Locating the Laramide Flat Slab
By: Christopher Harig
A flat slab subduction zone of some form or another has commonly been used to account for the unique features of the Laramide Orogeny. For instance, the Laramide Orogeny occurs on the order of 1000km away from the point of subduction, while more normal subduction zones today cause deformation much closer to the subduction zone. The flat slab subduction is thought to have several significant impacts different from normal subduction zones. The shallowing of the slab squeezes out the asthenosphere and mantle lithosphere which has the result of ceasing magmatism near the subduction zone and pushing it farther away. The shallow slab is also thought to couple with the lower crust of the overridding plate and induce a basal traction which would aid deformation further away from the subduction zone. When hypothesizing about the flat slab subduction in the Laramide, scientists have disagreed about the possible extents of the flat slab zone; estimates have varied from the entire western U.S. to very localized areas. An important tool in studying the Laramide flat slab is radiometric isotope geochemistry. Examining radiometric isotopes in volcanic rocks can tell us whether the rock erupted directly from the mantle through the continents or whether it passed through mantle lithosphere on its way. Studying this over a large area can tell us where there has been mantle lithosphere in the past, and where it was removed, possibly by flat slab subduction. In this site I examine some of the isotope geochemistry work and interpretations dealing with locating the Laramide flat slab.