New Zealand Journal of Geology and Geophysics abstracts
Possible Jurassic age for part of Rakaia Terrane: implications for tectonic
development of the Torlesse accretionary prism
PETER J. J. KAMP
Department of Earth Sciences
The University of Waikato
Private Bag 3105
Hamilton, New Zealand
Abstract Greywacke sandstone and argillite beds comprising
Rakaia Terrane (Torlesse Complex) in mid Canterbury, South Island, New Zealand,
are widely regarded as Late Triassic (Norian) in age based on the occurrence of
Torlessia trace fossils, Monotis, and other taxa.
This paleontological age assignment is tested using published
40Ar/39Ar mica and U-Pb zircon ages for these rocks and
published and new zircon fission track (FT) ages. The youngest U-Pb zircon ages
in the Rakaia Terrane rocks in mid Canterbury are Norian, whereas 10-20% of the
40Ar/39Ar muscovite ages are younger than Norian.
Numerical modelling of these mica ages shows that they cannot have originated
from partial thermal overprinting in the Torlesse prism if the thermal maximum
was short-lived and early in the prism history (210-190 Ma), as commonly
inferred for these rocks. The young component of mica ages could, however, be
explained by extended residence (200-100 Ma) at 265-290deg.C in the prism.
Early Jurassic (c. 189 Ma) zircon FT ages for sandstone beds from Arthur's
Pass, the Rakaia valley, and the Hermitage (Mt Cook) are interpreted not to
have experienced maximum temperatures above 210deg.C, and therefore cannot have
been reduced as a result of partial annealing in the Torlesse prism. This is
based on identification of a fossil Cretaceous, zircon FT, partial annealing
zone in low-grade schists to the west, and the characteristics of the age data.
The Early Jurassic zircon FT ages and the young component of
40Ar/39Ar mica ages are regarded therefore as detrital
ages reflecting cooling in the source area, and constrain the maximum
depositional age of parts of the Rakaia Terrane in mid Canterbury. The zircon
FT data also show the initiation (c. 100 Ma) of marked and
widespread Late Cretaceous cooling of Rakaia Terrane throughout Canterbury,
which is attributed to uplift and erosion of inboard parts of the Torlesse
prism due to continuing subduction accretion at its toe.
The critical wedge concept is proposed as a new framework for investigating
the development of the Torlesse Complex. The Rakaia Terrane may have formed the
core of an accretionary wedge imbricated against the New Zealand margin during
the Middle or Late Jurassic. Late Jurassic nonmarine sediments (e.g., Clent
Hills Formation) accumulated upon the inner parts of the prism as it enlarged,
emerged, and continued to be imbricated. Exhumation of Otago Schist from
c. 135 Ma may mark the development of a balance (steady state) between
sediments entering the prism at the toe and material exiting at the inboard
margin. The enlargement of the area of exhumation to all of Canterbury from
c. 100 Ma may reflect a dynamic response to widening of the prism through
the accretion of Cretaceous sediments. The model of a dynamic critical wedge
may help to explain the various expressions of the Rangitata Orogeny.
Keywords Torlesse Complex; Rakaia Terrane; fission track;
thermochronology; tectonics; Canterbury
New Zealand Journal of Geology & Geophysics, 2001, Vol. 44:
185-203
0028-8306/01/4402-0185 $7.00/0 (c) The Royal Society of New Zealand
2001
PDF file of entire paper: medium quality (1998K); (scanned from paper original: notes about this process)
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