| Summary: | Two examples of Pacific rim plate boundary deformation are presented. In the first
part of the thesis crustal models are derived for the northwestern part of the Vizcaino
block in California using marine seismic and gravity data collected by the Mendocino
Triple Junction Seismic Experiment. A northwest-southeast trending kink in the Moho is
imaged and interpreted to have formed under compression by reactivation of preexisting
thrust faults in the paleoaccretionary prism at the seaward margin of the Vizcaino block.
The study suggests that the deformation resulted from mainly north-south compression
between the Pacific-Juan de Fuca plates across the Mendocino transform fault and
predates late Pliocene Pacific-North America plate convergence.
In the second part, 195 earthquakes recorded during the duration of the Southern Alps
Passive Seismic Experiment (SAPSE) are analysed. Precise earthquake locations and
focal mechanisms provide unprecedented detail of the seismotectonics in the central
South Island. The short term (6 month) SAPSE seismicity is compared with long term (8
years) seismicity recorded by the New Zealand National Seismic network and the Lake
Pukaki network. The seismicity rate of the Alpine fault is low, but comparable to locked
sections of the San Andreas fault, with large earthquakes expected. Changes of the depth
of the seismogenic zone, generally uniform at about 10-12 km, occur only localised over
distances smaller than 30 km, suggesting that thermal perturbations must be of similar scale. This implies that the thermal effects of the uplift of the Southern Alps do not
change the seismogenic depth significantly and are not in accordance with most of the
present thermal models. Both the Hope and Porters Pass fault zones are seismically active
and deformation is accommodated near the fault zones and in the adjacent crust. North of
Mt Cook, a triangular shaped region along the Alpine fault is characterised by absence of
earthquakes. We interpret this as the result of the plate boundary shift from the Alpine
fault to the Hope and Porters Pass fault zones. The study region shows distributed
deformation in a 60-100 km wide zone on NNE-SSW trending thrust faults and strike-slip
mechanisms on transfer faults. === Graduation date: 2000 === Best scan available for black and white figures.
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