Constraints on melt content of off‐axis magma lenses at the East Pacific Rise from analysis of 3‐D seismic amplitude variation with angle of incidence

Abstract

We use 3‐D multichannel seismic data to form partial angle P wave stacks and apply amplitude variation with angle (AVA) crossplotting to assess melt content and melt distribution within two large midcrustal off‐axis magma lenses (OAMLs) found along the East Pacific Rise from 9°37.5′N to 9°57′N. The signal envelope of the partial angle stacks suggests that both OAMLs are partially molten with higher average melt content and more uniform melt distribution in the southern OAML than in the northern OAML. For AVA crossplotting, the OAMLs are subdivided into seven ~1 km2 analysis windows. The AVA crossplotting results indicate that the OAMLs contain a smaller amount of melt than the axial magma lens (AML). For both OAMLs, a higher melt fraction is detected within analysis windows located close to the ridge axis than within the most distant windows. The highest average melt concentration is interpreted for the central sections of the OAMLs. The overall low OAML melt content could be indicative of melt lost due to recent off‐axis eruptions, drainage to the AML, or limited mantle melt supply. Based on the results of this and earlier bathymetric, morphological, geochemical, and geophysical investigations, we propose that the melt‐poor OAML state is largely the result of limited melt supply from the underlying mantle source reservoir with smaller contribution attributed to melt leakage to the AML. We hypothesize that the investigated OAMLs have a longer period of melt replenishment, lower eruption recurrence rates, and lower eruption volumes than the AML, though some could be single intrusion events.

Distribution of Crustal Melt Bodies at the Hot Spot‐Influenced Section of the Galápagos Spreading Centre From Seismic Reflection Images

“…Our understanding about crustal accretion has been changing during the last decade as improved seismic reflection images from different settings provided evidence for more complex crustal magmatic systems. Recent evidence include segmentation of the AML (Carbotte et al, ; Marjanović et al, ), crustal axial melt sills below the AML (Arnulf et al, ; Marjanović et al, ), lower crustal sills in the near axis region (Canales et al, ), and off‐axis magmatic systems (Aghaei et al, ; Canales et al, ; Han et al, ).…”

Brine Formation and Mobilization in Submarine Hydrothermal Systems: Insights from a Novel Multiphase Hydrothermal Flow Model in the System H2O–NaCl

“…Axis perpendicular brine flow may also be related to off-axis melt lenses (OAML) close to the AML. Two mid-crustal OAMLs have been found within only 1.5 to 2.5 km lateral distance and at 200 m vertical distance from ridge axis at EPR 9°37.5′N to 9°57′N (Aghaei et al 2017;Han et al 2014). At the time of the seismic survey, these OAMLs had low melt fractions implicating that magma replenishment rates of the OAML are relatively low with respect to the AML.…”

The Role of On‐ and Off‐Axis Faults and Fissures During Eruption Cycles and Crustal Accretion at 9°50′N, East Pacific Rise

“…Both discontinuities have orientations resulting in an increasing distance from the ridge crest in the southward direction, and both have the fault throws that increase in the southward direction (Figure 17b). The fault at ∼9°48′N is oriented in a more oblique direction to the axis (Figure 17a) and its location is coincident with an OAML imaged by seismic reflection data (Aghaei et al, 2017).…”