## 3 Materials and Methods  
In magnetotelluric data, electric and magnetic fields at the surface of the Earth are related through \((E_{x}(\omega),E_{y}(\omega))^{T}=Z(\omega)*(H_{x}(\omega),H_{y}(\omega))^{T}\), where \(Z(\omega)\) is defined as the 2-order magnetotelluric impedance tensor [30]. \(E(\omega)\) is the horizontal electric field in V/m, and \(H(\omega)\) denotes the horizontal magnetic field components in A/m. The apparent resistivity of the subsurface is calculated based on the correlation between the measured electric and magnetic fields using the following formula, \(\rho_{a,il}=(1/\mu_{0}\omega)\left|Z_{ii}(\omega)\right|^{2}\), where \(\rho_{a,il}\) is the apparent resistivity in \(\Omega\)-m, \(\mu_{0}\) is the vacuum magnetic permeability, and \(\omega\) is the angular frequency [30]. The phase tensor (\(\phi\)) can be described as the quotient of the real and imaginary components of the complex impedance tensor, \(Z\). It signifies how phase associations vary with polarization in situations where the conductivity structure is three-dimensional [31]. The apparent resistivity and impedance phase are determined at several frequencies, with high frequencies measuring shallow structures and low frequencies sampling deep structures [32]. Magnetotelluric soundings were acquired using Phoenix Geophysics MTU-5A and Metronix ADU-07 and ADU-07e MT systems at various sample rates. A test survey was conducted at one site, with the three instruments recording simultaneously to test the agreement between the results [33]. The Metronix ADU systems were used as local sites measured at sampling frequencies of 2048 Hz and 16,384 Hz, while the Phoenix Geophysics MTU-5A system was placed 10 km away as a remote site and measured continuously at 150 Hz and intermittently at 2400 Hz and 24,000 Hz sampling rates. A notch filter was switched on at 50 Hz on the MTU-5A to eliminate noise from overhead electricity cables. The temporal variations of the magnetic field were measured using Phoenix Geophysics MTC-150L coils, with a detection range of 0.0001 Hz to 10 kHz, while MFS-07e coils, with an  
Figure 1: (**a**) A tectonic map of southern Africa during the Precambrian period delineates the geographical boundaries of Archaean cratons and Proterozoic Orogenic Belts (modified after [PERSON] et al. [28]; [PERSON] et al. [29]). GCB: Ghanzi-Chobe Belt, MD: Magondi Belt, LB: Limpopo Mobile Belt. The geological map of the study area is represented in (**b**). It includes the fault systems around the Kasane Hot Spring represented by the red star. The inset in (**b**) shows a red rectangular block representing the study area (modified after [PERSON] et al. [2]). acquisition range of 0.001 Hz up to 50 kHz, were used in Metronix systems. The electric field variations were measured using nonpolarized silver/silver-chloride (Ag-AgCl) electrodes for Metronix instruments and lead/lead-chloride (Pb-PbCl) electrodes for the Phoenix instrument. The dipole lengths were varied between 30 and 100 m for all the sites depending on the accessibility. The three instruments recorded 12 h per day synchronously to allow for a multivariate processing approach (discussed in the next section). We performed a multivariate single-site processing of data acquired in 2017 and 2019. The Metronix ADU-07 instrument was used to measure only horizontal electric field (_Ex_ and _Ey_) components, while the ADU-07e instrument recorded both horizontal electric and magnetic field components (_Ex_, _Ey_, _Bx_, _By_). The MTU-5A system recorded both horizontal and vertical components (_Ex_, _Ey_, _Bx_, _By_, _Bz_). In cases when data from a remote location were accessible, the magnetic fields from that remote site were employed to estimate the impedance at the local site. We collected a dataset comprising 32 magnetotelluric soundings distributed around the hot spring (Figure 2). The vertical magnetic component was recorded at 12 sites. Figure 2: MT survey coverage in the Kasane region. The blue-filled triangle indicates the location of the hot spring.