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

Abstract

S U M M A R YWe examine along-axis variations in melt content of the axial magma lens (AML) beneath the fast-spreading East Pacific Rise (EPR) using an amplitude variation with angle of incidence (AVA) crossplotting method applied to multichannel seismic data acquired in 2008. The AVA crossplotting method, which has been developed for and, so far, applied for hydrocarbon prospection in sediments, is for the first time applied to a hardrock environment. We focus our analysis on 2-D data collected along the EPR axis from 9• 29.8 N to 9• 58.4 N, a region which encompasses the sites of two well-documented submarine volcanic eruptions (1991-1992 and 2005-2006). AVA crossplotting is performed for a ∼53 km length of the EPR spanning nine individual AML segments (ranging in length from ∼3.2 to 8.5 km) previously identified from the geometry of the AML and disruptions in continuity. Our detailed analyses conducted at 62.5 m interval show that within most of the analysed segments melt content varies at spatial scales much smaller (a few hundred of metres) than the length of the fine-scale AML segments, suggesting high heterogeneity in melt concentration. At the time of our survey, about 2 yr after the eruption, our results indicate that the three AML segments that directly underlie the 2005-2006 lava flow are on average mostly molten. However, detailed analysis at finer-scale intervals for these three segments reveals AML pockets (from >62.5 to 812.5 m long) with a low melt fraction. The longest such mushy section is centred beneath the main eruption site at ∼9• 50.4 N, possibly reflecting a region of primary melt drainage during the 2005-2006 event. The complex geometry of fluid flow pathways within the crust above the AML and the different response times of fluid flow and venting to eruption and magma reservoir replenishment may contribute to the poor spatial correlation between incidence of hydrothermal vents and presence of highly molten AML. The presented results are an important step forward in our ability to resolve small-scale characteristics of the AML and recommend the AVA crossplotting as a tool for examining mid-ocean ridge magma-systems elsewhere.

Seismic Signatures of Hydrothermal Pathways Along the East Pacific Rise Between 9°16′ and 9°56′N

“…Elsewhere, the signal of low‐velocity anomaly is weak (<200 m/s) and in places, it is contrasting with the results from previous studies on melt distribution (Marjanović et al, ; Xu et al, ). For instance, while the AML segment extending between 9°40 and 42′N was characterized as a highly molten region, with bright reflection and shear velocities dropping to zero (Xu et al, ), the 2‐D waveform inversion shows only slight change in velocity (<100 m/s).…”

“…In addition to the upper crustal variations in velocities, we also image a subhorizontal, low‐velocity zone at ~1,500 m below the seafloor, visible north of 9°43′N that we attribute to the presence of the AML, a heat source for the onset and maintenance of hydrothermal activity (Figures b, , and S10). The strongest low‐velocity signal is observed around 9°50′N (about 500 m/s lower than the surrounding), where the AML is defined as mostly molten, with prominent patches of highly molten regions (Marjanović et al, ). It is also the area of hydrothermal discharge (Baker et al, ; Fornari et al, ), supporting the idea that permeability and thermal regime are equally important in determining the distribution of vent sites (Canales et al, ; Singh et al, ).…”

“…The blue stars indicate locations of the vents extracted from earlier studies (e.g., Haymon et al, ). The yellow, orange, and red rectangles represent interpretation of melt distribution along the EPR from Marjanović et al () where yellow represents solidified to mushy, orange mostly molten, and red highly molten portions of the AML segments defined by Carbotte et al (). The red and blue rectangles at the bottom show signal from ephemeral, oxidation‐reduction potential (ORP) and broadly dispersing nephelometric turbidity units (NTU), respectively, from Baker et al ().…”

“…An illustration of processes taking place at the EPR compiling results on the hydrothermal circulation (this study), presence of the AML, its magmatic segmentation (Carbotte et al, ), and melt distribution (Marjanović et al, ). The blue and red regions with arrows superimposed indicate the presence of downgoing and upgoing pathways of a hydrothermal cell (as in Figure 8c), present within the upper crust.…”

“…The analysis of the along‐axis data indicated the presence of a fine‐scale AML segmentation linked to the seafloor morphology (Carbotte et al, , ). In addition, it has been shown that some of the AML segments are highly molten (Marjanović et al, ; Xu et al, ). To further explore and characterize zero‐age upper crust along the EPR, we apply elastic 2‐D wave‐equation based techniques (wave equation tomography and waveform inversion) to redatumed axis‐centered seismic profile spanning the region between ~9°16′ and 9°56′N.…”