Improved image of a large Moho step in the continental lithosphere: the Taranaki-Ruapehu Line, New Zealand.

Abstract:

The Taranaki-Ruapehu Line (TRL) is a geophysical boundary that runs east-west adjacent to the southern end of arc-volcanism in New Zealand. The TRL is defined by gravity, magnetotelluric, and seismic attenuation observations, which may be related to a proposed Moho “step”, yet the geometry of this step is not well constrained. Here we present an improved image of the Moho interface across the TRL, assembled from teleseismic receiver functions at 8 new temporary and 2 regional seismic stations, which augment a network of 7 temporary stations originally used to image the step. A total of 790 receiver functions from 17 stations and 281 earthquakes were arranged into geographical bins, then stacked to form a north-south Common Conversion Point (CCP) profile 130 km long and approximately centered on the TRL. Consistent with previous investigators, 2D velocity effects from basin sediments and station elevation are compensated for with a static time shift of the receiver functions, which were first rotated into the LQT (i.e. P-SV-SH) domain. Ray tracing and pre-stack depth migration then operated on a simple 1D velocity model. The new 17 station CCP profile compares favorably with the original 7 station profile; the extra data density improves attenuation of P multiples and results in noticeably clearer Ps converted phases along the profile, including better delineation of the previously identified crustal step where the Ps conversion deepens from ~25 km to ~35 km over a distance of several kilometers. Synthetic receiver functions have also been calculated for station-event pairs and formed into CCP stacks, and we assess the potential of synthetics as an interpretational tool for a large change in crustal thickness over a relatively short distance. We hypothesize that such a change in Moho depth may be related to dynamic mantle instabilities, such as subduction interaction at depth or a Rayleigh-Taylor instability, both of which have been proposed to occur in the vicinity of this study area.