GROWTH OF NORMAL FAULTS TO THE SURFACE IN BASALT

Field observations of normal fault scarps on Kilauea Volcano combined with 2D boundary element analyses conclusively demonstrate that irreversible opening of fissures and voids and other non-elastic deformation plays a key role in the propagation of a normal fault to the surface in basalt. Adjacent monoclines and breached monoclines along scarps indicate that the faults grow up from depth, first flexing the surface then breaking it, rather than from the surface down. Across-fault profiles reveal deep gaping fissures on the footwall, buckles at the scarp base, and large cavities beneath monoclines along the scarp. The fissures open along pre-existing cooling joints. The formation of cavities involves delamination along flow contacts and vesicular zones within lava flows. Two-dimensional boundary element analyses indicate that the faults probably dip 60 to 75 degrees. Fault slip then enhances the surficial horizontal tension in the footwall and the surficial horizontal compression at the scarp base, consistent with the locations of fissures and buckles, respectively. The analyses strongly suggest that the fissures grow down from the surface, whereas the cavities initiate at depth above the fault tip and propagate with it up towards the surface. Opening of a footwall fissure doubles the compressive stress at the scarp base and promotes buckling. Stoped blocks derived from the fissure walls prop the fissures open as a fault propagates to the surface; otherwise the fissures would be closed. The propped fissures and voids must increase the hydraulic conductivity along such a normal fault system tremendously. Our findings should have some relevance to sedimentary materials because growth faulting and sedimentation in aqueous environments are kinematically analogous to faulting and volcanism at Kilauea, and sediments and lava flows are both layered. A mechanical analogy is most likely where sedimentary materials are well indurated and localized deformation (e.g., fracture) occurs in preference to the distributed deformation that is common in loose granular materials.