Teleseismic P wave delays and modes of shortening the mantle lithosphere beneath South Island, New Zealand

A high-speed zone in the mantle directly beneath the Southern Alps of New Z ealand is required by the recorded pattern of teleseismic P waves. Two para llel lines of 80 seismographs spaced at similar to 2 km intervals recorded three earthquakes from the western Pacific with epicentral distances of 52 degrees, 53 degrees and 78 degrees. Azimuthal bearings were all within 15 d egrees of the mean trends of the seismograph lines. Differences between mea sured delays and those predicted from the crustal structure reach 0.8 s alo ng one line and 1.0 s along the other, with the rays for the earliest arriv ing signals passing the depth of similar to 120 km beneath the center of th e island. Assuming these early arrivals are due to structure within the man tle shallower than 200 km, they imply that the core of the high-speed zone lies beneath the thickest crust, which has been shortened by similar to 100 km of convergence during the past 6-7 Myr. Although the shape and position of the high-speed body cannot be fixed uniquely, a roughly symmetric body centered about a depth of 120 km, 80-100 km wide, with a depth extent of 10 0 km and with a maximum speed advance of similar to 7% satisfies the observ ations. The pattern of residuals does not fit with those predicted by simpl e models of intracontinental subduction in which crust and mantle lithosphe re are detached and one slab of mantle lithosphere underthrusts the other. Rather, the residuals favor thickening of mantle lithosphere by a more homo genous straining of it, as if mantle lithosphere beneath continental crust behaved as a continuum. An excess mass in the mantle is also required by th e observed gravity anomalies, once allowance is made for the seismically de termined crustal thickness. This high-density mantle anomaly provides suffi cient force (per unit length) to maintain the crustal root, which is approx imately twice as thick as that necessary to support the topography.