Fault-controlled magma transport through the mantle lithosphere at slow-spreading ridges

Abstract

Abstract At the ends of spreading segments at slow-spreading ocean ridges the axial valleys are usually asymmetric and bounded by large valley-wall faults, in contrast to segment centres, which are usually symmetric, with relatively small faults. These morphological variations are believed to reflect differences in the thermal structure of the lithosphere, caused by focused upwelling of partially molten mantle beneath segment centres. This gives rise to a thinner crust but thicker, and consequently stronger, lithosphere at segment ends, which is likely to provide a barrier to melt migration in comparison with segment centres. High-resolution sidescan sonar images of parts of the Mid-Atlantic Ridge reveal that flattopped seamounts occur preferentially at segment ends. Many of these seamounts are situated asymmetrically within the axial valley, and show a spatial association with the large segment-end faults, raising the possibility that melt may be channelled through the lithospheric mantle by such faults. If this is the case, then one might expect to encounter gabbros within mantle shear zones. We here document one such case in the Lizard Ophiolite, Cornwall, Southwest England: the Carrick Luz shear zone, a 100m wide dykelike body of gabbro and gabbro mylonite occurring within mantle ultramafic rocks. The shear zone shows a progression from penetrative mylonite fabrics, discrete ultramylonitic shear planes, through to cataclasites and fault gouges. The penetrative mylonite fabrics themselves deform, and are in turn cut by, mafic dykes. Shear direction and sense is the same for all of the fault rock types, ductile and brittle, and after correction for the regional tilt of the ophiolite section, are consistent with normal faulting. Gabbro mylonitic shear zones such as this within the lithospheric mantle provide a mechanism for weakening the lithospheric mantle. They might be expected to have a high acoustic impedance contrast with the surrounding ultramafic rocks, and are strong candidates for dipping seismic reflectors observed in old oceanic crust.