MAKING THE COLUMBIA EMBAYMENT WITH OBLIQUE FLAT-SLAB SUBDUCTION

The Columbia Embayment formed from the Late Cretaceous to Neogene as tectonic blocks shuffled around. In the Late Cretaceous, prior to Siletzia accretion, the Intermontane Okanogan and North Cascades crustal blocks pulled away from their suture with the Blue Mountains and moved north (Wyld et al., 2007) leaving crustal fragments behind in the opening Columbia Embayment. This occurred on a slippery de-watering schist layer on top of an obliquely subducting flat Farallon slab, in a manner similar to the creation of the southern California Borderland. As a result, the Embayment was left with a thin crust, which causes its low elevation today. At ~50 Ma, Siletzia accreted across the western end of the Columbia Embayment (Wells et al., 2014), terminating Farallon flat-slab subduction and starting Cascadia subduction; the Cascade arc was well developed by 35 Ma. Beneath the continent interior, the far reaches of the stalled flat slab delaminated from beneath Montana and northeast Washington, rolling back toward the Embayment, and leaving a north-trending dangling slab fragment in the upper mantle beneath much of central Idaho (Schmandt & Humphreys, 2011), and an east-trending dangling slab fragment beneath east-central Washington. Farallon slab delamination caused asthenospheric ascent, initiating the ignimbrite flareup where the asthenospheric contacted the hydrated continental crust. Flat slab was left just west of the dangling slabs, beneath southeast Washington (Levander & Miller, 2012) and eastern Oregon; these areas remained amagmatic. The eastern Oregon flat slab delaminated during the Columbia River flood basalt event and it is imaged in the upper mantle beneath (Darold & Humphreys, 2013). Following Siletzia accretion, clockwise rotation of the Blue Mountains terrane and Siletzia was driven transtensional deformation associated with oblique Farallon subduction, and later by the northwest motion of the Sierra Nevada block (Wells et al., 1998). These events must have ripped apart the flat slab beneath the Embayment and also widened the Embayment. We attribute Eocene core-complex extension on the northern and eastern edges of the Embayment, and Clarno and Pasco Basin magmatism within the Embayment (Catchings & Mooney, 1988), to this transtensional disruption.