New Zealand Journal of Geology and Geophysics abstracts
Tectonic significance of Cretaceous bivergent extensional shear zones
in the Torlesse accretionary wedge, central Otago Schist, New Zealand
H. Deckert
U. Ring
Institut für Geowissenschaften
Johannes Gutenberg-Universität
55099 Mainz, Germany
N. Mortimer
Institute of Geological & Nuclear Sciences
Private Bag 1930
Dunedin, New Zealand
Abstract We describe two shear zones in the Otago
Schist of the Torlesse accretionary wedge, South Island, New Zealand: the
north-dipping Rise-and-Shine Shear Zone (RSSZ) and the south-dipping Cromwell
Gorge Shear Zone (CGSZ). Kinematic indicators (shear bands and asymmetric
folds) indicate top-north movement for the RSSZ and top-south transport for
the CGSZ. Back rotation of the shear zones into their Late Cretaceous orientation
and consideration of the relationship of the shear zones to arching of the
Otago Schist show that both shear zones are extensional. Offset of textural
zones suggests up to 15 km of dip-slip displacement on the RSSZ and probably
a similar amount of slip for the CGSZ. We speculate that the gold-mineralised
RSSZ may be the western continuation of the late Mesozoic gold-bearing Hyde-Macraes
Shear Zone in eastern Otago, forming a c. 100 km long extensional shear zone
on the northern flank of the Otago Schist. Age constraints suggest that shear
zone formation took place between 135 and 105 Ma. The shear zones aided the
final exhumation of the deeper parts of the Otago Schist. We discuss whether
normal shearing is related to syn-orogenic supercritical tapering of the
Torlesse wedge, or due to post-orogenic New Zealand-wide Albian rifting.
Keywords normal fault; accretionary wedge; rifting;
Torlesse Terrane; Dunstan Range; Otago Schist; New Zealand
G01024 Received 1 August 2001; accepted 8 July 2002 ; published 6 December
2002
New Zealand Journal of Geology & Geophysics, 2002, Vol. 45: 537-547
0028-8306/02/4504-0537 $7.00/0 © The Royal Society of New Zealand
2002
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