Fault-induced seawater circulation in the Seferihisar-Balçova Geothermal Basin, Western Anatolia, Turkey

The Seferihisar-Balçova Geothermal system (SBG), Western Anatolia, Turkey is characterized by complex temperature and hydrochemical anomalies manifested by hot springs along the major fault systems and contrasting salinity distributions in different areas of the basin. Thermal waters in the Northern Balçova are heated meteoric freshwater, whereas the hot springs of the southern Seferihisar region have a strong seawater contribution. Previous numerical simulations of coupled fluid flow and heat transport processes along a North-South transect indicated that the interaction between forced convection from the Seferihisar Highs and free convection in the faults (i.e. mixed convection) is likely the major flow mechanism. It turned out that focussed upsurge of hot water in the faults induces a convective-like flow motion in the surrounding reservoir-units, even at sub-critical Rayleigh conditions. Below the sea, these fault-induced cells stretch from the seafloor toward the inner part of the basin. The question arises whether the described fault-induced cells could be responsible for sweater encroachment in the SBG. In this contribution, this hypothesis is investigated by fully coupling salt transport to the thermally-driven flow models (i.e. thermohaline flow). New isotopes data are presented to support the numerical findings. The results show that, within the expected ranges of hydraulic permeability, fault-induced cells generate brine plumes which protrude from the seafloor toward the faults along the basement interface. At the fault intersections, seawater mixes with rising hot thermal waters. Driven by buoyant forces, the captured brines ascend along the fault flanks reaching near-surface aquifers. Shallow alluvial sediments play a major role in shaping brine plumes and controlling discharge areas. In Balçova, the thick alluvium deposits and the regional flow prevent ascending salty water from spreading at the surface, whereas the weak recharge flow in the thin alluvium unit of the southern SBG is not sufficient to flush the ascending hot brines toward the sea. Further controlling factors as permeable interfaces and minor faults cutting the seafloors are studied. Although permeability variation and local heterogeneities modify flow and chemical patterns, fault-induced cells persist. This process seems to be a plausible mechanism allowing inland seawater circulation. Because of parameters uncertainty, it is not possible to reproduce thermohaline flow at basin-scale. Nevertheless, this attempt to model seawater circulation in the SBG qualitatively supported the interpreted data and described the different fluid-dynamics of the basin. The suggested mechanisms and flow patterns could likely develop in several coastal hydrothermal systems of the world with similar tectonic features.