Seismic Imaging and Physical Properties of the Endeavour Segment: Evidence that Skew Between Mantle and Crustal Magmatic Systems Governs Spreading Center Processes

Abstract

We invert Pg, PmP, and Pn traveltimes from an active‐source, multiscale tomography experiment to constrain the three‐dimensional isotropic and anisotropic P wave velocity structure of the topmost oceanic mantle and crust and crustal thickness variations beneath the entire Endeavour segment of the Juan de Fuca Ridge. The isotropic velocity structure is characterized by a semicontinuous, narrow (5‐km‐wide) crustal low‐velocity volume that tracks the sinuous ridge axis. Across the Moho, the low‐velocity volume abruptly broadens to approximately 20 km in width and displays a north‐south linear trend that connects the two overlapping spreading centers bounding the segment. From the seismic results, we estimate the thermal structure and melt distribution beneath the Endeavour segment. The thermal structure indicates that the observed skew, or lateral offset, between the crustal and mantle magmatic systems is a consequence of differences in mechanisms of heat transfer at crustal and mantle depths, with the crust and mantle dominated by advection and conduction, respectively. Melt volume estimates exhibit significant along‐axis variations that coincide with the observed skew between the mantle and crustal magmatic systems, with sites of enhanced crustal melt volumes and vigorous hydrothermal activity corresponding to regions where the mantle and crustal magmatic systems are vertically aligned. These results contradict models of ridge segmentation that predict enhanced and reduced melt supply beneath the segment center and ends, respectively. Our results instead support a model in which segment‐scale skew between the crustal and mantle magmatic systems governs magmatic and hydrothermal processes at mid‐ocean ridges.

Evolution of Fine‐Scale Segmentation at Intermediate‐Spreading Rate Ridges

“…The disruption of the magma lens between AV1 and AV2 also marks a transition in the lava composition from normal MORB to transitional and enriched MORB. Similar relations between tectonic and magmatic discontinuities have been documented along the EPR (Carbotte et al, , ; Le Saout et al, ; White et al, ), and third‐order tectonic discontinuities have been interpreted to reflect variation in crustal melt volume, which decreases near discontinuities of the magma lens (e.g., Arnoux et al, ; Carbotte et al, ; White et al, ).…”

“…The two larger disruptions of the AML correspond to gaps of magma lenses over distances exceeding 2 km and are directly linked to the locations of the third‐order discontinuities identified in the bathymetry (Figure ). These third‐order discontinuities are also associated with variation in melt content with higher melt content in the center of AV2 evidenced by seismic tomography (Arnoux et al, ). The disruption of the magma lens between AV1 and AV2 also marks a transition in the lava composition from normal MORB to transitional and enriched MORB.…”

“…At the Alarcon Rise, we therefore expect a magma lens to be present along its entire length, or at least under the third‐order segment EA2, where the ridge is the most inflated and the youngest and most vigorous volcanic activity occurs (Clague et al, ). Based on the observation that third‐order segments are often underlain by individual volcanic plumbing systems (White et al, ) associated with higher mantle and crustal melt at the segment center (Arnoux et al, ), and on the correlations between tectonic and magmatic discontinuities observed along the Endeavour Segment and EPR segments (Carbotte et al, ; Han et al, ; Le Saout et al, ; Marjanović et al, ), the magma lens underlying the Alarcón Rise is most likely segmented. Disruptions of the magma lens probably lie beneath each third‐order discontinuity along the EA, which is associated with more recent volcanic activity.…”

Discovery of flat seismic reflections in the mantle beneath the young Juan de Fuca Plate

“…This change in the direction of the ridge propagation could be associated with the diminished melt supply to the ridge axis due to the transfer of the Heckle melt anomaly to the east flank 22 . Recently, active-source seismic studies showed that there is a lateral offset between the mantle and the crustal magmatic systems 28,29 (Fig. 1b), which is attributed to the difference in mechanisms of heat transfer that operate in the crust (advection) and mantle (conduction) 29 .…”

“…Recently, active-source seismic studies showed that there is a lateral offset between the mantle and the crustal magmatic systems 28,29 (Fig. 1b), which is attributed to the difference in mechanisms of heat transfer that operate in the crust (advection) and mantle (conduction) 29 . Interestingly, the most prominent low-velocity zones lie directly beneath the OSCs.…”