New Zealand Journal of Geology and Geophysics abstracts
Structure and neotectonics of the Blackstone Hill Antiform, Central Otago,
New Zealand
MICHELLE MARKLEY*
RICHARD J. NORRIS
Department of Geology
University of Otago
P.O. Box 56
Dunedin, New Zealand
*Present address: Department of Geology, Mount Holyoke College, South Hadley,
MA 01075, USA.
Abstract Blackstone Hill is a northeast-trending ridge of
schist basement rising between the Ida Burn and Manuherikia River valleys.
Deformation of young sediments and uplift of strath river terraces cut into
both sides of the ridge indicate that its rise is an ongoing, late Cenozoic
phenomenon. Deformation of poorly exposed cover sediments at the ridge margins
appears to be relatively localised and complex. In the schist basement, a new
generation of fractures has developed to accommodate late Cenozoic deformation.
The basement also displays a prominent lineation and foliation of Mesozoic age.
Lineation defined by quartz rodding trends c. 020deg.. The orientation of
foliation varies and defines the Blackstone Hill Nappe, a large synmetamorphic
fold of Mesozoic age with a hinge trending c. 167deg..
The Otago Peneplain, the regional unconformity between schist basement and
overlying sedimentary cover, was a subhorizontal erosion surface before the
onset of ridge formation. It is now warped around the late Cenozoic Blackstone
Hill Antiform hinge, which trends 035deg., rather than offset discontinuously
along a range-scale fault. The antiform is box-shaped, asymmetric, and open,
although the peneplain may be steeply tilted at depth. Data from a strath
terrace system indicate that, on the flanks of the antiform, the peneplain has
been progressively tilted during folding. Drainage patterns incised into the
peneplain suggest that the southeastern flank of the antiform began rising
before the northwestern flank. These observations are consistent with a
fault-propagation fold model for the Blackstone Hill Antiform, whose geometry
is best explained by reverse motion along two steep faults at depth. Variations
in basement fabric related to the Mesozoic Blackstone Hill Nappe have
mechanically influenced the development and topographic expression of the
antiform.
Keywords neotectonics; structural geology; active folding;
reverse faults; tectonic geomorphology; Central Otago; late Cenozoic;
Quaternary
New Zealand Journal of Geology and Geophysics, 1999, Vol. 42: 205-218
0028-8306/99/4202-0205 $7.00/0 (c) The Royal Society of New Zealand
1999
PDF file of entire paper: medium quality (4353K); (scanned from paper original: notes about this process)
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