Structure of massively dilatant faults in Iceland: lessons learned from high-resolution unmanned aerial vehicle data

Abstract

Abstract. Normal faults in basalts develop massive dilatancy in the upper few hundred meters below the Earth's surface with corresponding interactions with groundwater and lava flow. These massively dilatant faults (MDFs) are widespread in Iceland and the East African Rift, but the details of their geometry are not well documented, despite their importance for fluid flow in the subsurface, geohazard assessment and geothermal energy. We present a large set of digital elevation models (DEMs) of the surface geometries of MDFs with 5–15 cm resolution, acquired along the Icelandic rift zone using unmanned aerial vehicles (UAVs). Our data present a representative set of outcrops of MDFs in Iceland, formed in basaltic sequences linked to the mid-ocean ridge. UAVs provide a much higher resolution than aerial/satellite imagery and a much better overview than ground-based fieldwork, bridging the gap between outcrop-scale observations and remote sensing. We acquired photosets of overlapping images along about 20 km of MDFs and processed these using photogrammetry to create high-resolution DEMs and orthorectified images. We use this dataset to map the faults and their damage zones to measure length, opening width and vertical offset of the faults and identify surface tilt in the damage zones. Ground truthing of the data was done by field observations. Mapped vertical offsets show typical trends of normal fault growth by segment coalescence. However, opening widths in map view show variations at much higher frequency, caused by segmentation, collapsed relays and tilted blocks. These effects commonly cause a higher-than-expected ratio of vertical offset and opening width for a steep normal fault at depth. Based on field observations and the relationships of opening width and vertical offset, we define three endmember morphologies of MDFs: (i) dilatant faults with opening width and vertical offset, (ii) tilted blocks (TBs) and (iii) opening-mode (mode I) fissures. Field observation of normal faults without visible opening invariably shows that these have an opening filled with recent sediment. TB-dominated normal faults tend to have the largest ratio of opening width and vertical offset. Fissures have opening widths up to 15 m with throw below a 2 m threshold. Plotting opening width versus vertical offset shows that there is a continuous transition between the endmembers. We conclude that for these endmembers, the ratio between opening width and vertical offset R can be reliably used to predict fault structures at depth. However, fractures associated with MDFs belong to one larger continuum and, consequently, where different endmembers coexist, a clear identification of structures solely via the determination of R is impossible.

Mapping the fracture network in the Lilstock pavement, Bristol Channel, UK: manual versus automatic

“…Shapefiles of the fracture traces presented in this article are provided in the Supplement. The image files are published as Weismüller et al (2020) and available as download under https://doi.org/10.18154/RWTH-2020-069 (last access: 28 August 2020).…”

“…By comparing these areas, we give further estimates about the spatial variation within the larger fracture network. In an associate paper (Passchier et al, 2020), we present a manual fracture mapping study of the whole area, discuss the criteria for identifying different fracture generations, and study the spatial heterogeneity of different fracture generations.…”

Low‐Angle Shear Within the Exposed Mânzălești Diapir, Romania: Salt Decapitation in the Eastern Carpathians Fold‐and‐Thrust Belt

“…During recent years, the use of photogrammetry has become a key component of fieldwork in geosciences (i.e., Adamuszek et al, 2021;Fernandez et al, 2021;Mercuri et al, 2020;Weismüller et al, 2019). Tools for interpreting three-dimensional (3D) outcrops in Digital Outcrop Models (DOM) are emerging, and extracting structural data from outcrops that are not easy or safe to reach is now possible.…”

Mapping and evaluating kinematics and the stress and strain field at active faults and fissures: a comparison between field and drone data at the NE rift, Mt Etna (Italy)

“…As a parallel and complementary activity to the UAV survey, we collected several GCPs, distributed all over the area, essential to scale and reference the SfM-derived models as well as to avoid any bulging effect (James and Robson, 2012;Turner et al, 2012;Westoby et al, 2012;Smith et al, 2016;Vollgger and Cruden, 2016;James et al, 2017;Esposito et al, 2017). We targeted 34 natural targets, as already successfully performed by Bonali et al (2020), to speed up the GCP collection avoiding the deployment and recovery of artificial targets.…”