HR: 10:25h
AN: T32B-01 INVITED    [Abstracts]
TI: Segmentation of Axial Depth Along the East Pacific Rise: An Isostatic Response to Magmatic Differentiation that Results From Skew of Mantle Upwelling
AU: * Toomey, D R
EM: drt@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272, United States
AU: Hooft, E E
EM: emilie@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272, United States
AB: Within East Pacific Rise (EPR) ridge segments axial depth typically shoals away from a tectonic offset and toward a local minimum or intra-segment high. Axial depth minima are commonly associated with broad axial summits, intense seafloor hydrothermal activity and the eruption of MgO-rich lavas. A popular hypothesis for explaining these characteristics is that the supply of magma from the mantle to the crust is increased beneath intra- segment highs and reduced near segment offsets. If this conceptual model were correct, we would expect to observe either thickened crust beneath axial depth minima or evidence for the segment-scale, rise-parallel redistribution of magma. Motivated by geophysical studies that are inconsistent with either of these predictions, we propose an alternative model for the origin of rise parallel variations in seafloor depth along fast-spreading ridges. We attribute rise-parallel changes in on- and off-axis seafloor depth to changes in both the density and thickness of the crust, including the Mohorovicic transition zone. We use regional-scale bathymetry, gravity and seismic crustal thickness data from the EPR near 9°N to explore the magnitude of such density variations following two approaches: (1) we determine the variations in crustal density that fit the along-axis gravity, bathymetry and crustal thickness data, and (2) we isostatically estimate the variations in crustal density required to support seafloor depth anomalies. We find that variations in the density and thickness of oceanic crust are consistent with segment-scale, rise-parallel variations in seafloor depth and gravity near the EPR. As there is no evidence for either thicker crust or anomalously low density mantle localized beneath the axial depth minimum in this region, we conclude that crustal density plays a more important role in determining axial depth than heretofore thought possible. By this view, deeper on- and off-axis seafloor depth corresponds to more dense crust, whereas average crustal densities are expected to be less near axial depth minima or where off-axis seafloor depth shoals in the rise-parallel direction. Our modeling results, in combination with seismic observations and simple calculations, can be used to infer the origin of rise-parallel, segment-scale variations in crustal density. We attribute segment-scale variations in crustal density to magmatic differentiation that results from a skew between the axes of mantle upwelling and plate spreading. We use seismic imaging of crustal thickness and upper mantle structure combined with gravity data to provide novel constraints on the temporal evolution of mantle upwelling beneath the EPR and to infer that axial sites of intense volcanic and hydrothermal activity can persist for 10⁵ to 10⁶ years.
DE: 3035 Midocean ridge processes
DE: 7220 Oceanic crust
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8434 Magma migration and fragmentation
SC: Tectonophysics [T]
MN: 2007 Fall Meeting