Brittle tectonics of the Thingvellir and Hengill volcanic systems, Southwest Iceland: field studies and numerical modeling

Abstract

This paper focuses on field studies and numerical models of fracture development in the area of the Hengill Central Volcano and its northern fissure swarm containing the Thingvellir Graben, in Southwest Iceland. Apart from additional field data on normal faults, a new detailed map of the Holocene fractures in the Thingvellir Graben is presented and used as a basis for numerical models on normal fault development. Field observations and models yield four basic results. First, hyaloclastite mountains in the area (“soft mechanical inclusions”) tend to deflect or arrest propagating tension fractures and normal faults. Second, fractures with an en echelon arrangement and an overlapping configuration develop shear stress shadows and tend to prevent the linking up into larger fault segments. Third, fractures with an en echelon arrangement and underlapping configuration develop shear stress concentrations between nearby tips resulting in development of transverse shear faults (transfer fault). Fourth, colinear normal fault segments concentrate tensile stresses at their tips and encourage tip-to-tip growth into larger segments.