Spatial and temporal relations between early Tertiary shortening and extension in NW Washington, based on geology of the Pipestone Canyon Formation and surrounding rocks
Authors: Kriens Bryan J. , Hawley Diane L. , Chappelear F. Duke , Mack Peter D. , Chan Ahn F. 
Published: February 2004

Detailed mapping of the deformed Paleocene Pipestone Canyon Formation and surrounding Cretaceous rocks in NW Washington suggests that uplift, erosion, and unroofing of adjacent rocks was influenced by shortening at high angle to the continental margin during mid‐Cretaceous (circa 88–110 Ma) and early Tertiary (47–65 Ma) time. Although significant shortening is mid‐Cretaceous in age, additional shortening, expressed as reverse faults and major folds, is early Tertiary. In the map area and regions to the west and northwest (the Skagit‐Methow‐western Okanogan area), the shortening is essentially unmodified by extensional deformation, whereas in regions east and northeast of the map area (the Omineca and eastern Okanogan regions), widespread Eocene extension has obscured the shortening and caused additional unroofing as well. The conglomeratic Pipestone Canyon Formation lies unconformably on intruded Early Cretaceous rocks of the lower Methow sequence. Approximately 3–5 km of mid‐Cretaceous Methow sequence strata are missing at the unconformity yet are present 15 km to the west and northwest. These observations indicate Late Cretaceous erosion and unroofing in the southeastern Methow area prior to deposition of the Pipestone Canyon Formation. Furthermore, conglomerate clasts in the Pipestone Canyon Formation record sources resulting from relative uplift of the crystalline Okanogan Complex east of the basin and, slightly later, from relative uplift of the volcano‐sedimentary Methow sequence and structurally underlying migmatitic Skagit Complex west to northwest of the basin. Following deposition, which ended sometime between 55 and 65 Ma, the Pipestone Canyon Formation and underlying Methow sequence were tightly folded and faulted and juxtaposed with the Okanogan Complex by reverse faulting. The map pattern of these folds and faults suggests ENE–WSW shortening and concomitant uplift. Crosscutting relations north of the map area constrain this deformation to be pre‐47 Ma. Similarity of Pipestone Canyon fold geometry to folds elsewhere in the Methow and Skagit areas supports the interpretation that a significant fold belt of circa 47–65 Ma age developed in these areas and overprinted the mid‐Cretaceous folding and thrusting. Regional data suggest the circa 47–65 Ma folding formed a synclinorium in the Methow area and an anticlinorium in the Skagit area, roughly concurrent with cooling ages for deep (8–9 kbar) Skagit rocks. For the Skagit‐Methow‐western Okanogan region as a whole, the 47–65 Ma shortening and unroofing were accompanied by uncertain amounts of fold belt‐parallel strike‐slip faulting. Timing estimates for shortening in the Skagit‐Methow‐western Okanogan area are generally older but overlap with those established for extension farther east and northeast in the Okanogan Complex and Omineca Belt, supporting interpretations that the belt of extension began to develop during or very soon after regional shortening. Existing thermal rheological models of extension immediately after shortening point to the need for heat sources other than mantle conduction and crustal radioactivity to facilitate the extension. Apparently, the Skagit‐Methow fold belt failed to extend because it lacked the necessary heat.