HR: 16:10h
AN: S44A-01 INVITED     [Abstracts]
TI: Combined Active and Passive Seismology to Study Continental Collision; Central South Island of New Zealand
AU: * Stern, T
EM: Tim.Stern@vuw.ac.nz
AF: Institute of Geophysics, Victoria University of Wellington, P.O Box 600, Wellington, 6001 New Zealand
AU: Okaya, D
EM: okaya@usc.edu
AF: 2. Dept. of Geological Sciences, University of Southern California, South Science building, Los Angeles, CA 90089 United States
AU: Baldock, G
EM: guybaldock@yahoo.com
AF: Institute of Geophysics, Victoria University of Wellington, P.O Box 600, Wellington, 6001 New Zealand
AU: Baldock, G
EM: guybaldock@yahoo.com
AF: now at Schlumberger Technology Corporation, 16879 W. 141st St. S. Kellyville, Oklahoma, OK 74039 United States
AU: Scherwath, M
EM: mscherwath@ifm-geomar.de
AF: Institute of Geophysics, Victoria University of Wellington, P.O Box 600, Wellington, 6001 New Zealand
AU: Scherwath, M
EM: mscherwath@ifm-geomar.de
AF: Leibniz-Institute of Marine Sciences, IFM-GEOMAR Wischhofstr. 1-3, Kiel, 24148 Germany
AB: Central South Island of New Zealand is a continental region that has undergone both collision and strike-slip shear in the late Tertiary. From a tectonics, or rock-mechanics, view-point there is an interest in how the crust and mantle have been both thickened and sheared. The South Island Geophysical Transect (SIGHT) and the Southern Alps Passive Seismic Experiment (SAPSE) - both jointly funded US-NZ programs - studied these processes. Some of the most important findings came about by merging data from passive and active seismology. Three specific examples will be discussed: 1. Teleseismic P-wave delays from earthquakes in the Western Pacific are used to map a ~ 0.8-1 s speed-up in the mantle right beneath the region of thickest crust and highest topography of the collision zone. Forward modeling of this velocity anomaly shows that the amplitude of the anomaly can be explained by a 100 km-thick body that has a 7% in increase in P-wave speed. From the spatial pattern of the P-wave residuals we can also show that the high-speed body is about 80-100 km wide and roughly vertically disposed beneath the crustal root. The shape and position of the high-speed body beneath the seismically determined crustal root is consistent with it being thickened, and therefore cold, mantle lithosphere that has uniformly strained into a roughly symmetric root beneath the collision zone. 2. Pn anisotropy measurements from our onshore-offshore seismic shooting program allowed us to make mutually perpendicular determinations of Pn wave speeds at three localities. P-wave anisotropies of up to 11 ± 3%, 6.5 ± 2.5% and 0 ± 3%, were measured, depending on the distance of the measurement from the surface trace of the plate boundary (the Alpine Fault). These are necessarily minimum anisotropy values because it assumes that the two axes of measurement are those of minimum and maximum wave speed. Combining these results with SKS splitting values of ~ 2 s from passive seismology allowed us to make an estimate of thickness for the mantle lid. Assuming a ratio of P to S anisotropy of 1.4, a ~ 100 km thick mantle lid of constant anisotropy is required that will give the observed SKS-split and the observed Pn anisotropy. This is a plausible thickness for the mantle lid of a young continental area like New Zealand. 3. The width of mantle deformation is observed to be ~ 330 km in southern South Island based on the Pn anisotropy observations. In this domain there is a consistent 20° angle between the fast SKS direction and the direction of shear (taken as the strike of the Alpine Fault). These two observations can be reconciled with the total ~ 800 km of dextral shear in South Island if relative plate motion in the mantle is accommodated by simple shear rather than faulting.
DE: 7205 Continental crust (1219)
DE: 7218 Lithosphere (1236)
DE: 8108 Continental tectonics: compressional
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Seismology [S]
MN: Fall Meeting 2005