FLAT-SLAB BULLDOZING OF BASAL CONTINENTAL MANTLE LITHOSPHERE DURING THE LARAMIDE OROGENY

Flat-slab (horizontal) subduction occurs along ~10% of subduction margins, forms magmatic gaps, causes upper-plate deformation to migrate inland, and is thought to have caused the Laramide Orogeny. We investigate bulldozing of basal continental mantle lithosphere (CML) by the flat Farallon slab, using 2D numerical thermo-mechanical models that test effects of CML density and strength. Flat-slab subduction begins as the buoyant, conjugate Shatsky Rise enters the trench. The dense slab ahead detaches and continues to sink, and subduction erosion of the continental margin begins. A slab hinge is re-established at the front of the flat slab. The advancing flat slab erodes the basal ~25-50 km of continental mantle lithosphere (CML), thinning it. Buoyant (depleted) bulldozed CML can accumulate ahead of the slab hinge, in a growing, migrating wedge up to ~200 km thick, that is narrower if bulldozed CML is strong, or wider (up to ~700 km) if weak. We suggest that bulldozed wedges transmit contact stress into the upper plate, driving crustal deformation ahead of the flat slab itself. Dense, and/or strong, bulldozed CML is entrained with the sinking slab and would be imaged with it. It seems likely that bulldozed CML also may accumulate along the sides of the "contact corridor" between slab and upper plate. Bulldozed material (dense or buoyant) fills the asthenospheric wedge, which would end arc-type melting during flat-slab advance. Flat-slab rollback reopens the asthenospheric wedge, renews melting and leaves a step and/or a thickened keel in the CML at/ahead of the farthest slab extent.

Recent plate models suggest a narrow Farallon-North America contact corridor. This probably was bounded in S AZ-NM by a slab tear. Fossil basal CML steps/wedges may remain below North America and define the limit of flat-slab advance: We interpret an upper-mantle fast-velocity anomaly below SE NM and W TX as a dripping keel and a step in CML thickness below SW CO as a fossil step. If true, Farallon flat-slab contact ended 100s of km SW of the Laramide front in WY. Some Laramide magmatism occurred above the contact corridor: the CO mineral belt probably was above a slab tear. Laramide magmatism in W AZ and SE CA may reflect petit-spot type melting where the slab was flexed concave-up. Modern examples of bulldozed CML and its effects likely exist in South America.