HR: 1340h
AN: T33B-1361    [Abstracts]
TI: Segment-Scale Variations in Crustal Thickness Along the Southeast Indian Ridge From 2-D Body Wave Tomography
AU: * Holmes, R
EM: cholmes@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AU: Tolstoy, M
EM: tolstoy@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AU: Cochran, J R
EM: jrc@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AB: The Southeast Indian Ridge (SEIR) west of the Australian Antarctic Discordance (AAD) exhibits a nearly constant intermediate spreading rate (72-76 mm/yr) and dredge sample basalt geochemistry despite systematic changes observed in the depth and morphology of the ridge axis. These unique conditions make it possible to investigate how variations in melt production (or equivalently, crustal thickness) correlate with axial morphology along the SEIR. A total of six seismic refraction experiments were conducted along SEIR segments P1, P2, S1, and T as part of R/V Maurice Ewing cruise EW0114 (Dec 2001-Jan 2002). The 100-125 km lines were positioned above zero- age crust for all four segments as well as 20 km south of the axis (~525 ka) for segments P1 and P2. In each survey, four ocean bottom hydrophones deployed 15-20 km apart recorded shots fired by the 8480 in³ 20-gun airgun array. P wave travel times for crustal refractions (Pg) and Moho reflections (PmP) were hand-picked and assigned error values ranging from 15-150 ms based on the trace signal-to-noise ratio. Starting with 1-D crustal velocity-depth profiles, we constructed an overparameterized 2-D inverse problem and derived minimum structure solutions by regularizing the inversion with predefined damping and smoothness constraints. The forward step calculated ray travel times by combining shortest path (graph) method results with a beta-spline ray- bending refinement scheme. We applied a top-down methodology to the inversion, including picks from progressively larger instrument offsets with each iteration to account for the dominant influence of upper crustal heterogeneity on travel time variations. Our preferred models show a consistent thinning of the crust toward the AAD coincident with the transition from a robust axial high (P1) to a well-developed axial valley (T). We performed a direct assessment of model resolution using checkerboard tests, nonlinear Monte Carlo analysis, and a variable depth kernel weighting strategy. These tests indicate our results are robust, which suggests the relationship between melt production and ridge character behaves in a stepwise fashion where changes in melt supply and crustal thickness are expressed by abrupt shifts in the depth and morphology of the ridge axis.
DE: 3025 Marine seismics (0935, 7294)
DE: 3035 Midocean ridge processes
DE: 7245 Mid-ocean ridges
DE: 7270 Tomography (6982, 8180)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting