THE RECURRING MYTH OF FARALLON FLAT-SLAB SUBDUCTION

Geologists common invoke the presence of flat-slab subduction underneath western North America in the Late Cretaceous-Paleogene, to explain: 1) Cessation of coastal magmatism; and 2) Foreland block uplifts of the Rocky Mountains far from the continental margin (Laramide orogeny). This hypothesis is no longer viable. First, the Cretaceous Sierra Nevada magmatic arc contained an ~120 km deep eclogitic root that existed until the Miocene, eliminating the possibility of “flatness” to a subducting slab. Second, recent seismic topography results indicate the presence of two vertical slabs (slab walls) that occurred underneath the western US. Not only is there is no indication of a shallow slab, but the seismic tomography indicates that the Juan de Fuca (remnant of the Farallon slab) subduction is connected to a series of vertical slab walls, which continue into the mantle for at least a thousand km. This data effectively rules out the possibility of flat-slab Farallon subduction. Another problem is the proposed presence of yet another plate – the Resurrection plate – that was inboard from the Farallon plate.

The alternative explanation to shallow slab subduction is an endload on the western Cordilleran margin. A new understanding of the Idaho segment of the Cordilleran margin illuminates this alternative. The Sevier orogenic belt shows maximum foreland deposition immediately inboard of the active (~103-90 Ma) western Idaho shear zone (WISZ). Reconstruction on faults with known displacement returns the Insular terrane to the California/Oregon border (a minimum reconstruction) during activation of the WISZ. The Idaho batholith (Atlanta Lobe), active immediately after cessation of the WISZ, does not have any mantle component of magmatism: Rather, it appears to be a result of crustal melting in a continental plateau. Yet, the Idaho batholith is the northern extension of the Nevadaplano. If the Idaho batholith is caused by a terrane collision, the remainder of the Nevadaplano is likely caused by the same event. A terrane collision is required by the geology of the northern 1/3 of the US Cordillera, and is consistent with paleomagnetic constraints from the Insular terrane and seismic tomography of western North America; likely, it also caused the Laramide orogeny.