Seismological and Gravitational Constraints on the Lithospheric Geometry beneath Fiordland, New Zealand

Seismological and Gravitational Constraints on the Lithospheric Geometry beneath Fiordland, New Zealand

Meuwissen, Ankie
Furlong, Kevin
Govers, Rob

Department of Geophysics University Utrecht
Geodynamics Researcii Group, Department of Geoscience, Penn State University

Molengraaff Fonds

1998

Fiordland, located in the SW of the South Island of New Zealand, is part of the transition zone between the westward dipping subduction in the north (where the Pacific plate subducts beneath the Australian plate at the Hikurangi Trench) and the eastward dipping subduction in the south (where the Australian plate subducts beneath the Pacific plate at the Puysugur Trench). In Fiordland there is ocean-continent collision, with oceanic lithosphere on the Australian plate (west) and continental lithosphere on the Pacific plate (east), which typically leads to subduction of the oceanic plate. Deep seismicity (> 165 km) and a large gravity anomaly high-low pair have been observed beneath the Fiordland region. Offshore the free-air anomaly is as large as -160 mgal, onshore the Bouguer anomaly is as large as +160 mgal. These gravity anomalies must be explained by a model which is also consistent with other geophysical constraints, such as the seismicity. In this study we tested possible lithospheric geometries, developed from the seismological and kinematic constraints. Two possible geometries were found which fit the seismicity data: a subduction-like geometry and a collision-like geometry with a thickening of oceanic lithosphere and continental crust. These two models were then tested against the gravity constraints. The model with thickening of both plates does not fit the gravity constraints, whereas the subduction geometry provides a good fit. Applying constraints from plate kinematics (velocities, directions) to this subduction geometry supports the model of a geologically recent initiation to the subduction. During the Miocene relative plate motions in the area were nearly pure translational, changing to more transpressional during the past 5 Ma. It also appears that only a small flake of the Australian plate is subducting. This is supported by 3-D pictures of the earthquakes, that show a point-shaped Benioff zone at depth. Gravitational and isostatical calculations give evidence for a SW- NE orientated tear in the Australian plate at the north-end of the Benioff zone.

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Utrecht University

Geoscience Reports

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(c) A. Meuwissen; K. Furlong; R. Covers