Large salt accumulations as a consequence of hydrothermal processes associated with 'Wilson cycles': A review Part 1: Towards a new understanding

The formation of large salt deposits is observed especially in areas with a geological history of high tectonic activity. Over the last decade it has become a well-established fact that heavy brines form and solid salts precipitate, due to the thermodynamic and physico-chemical properties of seawater at high temperatures and pressures encountered within hydrothermal systems. This article reviews the modern theoretical and experimental research behind these findings, and also describes geological settings that most likely cause brine- and salt-forming hydrothermal processes to occur. This analysis has led to the identification of a set of specific conditions, properties, and processes (referred to as Conceptual elements) that are used to explain the often complex processes of brine behavior that leads to hydrothermal formation of solid salt. The main objective of this review is to present hydrothermal conditions known to occur during Wilson cycles: subduction, collision, and rifting, e.g., zones of repeated tectonic unrest, where brines (commonly derived from seawater) are concentrated into heavy brines and precipitate solid salts. The internal heat of the Earth and its interaction with deeply-circulating seawater in hydrothermal systems and also the immense recycling of crustal materials, including porous oceanic crust and serpentinite (hydrated) rocks via mantle processes may lead to the formation of salt accumulations. It is also acknowledged that such brines and solid salts may often be stored subsurface for long time periods, extending from one Wilson cycle to another. Thus, on the basis of this analysis, it is cautiously suggested that large amounts of salts 'hidden' inside subduction zones may appear on the surface during subsequent rifting and oceanization phases. In Part 2 of this review, the Conceptual elements, which are described and discussed herein (Part 1) are applied to selected cases, including the Andean Mountains, the East African Rift, the Red Sea Rift, and other locations.