Linked thrust and strike-slip faulting during Late Cretaceous terrane accretion in the San Juan thrust system, Northwest Cascades orogen, Washington

Abstract

The San Juan thrust system makes up the western elements of the Late Cretaceous Northwest Cascades orogen of Washington, a stack of metamorphic nappes with terranes ranging in age from early Paleozoic to middle Cretaceous. The tectonic development of the nappes is an unresolved issue important to understanding plate- convergent processes operating in the North American Cordillera in the Creta ceous as well as at other times. Two terrane-bounding fault zones were studied in this project, the Lopez and the Rosario fault zones. The fault-zone rocks were affected by two cataclastic-plastic, Late Cretaceous deformational events (D1 and D2) that included accretion and lateral motion during blueschist-facies metamorphic conditions. D1 produced upright folds and a subvertical axial-planar S1 foliation fabric in the Lopez fault zone, and southwest- vergent folds and gently northeast-dipping axial-planar S1 foliation in the Rosario fault zone. In the Lopez fault zone, D2 produced northwest-striking and subvertical strike-slip shear zones (S2) with dominant sinistral displacements, whereas in the Rosario fault zone, D2 generated low-angle S2 shear zones and various slip-crenulation fabrics formed by northwest-directed thrust motion. The D1 fabrics are inferred to be related to initiation of subduction-related accretion of the nappes by orogen-normal (southwest-northeast) shortening and accretionary tectonics. The subsequent D2 fabrics developed as temporally overlapping, subduction-related low-angle thrusts in the Rosario fault zone and high-angle, sinistral strike-slip faults in the Lopez fault zone in an overall transpressive setting due to oblique convergence. This model suggests that the high-angle Lopez fault zone (D2) was located above a gently northeast-dipping thrust detachment (S2) expressed by the Rosario fault zone that allowed the accreted nappes to move parallel to the orogen. The kinematic model cannot validate regional-scale, dextral terrane translation of the San Juan thrust system, as inferred by paleomagnetic data elsewhere in the western United States, but addresses important tectonic processes caused by oblique subduction and changing plate conditions.