The >200-km-long Ross Lake fault zone (RLFZ) forms the northeastern border of the crystalline core of the North Cascades. This Late Cretaceous(?) and Paleogene structure is an ∼15-km-wide system of faults and fault zones, several of which deform the Paleocene (∼65 Ma, U-Pb zircon) Oval Peak batholith.

The southern segment of the Gabriel Peak tectonic belt of the RLFZ is defined by strongly deformed rocks of the foliated southwestern margin of the Oval Peak batholith, structurally underlying pre-Late Cretaceous Twisp Valley Schist, and structurally highest part of the subjacent Skagit Gneiss, which here consists of Late Cretaceous (87 Ma) and Paleocene (60 Ma) orthogneiss. Kinematic indicators in moderately steeply northeast-dipping mylonitic orthogneiss record reverse slip. The strong solid-state flattening fabric in the foliated margin of the Oval Peak batholith formed during or shortly after emplacement and overprinted a magmatic fabric observed in the core of the batholith. Foliation and associated stretching lineation within the Gabriel Peak tectonic belt are oblique to the regional structural grain but concordant with the contacts of the batholith. These patterns in part record emplacement of the batholith as an expanding diapir into the tectonic belt. Reverse slip in the tectonic belt occurred before, during(?), and shortly after emplacement but ended by ∼55-58 Ma.

The core of the Oval Peak batholith intruded a large tongue of Twisp Valley Schist on the north. Porphyroblast microstructures and Ar-Ar and K-Ar geochronology indicate that amphibolite-facies dynamothermal metamorphism of the schist occurred during intrusion.

The Foggy Dew and Twisp River fault zones separate the batholith and its wall rocks on the northeast from weakly metamorphosed strata of the Methow basin. The Foggy Dew fault zone contains mylonitic gneiss derived from the Oval Peak batholith, Skagit Gneiss, and tonalitic rock that displays U-Pb zircon systematics suggestive of an Eocene age. Sillimanite schist and amphibolite are also present. Mylonites record dextral strike slip with a component of normal slip, down to the northeast. Movement in the fault zone continued subsequent to deformation in the Gabriel Peak tectonic belt and ended by 48 Ma. Mylonites in the Twisp River fault zone also reveal dextral strike slip, but apparently with less normal slip.

The RLFZ experienced a diachronous history from at least 65 to 45 Ma. This history was dominated by dextral strike slip, but there were significant components of normal slip and reverse slip on some structures. During the latter part of this interval (∼57-45 Ma), the RLFZ probably recorded a transtensional tectonic regime transitional between extension to the east in Omineca metamorphic core complexes and dextral strike slip to the west related to oblique convergence along the continental margin. Early Cenozoic plutonism, metamorphism, and ductile deformation have strongly overprinted any of the major Cretaceous deformation postulated in previous tectonic models for the RLFZ.

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