An examination of along-axis variation of magma chamber width and crustal structure on the East Pacific Rise between 13 degrees 30 ' N and 12 degrees20 ' N

We investigate the along-axis variations of magma chamber width and crustal structure along the East Pacific Rise (EPR) from 13 degrees 30'N to 12 deg rees 20'N through reprocessed common depth point (CDP) reflection profiles. The magma lens is, predominantly, a continuous feature in the study area w ith an average width of similar to 500 m as determined from migrated cross- axis CDP profiles. This value is similar to widths estimated elsewhere alon g the EPR, suggesting that the axial magma chamber (AMC) width is not sprea ding rate dependent once the threshold for a steady state magma chamber is reached. The axial morphology of the 13 degrees N area is generally not a g ood predictor of magma lens width or continuity. A fairly continuous melt l ens is imaged where the triangular axial topography might suggest waning ma gma supply. In fact, between 13 degrees 05'N and 13 degrees 01'N a shallow melt lens has been imaged which may be indicative of recent or impending er uptive activity. This shoaling is similar to that observed near the 17 degr ees 26'S region of the EPR where the rise axis summit is domed and highly i nflated. Generally, the thickness of seismic layer 2A beneath the ridge cre st is uniform and comparable to that estimated for 9 degrees N, 14 degrees S, and 17 degrees S on the EPR, suggesting that the axial extrusive layer i s invariant along fast spreading ridges. Uniformity of layer 2A thickness a long-axis implies that variations in magma chamber depth are directly attri buted to changes in thickness of the sheeted dike complex (seismic layer 2B ). Contrary to expectations of decreasing melt sill depth with increasing s preading rate, the average thickness of seismic layer 2B is slightly less ( similar to 165 m) at 13 degrees N than at the faster spreading, more robust 9 degrees N area. Finally, geochemical/petrologic boundaries, which may de lineate separate melt supply regions, occurring at the 13 degrees 20'N and 12 degrees 46'N devals (deviation in axial linearity) are observed to coinc ide with subtle changes in AMC and layer 2A reflection characteristics.