Fig. 1

Model assumptions describing the structure of the upper continental crust. A Temperature–depth profiles assuming basal heat fluxes of 75 mW m⁻², and temperature-dependent thermal conductivity representative of bulk crustal materials (Whittington et al. 2009). Measured temperatures from Kola superdeep (Popov et al. 1999) and German Continental Deep Drilling Program KTB (Clauser et al. 1997) are shown with green and orange lines. B Permeability–depth profile. The solid black line shows the initial permeability, which is depth-dependent (Ingebritsen and Manning 1999) in brittle rock, and temperature-dependent (Hayba and Ingebritsen 1997; Scott et al 2015) in ductile rock. After failure, permeability increases by two to four orders of magnitude (grey area) above the background depth-dependent value, shown by the dashed line. C Differential stress profile assuming optimally oriented shear failure for compressive effective normal stress conditions in critically stressed crust (Sibson 2017), and power-law creep (Kohlstedt et al. 1995) at ductile conditions. D Fluid pressure, failure pressure (thin-dashed-dot) and lithostatic pressure (thick-dashed) with depth. Initial fluid pressure is hydrostatic in the brittle upper crust and over-pressured in the ductile regime. In the upper crust, rock fails with small overpressure values corresponding to the tensile strength of the rock (Cox 2010); in the ductile crust, our model assumes that failure pressure increases as the differential stress relaxes. The grey arrows show the variations of properties during rock stimulation, including cooling by fluid injection (A), permeability enhancement (B), rock embrittlement (C), and pressurization (D)