The axial summit graben and cross-sectional shape of the East Pacific Rise as indicators of axial magma chambers and recent volcanic eruptions

Abstract

The axis of the East Pacific Rise (EPR) undulates up and down hundreds of meters over distances of 30–200 km along strike, the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers (OSCs). We have suggested that systematic variations in depth and cross-sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment [1]. A comparison of recently collected multichannel seismic (MCS) data [2] with our Sea Beam and SeaMARC II data have allowed us to test and advance this hypothesis. Along the EPR from 9° to 13°N there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment: the cross-sectional shape of the rise, the presence or absence of an axial summit graben, and the presence or absence of a shallow axial magma chamber (as interpreted from MCS data). Where the axial magma chamber is present, the cross-sectional shape of the ridge is broad and an axial summit graben is recognized along the axis. In contrast, where the cross-sectional shape of the rise is narrow and triangular, an axial magma chamber is not detected and an axial summit graben is absent. These ridge axis characteristics tend to occur along deeper portions of a given ridge segment, often near ridge axis discontinuities. We suggest that these systematic variations in ridge axis morphology (cross-sectional shape) and structure (presence or absence of an axial graben) reflect spatial and temporal variations in the magmatic budget of the ridge axis. Where the magmatic budget is waxing, shallow-level magma reservoirs in the crust and underlying upper mantle swell, creating a broad axial bulge with a summit graben. Where the magmatic budget is diminished, the crustal magma chamber is small (< 2 km wide) or absent and the ridge axis is characterized by a narrow triangular edifice that lacks a clearly defined and continuous axial graben. A summit graben is missing because the underlying axial magma reservoir is not large enough to produce a significant collapse structure or a caldera. This proposed correlation of shape, structure and magmatic parameters fails along only two short portions of the ridge. In these areas there is evidence for an axial magma chamber and the rise has a broad cross-sectional shape, but there is no summit graben. Bottom photographs and submersible results, however, show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting, suggesting that the summit graben has been recently filled in by lava flows, and the development of a summit graben (or a linear caldera) by volcano-tectonic collapse has not yet occurred.