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

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 10km of the axis of fracture zones. -Author