New Zealand Journal of Geology and Geophysics abstracts
Geometry of the folded Otago peneplain surface beneath Ida valley, Central
Otago, New Zealand, from gravity observations
Michelle Markley
Department of Earth and Environment
Mount Holyoke College
South Hadley, MA 01075, USA
email: mmarkley@mtholyoke.edu
Basil Tikoff
Department of Geology and Geophysics
University of Wisconsin
Madison, WI 53706, USA
email: basil@geology.wisc.edu
Abstract We present new Bouguer gravity data and detailed
cross-sections of the geometry of the deformed Otago peneplain surface beneath
the Ida valley, a northeast-southwest elongate basin in Central Otago, New
Zealand. Taking the strong regional gravity gradient into consideration,
a -15 mgal Bouguer anomaly results from the density contrast between Cenozoic
Ida Basin sediments and Mesozoic Otago Schist basement. Using gravity modelling
and geologic constraints, we present a series of northwest-southeast cross-sections
through the basin. Underneath the southeast side of the Ida Basin, adjacent
to the flank of North Rough Ridge, the peneplain surface dips gently. Underneath
the northwest side of the Ida Basin, adjacent to the flank of Blackstone
Hill, the peneplain surface is strongly deformed by a fault-propagation fold
associated with the tip of the Blackstone Fault. Along this margin, the orientation
of the peneplain surface varies dramatically along-strike. In the north,
the peneplain surface is overturned, and the fault-propagation fold is a
tight and gently inclined anticline-syncline pair. In the south, the peneplain
surface is upright, and the fault-propagation fold is an open and moderately
inclined anticline-syncline pair. Three possible explanations for the along-strike
variation in fold geometry are: (1) the dip of the fault varies dramatically
along-strike, (2) the fault tip at the northeast end of the study area has
an unexpectedly shallow depth, and (3) mechanical anisotropy of the basement
schists exerts a strong influence on fold shape.
Keywords fault-propagation folding; Otago peneplain;
Blackstone Fault; Central Otago; Otago Schist; Ida valley; Blackstone Hill;
North Rough Ridge
G02018; Received 23 March 2002; accepted 14 April 2003; online publication
date 10 September 2003
New Zealand Journal of Geology & Geophysics, 2003, Vol. 46: 449-456
0028-8306/03/4603-0449 $7.00/0 © The Royal Society of New Zealand
2003
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