Exploring the oceanic lithosphere-asthenosphere boundary (LAB) with active source seismic methods

A traditional view of the LAB is that it is a gradual rheological boundary defined by the temperature of the mantle adiabat. A smooth shear-wave decrease across the LAB, often reported from surface-wave inversion studies in oceanic areas, has reinforced that view. But work with converted and reflected seismic waves now show that the LAB can be characterised by a sharp change in P and S wave speeds. We report (1) P-wave reflections with frequencies as high as 14 Hz with explosive sources from what is interpreted to be the oceanic LAB of the subducted Pacific plate, at about 100 km deep beneath eastern New Zealand. A 14 Hz frequency implies that the “sharpness” of the interface can be constrained to a transition zone no more than 500 m thick. Moreover, active source studies, both on land (1) and offshore(2), report a 10-15 km thick channel at the LAB, and there is evidence that this channel thins and deepens with age. Amplitude analysis suggest a ~ 8 % drop in P-wave speed within the channel which is consistent with a layer with about 2% partial melt. The land-based study in New Zealand used ~ 850 seismographs spread over ~ 82 km, with a spacing between them of ~ 100 m. Explosive charges of 500 kg in 50 m deep bore holes were used. The advantage of very large shot sizes like this is that they generate a broad range of frequencies. We also recorded strong, 6 Hz, reflectivity from the LAB beneath eastern New Zealand with receiver gathers from onshore seismographs recording airgun shots from offshore (3). This method allows us to record at wider angles (incidence angles > 50 deg) where higher levels of reflection energy are generated. In the wide-angle data we interpret more complexity of the LAB channel than from the near vertical reflection data. In particular, we argue for a thin (~ 3 km) highly anisotropic layer to the top of the channel where P-wave speeds, parallel to the boundary are > 9 km/s. This is interpreted to be a 10% azimuthal anisotropy due to lattice-preferred orientation of olivine crystals resulting from finite strain accumulated at the LAB. Beneath this is about 8 km of low velocity zone where polarity analysis and relative moveout of the reflection data suggest melts may reside. --- Zoom connection info: Topic: Marine Geosciences seminar - Tim Stern - October 26th Time: Oct 26, 2022 11:00 AM Paris Join Zoom Meeting https://u-paris.zoom.us/j/82325338954?pwd=Vlowd25NenBqT1JxaWR6THBGTmRsdz09 Meeting ID: 823 2533 8954 Passcode: 975377 One tap mobile +33170379729,,82325338954#,,,,*975377# France +33170950103,,82325338954#,,,,*975377# France Dial by your location +33 1 7037 9729 France +33 1 7095 0103 France +33 1 7095 0350 France +33 1 8699 5831 France +33 1 7037 2246 France Meeting ID: 823 2533 8954 Passcode: 975377 Find your local number: https://u-paris.zoom.us/u/kbiq4Z0lK Join by SIP