A unique ~12 ka subaerial record of rift-transform triple-junction tectonics, NE Iceland

Derek Rust,Malcolm Whitworth

doi:10.1038/s41598-019-45903-8

Abstract

In northern Iceland the European-North American plate boundary is broad and complex but includes a remarkable subaerial triple-junction intersection between the Husavik-Flatey Fault (HFF) dextral transform and rifting in the Northern Volcanic Zone. Fortuitously, the triple junction occurs in a sheet of ~12 ka pahoehoe lavas; a tabula rasa recording innumerable fault features displayed in exquisite detail. High-resolution drone imagery, coupled with 120 field measurements of fault slip directions and opening amounts, made possible the mapping and analysis of this detail and, importantly, enabled recognition and exclusion of potentially misleading primary deformation features associated with emplacement of the lavas. Rift-transform interactions in this natural laboratory have remained spatially stable throughout post-glacial time, although with transform-affinity faults reactivated to accommodate rift extension and transform ‘encroachment’ into the rift domain. First-order en-echelon Riedel fault complexes are recognised, linked by transpressional faulting and compressional strike-slip relay ramps, as well as second-order R shears, R’ and P shears, and previously undescribed R’ Riedel-in-Riedel relationships. A pahoehoe flow front offset along a first-order Riedel fault complex records slip at ~3.8 mm a−1, which may be consistent with the published GPS-based current slip-rate estimate of ~6.8 mm a−1 across the HFF as a whole.

Highlights

Iceland lies astride the European-North American plate boundary, represented in the northern part of the island by the Northern Volcanic Zone (NVZ)
For the NVZ the most notable historical activity is the 1975-84 Krafla eruption south of the study area, which was accompanied by episodic rifting deformation amounting to several metres horizontally, and a few metres vertically, as well as by numerous M5.0–6.5 earthquakes[5,17,18,19,20]
The deformation structures are developed in an extensive sheet of pahoehoe lava flows, constrained to about 12.5 ka BP26,27, emitted from the Theistareykir central lava shield to the south of the study area (Figs 1 and 2)

Summary

Introduction

Iceland lies astride the European-North American plate boundary, represented in the northern part of the island by the Northern Volcanic Zone (NVZ). The principal onshore component of the TSZ, the Husavik-Flatey Fault (HFF), with a GPS-based current slip rate estimate of ~6.8 mm a−1, has an on-land extent of some 25 km that displays classic features of active strike-slip tectonics before intersecting the western margin of the NVZ, here marked by a belt of normal faulting and eruptive fissures known as the Theistareykir fissure swarm, to produce a remarkable subaerial exposure of ridge-transform related triple junction deformation features[2,7,8] (Figs 1 and 2) normally obscured within the oceanic realm[9,10,11]. Summarising: the overwhelming wealth of detail, some potentially misleading, demands a bird’s eye perspective and to be assembled and analysed in a map view, an essentially impossible task using conventional base maps or aerial photographs

Methods

Results

Conclusion