The Transition From Jointing to Faulting Observed at the Koa'e Fault Zone, Hawai'i Volcanoes National Park, Hawaii

Abstract

Fractures can exhibit mixed modes of displacement, that is, combinations of displacements parallel and perpendicular to the fracture plane, that make a displacement-to-length (D_max/L) scaling analysis challenging. However, such analysis is important for understanding the propagation and nature of mixed-mode fracture populations. In this study, we investigate the D_max/L scaling relationship for fractures involving opening and shearing modes from field measurements at the spectacularly exposed Koa'e Fault Zone in Hawai'i Volcanoes National Park, Hawaii. Its major structures have prominent fault scarps and display openings of up to several meters. They are locally accompanied by monoclines and sheared and pure joints. Through structural mapping and detailed field observations, we identify a morphological continuum along the structures representing different stages in the evolution of the faults. Contrary to previous studies, our observations support that faults are formed by the downward propagation of joints that transition to faulting at depth, then creating the monoclinal flexure. Our measurements allow us to investigate the D_max/L scaling behavior for the total mixed-mode displacement and their individual vector components, that is, reliefs and openings. The D_max/L scaling relationships for all structure types, including pure joints, sheared joints, and faults, show a power-law relation with a near-linear dependence of maximum displacement and length. The joint apertures scale to length with a nearly linear scaling relationship, not following the widely observed square root scaling relationship that all opening-mode fractures are believed to have.