Normal fault growth in layered basaltic rocks: the role of strain rate in fault evolution

Conceptual models for the evolution of dilatant faults in volcanic rift settings involve a step-wise growth pattern, involving upward propagation of subsurface faults, and surface monocline formation, which are breached by subvertical, open faults. Immature, discontinuous normal faults are considered representative of the early stages of mature, linked faults that accommodate extensional strains. We consider the evolution of surface-breaking normal faults using a comparison of the distribution and geometry of normal faults from two volcanic rift zones: the Koaʻe fault system, Hawaiʻi, and the Krafla fissure swarm, NE Iceland. Field mapping highlights similarities to current predicted geometries, but also prominent differences that are not reconciled by current models. Variable deformation styles record magma supply changes within the rift zones, which drive local strain rate gradients. Building on existing studies, we present a conceptual model of fault growth that accounts for spatial and temporal changes in strain rate within the deforming regions. We propose that faults in separate rift systems may not advance through the same stages of evolution and that faults within individual rift systems can show differing growth patterns. Variations in surface strains may be indicative of subsurface magmatic system changes, with important implications for our understanding of volcano-tectonic coupling.