HR: 1340h
AN: GP43A-0842    [Abstracts]
TI: Rotation of the Hikurangi Margin, East Coast, New Zealand: Reconciling Long-Term Deformation Patterns Indicated by Paleomagnetic and Magnetic Fabric Data With the Short-Term Velocity Field
AU: * Rowan, C J
EM: cjr01@soc.soton.ac.uk
AF: Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH United Kingdom
AU: Roberts, A P
EM: arob@soc.soton.ac.uk
AF: Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH United Kingdom
AB: A key question when studying continental tectonics is whether there is a practical length scale below which the continental crust is effectively rigid, allowing a 'microplate' treatment, or whether deformation is distributed at all length scales, requiring a quasi-continuous flow model. A common feature of continental deformation, the rotation of fault-bounded blocks about a vertical axis, can shed light on this question; any such blocks will have dimensions of the order of the dominant length scale of deformation. GPS and geodetic studies demonstrate that the Hikurangi Margin (East Coast, New Zealand) is actively rotating clockwise at 2-$4\deg$/ m.y.; the forearc region is acting as a linkage between regions of back-arc extension to the north and transpression to the south. Paleomagnetic studies indicate that these rotations have persisted at least since the Late Miocene, and have been used to further propose that the forearc is divided into a number of independently rotating domains. However, this partitioning is not observed in the short-term velocity field; the two datasets imply different dominant length scales of deformation. To investigate this discrepancy, we present the results of extensive sampling from 40 localities in the forearc of the Hikurangi Margin. In addition to stepwise demagnetisation to isolate characteristic remanences, the anisotropy of magnetic susceptibility was also measured. In relatively undeformed sediments the magnetic fabric is closely related to the regional stress field, and can therefore potentially be used as an independent rotation marker at sites where ancient paleomagnetic components are obscured by strong present day field overprints. The new data permit a better understanding of the manner and timing of rotations of the Hikurangi Margin. Rotations of the hitherto poorly-constrained southern part of the margin are consistent with sites further to the north, indicating that the whole margin may have rotated coherently since the Late Miocene, in agreement with the short-term velocity field. In addition, the data enable more accurate delineation of the boundary with the unrotated northern part of the margin, and allow better constraint of the structural mechanisms by which large differential rotations have been accommodated in this region.
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting