Abstract

Pre-Tertiary rock units on the western flank of the Cascades in Washington and southern British Columbia constitute the Northwest Cascades system (NWCS). This litho-tectonic package is characterized by predominantly oceanic lithologies subjected to high P/T metamorphism and structurally mixed to form a regional mélange. Although the focus of this report is on rocks of the NWCS as originally defined, survey of the regional geology suggests broadening of the limits of this system to include the San Juan Islands, the Pacific Rim complex, and pre-Tertiary rocks on the western flank of the central and southern Washington Cascades.

The Shuksan Metamorphic Suite, a relatively coherent and broadly exposed unit of the NWCS, contains a regionally consistent stretching lineation which provides evidence of east-northeast–directed underthrusting (subduction) during the Early Cretaceous blue-schist metamorphism. Metamorphic F2 folds are approximately normal to the stretching lineation and appear to have developed late in the same deformational event.

The mélange structure postdates the high-pressure regional metamorphism. It is characterized by a tectonic assemblage of diverse and unrelated rock units occurring as fragments as much as 10 or more kilometres in breadth and separated by a network of low- to high-angle north-northwest–striking faults. Pre-fault restoration is impossible. Argillaceous or serpentinitic material commonly occurs in deformation zones between more competent lithologies. Mylonite zones as much as several metres thick are widely developed and display stretched clast lineations, indicating a north-northwest–south-southeast transport direction for both low- and high-angle faults. The high-angle faults are therefore strike-slip in nature. Shear-sense indicators are mixed in thrust fault zones, but many indicate dextral movement for the strike-slip faults. K/Ar whole-rock ages of mylonites range from 20–127 Ma, but two Late Cretaceous ages of 87 and 91 ± 3 Ma are considered as best representing the time of faulting.

The inferred shift in direction of tectonic transport from east-northeast–west-southwest during Early Cretaceous metamorphism to north-northwest–south-southeast during Late Cretaceous faulting and mélange formation is correlated with a mid-Cretaceous change in plate interaction between the North American and Farallon/Kula plates from high- to low-angle convergence. The NWCS mélange is interpreted to have formed during large dextral transform motion along the continental margin and to have reached final emplacement by thrusting against the southern end of Wrangellia and the Coast Plutonic Complex.

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