Suzanne M. Carbotte, Lamont Doherty Earth Observatory

Comparisons of crustal properties at the EPR and Endeavour ISS reveal commonalities that bear on a number of long-standing questions including the origin and significance of ridge axis segmentation, influence of mantle melt anomalies on ridge processes, and driving forces for ridge propagation.

1. Influence of mantle melt anomalies on  ridge axis structure
Both the EPR and Endeavour sites are adjacent to off-axis seamount chains and proximity of these local melt anomalies is presumably linked to localization of magmatism at both sites. The Lamont seamounts are located immediately west of the EPR “bulls eye” (9°49-9°51’N). Here, the ridge axis and the underlying mid-crust magma lens reach the local shallowest point along the ridge, hydrothermal venting is clustered, and the lavas with highest Mg# have erupted [Perfit et al., 1994].  Similar relationships are observed at Endeavour segment where the hydrothermally and magmatically active portion of this segment coincides with the on-axis projection of the Heckle seamount chain. An axial magma lens is present only beneath the central shallow portion of the segment where active venting is also focused. Seismic data indicate thicker crust has been accreted within this central portion of the segment for the past 0.7 Ma coincident with timing of ridge intersection with the Heckle chain. An important prediction of the ridge-melt anomaly interaction apparent at these sites is that regions of locally enhanced axial magmatism are likely to persist for long time periods (10’s– 100’s of ka) and longevity in the axial hydrothermal system is also expected.

2. Fine-scale segmentation of crustal melt
On a local scale, fourth-order segmentation of the mid-crustal magma lens plays a key role in the variability in seafloor volcanism and hydrothermal activity at both sites. At the EPR, segmentation of the axial magma lens into discrete overlapping lenses with along-strike dimensions of 5-10 km is evident in a new 3D multichannel seismic reflection data set. The main clusters of hydrothermal venting at the EPR site (at 9°49’-50’ and ~9°46-47’) are underlain by discrete lenses. Fine-scale geochemical sampling within the region reveals variations in major element geochemistry, also linked to the underlying magma lens segmentation. Although the detailed geometry of the magma lens at Endeavour segment can not be confidently established without a 3D seismic survey, the existing 2D seismic reflection data indicate magma lens segmentation on a similar scale may be present, with a discrete lens beneath the Mothra vent, offset from a shallower lens beneath the vents to the north. Along-strike differences in geochemistry of axial lavas may be linked to this segmentation.

3. Structure of propagating offsets and driving forces for ridge propagation
Early observations of seafloor structure at modern propagating ridges indicated that the magmatic ridge tip is typically located 10-20 km behind the extensional rift tip. Recent observations of discordant zones left by propagating ridge offsets at both the Endeavour and EPR sites reveal a local zone of thicker crust is present at roughly the same location, and local melt anomalies associated with propagating ridge tips are inferred. On the JdF plate, a 10-20 km wide zone of thicker and possibly denser crust is found on the young crust side of pseudofaults left by former propagating offsets [Marjanovic et al., in prep.]. A sequence of bright ridge-ward dipping sub-Moho seismic reflections underlie the region of thicker crust and are interpreted as frozen melt sills at the base of the crust emplaced behind the propagating ridge tips [Nedimovic et al., 2005]. These frozen magma lenses are presumably the source magma bodies for the denser, iron-enriched crustal rocks found above and within the adjoining pseudofault zones. Similarly, at the southward propagating 9°03’N OSC at the EPR site an ~20 km wide band of crust that is both thicker and denser is located behind the v-shaped discordant zone left by the OSC propagation [Canales et al., 2002; Toomey and Hooft, 2008]. At the southern edge of this band of thick crust, Singh et al. [2008] find evidence for a large melt anomaly in the lower crust and anomalously thick crust at the propagating eastern ridge of the OSC. The presence of local melt anomalies beneath propagating ridge tips presumably contributes to the forces driving ridge propagation in these regions. These melt anomalies may arise from small shallow mantle compositional or thermal anomalies. Alternatively, as the propagating ridge advances into colder preexisting lithosphere, damming and accumulation of melts due to the strongly 3D topography at the base of the lithosphere may be important.