Locating the Laramide Flat Slab
By: Christopher Harig
While there are some who would prefer not to accept a flat slab as a mechanism for Laramide deformation, it is for the most part widely accepted. What is currently disputed are the extents and impact of a flat slab geometry. For instance, in their diagram shown before and seen here, Livaccari and Perry 1993 define a boundary near Las Vegas between mantle volcanics and mantle lithosphere volcanics. They draw this boundary by assuming that signatures of mantle are due to late Cenozoic extension. However, some of the samples to the east of this boundary (in the lithosphere zone) show epsilon Nd values very close to that of the mantle zone. As a result, this interpretation is up for debate.

Saleeby 2003 w/additions by C. Harig
1 - from Ducea and Saleeby 1998
2 - Lee et al., 2001
3 - from Miller et al. 2001
4 - Livaccari and Perry 1993
5 - from Farmer et al. 1989
The isotope geochemistry studies discussed previously have helped to define the area where mantle lithosphere persisted throughout the Laramide Orogeny and where it did not. When we examine the data together, as in the diagram below, we can begin to get a picture of what the geometry of the flat slab might have looked like in Laramide time. Throughout the Mojave Desert, post-Laramide volcanic rocks show isotopic signatures indicative of having been sourced in the mantle. To the north and to the southeast the rocks show signatures of a persisting mantle lithosphere.

Saleeby 2003 w/additions by C. Harig
As a result of these studies and other geophysical evidence, some have proposed a much narrower flat slab subduction zone than previously envisioned. Saleeby 2003 proposes that a slab originating from the latitude of the Mojave Desert with motion in the direction of the North America - Farallon relative plate motion could cause the deformation seen in Wyoming and Colorado. This interpretation also accommodates observed gradational features seen at the southern boundary of the Sierra Nevada Batholith. For example, there is a gradient in the igneous barometry of the edge of the SNB. So far this interpretation appears to fit well with observed data; however continued study of the region will likely continue to yield valuable data.


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