Gravity, magnetic and seismic surveys of the caldera complex, Lake Taupo, North Island, New Zealand

The Taupo volcanic centre lies at the southern end of the Taupo Volcanic Zone in the central North island of New Zealand. Major rhyolitic eruptions of 400 and 35 km(3) (dense rock equivalent) occurred from this centre 26.5 and 1.8 thousand years ago, respectively. As a result of these eruptions much of the volcanic centre has collapsed and now Lies beneath the waters of Lake Taupo. Gravity data measured on Lake Taupo show that the most intense (negative) Bouguer anomaly to occur anywhere within the Taupo Volcanic Zone lies over the northern part of the lake. This anomaly is interpreted to outline the area of caldera collapse caused by the 26.5 ka eruption which has been subsequently modified by the 1.8 ka eruption and filled by approximately 3 km of low-density, uncompacted, volcaniclastic material. A thick laminated sequence of seismic reflectors forming a distinct unit in the seismic sections can be recognized over most of the lake area and lies beneath material interpreted to have been erupted in the 1.8 ka eruption. This sequence of reflectors is interpreted to be the result of the lacustrine deposition of unconsolidated material immediately following the 26.5 ka eruption. A prominent magnetic anomaly within the western part of the collapse structure is interpreted to be caused by a body of more highly magnetised material lying at or near the base of the low density caldera fill. The laminated sequence of reflectors tilts and thickens towards this magnetic anomaly suggesting that the magnetised material occurs in the deepest, part of the 26.5 ka collapse structure. The southern margin of the intense low-gravity anomaly is marked by a zone of faulting observed in the seismic reflection data. Magnetic and seismic data in the southern part of the lake are interpreted as a welded ignimbrite sheet overlain in places by rhyolite lavas similar to the pre-26.5 ka surface exposed west of the lake. This surface appears to have been down-faulted by approximately 500 m and is covered by more recent low-density volcaniclastic material. The thickness of the volcaniclastic material increases northward as the pre-26.5 ka surface is tilted downwards towards the area of major collapse. The northward tilt of this surface suggests that subsidence in the southern part of the lake may also be a response to the 26.5 ka eruption. (C) 1998 Elsevier Science B.V.