Rock Control of Faulting: A Case Study from the Icelandic Rift Zone

Abstract

Abstract The work examines relationships between faulting and the physical properties of the host rock based on a study from the active rift zone in Iceland. Although compositionally identical, hyaloclastites, pillow lavas, and columnar‐jointed lavas have different mechanical properties that exert strong controls on the distribution of faults and strain within fault zones and the mechanical processes associated with displacement. In hyaloclastite, a fault zone comprises a braided array of lower‐order shear fractures. Veins of silica within gouge zones imply periodic dilational shear. In pillow lavas, reactivation of the near‐random arrays of primary fractures results in wide fault zones with diffuse margins. Displacement is achieved by the accumulation of small increments of slip on primary joints and by the development of through‐going shear fractures. Columnar lavas have the highest bulk strength and the fewest primary weaknesses. The subvertical cooling joints are favorably oriented for dilational‐shea...