Emma K Bramham, Tim J. Wright, Douglas Paton

Constraining the mechanisms of fault growth is essential for understanding extensional tectonics. In these dynamic systems the propagation of existing faults through recent syn-kinematic depositions is a poorly understood yet critical process. To understand how underlying structures influence faulting, we examine fault growth in a 10 kyr magmatically-resurfaced region of the Krafla fissure swarm, Iceland. We use a high-resolution digital (0.5 m) elevation model derived from airborne LiDAR to measure 775 fault profiles, with lengths ranging from 15 m to 2 km. For each fault, we measure the ratio of vertical displacement to length (Dmax/L) and the proportion of the fault that is un-displaced (“fissure-like”). We observe that many shorter faults (<200 m) retain fissure-like features with no vertical displacement for substantial parts of their displacement profiles; longer faults (> 200 m) are typically fully displaced and have Dmax/L at the upper end of the global population. We hypothesize that faults initiate at the surface as fissures in resurfaced material as a result of flexural stresses caused by displacements on underlying faults. Faults then accrue vertical displacement following a constant-length model and only grow by linkage or lengthening when they reach a classically-shaped displacement-length profile. This hybrid growth mechanism is repeated with the deposition of each subsequent syn-kinematic layer resulting in a remarkably wide distribution of Dmax/L. Whilst our observations are directly associated with magmatic resurfacing process, our results capture a specific period in the fault slip-deposition cycle that is equally applicable to fault zone characterization in sedimentary basins.