Evaluation of morphological indicators of magma supply and segmentation from a seismic reflection study of the East Pacific Rise 15°30′–17°N

Abstract

Hydrosweep bathymetry and multichannel seismic reflection data from two contrasting segments of the East Pacific Rise 15°30′–17°N are used to assess the relationship between crustal structure, morphological indicators of magma supply, and ridge segmentation. From stacked and migrated seismic profiles we evaluate the width and depth of the axial magma lens and the geometry of the seismically inferred extrusive crust, layer 2A. In contrast to other studies, correlation between axial morphology and magma lens characteristics is found with a lens that, on average, is twice as wide and slightly shallower (100–200 m) beneath the unusually shallow and broad southern segment and that shoals and broadens beneath the shallowest part of the northern segment. However, large local‐scale variations in lens depth and width are also observed that are not related to morphology. We conclude that magma supply contributes to the regional characteristics of the magma lens beneath a ridge segment, with large variations within a segment due to local processes of magma eruption and delivery. A negative correlation between zero‐age extrusive layer thickness and morphology is found with a thinner extrusive layer along the southern segment. In this innermost axial region above the magma lens, factors such as magma pressure, not magma volume in the lens, may govern the thickness of extrusives that accumulate. The width of the zone over which the extrusive layer is built is positively correlated with morphology with a wide zone where extrusives approximately triple in thickness characterizing the southern segment (5–8 km), and a narrower zone (2–5 km) of minor thickening (less than double) along the northern segment and toward ridge segment ends. This relationship could reflect changes in lava flow characteristics in these areas of contrasting magma supply. We find a close correspondence between discontinuities in the narrow axial summit trough and changes in magma lens presence and geometry, indicating a genetic link between the finest‐scale tectonic segmentation of the ridge and segmentation of the magma lens. In several locations, discrete magma lenses at different levels within the crust (offset by several hundred meters) are imaged beneath adjacent fourth‐order ridge segments.