Nature of the contact zone between the North Cascades crystalline core and the Methow Sequence In the Ross Lake Area, Washington: Implications for Cordilleran tectonics

Abstract

Previous analyses of the accretionary history of NW Washington‐SW British Columbia have suggested the possibility of large‐scale, dextral‐slip faulting events to explain the present day regional outcrop pattern. The contact zone between the crystalline Skagit Complex and the volcano‐sedimentary Methow sequence has been considered to be a major fault, named the Ross Lake fault by Misch (1966), and has been considered to be a possible large‐scale strike‐slip fault by later workers. Detailed mapping of this contact zone in the Ross Lake area shows that the brittle faults and mylonites thought by Misch to express dextral shearing along the contact are not continuous, throughgoing structures, and that the contact between Skagit orthogneisses and Methow sequence strata is primarily intrusive. Furthermore, the mylonites show both left‐ and right‐lateral shear. The Skagit‐Methow contact is therefore not a major terrane boundary but rather a tectonized intrusive contact that has not accommodated significant motion between the two regions. Based on these observations and regional synthesis of timing relations, deformation in the contact zone is interpreted to express regional ENE‐WSW shortening, not major dextral slip, in the early Tertiary. Since no terrane‐bounding fault exists, the Cretaceous‐Tertiary history of the Ross Lake area can be interpreted as follows: (1) Jurassic(?)‐Early Cretaceous forearc/rift basin sedimentation (deposition of the lower Methow sequence); (2) 110–95 Ma, imbrication of the lower Methow sequence and its basement of oceanic crustal rocks by east‐vergent thrusting, with concomitant development of a west‐derived foredeep in the upper Methow sequence; (3) 95–85 Ma, arc magmatism, with genesis of the Skagit migmatite from Methow and older basement protoliths; and (4) Paleocene‐Early Eocene folding, faulting, uplift, and exposure of a relatively intact 25+ km crustal section, termed the Skagit‐Methow crustal section by Kriens and Wernicke (1987).