Cooperative Institute for Research in Environmental Sciences

Collaborative Research: Deformation of the lower crust beneath strike-slip faults: Array studies of anisotropy and converted phases in the Marlborough Fault zone of New Zealand

Funded by the NSF Geophysics Program

Principal Investigators: Craig Jones, Anne Sheehan, and Peter Molnar

Project Summary

The contrast between narrow plate boundaries in oceanic regions and much wider ones within continental regions has long posed the questions of: Does deformation occur differently at depth? If so, how? Does the difference seen at the surface result from a weak continental lithosphere distributing strain over a broad region? Is distributed deformation an imperfect response of heterogeneous crust to broad deformation in the mantle? Is it, in fact, an accurate rendering of displacement within the mantle? The key difficulty has been our inability to construct a depth section through such a deforming regime, and in particular to map surface deformation down into the mantle. We will construct such a section through the broad deformation zone across the plate boundary in New Zealand.

We plan to install ten portable arrays, each consisting of eleven 3-component, dominantly short-period seismographs, in the Marlborough region at the northern end of the South Island of New Zealand to measure both crustal thickness and anisotropy in the crust and uppermost mantle. Our goal is to quantify the offset of the Moho below strike-slip faults and to determine whether shear has occurred over broad regions in the lower crust near such faults. Such shear is a predicted consequence of ductile deformation in the lower crust lying beneath blocks of upper crust that move with respect to one another along strike-slip faults and mantle lithosphere that is sheared over a laterally wide zone. Conversely, if faults passed directly from the upper crust into the upper mantle, we would expect little or no anisotropy in the lower crust, and possibly an offset of the Moho where strike-slip faults pass into the mantle. The Marlborough region constitutes an excellent laboratory because: it is active and large displacements have occurred on the faults; there is abundant local intermediate depth seismicity to provide high-frequency body waves with steep angles of incidence; preliminary results indicate anisotropy, though not its precise depth range; logistically it is an easy place to work; and local expertise in both field seismology and seismic anisotropy increases the likelihood of success.

A broader problem of interest is the extent to which mantle lithosphere beneath continents deforms by distributed continuous shear or by slip on localized shear zones that separate relatively rigid blocks of mantle lithosphere, analogous to plate tectonics. The demonstration of negligible lower crustal anisotropy, or of anisotropy but with an orientation of the faster S wave notably different from the strikes of the faults (which are parallel to relative plate motion), will suggest that faults in the upper crust continue as localized shear zones into the lower crust and probably into the mantle. A demonstration of an offset of the Moho at the faults will corroborate such an inference. Conversely, the demonstration of anisotropy implying shear on approximately horizontal planes (or planes dipping gently away from the faults) with a fast orientation parallel to the strikes of the faults would imply that deformation in the upper mantle is distributed and that it connects to upper crustal blocks by shear on sub-horizontal planes in the lower crust.

Studies elsewhere demonstrate that the polarization and amplitude of the S phase converted at a sharp interface where anisotropy is pronounced can vary markedly with the azimuth of the incident P wave. Such variation should permit detection of sharp interfaces in the lower crust. Moreover, digital recording of horizontal components of short-period S waves allows accurate estimates of shear-wave splitting. With small arrays of three-component seismographs, we will examine P and S waves, both those arriving directly and phases converted from the Moho and from other interfaces within the crust, if they exist. With arrays we can remove signal generated noise, which can obscure converted phases and prevent quantitative analysis of them.

Although the focus will be on studying the lower crust, we anticipate that the data obtained will also illuminate the subduction zone beneath the South Island and the upper mantle transition zone at greater depth. We also plan to study these regions using analyses of both direct and converted phases from teleseisms as well as phases from earthquakes within the down going slab.

See Also

Project site: New Zealand Plate Boundary Experiment

Proposal figures

Deployment photos

Field photos