THE CHIWAUKUM STRUCTURAL LOW AND THE TECTONIC HISTORY OF THE CENTRAL CASCADE RANGE

One of Eric Cheney’s favorite hypotheses is that clastic sections in the central Cascades are bound by unconformities that correlate across the region. Maps of these unconformities across the Chiwaukum Structural Low (CSL, a.k.a. Chiwaukum graben) depict a suite of WNW-striking, high-angle reverse faults and syncline-anticline pairs, with shortening further indicated by slickensided tectonic breccias. These structures deform the ca. 57-50 Ma (Swauk-Taneum) and ca. 48-43 Ma (Teanaway-Chumstick/Roslyn) clastic and volcaniclastic sections; the two are separated by an angular unconformity. The folds and reverse faults are cut by prominent NNW-striking strike-slip faults (e.g., the Entiat fault) with a minor dip-slip component. These are overlain by the Columbia River Basalt Group (CRBs), which are in turn folded and faulted. Key items are: (i) the topography across the region is largely late-to-post Miocene in age and several structures are still active, (ii) Eocene strata are more shortened than the Miocene and younger sections, (iii) notwithstanding hinge-perpendicular (Teanaway) dikes, pre-Miocene transtensional structures in the region are the predominantly strike-slip faults which largely postdate both the shortening structures and the Eocene clastic sections. Along the plate boundary, from ca. 60 to <42 Ma, thickened oceanic crust was subducted, terranes migrated northward as did the Kula/Resurrection-Farallon ridge, and Seletzia impinged on the margin, all more-or-less consistent with items i-iii. But did the ca. 60-45 Ma exhumation of the high-grade arc rocks (e.g., Skagit gneiss) occur in a period of significant extension/transtension, perhaps induced by the plate-boundary processes? There is nothing in the map pattern of the CSL — the most commonly cited evidence for this extension/transtension — to warrant this interpretation, unless one explicitly ties early Eocene clastic (Swauk-Taneum) deposition to now completely overprinted normal faults. This last hypothesis is thus the alternative to Cheney’s, and, barring independent lines of evidence, if ruled out then the patterns of sedimentation and exhumation in the central Cascades, and potentially other arc/forearc settings, cannot be attributed to regional extension or transtension.