New Zealand Journal of Geology and Geophysics abstracts

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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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