KINEMATIC ANALYSIS OF A CRETACEOUS THRUST ZONE, CASCADE CRYSTALLINE CORE, WASHINGTON

The Cascade Crystalline Core of Washington is a fault bounded block of high grade metamorphic and intrusive rocks at the SE end of the Cretaceous Coast Plutonic Complex. Previous workers have disputed if NW translation or SW-vergent thrusting is responsible for regional metamorphism and deformation. Mapping and structural analysis along a well exposed 1.5 km² segment of a thrust S/SW of the Cascade River displays evidence of 3 deformational events (D1, D2, D3), and a new interpretation of mapped units. Units include the ocean floor protolith Napeequa schist, and an accreted Triassic arc: the Marblemount Meta Quartz-Diorite (MMQD), and the Cascade River Unit (CRU). Fluke (1992, WWU MS thesis) mapped imbricate slivers of mylonitic MMQD in the hanging and footwall of a thrust of MMQD over Napeequa. Thin section analysis indicates a clastic protolith for the mylonite and we reinterpret these slivers as CRU. We interpret that the MMQD is thrust over the CRU, and CRU over the Napeequa.

D1 formed S1 foliation across the study area; S1 is visible in isoclinal folds of foliation in hand sample and thin section. Due to D2 overprint it is uncertain whether the timing, kinematics, and metamorphic grade of D1 is equivalent in all 3 units. D2 formed a NW striking, moderately NE dipping foliation with NE- and SW- plunging mineral lineations throughout the study area. S2 is axial planar to isoclinal folds of S1; moderately plunging fold axes range from NW to E. F2 folds are mostly symmetric, however asymmetric folds near the thrust show S to SW vergence. S-C fabric and asymmetric augen in MMQD near the thrust also show top to the S and SW. We argue that D2 is responsible for imbrication of CRU and Napeequa which were thrust beneath the MMQD. D3 structures are concentrated in a narrow region adjacent to the thrust. Asymmetric upright E to NE F3 folds of S2 in CRU and Napeequa are N and NW vergent. S-C fabric parallel to SE plunging L3 mineral lineations in the hanging wall of the D2 thrust also show top the NW kinematics. The data suggest regional S- to SW vergent structures are locally overprinted by NW-vergent structures. The timing and broader regional significance of each event are currently being investigated.