An examination of along‐axis variation of magma chamber width and crustal structure on the East Pacific Rise between 13°30′N and 12°20′N

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Abstract
We investigate the along‐axis variations of magma chamber width and crustal structure along the East Pacific Rise (EPR) from 13°30′N to 12°20′N through reprocessed common depth point (CDP) reflection profiles. The magma lens is, predominantly, a continuous feature in the study area with an average width of ∼500 m as determined from migrated cross‐axis CDP profiles. This value is similar to widths estimated elsewhere along the EPR, suggesting that the axial magma chamber (AMC) width is not spreading rate dependent once the threshold for a steady state magma chamber is reached. The axial morphology of the 13°N area is generally not a good predictor of magma lens width or continuity. A fairly continuous melt lens is imaged where the triangular axial topography might suggest waning magma supply. In fact, between 13°05′N and 13°01′N a shallow melt lens has been imaged which may be indicative of recent or impending eruptive activity. This shoaling is similar to that observed near the 17°26′S region of the EPR where the rise axis summit is domed and highly inflated. Generally, the thickness of seismic layer 2A beneath the ridge crest is uniform and comparable to that estimated for 9°N, 14°S, and 17°S on the EPR, suggesting that the axial extrusive layer is invariant along fast spreading ridges. Uniformity of layer 2A thickness along‐axis implies that variations in magma chamber depth are directly attributed to changes in thickness of the sheeted dike complex (seismic layer 2B). Contrary to expectations of decreasing melt sill depth with increasing spreading rate, the average thickness of seismic layer 2B is slightly less (∼165 m) at 13°N than at the faster spreading, more robust 9°N area. Finally, geochemical/petrologic boundaries, which may delineate separate melt supply regions, occurring at the 13°20′N and 12°46′N devals (deviation in axial linearity) are observed to coincide with subtle changes in AMC and layer 2A reflection characteristics.