Structural Geology Projects

Deformation in the mantle wedge associated with Laramide flat-slab subduction

Early Tertiary crustal deformation preserved ~1500 km from the plate boundary in the western U.S. is considered by most to be related to a narrow segment of shallow Farallon-slab subduction, similar to the modern Pampean flat-slab of the central Andes.  Evidence that the slab shallowed enough to penetrate several hundred kilometers inboard of the plate boundary includes a) shearing off of lithosphere and underplating of schists derived from the accretionary wedge beneath the volcanic arc; b) a cessation of arc magmatism and eastward sweeping of the magmatic front; and c) mid-Tertiary eruptions as far east as the Four Corners region of serpentinized ultramafic microbreccia (SUM) sourced from very cold, hydrated mantle lithosphere.

Schematic representation of Laramide flat-slab subduction (modified from Humphreys et al. 2003 Int. Geo. Rev.). The mantle inclusions examined here are sourced from the mantle wedge above the slab and from a serpentinite melange along the slab interface.

Included within the SUM diatremes are eclogites interpreted to represent fragments of the slab itself and/or remnants of older rock from the mantle wedge metasomatized and recrystallized to eclogite along the top of the slab.  Also included within the SUM diatremes are deformed peridotites that represent pieces of the variably hydrated mantle wedge as well as tectonically eroded and entrained fragments of the plate interface. These include weakly deformed to strongly foliated tectonites, spectacularly sheared mylonites and ultramylonites, and cataclasites, formed at temperatures ranging from 500-650°C. We are currently investigating the rheological and seismic properties of several peridotite samples from the Navajo Volcanic Field in Arizona using detailed microstructural and petrological analyses.

Mantle peridotite tectonite from the Navajo volcanic field.

Serpentine-bearing mylonite from the Navajo Volcanic Field



Behr, W.M., Smith, D., 2013.  Deformation in the mantle wedge associated with Laramide flat-slab subduction.  American Geophysical Union Annual Meeting in San Francisco, CA.