Structural electrical anisotropy in the crust at the South-Central Chilean continental margin as inferred from geomagnetic transfer functions

Induction vectors, as a visualization of geomagnetic deep sounding transfer functions, display an unique pattern at the South Chilean continental margin between latitudes 38°–41°S and longitudes 71°–74°W: at long periods of approx. 3000 s their real parts are uniformly deflected from the W–E direction (which would be expected due to the coast effect and/or anomalies beneath the roughly N–S striking Andean mountain chain) to the NE. Attempts to model this behavior with simple and geologically realistic 3-D models failed, but a reasonable data fit was obtained by employing 2-D models with a structurally anisotropic, lower crust. This anisotropy hints at a deeply fractured, fluid-rich crust with a major strike direction of 40°–50° (SW–NE), oblique to the continental margin and in accordance with the regional stress field in the region of the volcanic arc. A surprising result is that the anisotropy persists in the forearc and may even reach until the continental slope near the trench.