New Zealand Journal of Geology and Geophysics abstracts
A late Quaternary extension rate in the Taupo Volcanic Zone, New Zealand,
derived from fault slip data
PILAR VILLAMOR
KELVIN BERRYMAN
Institute of Geological & Nuclear Sciences
P.O. Box 30368
Lower Hutt, New Zealand
email: p.villamor@gns.cri.nz
Abstract A northwest-southeast oriented extension rate from
faulting for a time-averaged period of c. 50 000 yr
(10 000-64 000 yr), across the Ngakuru-Waikite depression (modern
Taupo Fault Belt, central Taupo Volcanic Zone), has a best estimate of 1.9
mm/yr (in a range of 1.2-2.8 mm/yr) in the near surface, but increases to a
best estimate of 6.4 mm/yr (in a range of 3.6-10.2 mm/yr) at seismogenic depths
of 6-10 km. We obtain this result by summing the vertical components of fault
displacement across known-age surfaces, or as the vertical component of
displacement in stratigraphic units of known age, within the 14 km wide zone of
active normal faulting. We convert the summed vertical slip rate of 7.2 +/- 0.4
mm/yr to dip-slip displacement rate and to northwest-southeast extension by
estimating a range of possible fault plane dips at the surface and at
seismogenic depth. Fault displacement at seismogenic depth in large events is
on average 1.6 times larger than at the surface, and for earthquake magnitudes
of M6.8 and smaller, about one-third of the displacement occurring with
the whole Gutenburg & Richter distribution of earthquakes in the modern
Taupo Fault Belt will not rupture to the ground surface. Fault dip averages c.
75deg. in the near surface, but is poorly constrained at seismogenic depth in
the Taupo Fault Belt. From a variety of local and literature considerations, we
propose a dip of c. 60deg. at seismogenic depth in the Taupo Fault Belt. Our
observations suggest only a minor component of extension at the surface (c. 5%)
is contributed by small scale faulting below our observation threshold of
0.1-0.5 m of fault slip. The c. 4.5 mm/yr difference in extension rate between
seismogenic depth and the ground surface may represent the surface extension
rate caused by a combination of opening of extension fractures and penetrative
grain-scale extensional deformation.
Keywords normal fault; slip rate; extension; earthquake
geology; active tectonics; Taupo Fault Belt; Ngakuru-Waikite depression; Taupo
Volcanic Zone
New Zealand Journal of Geology & Geophysics, 2001, Vol. 44:
243-269
0028-8306/01/4402-0243 $7.00/0 (c) The Royal Society of New Zealand
2001
PDF file of entire paper: medium quality (5359K); (scanned from paper original: notes about this process)
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