Abstract

The Ingalls Complex is the largest and most complete of several Middle to Late Jurassic ophiolites in northwestern Washington. It structurally overlies the Skagit metamorphic core of the North Cascades and is intruded by the Late Cretaceous Mount Stuart Batholith. The last truncates major structures of the ophiolite and divides it into two outcrop areas, a main area south of the batholith and a smaller area in a roof pendant. In the main area, the internal structure of the ophiolite is dominated by two wide fault zones characterized by steeply dipping serpentinite mélange. Displacement probably was a combination of strike slip and dip slip, with the latter apparently related to diapiric rise of serpentinite. Deformation probably occurred in an oceanic transform fault zone.

Ultramafic tectonites predominate in the main outcrop area, including from south to north: (1) harzburgites and dunites; (2) mylonitic iherzolites and hornblende peridotites; and (3) Iherzolites. Unit 2 represents a high-temperature shear zone that may record early movement in the deeper levels of the postulated oceanic fracture zone. Mafites generally occur as tectonic blocks; in the southern fault zone, they underwent ocean-floor-type metamorphism, but to the north, metagabbros experienced dynamothermal amphibolite-facies metamorphism prior to incorporation into the serpentinite mélange. Gabbros in the southern fault zone locally intrude ultramafites and in places pass upward into a diabase dike complex. Gabbros and diabases intrude partly pillowed basaltic flows and pillow breccias, with interbeds of argillite, chert, and ophiolite-derived sedimentary breccia. Another sequence of basalt, argillite, chert, and minor limestone is penetratively deformed and may be exotic.

In the pendant on the Mount Stuart Batholith, the north-directed Windy Pass thrust (WPT) places the Ingalls on the pelitic Chiwaukum Schist of the Skagit metamorphic core. The Ingalls is imbricated with amphibolite-facies metasedimentary and metaplutonic slices that may be a part of the Skagit core not exposed in the lower plate. Imbrication was synchronous with amphibolite-facies metamorphism of the ophiolite in the pendant, but the postmetamorphic sole thrust (WPT) truncates both the imbricate slices and fabric elements in the Chiwaukum Schist. Thrusting was mid-Cretaceous, apparently postdating high-angle faulting within the main portion of the Ingalls.

The WPT marks a major Cretaceous boundary between an oceanic and a metamorphic terrane (Skagit core), and its upper plate represents the only rocks preserved above the Skagit core. The Ingalls is similar in age to ophiolites in California, Oregon, and British Columbia, as well as those in northwestern Washington. The California and Oregon ophiolites (Coast Range, Smartville, Josephine) apparently originated in small, short-lived marginal basins, whereas the Ingalls was deformed in a fracture zone in a wider, longer-lived marginal basin or large ocean.

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