Structure of massively dilatant faults in Iceland: lessons learned from high resolution UAV data

Abstract

Normal faults in basalts develop massive dilatancy up to several tens of meters close to the Earth's surface and show corresponding interactions with groundwater and lava flow. These massively dilatant faults (MDF) are widespread in 15 extensional settings like Iceland or the East African Rift, but their detailed geometry is not well understood, despite their importance for fluid flow in the subsurface, geohazards or geothermal energy. We present a large set of digital elevation models (DEM) of the surface geometries of MDF with 5-15 cm resolution, acquired along the Icelandic Rift zone using unmanned aerial vehicles (UAV). UAV provide a much higher resolution than aerial/satellite imagery and a much better overview than ground-based fieldwork, thus bridging the gap between outcrop scale and regional observations. 20Our data present representative outcrops of MDF, formed in basaltic sequences linked to the Mid Ocean Ridge. We acquired photosets of overlapping images along about 20 km of MDF and processed these using photogrammetry to create high resolution DEMs and ortho-rectified 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. 25Mapped vertical offsets show typical trends of normal fault growth by segment coalescence. However, opening widths in mapview show variations at much higher frequency, caused by segmentation, collapsed relays and tilted blocks. These effects cause a commonly 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 MDF: (i) dilatant faults with opening width and vertical offset, (ii) tilted blocks (TB), and (iii) opening mode 30 (mode I) fissures. Field observation of normal faults without visible opening invariably shows that these have an opening filled by recent sediment. TB dominated normal faults tend to have a largest opening width with respect to vertical offsets. Fissures Opheim and Gudmundsson, 1989). The bedrock is mostly of volcanic origin with ages increasing with distance from the rift (Fig. 2). The succession of lava flows with cooling joints, paleo-soils and hyaloclastite result in locally complex mechanical stratigraphy. Faults and fissures crosscutting this mechanically heterogeneous section reactivate the pre-existing cooling joints close to the surface, leading to complex geometries (Forslund and Gudmundsson, 1991;Gudmundsson, 1987a; Gudmundsson 5 and Bäckström, 1991;Hatton et al., 1994).