HR: 1340h
AN: T13B-1366    [Abstracts]
TI: Axis-Parallel Depth and Gravity Profiles: Implications for Intra-Segment Crustal Thickness Variations and Near-Axis Tectonic Disruption
AU: * Cochran, J R
EM: jrc@ldeo.columbia.edu
AF: Lamont Doherty Earh Obs., 61 Route 9W, Palisades, NY 10964
AB: Axial depths within a ridge segment generally increase regularly along the axis away from an intrasegment minimum. These along-axis depth variations are widely assumed to be an isostatic response to along-axis intrasegment variations in crustal thickness, particularly in the Atlantic where MBA gravity lows are consistently found at the shallowest portion of segments, leading to the widespread use of gravity anomalies as a proxy for along-axis crustal thickness variations. However, Neumann and Forsyth [1993] have argued that the ridge axis cannot be locally compensated because the axial valley is a dynamic feature. To the extent that along-axis depth variations reflect crustal thickness variations, they should persist onto the ridge flanks as the isostatic expression of the changes in crustal thickness. Similarly MBA gravity anomalies resulting from crustal thickness variations should continue onto the ridge flanks. We examined ridge-parallel depth and gravity variations, both on-axis and at various distance from the axis, at the EPR, segments of the SEIR with axial highs, and at the MAR. The areas on the EPR and SEIR were chosen because they have large along-axis depth variations.. We picked the ridge axis location at 1' intervals and sampled the bathymetry and gravity grids at 1 km intervals along the flow lines through the ridge axis locations. The bathymetryprofiles were filtered using several techniques to remove the abyssal hill relief and were used to construct ridge parallel profiles at various distances from the axis. Along-axis depth variations at intermediate- and fast-spreading ridges do not extend out onto the ridge flank. Within segments where there is significant relief along the axis, axis-parallel profiles on the ridge flanks are either nearly flat or have a steady long-wavelength regional gradient across the segment. Profiles across the axis demonstrate that changes in axial depth result from variations in the form and relief of the axial morphology. This is particularly clear at intermediate spreading ridges where there is greater along-axis depth variation, but can also be seen at 13-15\deg N portion of the EPR where there is significant along-axis depth variation. At a few segments on the SEIR, a small (100 m) along-axis off-axis bathymetric high is found along the same flow line as the much larger axial bathymetric high. Along-axis depth and MBA gravity profiles at the slow-spreading MAR show a close correlation. However, he characteristic humped axial morphology does not extend off-axis. The interior of segments on the ridge flanks is shallower than the ridge discontinuities, but the shallowest off-axis depths are not in the center of the segment but rather near the ends. The center of the segment, the location of the shallowest axial depth, is several hundred meters deeper than near the segment ends. This appears to be the result of tectonic processes near the segment ends, which form segment-bounding ridges parallel to the axis. These ridges are most prominent on the "inside corners", but are usually present at both on both sides of the axis. At the 33\deg S "Plume" segment on the southern MAR, the off-axis MBA profiles have the same shape as the axial profile, although with slightly smaller amplitude. At shorter segments, the off-axis gravity anomalies do not show a coherent pattern. It thus appears that, at short segments, tectonic activity as the crust is transported up and out of the rift valley may disrupt the crustal thickness distribution established at the ridge axis. At longer segments, disruption by faulting is limited to the segment ends leaving the gravity pattern in the center of the segment intact.
DE: 8130 Heat generation and transport
DE: 3010 Gravity
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting