Quantifying palaeostress and burial depth in the upper crust using magnetic anisotropy data

Quantifying palaeostress and burial depth in the upper crust is fundamental to reconstructing geomorphic and tectonic histories, yet remains challenging in rocks lacking macroscopic piezometric indicators. Here, we demonstrate a calibrated magnetic proxy for differential palaeostress using the stress sensitivity of magnetic fabrics in columnar basalts. Using the Seljadalur basalts from southwest Iceland (~7 Ma) as a natural laboratory, we combine anisotropy of magnetic susceptibility (AMS), three-dimensional X-ray micro-computed tomography (micro-CT) and laboratory stress calibration to decouple primary flow fabrics from stress-induced magnetic signals. Micro-CT analyses show that the grain-shape preferred orientations of both silicate and magnetic grains preserve a primary sub-horizontal flow alignment across the column, whereas the principal AMS axis Kmax rotates toward the vertical at column edges. We argue that this rotation reflects a piezomagnetic response of mixtures of single-domain and single-vortex titanomagnetite grains to differential stress. Using calibration data of Kmax rotation versus stress, yields a median differential palaeostress of 49 ± 6 MPa, corresponding to a lithostatic burial depth of 1.7 ± 0.2 km, reduced to 1.4 ± 0.2 km if we allow for possible glacial loading.