HR: 09:40h
AN: T51B-07    [PDF]
TI: A causal relationship between the migration of oceanic spreading centers and the magmatic segmentation of ridges
AU: * Carbotte, S
EM: carbotte@ldeo.columbia.edu
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AU: Small, C
EM: small@ldeo.columbia.edu
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AU: Spiegelman, M
EM: mspieg@ldeo.columbia.edu
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AU: Ryan, W B
EM: billr@ldeo.columbia.edu
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AU: Haxby, W
EM: bill@ldeo.columbia.edu
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AU: Buck, R
EM: buck@ldeo.columbia.edu
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AB: The elevation of the world's mid-ocean ridges (MORs) displays pronounced undulations which subdivide the MOR into discrete spreading segments bounded by transform faults and smaller offsets of the axis. These morphological changes have been attributed to spatial variations in the supply of magma from the oceanic mantle but the cause of these variations has not been explained. Shallow and broad ridge segments are believed to reflect enhanced magma supply while deeper, more tectonized segments are believed to be magma-starved. Analysis of morphological changes along the ridge crest in the context of absolute plate motions reveals a simple relationship between the magmatic segmentation of the ridge and ridge migration direction. Currently all spreading centers are migrating relative to the "fixed" hotspot reference frame at a rate and direction determined by the absolute motions of the bounding plates. Along the northern East Pacific Rise (NEPR), where the spreading center migrates to the northwest, depth profiles show that the shallow robust ridge segments are consistently offset in the direction of ridge migration relative to the deeper adjoining segments. This relationship is observed at all transform offsets as well as at smaller discontinuities with offsets typically $<15$ km. Analysis of other fast and intermediate spreading portions of the world's MOR reveals changes in topography consistent with the ridge migration direction at the majority of transform faults and second-order discontinuities. We suggest that these variations in ridge morphology may be a consequence of a three-dimensional pattern of melt focussing from a broad zone of mantle upwelling in the presence of a migrating spreading center. In the case of a segmented spreading center, focussing of melts should occur not only from the mantle upwelling zone beneath each ridge segment, but also across ridge axis discontinuities. Indeed, melt may be preferentially entrained across discontinuities where horizontal flow paths are shorter to the adjoining ridge axis leading to extraction of melts at one segment which originated in the upwelling zone of the adjoining segment. Simple kinematic models of mantle upwelling in the presence of a migrating spreading center predict faster mantle upwelling and resultant higher melt production by decompression beneath an advancing plate (Davis and Karsten, 1986; Schouten et al., 1987). A consequence of this asymmetry is that the pattern of mantle upwelling and melt production across discontinuities will depend on ridge geometry with more melt available for lateral entrainment across discontinuities to leading segments than to the trailing segments.
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting