Unstable Rifts, a Leaky Transform Zone and a Microplate: Analogues from South Iceland

Abstract

A structural analysis was undertaken in the South Iceland Seismic Zone (SISZ) transform zone, and in the Hreppar Microplate (HMP) located between the propagating Eastern Rift Zone (ERZ) and the receding Western Rift Zone (WRZ). The age of the oceanic crust in these areas is 3.4 Ma to present. About 20,000 fracture segments on aerial images reflect the dominance of NNE extensional structures in the WRZ. Around 9,000 basement faults, intrusions, secondary fractures, surface ruptures of earthquakes, and leakages were mapped in the outcrops of the HMP and the SISZ. About 23% of these fractures strike NNE, while 77% are dominantly northerly dextral and ENE sinistral, and secondarily E-W, WNW and NW sinistral strike-and oblique-slip structures, forming a Riedel shear pattern typical of a transform zone. Dyke injections into Riedel shears indicate a leaky transform zone. Fractures reactivated, accumulated slip, and re-opened for fluid flow. The ENE faults dip mostly to the southeast and could be the present boundary of the SISZ to the north. A 10 -30 km wide ENE structural zone hosts a valley to the east, which could be deeper in the west. This ENE zone contains all the earthquakes, dominant ENE rivers, frequent ENE secondary fractures, and is likely the active part of the SISZ. The HMP does not show rotation since 3.4 Ma despite being between two rift segments. Future propagation/recession of the rift segments along their N55˚E sections would cause a migration and a clockwise rotation of the SISZ from ENE to E-W. The boundary faults of the SISZ would then be E-W, with unchanged internal Riedel shears, compensating its sinistral motion. Insights into complexities of diverging plate boundaries are critical for resource management in such tectonic contexts. Figure 1. Geological settings of Iceland and the study area. (a) Active and extinct plate boundaries and microplates in Iceland (modified from [55]); (b) Main geological elements of the study area. Fissure swarms and intraplate volcanism on (a), and the geology and tectonics on (b) are from [122], the petrology of Holocene and Late Pleistocene volcanic systems is from [109], and the earthquakes are from the SIL network of the Icelandic Meteorological Institute (IMO). The rate of spreading is from [121]. M. Khodayar et al.and results in this case study will also benefit exploration and resource management at other diverging oceanic plate boundaries. Geodynamics of Diverging Plate BoundariesAs oblique rifting is absent in the study area, an overview of the key features associated with rift/ridges, transform zones, and microplates is presented below.  Continental rifts and oceanic ridges trend orthogonal to spreading. From initial continental break-up, extension is accommodated by rift boundary normal faults, but intra-rift complexities appear during rift evolution with half-grabens, subsided central grabens/axial valleys, uplifted rift shoulders, rotated fault blocks and detachments [8] [9], as well as ridge segmentation [10] and ridge instability [11] [12]. During thei...

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

“…Gudmundsson (2007) proposed a structural strength barrier of the Hreppar anticline for a throughgoing~E-W rupture in the SISZ rather than a general strength anisotropy of the crust (Bjarnason, Cowie, et al, 1993). The fault pattern in the Hreppar formation is similar to strike-slip faulting in the RP (Gudmundsson, 2017;Karson, 2017;Karson et al, 2018Karson et al, , 2019, where the Riedel shear analogy fits well (Clifton & Schlische, 2003;Khodayar et al, 2018). Detailed mapping of the destruction zones marked by damage to housing during the 6 September 1896 earthquake, extending between SISZ and Hreppar, indicates ESE-WNW as well as~N-S destruction (Figure 3 in Björnsson, 1976).…”

“…It is to be noted that the fractures mapped here probably do not show all the fault sets and traces present in the area. For instance, Khodayar et al (2020) finds that 23% of fractures in HB and SISZ strike NNE, while 77% are primarily ~N dextral and ENE sinistral and secondarily E‐W, WNW, and NW sinistral strike‐ and oblique‐slip structures, forming a leaky transform zone.…”

“…The 70-80°trending plate boundary in RP is oblique to the plate motion direction (Árnadóttir et al, 2006;Brander et al, 1976;Einarsson, 1991), where divergence occurs along northeast-trending faults and fissures (Clifton & Schlische, 2003;Gudmundsson, 2017). The sinistral shear is accommodated by ductile deformation below a locking depth of 6 km (Hreinsdóttir et al, 2001) and along N to NNE-striking right-lateral strike-slip faults closer to the surface (Keiding et al, 2008;Khodayar et al, 2018). However, seismicity in the peninsula mostly manifests as normal-faulting seismic swarms along the rift in the west and as dextral earthquakes on the~N-S faults in the east toward the SISZ (e.g., Árnadóttir et al, 2004;Keiding et al, 2009).…”

“…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).…”

A New Model for the Evolution of Oceanic Transform Faults Based on 3D Broadband Seismic Observations From São Tomé and Príncipe in the Eastern Gulf of Guinea

“…OTF studies have primarily focused on currently active zones of oceanic spreading using a variety of geological and geophysical methods. These include in‐situ observations of OTF's in marine settings through submersible dives (Mamaloukas‐Frangoulis et al., 1991; Pastouret & Cyamex Scientific Team, 1981) as well as outcropping active transform faults (Gudmundsson, 1995; Khodayar et al., 2020) and remnants of oceanic crust in ophiolites (e.g., Fagereng & MacLeod, 2019), multibeam bathymetry (Embley & Wilson, 1992; Fornari et al., 1989; Searle, 1986), active source (Gasperini et al., 2017; Marjanović et al., 2020; Van Avendonk et al., 1998), seismicity and passive seismic studies (Kuna et al., 2019; Wolfson‐Schwehr & Boettcher, 2019; Wolfson‐Schwehr et al., 2014), as well as potential field data‐based studies (Gregg et al., 2007; Wessel et al., 2015). The magmatic evolution of OTZ/OFZ features has also been directly sampled through dredges and submersibles (Dick et al., 2008).…”