Evidence for dyke-parallel shear during syn-intrusion fracturing

Dyke intrusion is a highly dynamic process with seismicity preceding and accompanying magma emplacement on timescales of hours to days. Recent surveys of microseismicity indicate shear failure along fault planes parallel to the orientation of intruding dykes. However, the precision of earthquake hypocentre locations is typically limited to tens or hundreds of meters and cannot resolve whether the hypocentres relate to strain of wall rock fragments within the dykes, fault damage around the intrusions or peripherally in the country rock. Here we present high-resolution three-dimensional (3D) reconstructions of outstanding coastal exposures of a swarm of 19 dolerite dykes, near Albany, Western Australia using an unmanned aerial vehicle and Structure-from-Motion photogrammetry. It is observed that the number of faults and joints increases towards the dyke swarm, which, alongside mutually overprinting relationships, indicate that dyke emplacement and faulting were coeval. The faults contain cataclasites and are parallel to the dykes. In contrast, Mohr–Coulomb theory predicts shear failure on strike-parallel faults inclined ∼30° to the dyke plane. The faults and joints form a damage zone associated with the dyke swarm, even though the dykes themselves occupy Mode I extension fractures. These results confirm the recent geophysical evidence for dyke-parallel shear failure that can occur in the host rocks around intruding dykes. We suggest that coeval dyke-parallel seismicity both reactivated existing fracture networks and nucleated new fractures. Based on the premise that host-rock fracturing induces changes in elastic properties, remote stresses can reorientate locally leading to shear failure. This model provides for the first time an explanation for the origin of double-couple failure that is parallel in 3D (strike and dip) to dykes during their emplacement.