Nathan Alan Simmons

Thermochemical structure and dynamics of the African superplume

[1] We present a new three-dimensional (3-D) model of the thermochemical structure of the African superplume region obtained from simultaneous inversion of global seismic and convection-related data. Convection-related observations include the global free-air gravity field, tectonic plate motions, dynamic surface topography and the excess ellipticity of the core-mantle boundary. A 3-D image of the chemically-induced density perturbations provides direct evidence that intrinsically-dense material is entrained within the superplume and concentrated into a rounded structure � 1000 km above the core-mantle boundary. The thermally-induced density perturbations are greater in magnitude than the chemically-induced implying overall positive buoyancy throughout the superplume. The observed morphology and density signatures are consistent with a thermochemical plume that has risen from a compositionally-distinct ‘pile’ at the base of the mantle and may be currently deforming under the influence of its intrinsic negative chemical buoyancy. Citation: Simmons, N. A., A. M. Forte, and S. P. Grand (2007), Thermochemical structure and dynamics of the African superplume, Geophys. Res. Lett., 34, L02301, doi:10.1029/2006GL028009.

Dynamic Topography Change of the Eastern United States Since 3 Million Years Ago

By the Sea Side The Atlantic coastal plain of North America has been thought of as a passive margin, responding mostly to the weight of deposited sediments. As a result, the fine-scale stratigraphy of the sediments has been used to infer changes in global sea level through the Cenozoic. However, recent work has shown that the coastal plain has deformed in response to flow in Earths mantle. Rowley et al. (p. 1560, published online 16 May) used a model of flow in the mantle to show that the topography of the mid-Atlantic and Southern United States coast varied by 60 meters or more during the past 5 million years. Mantle flow has deformed the presumed passive eastern margin of North America by up to 60 meters during the past 5 million years. Sedimentary rocks from Virginia through Florida record marine flooding during the mid-Pliocene. Several wave-cut scarps that at the time of deposition would have been horizontal are now draped over a warped surface with a maximum variation of 60 meters. We modeled dynamic topography by using mantle convection simulations that predict the amplitude and broad spatial distribution of this distortion. The results imply that dynamic topography and, to a lesser extent, glacial isostatic adjustment account for the current architecture of the coastal plain and proximal shelf. This confounds attempts to use regional stratigraphic relations as references for longer-term sea-level determinations. Inferences of Pliocene global sea-level heights or stability of Antarctic ice sheets therefore cannot be deciphered in the absence of an appropriate mantle dynamic reference frame.

Mantle convection and the recent evolution of the Colorado Plateau and the Rio Grande Rift valley

The Colorado Plateau contains Late Cretaceous marine strata that are at a mean elevation of ~2 km. The timing and amount of uplift since the Cretaceous has generated considerable debate. With the exception of a few studies, topography supported by vertical stresses generated by viscous fl ow in the mantle has not been explicitly considered to contribute to the elevation of this region. Herein we compute the viscous fl ow beneath North America that is driven by density anomalies inferred from joint seismic-geodynamic modeling. We fithat the Colorado Plateau overlies a strong mantle upwelling that is coupled to the sinking Farallon slab, currently beneath the eastern United States. Consequently, the Colorado Plateau is currently a focused dynamic topography high within the western U.S. Cordillera. Moreover, this strong upwelling impacts the base of the lithosphere at an oblique angle east of the plateau directly below the Rio Grande Rift. We attribute this fl ow as being responsible for some of the recent magmatic activity along the Jemez lineament as well as contributing to the recent rifting process in the Rio Grande Rift valley.

Evidence for long-lived subduction of an ancient tectonic plate beneath the southern Indian Ocean

In this study, ancient subducted tectonic plates have been observed in past seismic images of the mantle beneath North America and Eurasia, and it is likely that other ancient slab structures have remained largely hidden, particularly in the seismic-data-limited regions beneath the vast oceans in the Southern Hemisphere. Here we present a new global tomographic image, which shows a slab-like structure beneath the southern Indian Ocean with coherency from the upper mantle to the core-mantle boundary region—a feature that has never been identified. We postulate that the structure is an ancient tectonic plate that sank into the mantle along an extensive intraoceanic subduction zone that migrated southwestward across the ancient Tethys Ocean in the Mesozoic Era. Slab material still trapped in the transition zone is positioned near the edge of East Gondwana at 140 Ma suggesting that subduction terminated near the margin of the ancient continent prior to breakup and subsequent dispersal of its subcontinents.

Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling

Longitudinal stability of East Pacific Rise reflects coupling of deep-mantle buoyancy, mantle-wide flow, and seafloor spreading. Earth’s tectonic plates are generally considered to be driven largely by negative buoyancy associated with subduction of oceanic lithosphere. In this context, mid-ocean ridges (MORs) are passive plate boundaries whose divergence accommodates flow driven by subduction of oceanic slabs at trenches. We show that over the past 80 million years (My), the East Pacific Rise (EPR), Earth’s dominant MOR, has been characterized by limited ridge-perpendicular migration and persistent, asymmetric ridge accretion that are anomalous relative to other MORs. We reconstruct the subduction-related buoyancy fluxes of plates on either side of the EPR. The general expectation is that greater slab pull should correlate with faster plate motion and faster spreading at the EPR. Moreover, asymmetry in slab pull on either side of the EPR should correlate with either ridge migration or enhanced plate velocity in the direction of greater slab pull. Based on our analysis, none of the expected correlations are evident. This implies that other forces significantly contribute to EPR behavior. We explain these observations using mantle flow calculations based on globally integrated buoyancy distributions that require core-mantle boundary heat flux of up to 20 TW. The time-dependent mantle flow predictions yield a long-lived deep-seated upwelling that has its highest radial velocity under the EPR and is inferred to control its observed kinematics. The mantle-wide upwelling beneath the EPR drives horizontal components of asthenospheric flows beneath the plates that are similarly asymmetric but faster than the overlying surface plates, thereby contributing to plate motions through viscous tractions in the Pacific region.