SPATIAL AND TEMPORAL RELATIONS BETWEEN EARLY TERTIARY SHORTENING AND EXTENSION IN NW WASHINGTON, BASED ON GEOLOGY OF THE PIPESTONE CANYON FORMATION AND SURROUNDING ROCKS

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 shortenin g at high angle to the continental margin during mid-Cretaceous (circa 88-110 Ma) and early Tertiary (47-65 Ma) time. Although significant s hortening is mid-Cretaceous in age, additional shortening, expressed a s reverse faults and major folds, is early Tertiary. In the map area a nd regions to the west and northwest (the Skagit-Methow-western Okanog an area), the shortening is essentially unmodified by extensional defo rmation, whereas in regions east and northeast of the map area (the Om ineca and eastern Okanogan regions), widespread Eocene extension has o bscured the shortening and caused additional unroofing as well. The co nglomeratic Pipestone Canyon Formation lies unconformably on intruded Early Cretaceous rocks of the lower Methow sequence. Approximately 3-5 lan of mid-Cretaceous Methow sequence strata are missing at the uncon formity yet are present 15 km to the west and northwest. These observa tions indicate Late Cretaceous erosion and unroofing in the southeaste rn Methow area prior to deposition of the Pipestone Canyon Formation. Furthermore, conglomerate clasts in the Pipestone Canyon Formation rec ord sources resulting from relative uplift of the crystalline Okanogan Complex east of the basin and, slightly later, from relative uplift o f the volcano-sedimentary Methow sequence and structurally underlying migmatitic Skagit Complex west to northwest of the basin. Following de position, which ended sometime between 55 and 65 Ma, the Pipestone Can yon Formation and underlying Methow sequence were tightly folded and f aulted and juxtaposed with the Okanogan Complex by reverse faulting. T he map pattern of these folds and faults suggests ENE-WSW shortening a nd concomitant uplift. Crosscutting relations north of the map area co nstrain this deformation to be pre-47 Ma. Similarity of Pipestone Cany on fold geometry to folds elsewhere in the Methow and Skagit areas sup ports the interpretation that a significant fold belt of circa 47-65 M a age developed in these areas and overprinted the mid-Cretaceous fold ing and thrusting. Regional data suggest the circa 47-65 Ma folding fo rmed a synclinorium in the Methow area and an anticlinorium in the Ska git area, roughly concurrent with cooling ages for deep (8-9 kbar) Ska git rocks. For the Skagit-Methow-western Okanogan region as a whole, t he 47-65 Ma shortening and unroofing were accompanied by uncertain amo unts of fold belt-parallel strike-slip faulting. Timing estimates for shortening in the Skagit-Methow-western Okanogan area are generally ol der but overlap with those established for extension farther east and northeast in the Okanogan Complex and Omineca Belt, supporting interpr etations that the belt of extension began to develop during or very so on after regional shortening. Existing thermal theological models of e xtension immediately after shortening point to the need for heat sourc es other than mantle conduction and crustal radioactivity to facilitat e the extension. Apparently, the Skagit-Methow fold belt failed to ext end because it lacked the necessary heat.