Propagation of normal faults to the surface in basalt, Koae fault system, Hawaii

Abstract

Normal fault scarps of the Koae fault system on Kilauea volcano consistently display locally breached monoclines underlain by prominent cavities, deep gaping fissures on the footwall, finer fissures on the hanging wall, and buckles at the scarp base. Elastic analyses reveal that this assemblage forms as a fault propagates up towards the surface rather than down from it. Models of a planar blind normal fault with a dip exceeding 60° yield a monocline with a tensile stress concentration at the surface where gaping fissures occur, a stronger subsurface tensile stress concentration near the blind fault tip, where cavities occur, and a compressive stress concentration at the surface where buckles occur. The footwall fissures grow down from the tensile stress concentration at the surface and link with a fault as its scarp grows. In contrast, the cavities initiate at depth near the fault tip and propagate with it up towards the surface. The hanging wall fissures apparently open in response to slip on late-forming blind antithetic faults near the surface. Stoped blocks derived from footwall fissure walls help prop the footwall fissures open as a normal fault breaches the surface. The fissures, cavities, and scarp rubble provide highly conductive hydraulic pathways.