Seismic Studies of the Structure Beneath Lake Taupo

Abstract

Lake Taupo and some of the lakes of the Rotorua region were surveyed using a 0.5 kJ sparker sound source and the continuous seismic profiling (CSP) technique. Lake Taupo was also surveyed using a 230 cm3 airgun sound source and the CSP and refraction profiling techniques. The equipment was developed to be suitable for use from a 6 m launch. This entailed some experimentation to improve the signal to noise ratio and to provide interpretable graphic seismic records. The final equipment layout provided data to a depth of up to 200 m from sparker CSP records and over 1 km from airgun CSP and refraction records. Together with gravity, magnetic, heat flow, geodetic, seismological and geological data, the seismic data enables detailed structural modelling of the Taupo Volcanic Centre to depths of up to 2 km. Rhyolitic and ignimbritic seismic velocities are encountered at depths of 300 m to 1000 m and the sediment column has two distinct seismic velocities. A number of faults have been identified and on this basis Lake Taupo may be divided into four structural regions. Central and western Lake Taupo are calderas relating to Holocene tephra and Pleistocene ignimbrite eruptions respectively. The northern bays and southern Lake Taupo are graben structures relating to the Taupo Fault Belt. Most of the caldera and graben faults have throws in excess of 500 m over the past 0.33 My. The NW trending Horomatangi Fault bisects the lake and offsets the Taupo Fault Belt sinistrally. At the intersection of the Horomatangi Fault and the Lower Pleistocene Taupo-Rotorua Depression is the source vent of the Whakamaru and earlier ignimbrites. The associated magnetic anomaly indicates a pipe intrusion at a depth of 2 km with a diameter of 4 km at the geometric centre of Western Bay. At the intersection of the Horomatangi Fault and the Upper Pleistocene Taupo-Reporoa Basin there are the source vents of all the major (>10 km3) tephra eruptions of the past 20,000 years, associated with the Taupo Volcanic Centre. There is an active geothermal field near Horomatangi Reefs. In central Lake Taupo the Horomatangi Fault has an offset of 30 m at the lake bed, 250 m in the 20,000 y sediment horizon, and at least 400 m in the 0.33 My horizon. There is over 370 km3 of sediment that has accumulated in Lake Taupo since 0.33 My BP of which 100 km3 was deposited since 20,000 y BP. In central Lake Taupo the ignimbrite surface is over 1 km below lake level and basement is over 3 km below lake level. Only 20% of the warping of the Whakamaru-Rangitaiki Ignimbrite surface may be accounted for by subsequent eruptive products. The balance of the warping may have been caused by extension in the NW-SE direction. If it is assumed that the crustal thickness is 10 km, then the extension has been at a rate of between 4.5 and 12 mm/y. The lake level was reduced to at least 110 m below the present level, and subsequently rose to 32 m above the present level, following the Taupo eruption of 1850 y BP. The Kawakawa eruption of 20,000 y BP emptied the lake, which subsequently rose to 120 m above the present level. Magnetic anomalies of the Central Volcanic Region show that the Taupo Volcanic Zone is broadest between Lakes Taupo and Rotorua. This portion is also one of high heat flow, massive rhyolitic volcanism, and major graben development. These effects may be a result of hot rock beneath the area, being a remnant of the older (22 to 6 My BP) NW trending Northland-Coromandel volcanic system.