Mantle Anisotropy at a Continental Convergence Zone: South Island, New Zealand
Abstract
We examine mantle anisotropy offshore and on the South Island of New Zealand via shear wave splitting. Offshore measurements yield fast orientations off the west coast parallel to the Alpine fault whereas offshore the east coast, fast directions are nearly perpendicular to the fault. Delay times approximately 100 km off the west coast are around two seconds, decreasing to around one second or less past 200 km. West coast stations over approximately 300 km from shore yield null measurements. East coast stations have delay times around one second. On land, stations close to the Alpine Fault also exhibit fault parallel fast directions with delay times around two seconds, with fast directions becoming more oblique farther from the fault. These new results suggest that deformation of the mantle lithosphere spans a zone at least 250 km wide. Thus, west coast anisotropy seems associated with active fault deformation, with other observed anisotropy in the east possibly due to fossil lithospheric anisotropy or due to current absolute plate motion. These observations lend insight into the behavior of mantle lithosphere and the underlying asthenosphere during deformation, a key feature of continental convergence zones that remains inadequately understood. Shear wave splitting measurements were made on SKS-type phases, as well as teleseismic S phases, recorded on the Marine Observations of Anisotropy Near Aotearoa (MOANA) ocean bottom seismic array and the land stations from the New Zealand National Seismograph Network. An adaption of the Silver and Chan routine was utilized to make the measurements. In regions where microseismic or ocean bottom noise prohibited adequate splitting measurements, we beamformed data from adjacent stations before performing the measurements.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMDI21B2360Z
- Keywords:
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- 7200 SEISMOLOGY