Magmatic faults: Challenges, progress, and possibilities

It is increasingly recognized that plutons are rich in magmatic structures that provide powerful tools for unraveling hypersolidus histories. However, one group of structures, magmatic faults recognized almost a century ago, has not been systematically examined thus far and is the focus of this review. Magmatic fault styles resemble those in solid materials, encompassing a wide range of orientations, kinematics, and mechanical attributes. But unlike their sedimentary and metamorphic counterparts, studies of magmatic faults face several challenges, leading to their underrepresentation in plutonic histories. Key parameters needed to study magmatic faults include the fault orientation and timing relative to the magmatic fabric(s) in the host, the offset and kinematic styles of faulting, and the recognition of uniquely magmatic structures within the fault such as shear folding, melt migration, and deflection of magmatic fabrics to determine deformation style. The preservation of early faulting under melt-rich conditions is relatively rare and is instead biased towards conditions of higher crystallinity; even these latter faults can be hard to see unless compositional defined markers are offset across the faults. Our synthesis to date suggests that magmatic faults are probably much more widespread than presently recognized and that the displacement of crystal mushes along these internal faults may be an effective, though commonly cryptic, mechanism for strain accommodation in magmatic columns. Heterogeneous strain in mushes can result in highly varied fault styles; however, the characteristics of different 'fault packages' in plutons may provide a means of evaluating causes of faulting in plutons (e.g., roof and sidewall magma crystallization, recharge and defrosting, thermal convection, emplacement, magmatic differentiation, tectonics). Studies of magmatic fault packages are in their infancy and future research should incorporate systematic, pluton-wide approaches designed to establish reliable criteria for determining the driving force(s).