Influence of magma supply and spreading rate on crustal magma bodies and emplacement of the extrusive layer: Insights from the east Pacific rise at lat16 degrees N

Seismic reflection data from the East Pacific Rise at lat 16 degrees N, which is spreading at the high end of intermediate rates, suggest that the depths at which axial magma chambers reside do not vary smoothly as a function of spreading rate, Rather, magma-chamber depths form two distinct populations, each associated,vith a distinct axial morphology and with an abrupt transition occurring,within the intermediate-spreading-rate range. Our data (1) show that the melt lens at high-intermediate-spreading ridges lies at a shallow level similar to lens depths at faster-spreading ridges, and (2) provide further support for the spreading-rate invariance of ridges with axial highs noted in other ridge properties. The axial morphology of the two ridge segments within the study area differs markedly, and a large contrast in magma supply is inferred. The ridge segment with greater magma supply is associated with a broader and more continuous melt lens, a wider region over which the extrusive crust accumulates, and a thicker extrusive layer off-axis where supply to the ridge segment appears to be centered. However, on-axis, the extrusive layer is thinnest where magma supply is robust and a shallower melt lens is observed, consistent with a model in which magma pressure controls the thickness of the extrusive layer accumulated above the magma lens.