the study area include: 1) the Napeequa schist, derived from an ocean floor protolith, and 2) an accreted Triassic arc, which includes the Marblemount Meta Quartz-Diorite (MMQD) and metamorphosed volcanic and sedimentary rocks of the Cascade River Unit (CRU). Two plutonic bodies, the Jordan and Cyclone Lakes, intruded the accreted terranes and display limited tectonic deformation. Previous workers mapped two imbricate slivers of mylonitic MMQD in the footwall of the Razorback thrust, which emplaced MMQD over the Napeequa unit. Thin section analysis of new samples indicates both a clastic and plutonic protolith for these imbricate slivers, and thus the slivers represent both the CRU and mylonitic MMQD. Mapping and structural analysis adjacent to and along the Cretaceous Razorback thrust zone leads to grouping of deformational events into three distinct phases (D1, D2 and D3). The D1 events took place during peak metamorphism and are preserved in the MMQD, within hinges of folds in the CRU and Napeequa unit, and within garnet porphyroblasts of the Napeequa unit. Textures and shear sense indicate that both orogen-normal and orogen-parallel displacements were active during this time. D2 events formed and/or reoriented the predominant foliations of the study area by southwest-vergent thrusting along the Razorback thrust and northwest-trending upright folds north of the thrust; these events indicate orogen-normal contraction during D2. Thrusting imbricated the CRU with the Napeequa and both units were thrust beneath the MMQD. North of the thrust zone, the timing of orogen-parallel strike-slip deformation relative to D2 events is uncertain; if active during D2 it suggests deformation was transpressive during the latter stages of D2. D3 features are concentrated between the Razorback thrust and northern margin of the Jordan Lakes pluton. D3 events record orogen-parallel folding of D2 fabric and sinistral shearing; folding and shearing are north and north-northwest-vergent. Newly and previously reported radiometric ages constrain the timing of deformational and metamorphic events. A previously reported 94 (+- 2.5) Ma K-Ar cooling age of muscovite in the MMQD provides a younger age limit to peak metamorphism and D1 events. A new 81 Ma U-Pb zircon age in the Cyclone Lakes pluton with a sub-solidus foliation concordant to D2 thrust fabric provides a possible lower age limit of D2 thrusting. New Ar-Ar cooling ages of biotite and muscovite are 72,60,59 and 58 Ma; these ages and the previously reported Jordan Lakes age (74 Ma, U-Pb) bracket D3 events as post-74 Ma to 58 Ma. These findings indicate that southwest-vergent thrusting postdates peak metamorphism and is overprinted by orogen-parallel deformation. Ages and deformational phases reported in this study have both similarities and differences with other studies throughout the crystalline core. Collectively these studies indicate widespread similarities in the early to mid-Cretaceous orogen-normal accretionary events, and Late Cretaceous to early Tertiary orogen-parallel deformational events. Interpretations in between these times, during the mid- to Late Cretaceous, are much more variable suggesting that orogenic processes within the study area may be localized during this time. Furthermore the differences among the various studies suggest that deformational processes throughout crystalline core are diverse during this time