A seismic reflection image for the base of a tectonic plate

Abstract

Plate tectonics successfully describes the surface of Earth as a mosaic of moving lithospheric plates. But it is not clear what happens at the base of the plates, the lithosphere-asthenosphere boundary (LAB). The LAB has been well imaged with converted teleseismic waves, whose 10-40-kilometre wavelength controls the structural resolution. Here we use explosion-generated seismic waves (of about 0.5-kilometre wavelength) to form a high-resolution image for the base of an oceanic plate that is subducting beneath North Island, New Zealand. Our 80-kilometre-wide image is based on P-wave reflections and shows an approximately 15° dipping, abrupt, seismic wave-speed transition (less than 1 kilometre thick) at a depth of about 100 kilometres. The boundary is parallel to the top of the plate and seismic attributes indicate a P-wave speed decrease of at least 8 ± 3 per cent across it. A parallel reflection event approximately 10 kilometres deeper shows that the decrease in P-wave speed is confined to a channel at the base of the plate, which we interpret as a sheared zone of ponded partial melts or volatiles. This is independent, high-resolution evidence for a low-viscosity channel at the LAB that decouples plates from mantle flow beneath, and allows plate tectonics to work.

Using large dynamite shots to image the structure of the Moho from deep seismic reflection experiment between the Sichuan basin and Qinling orogen

“…The structure of the Moho could be seen on the profiles from all five large dynamite shots, and the frequency range was 6-30 Hz, consistent with the frequency of other reported deep seismic reflections from large dynamite shots (Stern et al 2015;Jarchow et al 1990;Steer et al 1998). One large dynamite shot (Station No.…”

“…Developing the correct processes and selecting the correct parameters based on the characteristics of the data from the large dynamite shots are the key to imaging the lower crust and Moho. Many geophysicists have achieved good results in processing data from dynamite shots (e.g., Levander et al 1994;Klemperer et al 1986;Kawamura et al 2003;Stern et al 2015).…”

“…Kawamura et al (2003) revealed the deep crustal structure of the eastern central tectonic line using two large dynamite shots. Stern et al (2015) determined the structure of the Moho and the lithosphere-asthenosphere boundary beneath North Island, New Zealand, using large dynamite shots.…”

Geophysical constraints on geodynamic processes at convergent margins: A global perspective

“…the North Island of New Zealand (Reyners et al, 2006;Stern et al, 2015). A recent controlledsource seismic experiment provided a high-resolution image of the base of a subducting oceanic plate which is dipping at ca.…”

Application of thermodynamic modelling to natural mantle xenoliths: examples of density variations and pressure–temperature evolution of the lithospheric mantle

“…Moreover, it is important to emphasize that our calculations assume no volatile-bearing phases and only subsolidus conditions (and hence do not consider the possible presence of melt at the LAB or the presence of hydrous phases; cf. Green et al 2010;Sakamaki et al 2013;Stern et al 2015). Figure 3 depicts these geothermal gradients together with two examples of predicted density distribution in P-T space.…”