REVISITING THE LEAVENWORTH FAULT, SOUTHEASTERN NORTH CASCADE RANGE, WASHINGTON

The SW boundary of the ~49-45 Ma fluvial Chumstick Fm in central Washington has been mapped as the Leavenworth fault: a steeply dipping feature, perhaps with largely dextral strike slip, active during regional Paleogene wrench faulting related to accretion of Siletzia. Older crystalline rocks and fluvial strata of the ~63-51 Ma Swauk Fm lie SW of the Leavenworth fault. Farther NE, the steep, NW-striking, dextral Entiat fault separates Chumstick strata from crystalline rocks.

Ongoing geologic mapping of the Tiptop 7.5’ quadrangle, calculation of strikes and dips from bedding traces evident in lidar topography, and a focus on developing a restorable structural interpretation lead to several conclusions: (1) In the Tiptop quadrangle, the Leavenworth fault is a SW-dipping thrust that places complexly-folded Swauk Fm above a thick W-dipping homocline of Chumstick Fm. (2) Coarse monolithologic fanglomerates mapped by earlier workers as Chumstick Fm and cited as evidence for syn-Chumstick displacement on the Leavenworth fault are better interpreted as part of the Swauk Fm. (3) On Tumwater Mtn NW of Leavenworth, the “Leavenworth fault” mapped by earlier workers is subparallel to bedding in the adjacent polymict conglomerate and is evidently the basal contact of the Chumstick Fm. (4) E and SE of Tumwater Mtn, the NW-plunging, 2+ km-amplitude Peshastin syncline is rootless, indicating an underlying decollement which I interpret as the up-dip continuation of the Leavenworth thrust in the Tiptop quad. (5) Down-plunge projection of this geology suggests that crystalline rocks on Tumwater Mtn and the east end of Icicle Ridge overlie Chumstick Fm.

SW-NE shortening of the Chumstick Fm predates ~35 Ma deposition of the overlying Wenatchee Fm, as evinced by a profound angular unconformity beneath Wenatchee strata. Shortening continued after deposition of the Wenatchee Fm, as it is involved in NE-verging thrusts south of the city of Wenatchee. Trends of fold axes associated with the Leavenworth thrust are parallel to the strike-slip Entiat fault, indicating either interludes of NE-SW compression during Paleogene wrench faulting or—more likely—nearly complete partitioning of transpressive strain into domains of NE-SW shortening and NW-SE strike slip.