Origins of heat and solutes in springs in a non-volcanic tectonic region, South Island, New Zealand
|Location||International Geological Congress,oslo 2008|
|Author||Reyes, Agnes G.|
|Holding Date||11 October 2008|
South Island is dominated by oblique collision of the Pacific and Australian plates, marked by the 850 km long dextral strike slip Alpine Fault where displacement of 480 km has been accommodated since Miocene. Along the Alpine Fault seismic activity is high in the NE and SW ends of South Island where the Pacific plate in the NE is subducted under the Australian plate and the Australian plate in the SW is subducted beneath the Pacific plate. Where continent to continent collision is paramount along the Alpine Fault, seismicity is relatively low but the exhumation rate, conductive heat flow and displacement rates are highest along the deformation front. About 42 of the mineral springs in South Island are thermal.
Thermal waters occur in six tectono-geographic regions including the coastal Canterbury Plains and Taieri Basin, located about 100 km E of the Alpine Fault, and Southland on the Australian plate; three sectors along the Alpine Fault including the Marlborough Fault Zone (MFZ) in the NE, northern Alpine Fault Zone (NAFZ) in the middle and Fiordland Alpine Fault Zone (FAFZ) in the SW; and the West Coast. At present only two of the spring systems, Maruia and Hanmer, located along the MFZ, have been developed, using 54-55oC hot waters mainly for swimming pools and heat exchangers. Thermal spring discharge temperatures range from 17o-66oC with the highest found along the northern half of the Alpine Fault. Estimated subsurface temperatures range from <60oC in Canterbury and the FAFZ to about 250oC in the NAFZ. Discharge temperatures in the NAFZ springs are lower than in the MFZ but the estimated subsurface temperatures are higher due to differences in permeability, degree of conductive heating and depth of fluid source. The highest HCO3/Cl ratios are confined in springs located along the MFZ. In contrast spring waters along the NAFZ and FAFZ, with lower HCO3/Cl ratios, are farther away from equilibrium, suggesting differences in permeability, meteoric water throughput, storage time and circulation among the various sectors of the Alpine Fault and to a certain extent, reflecting variations in water-rock interaction and changes in bedrock composition. D and 18O values indicate that the waters are meteoric and C isotopes show crustal, organic and metamorphic contributions. However, along the Alpine Fault meteoric waters gain solutes from interaction with fault comminuted rock, contributions from subducted slabs, metamorphic fluids and serpentinisation. Gases in Taieri and a spring in the FAFZ have isotopic He signatures indicating entrainment of about 83% and 50%, respectively, of mantle volatiles. Anomalous heat along the Alpine Fault is attributed to rapid uplift and exhumation, influx of hot waters from depth and possibly serpentinisation. Conductively-heated groundwaters circulate in Canterbury and Taieri although in the latter some of the heat is probably generated from hot mantle at depth.