Fluid trapping at the brittle–ductile transition re‐examined

Abstract

The brittle–ductile transition has been suggested to provide a mechanical trap to deep crustal fluids. The mechanism was advanced as a way of reconciling the geophysical case for a wet lower crust, founded on the revelation of deep crustal electrical conductors and seismic reflectors, with the problem of maintaining interconnected, low‐density fluids in stable crust for geologically significant timescales. Although some deep crustal conductors are now attributed to graphite, the hypothesis of fluid trapping at the brittle–ductile transition has been widely adopted in electromagnetic literature, with no regard to tectonic regime, and in association with standardized temperatures of 300–450°C. Meanwhile, petrologists continue to argue that the lower crust is dry. This paper re‐examines the arguments on which the hypothesis of fluid trapping at the brittle–ductile transition has been founded, and concludes that there is a geophysical case for a dry lower crust based on electromagnetic studies. The magnetotelluric (MT) technique yields electrical conductances (conductivity–thickness products) that are direction dependent (or anisotropic). The necessity of considering direction‐dependent conductances, rather than a bulk conductance, is demonstrated using data from Saxothuringia, Germany. A quantitative model is developed to facilitate joint interpretation of the maximum conductance and the anisotropy of conductance (ratio of maximum to minimum conductance). The model yields quantitative arguments against fluids being the principal cause of deep crustal electrical conductivity, because unreasonably thick layers and unreasonably high porosities are required.

Fluid focusing and its link to vertical morphological zonation at the Dajishan vein-type tungsten deposit, South China

“…Generally, the BDT depends on temperature as well as mineralogy and strain rate (Fournier, 1999;Ingebritsen, 2012;Simpson, 2001). For simplicity, we consider the contact zone Є-1 as permeable rock when magmatic volatiles were expelled.…”

Relocation of aftershocks of the 2001 Bhuj earthquake: A new insight into seismotectonics of the Kachchh seismic zone, Gujarat, India

“…Under a normal intraplate stress and strain regime, the temperatures at which orthopyroxene undergoes the transition from brittle to ductile rheology, varies from 610 to ∼700 • C (Simpson, 2001). At such high temperatures (630-690 • C; John and Schenek, 2003) and pressures, mafic volcanic magmas can be partially transformed into eclogite through the process of regional metamorphism.…”

Upwarped high velocity mafic crust, subsurface tectonics and causes of intraplate Latur-Killari (M 6.2) and Koyna (M 6.3) earthquakes, India – A comparative study

“…Further, since P-wave velocities (6.4-6.9 km/s) and resultant densities (2.87-3.01 g/cm 3 ) are significantly high at subsurface depths, the presence of appreciable porosity or permeability within in situ rocks, characterizing seismogenic depth range, is unlikely (Simpson, 2001). In such case, the conductive anomaly in all likelihood would reflect to upwarp mafic crust, rather than fluid filled rock matrix, as suggested earlier by many MT workers.…”