S11B-05
WIDE ANGLE SEISMIC IMAGING OF (SERPENTINITE ?) FAULT ZONES THAT PASS THROUGH THE MOHO IN A COLLISIONAL- FORELAND BASIN SETTING
Monday, 14 December 2015: 09:00
307 (Moscone South)
Tim A Stern1, Stuart A Henrys2, David A Okaya3, Jesse Dimech1, Hiroshi Sato4 and Takaya Iwasaki4, (1)Victoria University of Wellington, Wellington, New Zealand, (2)GNS Science-Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand, (3)University of Southern California, Los Angeles, CA, United States, (4)University of Tokyo, Bunkyo-ku, Japan
Abstract:
We report strong, wide-angle, seismic reflections from the mantle beneath the Taranaki Fault zone, southwest North Island, New Zealand. We attribute these reflections to a serpentinised fault zone that accommodated large amounts (~100km) of shortening during the Miocene. Other manifestations of the collision zone are a fold and thrust belt in the overriding Australian plate, a 3-4 km deep foreland basin and fragments of serpentine outcropping at the surface. The 2011 SAHKE experiment carried out multi-component, active-source seismic surveys along a length of about 300 km. The middle 100 km of the line is on land. Twelve large explosions (500 kg weight) were detonated onshore into an array of ~ 900 vertical component and 300 horizontal component seismographs in the on land section. Deep reflections are evident on both the land and marine seismic data. At 30 s two way travel time (twtt) are ~ 100 km deep reflections that have been interpreted to be due to arise from the base of the subducting Pacific plate*. But at 20 s twtt there are back-dipping reflections that when migrated are located in the mantle wedge of the overriding Australian plate. These reflections are in the depth range of 40-70 km and dip ~45 degrees to the southeast. Coincident S-S reflections are also seen on the horizontal geophones of the land array coming in at ~ 34 s twtt. This timing implies a Vp/Vs ratio = 34/20= 1.7 for the time averaged reflection path. Zoeprittz equation solutions show that if both P-P and S-S arrivals are generated from a common reflector a negative impedance contrast is required. We suggest serpentinite is the most likely cause of the reflectivity. Marine reflection data also show low-frequency reflections around the Moho depths that we also associate with both serpentinite, and recently documented slow-slip events on the subducted plate interface. * Stern T.A. et al (2015). A seismic reflection image for the base of a tectonic plate, Nature, 518, pp. 85-88, doi10.1038/Nature14146.
