Kinematics of transition from subduction to strike-slip: an example from the New Zealand plate boundary (Invited)
Abstract
We develop a kinematic model for the transition from oblique subduction beneath the North Island to strike-slip in the South Island, New Zealand, constrained by GPS velocities and onshore and new offshore active fault slip rate and location data. To interpret these data, we simultaneously invert the kinematic data for poles of rotation of tectonic blocks (block boundaries chosen to coincide with known active faults) and the degree of interseismic coupling on faults in the region, including the Hikurangi subduction thrust. Our best-fitting kinematic models require that the northeastern corner of the South Island undergoes significant vertical axis rotation as a part of the rapidly rotating (3-4°/Myr) North Island forearc block, while the remainder of the tectonic blocks in the South Island undergo negligible vertical axis rotation relative to the Pacific Plate. This result agrees well with clockwise paleomagnetic declinations from ~4 Ma rock samples in the northeastern South Island, and suggests that paleomagnetically-observed rotation of that region is continuing today. The poles of rotation between the more rapidly rotating northeastern South Island blocks and the non-rotating South Island blocks (further south) coincide with the boundary between the rotating and non-rotating domains of the South Island. This result is consistent with structural mapping of the rotation/non-rotation boundary by Little and Roberts (1997), and suggests that the transition from the rapidly rotating forearc of the Hikurangi subduction margin to a strike-slip dominated plate boundary in the South Island is accommodated by a crustal-scale kink or hinge in the upper plate of that subduction zone. This result highlights a remarkable consistency between datasets spanning decades (GPS), thousands of years (active fault data), and millions of years (paleomagnetic data and bedrock structure) in the northeastern South Island. Although the interseismic coupling coefficients that we estimate for the southern Hikurangi subduction thrust are similar to those from previous studies (Wallace et al. 2004), the slip rate deficit in this study is ~10 mm/yr lower than previously obtained, and has implications for the subduction thrust earthquake hazard at the southern Hikurangi margin. This change in slip rate deficit is due to newly improved constraints from the offshore active fault data regarding how slip is transferred from the strike-slip fault system in the South Island onto the subduction thrust in the North Island. This result highlights the importance of having accurate fault location and slip rate data from geological studies to avoid the inherent trade-offs between elastic strain related to the subduction thrust coupling and slip on upper plate faults in the interpretation of GPS velocity fields adjacent to subduction margins.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.G23D..03W
- Keywords:
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- 1209 GEODESY AND GRAVITY / Tectonic deformation;
- 8011 STRUCTURAL GEOLOGY / Kinematics of crustal and mantle deformation;
- 8111 TECTONOPHYSICS / Continental tectonics: strike-slip and transform;
- 8170 TECTONOPHYSICS / Subduction zone processes