HR: 09:30h
AN: T11G-07    [Abstracts]
TI: A plate kinematic explanation for the magmatic segmentation of mid-ocean ridges
AU: * Carbotte, S M
EM: carbotte@ldeo.columbia.edu
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AU: Small, C
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AU: Donnelly, K
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AU: Katz, R
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AU: Spiegelman, M
AF: LDEO, 61 Rte 9W, Palisades, NY 10964 United States
AU: Supak, S
AF: Geological Sciences, UCSB, Santa Barbara, CA 93106 United States
AB: Along fast and intermediate spreading mid-ocean ridges (MOR), a direct relationship is found between magmatic segmentation inferred from ridge morphology and the geometry and migration velocity of the plate boundary. All MOR are migrating in the "fixed" hotspot reference frame. Analysis of bathymetric undulations along 9,500 km of ridge crest reveal that segments which are offset in the direction of ridge migration (leading) across transform and non-transform ($>$5km) discontinuities overwhelmingly correspond with shallow, magmatically robust segments. Differences in ridge elevation are similar (10-500 m) in spite of order of magnitude differences in discontinuity length, ruling out cold edge effects to explain observed changes in ridge morphology. Furthermore, morphological contrasts between adjacent segments diminish for the longest transform faults ($> $~150 km), with offset lengths comparable to the half width of the mantle upwelling zone predicted from passive mantle flow models. We attribute these relationships to asymmetric mantle upwelling and melt production beneath migrating ridges, with entrainment of melts generated in the upwelling zone of adjacent segments across discontinuities. Where transform offsets are less than the width of the mantle melt generation zone, this model predicts leading segments can tap a greater portion of melts generated beneath adjacent trailing segments giving rise to differences in magma delivery with plate geometry. In addition to bathymetric changes, geochemical differences in erupted lavas are commonly observed across discontinuities, which indicate different mantle source compositions often attributed to small-scale mantle heterogeneities. Our model provides a mechanism whereby early melting heterogeneities and melts originating deeper in the melting column from the upwelling zone of an adjacent advancing plate could be preferentially entrained beneath leading ridge segments. Along the NEPR 8-18degN, the most enriched and diverse lavas as indicated by incompatible trace element ratios, as well as those derived from deepest melting are found along leading segments, consistent with the notion that geochemical variability along this ridge may reflect the influence of plate kinematics on mantle melt production.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8150 Plate boundary--general (3040)
DE: 8158 Plate motions--present and recent (3040)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting