Some related papers and abstracts are on the western U.S. uplift page.
Jones, C. H., G. L. Farmer, B. Sageman, and S. Zhong, Hydrodynamic Mechanism for the Laramide Orogeny, Geosphere,7, 183-201, doi: 10.1130/GES00575.1, 2011
The widespread presumption that the Farallon plate subducted along the base of North American lithosphere under most of the western United States and ~1000 km inboard from the trench has dominated tectonic stud- ies of this region, but a number of variations of this concept exist due to differences in interpretation of some aspects of this orogeny. We contend that five main characteristics are central to the Laramide orogeny and must be explained by any successful hypothesis: thick-skinned tectonism, shutdown and/or landward migration of arc magmatism, localized deep foreland subsidence, deformation landward of the relatively undeformed Colorado Plateau, and spatially limited syntec- tonic magmatism. We detail how the first two elements can be well explained by a broad flat slab, the others less so. We introduce an alternative hypothesis composed of five particular processes: (1) a more limited segment of shallowly subducting slab is created by viscous coupling between the slab and the Archean continental keel of the Wyoming craton, leaving some asthenosphere above most of the slab; (2) dynamic pressures from this coupling localize subsidence at the edge of the Archean Wyoming craton; (3) foreland shortening occurs after the subsidence of the region decreases gravitational potential energy, increasing deviatoric stresses in lithosphere beneath the basin with no change to boundary stresses near the subduction zone or changes to basal shear stress; (4) shear between the slab and overriding continent induces a secondary convective system aligned parallel to relative plate motion, producing the Colorado Mineral Belt above upwelling aligned along the convection cell; (5) the development of this convective system interrupts the flow of fresh asthenosphere into the arc region farther west, cutting off magmatism even in segments of the arc not over the shallowly dipping slab.
A large version of the RMF station map (135k)
Harig, C., S. Zhong, C. H. Jones, Controls on subduction dip angles and their effects on deformation in the overriding plate, Geol. Soc. Am. Abstr. Prog., 42 (5), 78 (Abstract 24-1), 2010
Subduction processes have important impacts on lithospheric and crustal deformation at the surface including orogeny processes. For example, shallow dipping subduction of the Farallon plate is a potential mechanism to explain surface deformation far from the plate boundary during the Laramide orogeny. It is not clear, however, why some subducted slabs have different dip angles, and how the subduction affects topography and deformation in the overriding plate.
One approach for understanding subduction has been to assume that plate geometry is the result of steady state processes. Deformation and stresses within the mantle and in the overriding plate can then be infered as an application of fluid dynamic corner flow. Many authors have used this method to study the dips of subducting plates, stresses in the wedge and at the base of the overriding lithosphere, and seismic anisotropy resulting from flow within the mantle wedge.
Although kinematic models of subduction are very informative of the basic physics, the assumption of a steady state subduction process is likely inappropriate when many studies indicate time dependent slab behavior and more complex mantle flow. Laboratory experiments and numerical models that consider subduction as a dynamic time dependent process account for the internal deformation and buoyancy of slabs, and are hence more complete.
We examine the process of subduction to understand what causes slabs to descend at shallow dips. Our particular focuses are on the role of continental keels adjacent to subduction zones and overriding plate motion. The presence of a nearby continental keel can be expected to alter the pressure field in the wedge. Overriding plate motion can control the rate at which material subducts in relation to the rate at which it sinks into the mantle. However, it is unknown to what degree these factors can affect slab dip. We perform dynamic numerical calculations to quantify the subduction process and study the relation between keels, overriding plate motion, and shallow angle subduction.
Jones, C. H., K. Mahan, and G. L. Farmer (2011), Post-Laramide Epiorogeny through Crustal Hydration?, EOS (Trans. Am. Geophys. Union), 92 (Fall Suppl.), abstract T11B-2309. (ePoster link)
The most perplexing part of the Cordilleran orogen in the western U.S. has been the Cenozoic uplift of broad regions with insufficient crustal shortening to produce the change in elevation following retreat of the Western Interior Seaway. These regions (most notably the High Plains, Wyoming craton, and Colorado Plateau) generally also have heat flow values comparable to much of the tectonically inactive (and low) parts of the U.S. Explanations have included dynamic effects, erosion of mantle lithosphere, cryptic crustal thickening, and hydration of the mantle lithosphere. We suggest that an alternative worthy of investigation is the hypothesis that a garnet-rich lower crust throughout the region was hydrated, producing increased buoyancy capable of driving uplift. A profile from Canada to southernmost Wyoming contains coincident increases in lower crustal hydration, decreases in lower crustal wavespeed, and increases in elevation. Xenoliths from near the Canadian border in Montana are pristine and lack hydrous alteration. Similar xenoliths from the lower crust at the 50 Ma Homestead kimberlite in central Montana have been altered such that garnet+feldspar is partially replaced by a chlorite-calcite-albite assemblage that may have occurred under high-pressure conditions, reducing the rock density from 3.19 Mg/m3 to 3.05 Mg/m3. Farther south, lower crustal hornblende granulite xenoliths from Quaternary volcanic rocks in the Leucite Hills lack garnet and exhibit evidence for hydration reactions, some of which are late Archean. Along the same general trend, the DeepProbe seismic profile yielded a ~20 km thick lower crustal layer with wavespeeds decreasing from 7.7 km/s in Canada to ~7.2 km/s in central Wyoming to <7.0 km/s in southern Wyoming (Gorman et al., 2002). If this variation coincides with a 5-10% decrease in density of this layer, 1-2 km of topography should be produced, comparable to the ~1.5 km difference observed. Evidence for late-stage deep crustal hydration has also been described from xenoliths in the Four Corners region of the Colorado Plateau (Broadhurst, 1986; Selverstone et al., 1999). The presence of a partially hydrated high-wavespeed layer at the base of the crust could complicate attempts to define the Moho using receiver functions, a problem encountered in several areas in Wyoming and the Colorado Plateau.The timing of the observed lower crustal hydration is unknown, but if related to Cenozoic uplift this implies that fluids were added in Late Cretaceous to Early Tertiary, potentially via dehydration of shallowly subducting oceanic lithosphere. If correct, this idea requires some means of passing significant amounts of fluid to the lower crust through the lithospheric mantle.
Jones, C.H., Was there a Laramide "flat slab"?, AGU Fall meeting, abstract T22B-03, 2014
Slab-continent interactions drive most non-collisional orogenies; this has led us to usually anticipate that temporal changes or spatial variations in orogenic style are related to changes in the slab, most especially in the slab's dip. This is most dramatically evident for orogenies in the foreland, well away from the trench, such as the Laramide orogeny. However, the physical means of connecting slab geometry to crustal deformation remain obscure. Dickinson and Snyder (1978) and Bird (1984) laid out a conceptually elegant means of creating foreland deformation from shear between a slab and overriding continental lithosphere, but such strong shear removed all of the continental lithosphere in the western U.S. when included in a numerical simulation of flat slab subduction (Bird, 1988), a removal in conflict with observations of volcanic rocks and xenoliths in many locations. Relying on an increase in edge normal stresses results, for the Laramide, in requiring the little-deformed Colorado Plateau to either be unusually strong or to have risen rapidly enough and high enough to balance edge stresses with body forces. Early deformation in the Plateau rules out unusual strength, and the accumulation and preservation of Late Cretaceous near-sea level sedimentary rocks makes profound uplift unlikely (though not impossible). Relying on comparisons with the Sierras Pampeanas is also fraught with problems: the Sierras are not separated from the Andean fold-and-thrust belt by several hundred kilometers of little-deformed crust, nor were they buried under kilometers of marine muds as were large parts of the Laramide foreland. We have instead suggested that some unusual interactions of an obliquely subducting plate with a thick Archean continental root might provide a better explanation than a truly flat slab (Jones et al., 2011). From this, and given that several flat-slab segments today are not associated with foreland orogenesis and noting that direct evidence for truly flat-slab subduction is limited in cases like the Laramide, arguably flat slabs are neither necessary nor sufficient to drive foreland deformation. Clearly additional aspects of continent-ocean interaction (such as, perhaps, interaction of continental keels) need to be brought into play to understand non-collisional orogenies.
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Last modified at October 15, 2016
