A uniformitarian view on intra-continental phenomena using phase separation between silicate network and aqueous fluid in the lower crust

Abstract:

The continental crust has its unsolved problems: the continuous compressive state of the continental crusts (e.g., Moore, 1970) and the emergence of the surrounding seawater (e.g., Rubey, 1951) in the geological past, and intra-continental earthquakes induced evidently by aqueous fluid (e.g., Sibson, 1992) under horizontal compression on the crust (e.g., Zoback et al., 1989) in the present. The keys to connect these phenomena are compressive stress on the crust and aqueous fluid, but their origins in the continental inlands and their relation are unknown. Revealing these is seem to be difficult due to the assumption that the continental crust is static on the rigid tectonic plate. If we can assume a mechanism in the continental crust itself independently of plate tectonic processes, we may obtain an explanation for the stress and fluid, and a continuous view between the phenomena in the long and short timescales. Here, I propose a simple idea of the mechanism using a phase transition in multicomponent mixtures: phase separation between silicate and aqueous fluid (Hack et al., 2007), which are the two basic components in the crust. If assuming this phenomenon in the enigmatic continental lower crust (Koyama, JpGU2019), we can roughly evaluate that the silicate can form its amorphous 3-dimensional network in the fluid during the separation and that the network can elastically shrink with outward diffusion of the fluid (see e.g. Tognonvi et al., 2011 and “viscoelastic phase separation”: Tanaka, 2017, Koyama and Tanaka, 2018). This network shrinking with fluid diffusing in the crustal deep can contract the upper crust of usual rocks from the bottom and can pressurize the fluid into the upper crust along the cracks (Koyama, JpGU2019). If these continue from the geological past, the network must have stabilized the crust through the compression and the fluid must have transformed into the seawater (Koyama, JpGU2012) through various processes to the surface including inland-earthquakes (and inland-volcanisms). In other words, the phase separation between the shrinking network and diffusing fluid can be a unified origin of the stress and fluid for a uniform view on the intra-continental phenomena through from the past to the present, and maybe, until a time in the future.