Reconstructed seismic velocity structure of the Lewis Hills Massif and implications for oceanic fracture zones

Abstract

The Lewis Hills Massif of the Bay of Islands Ophiolite Complex provides a lower crustal to upper mantle cross section of an oceanic fracture zone segment and adjacent oceanic lithosphere. The structural and petrologic character of the fracture zone region is distinctly different from the comparatively simple layered structure of the rest of the Bay of Islands complex. Results of detailed field mapping, petrography, and laboratory seismic velocity measurements have been combined in order to reconstruct the seismic velocity structure of this slice of oceanic lithosphere. Velocities and velocity anisotropies of each of the major rock units in the massif are restored to their preobduction geometries relative to different stratigraphic levels in the complex and proximity to the fracture zone region. Although all oceanic fracture zones probably have unique structural and petrologic characteristics in detail, from the perspective of the present study it is suggested that, in general, seismic properties differing from that of ‘normal’ oceanic crust and upper mantle may be expected within approximately 10 km of the axis of fracture zones. In these regions, lower crustal as well as upper mantle anisotropy is expected. The Moho might be expected to shallow due to the presence of abundant ultramafic cumulates, resulting in crustal thinning, and significant velocity inversions may occur with depth where large cyclic mafic layers occur within cumulate ultramafics. In oceanic lithosphere which has passed or is passing through a transform fault domain, enhanced mantle anisotropy might occur. The middle to lower crust in these regions should be composed of highly anisotropic metamorphic rocks, which grade upward into serpentinites and highly fractured gabbros, diabases, and basalts with relatively low velocities. It is suggested that velocities as well as velocity anisotropies produced by these types of structures might be determined in situ by seismic refraction experiments and may be used to map the extent of deformation in the oceanic lithosphere near fracture zones.