COLLISION, SLAB FAILURE MAGMATISM, CORDILLERAN BATHOLITHS AND THE ORIGIN OF CONTINENTAL CRUST

Sam Bowring has long been interested in Cordilleran batholiths, arc magmatism, and the origin of continental crust. Cordilleran batholiths are considered by most researchers to form beneath arc terranes in thickened crust, but a survey of modern continental arcs reveals most to be regions of normal to thinned crust, as Sam and RSH argued in 1984 (Geology). Thus, the origin of Cordilleran batholiths is enigmatic. We suggest that these batholiths form mostly during and after arc-continent collision as the result of slab failure. Slab failure is a consequence of arc-continent collision because oceanic lithosphere is readily subducted, whereas continents are buoyant and difficult to subduct. Once the subducting slab tears, rocks of the partially subducted continental margin rise due to buoyancy and the collision zone is exhumed. The failure also allows asthenosphere to upwell through the tear, melt adiabatically, and rise into the subcontinental lithosphere and crust of the upper plate. Slab failure magmas are geochemically distinct from arc magmas because slab failure magmas form from melting of garnet bearing sources whereas arc magmas are derived from melting of spinal peridotite.

Cretaceous examples of slab failure magmatism include the La Posta and Sierran Crest magmatic suites of the Peninsular Ranges and Sierran batholiths, which formed due to closure of the Bisbee-Arperos seaway, a marginal basin that opened along the western margin of the Cordilleran Ribbon Continent at about 135 Ma and closed at ~100 Ma. Slab failure rocks of the ~125 Ma Sevier event – which occurred in the Great Basin sector of the US – outcrop in the Omineca belt and Selwyn basin of Canada where they were transported after 70 Ma during the 82-58 Ma Laramide event. The Laramide event also had a period of slab failure magmatism that extended from southern Mexico to Alaska. Late Cretaceous-Early Cenozoic slab failure magmas were emplaced into a zone of uplift and exhumation in the Sonoran batholith of Mexico and Arizona, the Transverse Ranges of southern California, the Idaho and Boulder batholiths, and the Coast Range batholith of British Columbia.

Most of the Cretaceous plutons in the North American Cordillera appear to be slab failure bodies, not arc plutons, and so we suggest that substantial volumes of continental crust formed by slab failure magmatism.