AUTO-SUBDUCTION MODEL FOR THE SEVIER-LARAMIDE (~125-50 MA) OROGENY OF WESTERN NORTH AMERICA

Slivers of oceanic slab in the form of the Rand-Pelona-Orocopia schist and related metamorphic complexes exhumed in area’s of extreme Neogene continental extension in the western cordillera of North America provide irrefutable evidence of subduction. A peculiarity of the complexes however, not explained by either east-dipping (back-arc) or west-dipping (fore-arc or ribbon continent) subduction models is that the complexes are nearly devoid of the kinds of rocks that would be expected to scrape off the top of a subducting oceanic slab: ophiolites, parts of ophiolites, and hemi-pelagic sediments. Instead they represent a melange of two main lithologies: voluminous siliciclastic turbidites of probable continental margin origin containing abundant youthful (90-55 Ma) detrital zircons as host to subordinate yet ubiquitous lozenges of lithospheric mantle-wedge ultramafic rock.

An alternative way to deliver these kinds of oceanic rocks to the base of the continental crust is to invoke very steep and possibly overturned subduction of the leading edge of the continent whose margin deforms into a fore-arc style fold-thrust belt. The continent overrides and breaks off its own (formerly connected) oceanic slab whose vertical to slightly inverted orientation allows slivers of its under side (mantle-wedge) to be scraped onto the base of the continental crust. As it descends, the easterly subducted, albeit originally west-dipping slab, produces the dehydration (arc) and asthenospheric upwelling (slab break-off) melts that penetrate the fold thrust belt.

Mantle tomographic imagery of the “Laramide” slab in its current, nearly vertical position beneath the eastern seaboard (Sigloch and Mihalynuck, 2013, Nature, v. 456) supports this model.