Rift‐Parallel Strike‐Slip Faulting Near the Iceland Plate Boundary Zone: Implications for Propagating Rifts

Abstract

Strongly lineated terrain outside of Iceland's active plate boundary zones is created by faults and dikes aligned with the rift zones where they formed, similar to the spreading fabric defined by abyssal hills generated at mid‐ocean ridge spreading centers. As expected, rift‐parallel normal faults and fissures dominate in the active rift zones, but in older crust to the east and west, faults with strike‐slip and oblique‐slip displacements dominate. Some areas have widespread, small‐scale, strike‐slip, and oblique‐slip faults, while others have more widely spaced, larger, strike‐slip fault zones. In most cases, the strike‐slip and oblique‐slip faults strike subparallel to nearby older dikes and normal faults assumed to indicate the orientation of the rift zones where they formed. Strike‐slip displacements overprinting normal faults and along dike margins suggest reactivation of spreading‐related zones of weakness. More complicated fault geometries and kinematics occur near the oblique rifts and the major transform fault zones. The sense of movement on the strike‐slip and oblique‐slip faults is broadly systematic with respect to the active Northern and Eastern Rift Zones supporting the interpretation that they are the result of crustal block rotations on either side of rift zones that propagate to the north and south away from the center of the Iceland hot spot. Similar fault kinematics may occur along mid‐ocean ridges and other magmatic rifts where rift propagation occurs on a range of scales.

Microstructures of bedding-parallel faults under multistage deformation: Examples from the Southeast Basin of France

“…On the contrary, we propose that reactivation is promoted at boundaries between materials of different nature or inherited mechanical heterogeneities, i.e., the vein/clay edges in this study. A similar process occurs for dyke walls that are often the locus of faulting in volcanic areas (e.g., Karson et al, 2018). It is also in agreement with the idea that preferential growth of fault is not positively correlated with the intrinsic weakness properties of faults (Walsh et al, 2001).…”

Seismicity on Conjugate Faults in Ölfus, South Iceland: Case Study of the 1998 Hjalli‐Ölfus Earthquake

“…A significant part of the sinistral transform motion on SISZ is accommodated by large earthquakes (Roth, 2004) on N‐S dextral faults exhibiting “bookshelf‐like” characteristic along the E‐W trending SISZ (Einarsson et al, 1981). This zone is part of a larger region called the Hreppar formation that has older exposed bedrock north of SISZ exhibiting ENE trending sinistral and N‐S to NNE trending dextral faults, with a few NE trending sinistral faults (Gudmundsson, 2017; Karson et al, 2018; Khodayar et al, 2020; Khodayar & Franzson, 2007). They have been mostly aseismic during recorded history with some exceptions (Thorbjarnardóttir et al, 2018).…”

Investigating Strike‐Slip Faulting Parallel to the Icelandic Plate Boundary Using Boundary Element Models

“…We modeled the three‐dimensional geometry of faults by projecting their surface traces to 10 km depth, then rotating to an average dip of the directly associated fault region in the Karson et al. (2018) data or, for faults not included in that data set, a dip of 70°, reflecting patterns of slightly steeper dips found in the Karson et al. (2018) data than predicted for normal faults (Anderson, 1905).…”

“…Faults in Iceland mainly strike subparallel to the active rift zone (Figure 1) and are dominated by normal faulting in the active rift zones as well as strike‐slip faulting in the major transform zones (Karson et al., 2018). Surprisingly, widespread rift‐parallel strike‐slip and oblique slip faulting have also been found outside the active transform zones (Bergerat et al., 2000; Gudmundsson et al., 1992; Karson et al., 2018; Plateaux et al., 2012). This presents a curious feature of faulting kinematics in the region as, in a divergent setting, primarily normal slip is expected on faults that strike parallel to the boundary.…”

“…These studies also identify similarly oriented axes of maximum extension for both strike‐slip and normal faults, indicating that the strike‐slip faults are likely also related to rifting (Plateaux et al., 2012). Constraints on ages of faulting are limited to mostly occurring within the last 9–11 Myr since much of the bedrock is 9 Ma or older, leaving few significant and consistent cross‐cutting relationships other than the constraint that faulting must have occurred after the rock formed (Karson et al., 2018). However, patterns of multiple sets of slickenlines, representing a shift between normal and strike‐slip faulting, while rare, are present within the data and some faults do cut historic lavas, which indicates that some slip is relatively modern (Karson et al., 2018).…”