A new model for the growth of normal faults developed above pre-existing structures

Constraining the mechanisms of normal fault growth is essential for understanding extensional tectonics. Fault growth kinematics remain debated, mainly because the very earliest phase of deformation through recent syn-kinematic deposits is rarely documented. To understand how underlying structures influence surface faulting, we examined fault growth in a 10 ka magmatically resurfaced region of the Krafla fissure swarm, Iceland. We used a high-resolution (0.5 m) digital elevation model derived from airborne lidar to measure 775 fault profiles with lengths ranging from 0.015 to 2 km. For each fault, we measured the ratio of maximum vertical displacement to length (Dmax/L) and any nondisplaced portions of the fault. We observe that many shorter faults (<200 m) retain fissure-like features, with no vertical displacement for substantial parts of their displacement profiles. Typically, longer faults (>200 m) are vertically displaced along most of their surface length and have Dmax/L at the upper end of the global population for comparable lengths. We hypothesize that faults initiate at the surface as fissure-like fractures 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 grow by dip and strike linkage or lengthening when they reach a bell-shaped displacement-length profile. This hybrid growth mechanism is repeated with deposition of each subsequent syn-kinematic layer, resulting in a remarkably wide distribution of Dmax/L. Our results capture a specific early period in the fault slip-deposition cycle in a volcanic setting that may be applicable to fault growth in sedimentary basins.