Controls on terrestrial hot spring distributions

GUIDO, D.; CAMPBELL, K.

Workshop; Early Earth, Hydrothermal Activity and Life, and the Implications for Life on Mars and Elsewhere; 2015

Controls on terrestrial hot spring distributions are proposed based on comparative studies from the fossil examples in the Deseado Massif (Middle to Late Jurassic in age), Argentinean Patagonia, and the fossil and active Coromandel and Taupo volcanic zones (Miocene to Quaternary) from New Zealand. It is well known that hot springs occur in active volcanic regions (Sillitoe, 1993). They concentrate in pauses of explosive activity, with development of hydrothermal fluid convection cells related to heat anomalies at depth, normally associated with shallow intrusions (nonexplosive volcanic activity). These pauses in explosive volcanic activity, and proximity to volcanic emission centers, are the major controls on the formation of a geothermal system, and the related surface expressions: hot spring sinters, travertines or acidic features (depending on the nature of the hydrothermal fluid). Consequently, hot spring deposits are normally related to quiescent, volcaniclastic, water-reworked environments, in lacustrine, fluvial or fluviolacustrine settings, and affiliated with volcanic edifices, caldera margins, or dome complexes, as well as in closely associated with volcanic domes and hydrothermal eruption craters. In this particular geological setting, hot spring distribution is mainly controlled by both structure and topography. Regional and local faults strongly influence the main circulation of the hydrothermal fluids, which typically manifest in topographically low areas, where thephreatic water level intercepts the surface. Then, the geothermal fluids spread-out from thespring vents, cooling as they discharge along channels, into pools, over terraces, and distalwetlands. The final architecture of the deposit, each unique to its particular local setting, ismainly controlled by the composition of the fluid, the amount of fluid flow and the landscape. Finally, hot spring deposits have variable preservation potential in the geologic record, depending on later volcanic/hydrothermal events, and burial/erosional cycles; they frequently outcrop in the geologic record in high areas of the landscape due to inversion relief, especially when they are siliceous (sinter or silica residue) or silicified travertines.There are strong similarities between the Jurassic volcanic Deseado Massif region and the Tertiary to Recent Coromandel and Taupo volcanic zones. The Jurassic hot spring deposits of Patagonia are distributed over a vast back-arc volcanic region that is approximately the same size as the geographic extent of geothermal manifestations in the combined Coromandel and Taupo volcanic zones (CVZ, TVZ, respectively), and was of similar duration (~20 Ma, Guido and Campbell, 2011). In most sites for which we have completed detailed mapping at the Deseado Massif and CVZ/TVZ, hot springs are related to volcaniclastic deposits of a fluvial or lacustrine setting, which are related to pauses in the explosive volcanic activity, related to volcanoes or calderas, and in proximity of lava domes and different types of breccias bodies, in particular phreatic types like hydrothermal eruption breccias. The locations of hot spring deposits in both provinces are aligned with major faults, and particularly at the intersections of these faults: NNW and WNW regional lineaments for the Jurassic deposits of the Deseado Massif, as was interpreted by Guido and Campbell (2011) using site locations and regional geophysical data, and extensional NE striking lineaments and WNW basement faults for the TVZ active and preserved (<100 kyr old) hot spring deposits, as interpreted from geophysical data (Rowland and Sibson, 2004).