Flattening the slab: Farallon plate subduction and the Laramide orogeny

• Author / Creator

  Liu,Sibiao

• The Laramide orogeny (~80-50 Ma) was an anomalous period of mountain-building in the western United States that took place >1000 km inboard of the Farallon Plate subduction margin. The orogeny was preceded by the development of the Western Interior Seaway and eastward migration of the volcanic arc. Thus, it is believed that this marked a time of flat (subhorizontal) subduction. However, the factors that caused the Farallon Plate geometry to evolve from a steep geometry to flat subduction are not well-understood. Three mechanisms have been proposed: (1) a westward (trenchward) increase in North America plate motion, (2) an increased slab suction force owing to the presence of thick Colorado Plateau lithosphere, and (3) subduction of a buoyant oceanic plateau. This study uses 2D upper-mantle-scale numerical models to investigate these mechanisms. The models show that trenchward continental motion provides the primary control on subduction geometry, with decreasing slab dip as velocity increases. However, this can only create low-angle subduction, as the Farallon Plate was old (>100 Ma) and therefore much denser than the mantle. A transition to flat subduction requires: (1) subduction of a buoyant oceanic plateau which does not undergo eclogitization, and (2) a slab break-off at the landward side of the plateau. The break-off removes the dense frontal slab, and flat subduction develops as the oceanic plateau pulls the slab upward to a subhorizontal trajectory. These results are confirmed through a quantitative comparison of the models based on the slab dip and oceanic plate buoyancy. In the preferred model, the oceanic plateau is 400 km wide with 18 km thick metastable crust and an underlying layer of depleted, harzburgitic mantle. With a continental velocity of 4 cm/yr, flat subduction develops within 14 Ma of plateau subduction. The flat slab segment underthrusts the continent at ~200 km depth and removes the lower ~40 km of the Colorado Plateau mantle lithosphere. It extends >1500 km inboard of the trench to underlie the Rocky Mountain foreland region, which was the locus of Laramide deformation. Overall, the models show that the Farallon Plate could have developed a flat slab geometry in the Late Cretaceous. This is also the first study to demonstrate the importance of a slab break-off in creating flat subduction of an old, dense oceanic plate.

• Subjects / Keywords

  • Numerical model
  • Laramide orogeny
  • Farallon plate subduction
  • Flat slab

• Graduation date

  Fall 2014

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3NZ80X0J

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.