Mid‐Cretaceous strike‐slip and contractional fault zones in the western intermontane terrane, Washington, and their relation to the North Cascades‐Southeastern Coast Belt Orogen

Abstract

Structural and geochronologic data from the 111–114 Ma Okanogan Range batholith in north central Washington are used to characterize the timing and style of deformation during the early stages of the mid‐to Late Cretaceous North Cascades‐southeastern Coast Belt (NC‐SECB) orogen. The Pasayten fault zone bounds Jurassic‐Cretaceous sedimentary and volcanic rocks of the Methow basin to the west against predominantly Mesozoic igneous rocks of the Intermontane terrane to the east. The Pasayten fault zone accommodated intrusion of the western units of the Okanogan Range batholith during high‐angle slip, then approximately 20 km of left‐lateral strike‐slip at 109–95 Ma. South of the Pasayten fault zone, the Red Shirt and Methow River thrust zones accommodated (1) west vergent contraction at amphibolite grade that began by 113 Ma and ended by 112 Ma, based on new U‐Pb zircon dates for deformed and undeformed granitoids, and (2) northwest vergent contraction in the Methow River thrust zone followed by west vergent contraction in the Red Shirt thrust zone, both at greenschist grade prior to 104 Ma. These contractional displacements thrust the Methow basin 10–20 km southeastward beneath the western Okanogan Range batholith. Deformation in the NC‐SECB thus began by earliest Albian time, earlier than previously thought, and was characterized by coeval sinistral transcurrent and west vergent contractional faulting. Sinistral strike‐slip in the Pasayten fault zone may have reflected the pre‐100 Ma sense of oblique plate convergence or relatively minor southward tectonic escape of the adjacent Methow basin during contractional deformation. The slip history of the Pasayten fault zone is undocumented between approximately 80 Ma and 60 Ma when, on the basis of paleomagnetic data, some workers have proposed that the Methow basin moved 1700 km northward to its present position relative to the Intermontane terrane. However, significant Late Cretaceous to Early Eocene dextral slip on the Pasayten fault zone is unlikely because the western intrusive units of the Okanogan Range batholith continue without disruption across its southeastward trend.