Fluid Compositions, Heat Sources and Fracture Interconnectivity in Geothermal Systems Along an Active Subduction Zone

Abstract

It is not common for the forearc of an active subduction zone to have hot springs or localized anomalously high thermal gradients. Among the few in the world are the Nankai subduction zone of Japan, Cascadia in the Pacific Northwest of the USA and the Hikurangi Subduction Zone in eastern North Island in New Zealand where part of the forearc exposed above sea level is referred to as the Subaerial Hikurangi Forearc (SHF). Conductive heating in the SHF is superimposed by heat exothermically generated from regional serpentinization at depth. Increased heat circulation and heat transport in northern SHF, evinced by higher surface conductive heat flow values and the presence of hot springs, is enhanced by (1) a thinner sedimentary cover resulting to a shorter distance between the source of exothermic heat (serpentinizing basalt) and the surface, (2) increased fluid production at depth and (3) higher permeability and faster aqueous fluid ascent rates causing more vigorous fluid circulation (convective rather than conductive). Surface discharge temperatures of the two hot springs in the SHF may be high but subsurface temperatures of pore fluids are similar to that of the cold springs. However, structures in the two hot springs have more direct pathways from depth to surface than in cold springs; and aqueous fluid ascent rates are 23-270x that of cold springs leading to a decrease in heat dissipation. The relative proportion of the four main fluids ascending from depth in the SHF (seawater, water of clay dehydration, water of serpentinization and subducted water) and the mantle gas% in the discharges, whether hot or cold, is associated with the depths of penetration, permeability and interconnectivity at depth of the various structures/faults feeding the surface springs.