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

Abstract

A high‐speed zone in the mantle directly beneath the Southern Alps of New Zealand is required by the recorded pattern of teleseismic P waves. Two parallel lines of 80 seismographs spaced at ∼2 km intervals recorded three earthquakes from the western Pacific with epicentral distances of 52°, 53° and 78°. Azimuthal bearings were all within 15 degrees of the mean trends of the seismograph lines. Differences between measured delays and those predicted from the crustal structure reach 0.8 s along one line and 1.0 s along the other, with the rays for the earliest arriving signals passing the depth of ∼120 km beneath the center of the island. Assuming these early arrivals are due to structure within the mantle shallower than 200 km, they imply that the core of the high‐speed zone lies beneath the thickest crust, which has been shortened by ∼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 100 km and with a maximum speed advance of ∼7% satisfies the observations. The pattern of residuals does not fit with those predicted by simple models of intracontinental subduction in which crust and mantle lithosphere are detached and one slab of mantle lithosphere underthrusts the other. Rather, the residuals favor thickening of mantle lithosphere by a more homogenous straining of it, as if mantle lithosphere beneath continental crust behaved as a continuum. An excess mass in the mantle is also required by the observed gravity anomalies, once allowance is made for the seismically determined crustal thickness. This high‐density mantle anomaly provides sufficient force (per unit length) to maintain the crustal root, which is approximately twice as thick as that necessary to support the topography.