Architecture of on- and off-axis magma bodies at EPR 9°37–40′N and implications for oceanic crustal accretion

Abstract

3D multi-channel seismic imaging crustal accretion mid-ocean ridges East Pacific Rise volcanism Crustal accretion at fast-spreading mid-ocean ridges is believed to be concentrated in a narrow zone up to a few kilometers wide centered beneath the ridge axis. However, there is increasing evidence for off-axis magmatism occurring beyond this narrow zone. Here, we present 3D multichannel seismic (MCS) images from the East Pacific Rise 9 • 37-40 N extending to 11 km on the ridge flanks. In the axial region, two offset axial magma bodies underlie a small ridge-axis discontinuity at ∼9 • 37 N, displaying an overlapping geometry similar to that of the seafloor structures above. On the ridge flanks, a series of off-axis magma lenses (OAML) are imaged: they are located 2-10 km from the ridge axis, at 700 to 1520 ms two-way travel time below seafloor (bsf) (∼1.6 to 4.5 km bsf), with variable areas ranging from 0.5 km 2 to 5.2 km 2. The largest body is centered 4 km east of the ridge axis and is composed of a large, continuous, flat-topped lens and a series of small, discontinuous, westward-dipping bodies along its western edge. The flat crest of the OAML lies at approximately the same depth beneath layer 2A as the axial magma lens and we infer that this OAML has formed by aggregation of ascending melts that accumulate at the base of the sheeted dike section. A cluster of reflections underlying the OAML at 1260-1510 ms bsf are observed that may be deeper lenses feeding melts to the upper lens. This largest OAML is associated with Moho travel time anomalies of 120-260 ms within a zone that extends up to 2 km from the edge of the OAML, suggesting a lower crust that is partially molten with lower crustal velocities reduced by 8-18% and/or thicker than normal by up to 1 km. Local volcanic edifices are found above two of the three OAMLs imaged in our study area and are inferred to be the eruptive products of the OAMLs. From the volume of these edifices and the Moho travel time anomalies we estimate the potential contribution of off-axis magmatism to the total volume of the crust to be ∼0.01-3%. The OAMLs imaged in our study area are present over roughly the same distance range as the zone of formation of near-axis seamounts. We speculate that OAMLs and the volcanic edifices found above them are smallscale manifestations of the off-axis magmatism that gives rise to near-axis seamounts.

Formation of fast-spreading lower oceanic crust as revealed by a new Mg–REE coupled geospeedometer

“…The upward decrease in the initial temperature suggests that the deeper section (>2000 m) confines the temperature of melts delivered shallower. Many small-scale temperature variations in the deeper section may be attributed to vigorous convection of a large, deepseated cooling magma chamber, which, however, is against the 3-D seismic observations at EPR (e.g., Han et al, 2014). More likely, these small-scale variations indicate accumulation and in situ solidification of replenished melts in small magma bodies (e.g., tens to hundreds of meters) at various depths, as advocated by the multiple-sill model (e.g., Nicolas et al, 1988;Kelemen et al, 1997;Lissenberg et al, 2004;Maclennan et al, 2005;VanTongeren et al, 2008VanTongeren et al, , 2015Natland and Dick, 2009).…”

“…Anomalously rapid cooling rates (0.372-10.2 • C/yr) at 2198-3193 m may be attributed to (1) localized hydrothermal flows induced by faults/shear zones and/or (2) small magma bodies emplaced in the colder off-axial lower crust possibly related to the brittle-ductile transition. Both are equally feasible mechanisms, but recent 3-D seismic observations at EPR (e.g., Han et al, 2014) favor the latter. Although significant uncertainties are likely involved in the thermometer of Faak et al (2013) at lower temperatures (<1100 • C; see inset in Fig.…”

“…7), which involves axial and off-axial magma bodies (e.g., Han et al, 2014) and a crustal-scale mush zone (e.g., Lissenberg et al, 2013;Lissenberg and MacLeod, 2016). As indicated by the stratigraphic variations of cooling histories, the mush zone is supplied by mantle-derived primary melts from the bottom and is efficiently cooled by hydrothermal circulations across the entire lower crust.…”

Distribution of melt along the East Pacific Rise from 9°30′ to 10°N from an amplitude variation with angle of incidence (AVA) technique

“…In addition to the main ridge-perpendicular survey acquired for 3-D imaging of crustal structure (Canales et al 2012a,b;Aghaei et al 2014;Han et al 2014), an along-axis swath survey (Carbotte et al 2013;Marjanović et al 2014;Xu et al 2014) was conducted (Fig. 1a).…”

The hydrothermal power of oceanic lithosphere

“…As this fit to a seismic model reflects the Nu number, the seismic model supports the choice of Nu ≈ 10 rather than significantly higher values suggested elsewhere (e.g., Cochran and Buck, 2001;Spinelli and Harris, 2011). On the other hand, Han et al (2014), based on the observation of off-axis magma lenses in regions Dunn et al (2000) expected to be cool, suggested that Dunn's model may be inaccurate. However, it is not clear what ambient thermal structure is consistent with the presence of off-axis magma lenses, as these may be anomalous, even if frequent, features.…”