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...