Evaluating Strike-Slip Faulting Parallel to the Icelandic Plate Boundary Using Fault Slip Measurements, Crustal Seismicity, and Boundary Element Models

Abstract:

Most faults in Iceland strike roughly parallel to the plate boundary, which would be expected to lead to primarily normal faulting. However, several studies have found a significant component of plate boundary-parallel, strike-slip faulting in Iceland and proposed varying explanations. To evaluate these different mechanisms, we model fault slip and crustal stress patterns in Iceland arising from a variety of processes, focusing here on mechanical fault-hotspot interactions. We use a boundary element model of the Icelandic tectonic system that includes a spherical hotspot and stress derived from geodetic strain patterns that reflect rifting. On a network of faults, we estimate the fault slip required to relieve traction imposed by the hotspot and remote stress. We compare the modeled fault kinematics to moment tensors derived from published fault slip measurements and principal stresses from earthquake focal mechanisms and slip data. We note similarity between the model-predicted slip and fault slip measurements, with both identifying significant components of normal and primarily dextral strike-slip faulting. This occurs in a reference model using an applied rifting angle consistent with the Nuvel-1A value of 105°, and supplementary influence from the hotspot, with a stronger hotspot contribution causing sinistral slip in northwest Iceland. The preliminary results compared to principal stresses, using the same model parameters, suggest agreement with σ₃, with an average trend of 102° as compared to 110°, and σ₁ /σ₂ results that either also agree or appear to be permuted. Models based on a GPS velocity field, in which the hotspot is not explicitly modeled but is considered to impact the velocity field, also agree well with the reference model, except for a switch from dextral to sinistral slip on the Reykjanes Peninsula and the Húsavík-Flatey Fault. Our results suggest that current deformation of Iceland, tracked by geodetic data, is consistent with fault kinematics inferred from both recent seismicity and longer-term records of geologic fault slip. When synthesized in our models, we interpret that these data illustrate that a significant portion of strike-slip faulting in Iceland is likely driven by tectonic rifting, hotspot impacts, and mechanical interactions across the fault network.