DEMISE OF FLAT-SLAB SUBDUCTION AT THE END OF THE LARAMIDE OROGENY

Increasingly constrained interpretations for how the Laramide orogeny ended, and how the ignimbrite flareup followed, are derived from recent upper mantle tomograms in conjunction with the volcanic and tectonic record. Tomography is used to identify subducted slab, magmatic re-initiation indicates removal of the Laramide flat slab from the base of North America, and vertical motions are used to infer the evolving sub-crustal density distribution. The general story is not so much one of flat-slab rollback, but rather one of progressive abandonment of large fragments of ocean lithosphere at North America’s base and a subsequent foundering of this slab.

I infer the following history. Subduction of a large ocean plateau (Shatsky conjugate) occurred beneath southern California, the Colorado Plateau and Wyoming, where it stopped ~75 Ma and where it resides today (below ~150 km of Wyoming craton). Its buoyant lithosphere has elevated Wyoming; its (dense eclogite) crust is absent, probably lost at ~75 Ma when rapid subsidence of Wyoming ended, followed by uplift. The next major event was the ~53 Ma accretion of Farallon ocean lithosphere to the margin of NW U.S. and beneath much of NW U.S. This accretion ended the Laramide orogeny at this latitude, from where it propagated southward. Immediately, thrusting switched to core complex extension and magmatic quiescence switched to Challis-Absoraka volcanic flareup as the slab delaminated from North America. This slab is imaged as a vertical curtain extending from the Idaho batholith down into the transition zone. Subduction jumped outboard to create Cascadia, and the Cascade arc soon followed, indicating normal-dip subduction. Flat-slab subduction continued south of central Oregon, necessitating a slab tear across central Oregon. The volcanic flareup propagated south from the tear across the northern Basin and Range, indicating a progressive N-to-S removal of the flat slab, probably by sideways rollback. An absence of major change in California subduction presumably indicates that while the flat slab fell off of North America east of California, it remained at the base of California. More recently, the arrival of Yellowstone beneath S. Oregon triggered a delamination of the flat slab remaining beneath N. Oregon, drawing flood basalt activity north.