MAP-BASED CONSTRAINT ON DEXTRAL OFFSET ACROSS THE HOZAMEEN FAULT, NORTH CASCADES, WASHINGTON

The Hozameen/North Creek fault is one of several sub-vertical, N- to NW-trending, dextral oblique-slip faults in the North Cascades of Washington and British Columbia. These faults are generally recognized to have been active in Late Cretaceous and Early Tertiary time, when they facilitated unroofing of the Cascades metamorphic core during a period of oblique plate convergence. The analysis described herein builds on previous work which characterized the eastern Cascades foldbelt as an E-vergent fold-thrust belt that deformed Mesozoic strata in the Methow basin during mid-Cretaceous orogeny. That work demonstrated that the oceanic Hozameen terrane exposed west of the Hozameen fault (Jack Mountain allochthon) appears to continue in the subsurface east of the fault as a tectonic wedge beneath an uplifted panel of east-dipping Jurassic strata. The eastern terminus (leading-edge) of the Hozameen allochthon can be inferred on structural cross sections. The southern limit of Hozameen rocks is exposed west of the Hozameen fault, but to date its offset counterpart has not been reported east of the Hozameen fault.

A new cross section located in the southern Methow basin near Twisp, WA presented here suggests a likely southern limit of the Hozameen allochthon within the Methow subsurface. The southern basin, SE of the Boesel fault, is distinctive in several ways: 1) the sedimentary section is thinner, 2) several unconformities punctuate the condensed section there, and 3) the structural style is characterized by W-vergent, thick-skinned thrust faults. Taken together, these observations suggest that during Early Cretaceous time, prior to the onset of east-west crustal shortening across the region, the southern Methow basin was in a proximal depositional setting where structurally elevated, extensional fault blocks flanked a deeper water basin to the north and west. The Twisp cross section does not contain an east-dipping panel of older strata supported by an underlying Hozameen tectonic wedge. If the subsurface Hozameen allochthonous wedge is absent in the southwestern Methow as we contend, dextral offset on the Hozameen fault is constrained to be less than about 50km, such that the oceanic rocks on Jack Mountain would restore no further south than about the latitude 48⁰30’.