Rifting of passive margins has, in some places, led to the development of extensive submerged continental platforms. Crustal thinning by a factor of two or more is at least partially accommodated by high-angle normal faults that extend through the brittle upper crust to the ductile parts of the lower crust. However, an approximately horizontal detachment zone has been postulated to explain the width of the shelf and differences between the amounts of upper crust and lower lithosphere extension. Here, we present magnetotelluric (MT) and geomagnetic depth sounding (GDS) measurements across the Exmouth Plateau, a deep (~1500 m) and wide (>500 km) continental platform on the Northwest Shelf of Western Australia. MT responses across all of the Plateau in the bandwidth 102--104 s exhibit a pronounced apparent resistivity low of less than 1 Ω.m at ~2 ¿ 103 s. Two-dimensional smooth inversion of all MT and GDS data images a very low resistivity layer (~0.1 Ω.m) at a depth of 10--15 km, dipping less than 1¿ landward from the continent-ocean boundary. Below this layer, resistivity increases to >103 Ω.m beneath the Moho at 20 km depth. The low-resistivity mid-crustal layer coincides with a laterally extensive seismic reflector at 10--15 km observed through much of the Exmouth Plateau. We suggest that this layer represents a decoupling zone between ductile and brittle continental crust, with a concentration of conducting mineralisation along shear zones. The most probable conducting mineral is graphite; this is known to precipitate along shear zones, and would significantly facilitate extensional strain.