The depth extent of mantle processes contributing to surface topography of the western U.S., as constrained by the 410 km and 660 km discontinuities

Melissa McMullen, Purdue University

Abstract

The depth extents of both regional and plate-scale processes within the upper mantle can be assessed by using the 410 and the 660 discontinuities to determine if these processes extend to the base of the upper mantle. Plate-scale processes have been proposed to extend to the core-mantle boundary, while regional-scale processes may be confined to shallower depths above the transition zone. Interpreting depth variations along the discontinuities as thermal perturbations and using tomographic studies to assess the vertical extent of those perturbations through the upper mantle, we can evaluate the potential sources of convective processes influencing both the transition zone and surface topography. This study is the first of its kind to analyze the upper mantle structure of entire active western United States and a comparable portion of the stable craton. Our study provides a context in which focused regional studies can be compared and adds a level of detail that is not usually seen in continental scale investigations. Topography along the upper mantle discontinuities, recorded by P-to S-wave receiver functions demonstrated a clear distinction between the processes controlling topography of the 410 and the 660 discontinuities. The 410 shows strong contributions from processes within the upper mantle and a clear signature that changes between the tectonically active western US and the stable craton, suggesting a connection between surface tectonics and convective processes influencing the 410. Conversely, the 660 lacks this distinction and illustrates more of an association with processes near the base of the transition zone and within the lower mantle. As a result topography of the discontinuities seems to reflect a level of separation between the top of the transition zone and the base. Plate-scale processes represented by the subduction of the Farallon plate, are observed to influence both the 410 and the 660, supporting penetration of the slab into the lower mantle. Evidence of plate-scale processes on the 410 is inferred by the presence of three large topographic domains. 1) A deepened western domain representing a 5–10 km response to upwelling material thought to have been transported across the discontinuity with the detachment of the flat slab. 2) An eastern domain which may represent the current location of the Laramide slab, inducing a ∼10 km shallowing of the 410. 3) Finally, a central domain with a potentially cratonic signature, resulting from the lack of influence by the warm or cool material of the other two domains. Plate-scale processes are potentially present on the 660, but appear as regional features and do not produce large areas of consistent topography. Regional-scale processes are predominantly restricted to depths within the transition zone or above. These features on the 410 and 660 are placed in context with processes contributing to surface topography through tectonic models of seven physiographic provinces. The 410 beneath the Colorado Plateau and the Rio Grande rift exhibit signatures suggestive of the influence of warm material emplaced following slab removal and lithospherically induced convection. Cascadia and the Columbia Plateau both show a close affiliation with the Yellowstone plume and the results of the plume interacting with the subducting slab. Negatively correlated discontinuity topography associated with the location of the Yellowstone plume, suggests a source deeper than the transition zone, but a thermal perturbation of only ∼200K is not consistent with a plume originating at the D” layer. Only the Basin and Range and Rocky Mountain provinces do not exhibit evidence of regional processes contributing to surface tectonics and extending to the transition zone.

Degree

M.S.

Advisors

Gilbert, Purdue University.

Subject Area

Geology|Geophysics

COinS