Crustal structure of the northern Hikurangi margin and Bay of Plenty from marine seismic reflection imaging and double-sided onshore-offshore seismic tomography
Abstract
We present first results of a marine multichannel seismic (MCS) and onshore-offshore active-source seismic transect that spans the northern Hikurangi margin from offshore Gisborne, across the Raukumara peninsula, to the Bay of Plenty (BoP), acquired during the Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE). In a tomographic inversion, we invert wide-angle P-wave (VP) traveltime arrivals recorded on 27 ocean-bottom seismometers (OBS's) and 23 IRIS/PASSCAL seismometers, deployed every 9 km along the 400 km long line, to estimate VP structure to depths <30 km.
Analysis of wide-angle crustal and mantle seismic phases suggests that the Hikurangi plateau is 11-15 km thick with elevated VP in the lower crust ( 7.5 km/s) relative to normal oceanic crust ( 7 km/s). A <1.5 km thick layer of Oligocene and younger hemipelagic and trench fill sediments covers the Hikurangi plateau and thins with proximity to seamounts. MCS data and elevated VP reveal the Puke seamount at the toe of the slope and possibly a second subducted seamount 40 km from the trench. OBS's and onshore receivers show low VP in the shallow prism ( 1.6-3 km/s) and a shadow zone in seismic arrivals landward of Tuaheni ridge, the site of a major thrust fault. The shadow zone (caused by a seismic low velocity zone) coincides with a reflective interval near the subduction interface. We attribute wide-angle reflections beneath the shadow zone to the subduction interface, Hikurangi plateau, and Moho. The shadow zone may result from under-compacted, basally accreted sediments and/or elevated pore-fluid pressures in the prism. In the BoP, MCS data reveal extension focused in the Taupo Rift, with several normal faults that sole into deep crustal reflectivity. Crustal thickness in the BoP ranges from 10-15 km and crustal VP is consistent with extended and intruded continental crust ( 3.5-6.5 km/s). Upper-plate mantle velocities are relatively low ( 7.5 km/s), possibly due to high temperatures and melt. The coincidence of subducted seamounts, a shadow zone, and high reflectivity near the subduction interface in an area of recurring shallow slow slip events indicates that rough subducted topography, sediment underplating and/or high pore-fluid pressures promote conditions that favor slow earthquakes at the Northern Hikurangi margin.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T53C..06G
- Keywords:
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- 1207 Transient deformation;
- GEODESY AND GRAVITYDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8170 Subduction zone processes;
- TECTONOPHYSICSDE: 8185 Volcanic arcs;
- TECTONOPHYSICS