ACCRETING SILETZIA: REGIONAL MAGMATIC AND TECTONIC CONSEQUENCES

Teleseismic tomography images a large high-velocity slab extending vertically beneath the ~50 Ma Challis magmatic trend from the base of North America to depths of 230–600 km. The only reasonable origin of this slab is the leading portion of the Farallon lithosphere that accreted to North America 50-55 Ma, along with the Siletzia ocean plateau that resides on the western portion of accreted Farallon lithosphere. We infer flat-slab subduction beneath most of western North America by 55 Ma, to which we attribute Laramide-age magmatic quiescence and Rocky Mtn contraction. Accretion then requires a tear in the flat slab separating the accreted Farallon lithosphere to the north of central Oregon and the subducting flat slab to the south. With Siletzia accretion, the ignimbrite flareup (Challis) and core complex extension began in Idaho and westernmost Montana, beneath the imaged mantle slab. This indicates flat-slab foundering. Simultaneously, a switch from trench-normal compression to tension occurred as the new Cascadia subduction zone rolled backward. That is, Siletzia accretion and the resulting reconfiguration of subduction ended the Laramide orogeny, initiated core-complex extension and the ignimbrite flareup. This tectonic-magmatic transition began in the Pacific Northwest and then propagated to the south as the torn northern margin of the subducting flat slab foundered beneath the Great Basin.

Siletzia accretion occurred as British Columbia moved northward, creating the Columbia Embayment. The accreted, Farallon slab that resided beneath the Columbia Embayment (and regions to the north and south) must have been ripped apart as the ocean plateau portion of Farallon lithosphere rotated clockwise many 10s of degrees. We suggest that Clarno-Pasco-Okanogan volcanism is a product of this slab rupture, with the Pasco Basin forming where continental crust was absent.