Bull Volcanol (2005) 67:186–193 DOI 10.1007/s00445-004-0373-7
RESEARCH ARTICLE
Ian A. Nairn · Jeffrey W. Hedenquist · Pilar Villamor · Kelvin R. Berryman · Phil A. Shane
The ~AD1315 Tarawera and Waiotapu eruptions, New Zealand: contemporaneous rhyolite and hydrothermal eruptions driven by an arrested basalt dike system? Received: 17 July 2003 / Accepted: 2 April 2004 / Published online: 7 July 2004 Springer-Verlag 2004
Abstract A series of large hydrothermal eruptions occurred across the Waiotapu geothermal field at about the same (prehistoric) time as the ~AD1315 “Kaharoa” rhyolite magmatic eruptions from Tarawera volcano vents, 10–20 km distant. Triggering of the Waiotapu hydrothermal eruptions was previously attributed to displacement of the adjacent Ngapouri Fault. The Kaharoa rhyolite eruptions are now recognised as primed and triggered by multiple basalt intrusions beneath the Tarawera volcano. A ~1000 t/day pulse of CO2 gas is recorded by alteration mineralogy and fluid inclusions in drill core samples from Waiotapu geothermal wells. This CO2 pulse is most readily sourced from basalt intruded at depth, and although not precisely dated, it appears to be associated with the Waiotapu hydrothermal eruptions. We infer that the hydrothermal eruptions at Waiotapu were primed by intrusion of the same arrested basalt dike system that drove the rhyolite eruptions at Tarawera. This dike system was likely similar at depth to the dike that generated basalt eruptions from a 17 km-long fissure that formed across the Tarawera region in AD1886. Fault ruptures that occurred in the Waiotapu area in association with both the Editorial responsibility: J. Donnelly-Nolan I. A. Nairn ()) RD5, Rotorua, New Zealand e-mail:
[email protected] Tel.: +64-7-3628081 Fax: +64-7-3628354 J. W. Hedenquist Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, USA P. Villamor · K. R. Berryman Institute of Geological and Nuclear Sciences, PO Box 30368, Lower Hutt, New Zealand P. A. Shane Geology Dept, University of Auckland, PO Box 90129, Auckland, New Zealand
AD1886 and ~AD1315 eruptions are considered to be a result, rather than a cause, of the dike intrusion processes. Keywords hydrothermal · rhyolite · eruptions · basalt · dikes · faulting · CO2
Introduction The triggering of a series of large hydrothermal eruptions across the Waiotapu geothermal field (Figs. 1, 2) has been previously linked to fault displacement associated with the ~AD1315 (prehistoric) “Kaharoa” rhyolite eruptions from Tarawera vents at 10–20 km distance (Lloyd 1959; Cross 1963). Here we combine Waiotapu hydrothermal eruption and geothermal well data with our recent findings on the Kaharoa eruptions, and the faulting history of the Waiotapu-Tarawera region. This synthesis provides new insights into the relative influences of tectonic and magmatic processes, and we propose that both the Kaharoa and Waiotapu eruptions were primed by volatiles and heat from an arrested basalt dike system. Juvenile basalt inclusions were erupted with the Kaharoa rhyolites, but no basalt has been found at the shallow depths sampled by eruption and drilling at Waiotapu. Here the evidence for a link to basaltic magmatism remains circumstantial, but compelling.
Regional tectonics in the Taupo Volcanic Zone The ~15 km-wide, NE-trending Taupo Fault Belt (Fig. 1) is the site of active rifting within the Taupo Volcanic Zone (TVZ), with spreading rates estimated at ~1 cm/yr at seismogenic depths (Beanland et al 1990; Darby et al 2000; Villamor and Berryman 2001). The latest faulting occurred during the 1987 Edgecumbe M6.3 earthquake (Fig. 1), with up to 2.5 m normal and 1.8 m extensional displacement occurring on >20 km of multiple surface fault traces.
187 Fig. 1 Digital terrain map (data from LINZ Series 260 1:50,000 scale maps) of the central TVZ showing caldera outlines, major faults (F.), and the Haroharo and Tarawera volcanic complexes within Haroharo caldera. OVC is Okataina Volcanic Centre. Crosses (R) to the west of Haroharo mark the 3.5 ka Rotokawau basalt vents. The white box shows area of Fig. 2. Inset shows Fig. 1 location within the TVZ, epicentre of the 1987 Edgecumbe earthquake (star), and the Kaharoa Tephra 2 cm isopach (dashed)
Fig. 2 Map of the RotomahanaWaiotapu geothermal system, with margins approximately defined by the light-grey stippled 20–50 Wm iso-resistivity zone (from Stagpoole and Bibby 1998). The 5 and 10 Wm isoresistivity contours are also shown. The named young hydrothermal eruption craters are: 1, Okaro; 2, Opal; 3, Ngahewa; 4, Opouri; 5, Tutaeinanga; 6, Champagne; 7, Whangioterangi; 8, Ngakoro; 9, Haumi. T=fault trench excavation site
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The Okataina Volcanic Centre (OVC) lies across the Taupo Fault Belt, and includes Haroharo caldera (Figs. 1, 2) in which the Haroharo and Tarawera volcanic complexes have grown during nine rhyolite and two basalt episodes over the last 26 kyr (Nairn 1989, 2002). The eastern margin of the Taupo Fault Belt is defined by the 40 km-long Paeroa Fault (Fig. 1), with the Ngapouri Fault branching eastward as a poorly defined trace displacing >0.23 Ma ignimbrites (Nairn 2002). A step fault displaces Holocene sediments about 0.5 km northwest of the main Ngapouri scarp near the Lake Opouri crater (Fig. 2). No continuous fault trace occurs northeast of Opouri; here the Ngapouri Fault is mapped along a 5 km alignment of hydrothermal craters and pits that terminates at Opal crater (Fig. 2). The Ngapouri Fault may continue northeast at depth, to merge with the 3 kmlong Rotomahana Fault that splays from the southern margin of Haroharo Caldera (Fig. 2).
Kaharoa eruption episode The ~AD1315 Kaharoa eruptions form the latest of four rhyolitic episodes that have built the Tarawera complex over the last 18 kyr (Nairn et al 2001, 2004). The ~4 km3 (dense rock equivalent, d.r.e.) Kaharoa episode was primed and triggered by repeated basalt intrusions into a pre-existing body of rhyolite magma at ~5–7 km depth (Leonard et al 2002; Nairn et al 2004). The seven Kaharoa vents define a NE-trending (052 N) 8 km lineation (Fig. 2) that has since been overprinted by en echelon rift craters formed during the AD1886 Tarawera basalt rift eruption (Nairn and Cole 1981). The Kaharoa vent lineation, and the AD1886 rift, lie between the Paeroa and Fig. 3 Aerial view to NE with Okaro crater occupied by a lake in right foreground; white steam rises from Waimangu AD1886 crater lakes in middle left foreground; Lake Rotomahana occupies very large AD1886 craters in photo centre, with Tarawera volcano massif beyond. Kaharoa rhyolite domes (bare ground) form the Tarawera summit area, and are cut by large craters formed during the AD1886 eruption. Distance from Lake Okaro to Tarawera summit is 12.5 km. Photo by D.L. Homer
Ngapouri-Rotomahana faults extrapolated into the caldera. Neither volcanic lineation is a direct continuation of any fault outside the caldera. Early Kaharoa plinian tephra falls were dispersed to the southeast in narrow lobes during the passage of a single weather system (Sahetapy-Engel et al 2000). Later falls were widely dispersed into northern sectors (Fig. 1) over a much longer time, while a small lava dome was extruded. Three large domes (Figs. 2, 3) grew in the final phase of the Kaharoa episode, which had an estimated duration of ~5 years, based on the extrusion rates of observed lava dome eruptions (Nairn et al 2001). Waiotapu lies on upwind or crosswind azimuths from all Kaharoa tephra dispersal lobes so that very little tephra fell in this area. The tephra is found at Waiotapu only where it was rapidly buried by hydrothermal eruption deposits from Okaro crater – the closest Waiotapu crater to Tarawera (Figs. 2, 3, 4). Composition of the Kaharoa rhyolites changed during the eruption (Leonard et al 2002; Nairn et al 2004). The early plinian eruptives (with d.r.e volume 2 km3) have low Zr in whole pumice; low Ca/K ratio in glass, and biotite with low Mg/Fe ratios (Shane et al 2003). This composition is denoted Type 1 (Nairn et al 2004). Late stage Kaharoa pumice and lava eruptives have higher Zr in pumice, Ca/K in glass and Mg/Fe in biotite (Fig. 5), defining Type 2 composition.
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Fig. 4 Okaro hydrothermal eruption ejecta (Ok), >6 m thick at this Okaro crater location, directly overlies 1–2 cm thick Kaharoa ash (Ka), which overlies a weathered paleosol in 1.8 ka Taupo ignimbrite (Tp). White and black scale bars are 10 cm long
Fig. 5 Biotite crystals from early-erupted Type 1 (T1) and lateerupted Type 2 (T2) proximal Kaharoa pumices and lavas define two compositional fields, B1 and B2. K131 biotites are from the Kaharoa Tephra that underlies the hydrothermal eruption ejecta at Okaro crater (see Fig. 4), and overlap the B2 field. Analyses by electron microprobe at Auckland University (Shane et al 2003)
depth (Lloyd 1959; Hedenquist and Henley 1985). However, two of the largest craters (Ngahewa and Okaro) lie 0.5 km northwest of the Ngapouri and Rotomahana fault traces respectively; other young craters lie 3 km south of the Ngapouri Fault. Large craters were also formed by phreatomagmatic and hydrothermal eruptions at Rotomahana and Waimangu (Fig. 3) during the AD1886 Tarawera basalt eruption (Nairn 1979; Simmons et al 1993), but no Waiotapu eruptions occurred at that time. Eight geothermal wells (Fig. 2) at Waiotapu have a maximum depth and temperature of 1100 m and 295 C. No vent-proximal volcanic rocks, or basalts, were intersected by drilling. Hydrothermal minerals and fluid-inclusions in drill core samples record large variations in total CO2 content of the geothermal fluids over time, with recent cooling on some margins of the system (Hedenquist 1983; Hedenquist and Browne 1989). An adularia-albite mineral assemblage was formed during an early low-CO2 stage (~0.01 molal CO2). This was followed by a period of higher (>10) CO2 concentrations that produced an illite overprint. The subsequent deposition of abundant bladed calcite occurred from a boiling fluid in which fluid-inclusion data indicate a relatively high concentration of CO2 (to 0.59 molal), and temperatures locally 20–40 C higher than measured now. The present Waiotapu fluids have intermediate CO2 values (0.05 molal), and are apparently undersaturated with calcite, but are still illite stable. Due to the relatively slow alteration process, much of the reaction recorded by the change in alteration mineralogy (from adularia-albite to illite, with an interval of calcite deposition) occurred while the CO2 contents were decreasing from their maxima to the present concentrations. Timing of the CO2 pulse cannot be precisely defined, but the illite overprint common in Waiotapu well core and cutting samples from 100 to 1000 m depths (Hedenquist and Browne 1989) is rare in the hydrothermal eruption ejecta (Hedenquist 1983). This indicates a short pre-eruption period of elevated CO2 concentrations, and that most of the alteration associated with the CO2 pulse occurred after the hydrothermal eruptions. The CO2 pulse must have either been associated with the hydrothermal eruptions, or have occurred early during the relatively short time (
