DISTRIBUTION OF MAGMA BENEATH THE EAST PACIFIC RISE NEAR THE 9-DEGREES-03'N OVERLAPPING SPREADING CENTER FROM FORWARD MODELING OF COMMON DEPTH POINT DATA

We have reprocessed six cross-axis and three along-axis common depth p oint (CDP) profiles near the 9-degrees-03'N overlapping spreading cent er (OSC) to understand the relationship between axial magma chamber (A MC) width and seafloor morphology. Travel time modeling of the AMC ref lector reveals an asymmetric distribution of melt across the 9-degrees -03'N OSC. The variation of modeled AMC width beneath either OSC limb is minimal, but the width increases nearly fourfold across the offset attaining an estimated maximum width of 4.15 km near the 9-degrees-17' N ridge axis discontinuity. Additionally, melt distribution underlying the eastern rise limb is not symmetric with respect to the rise axis/ neovolcanic zone but is displaced toward the western rise flank. Depth migration, based on a continuum velocity model consistent with postcr itical reflections from the base of layer 2A, places the skewed AMC re flector beneath a nearly constant thickness sheeted dike section which dips approximately 10-degrees away from the rise axis. To confirm AMC continuity beneath the western rise flank, we use the Maslov syntheti c seismogram method to show that amplitude enhancement of the AMC refl ector is consistent with a continuous melt body underlying a thickenin g extrusive layer. Analysis of along-strike CDP profiles indicates an AMC which is neither overlapping nor discontinuous when projected onto the along-strike plane. Identifying intracrustal events on along-axis CDP lines. however, requires extreme caution; we have modeled out-of- plane scattering using a Kirchhoff formulation, and we show that a coh erent event identified beneath the overlap basin results from diffract ion off the AMC which lies nearly 3 km to the west of the profile. We attribute the asymmetric pattern of melt to a decoupling of melt suppl y from preexisting weaknesses in the brittle upper crust In this model , melt ascends upward (buoyancy forces) until deflected by the imperme able sheeted dike complex; melt then migrates updip, beneath the base of the sheeted dikes, toward the neovolcanic zone where fissuring prod uces a temporary conduit for emplacement. Discrete jumps in modeled AM C width toward the overlap basin represent a further displacement/defo cusing of melt supply (western AMC edge) relative to the neovolcanic z one (eastern AMC edge). The asymmetric pattern of melt therefore repre sents a gradual, en-echelon accommodation of melt supply across the 9 km of ridge axis offset at 9-degrees-03'N. Thus for asymmetric configu rations, AMC width may not correlate solely with magmatic robustness b ut may signify the amount of decoupling which exists between melt supp ly and extrusive emplacement within the neovolcanic zone. Here we pres ent a new model for OSC development which invokes a significant compon ent of cross-axis melt migration. Moreover, abrupt changes in AMC widt h near ridge axis discontinuities (e.g., 9-degrees-17'N deviation in a xial linearity) suggest that any along-axis melt migration is confined to subsegments of the ridge and seem to preclude the segment length m igration of melt proposed in some current models. The transition of me lt supply beneath the overlap basin might favor a continuous low-veloc ity zone underlying this feature; if true, basin development may be re lated to the subsidence of a mechanically weak crustal lid. The propos ed model for OSC development therefore views ridge axis discontinuitie s as the surficial response of misalignment and/or defocusing of melt supply in the uppermost mantle.