SEISMIC STRUCTURE AND INDICATORS OF MAGMA BUDGET ALONG THE SOUTHERN EAST PACIFIC RISE

In this paper we re-examine the relationship between seismically const rained variations in crustal structure along the southern East Pacific Rise (SEPR) and the segment-scale variations in axial depth, morpholo gy, basalt geochemistry, and hydrothermal activity that have often bee n attributed to along-axis differences in the supply of magma to the m id-ocean ridge. Along >800 km of the East spreading SEPR, good correla tions exist between axial depth, ridge cross-sectional area, mantle Bo uguer anomaly, and the MgO weight percent of basalts recovered from th e rise axis. These correlations indicate along-axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of similar to 100,000 years. In contrast, we show that the depth and width of the midcrustal magma sill, the thickness of sei smic layer 2A, and the intensity of hydrothermal venting are poorly co rrelated with regional variations in ridge depth and cross-sectional a rea. We suggest that the emplacement geometry (width of the intrusion zone and flow lengths), not magma supply, controls extrusive layer (se ismic layer 2A) thickness. We hypothesize that magma lens properties a nd hydrothermal activity are closely linked to spreading events (dike intrusion, eruptions, faulting) which occur on much shorter timescales (similar to 10-100 years) than the longer-term variations in ma,oma s upply reflected in along-axis changes in the shape and depth of the ri dge axis.