摘要：Normal faults in basalts develop massive dilatancy in the upper few hundredmeters below the Earth's surface with corresponding interactions withgroundwater and lava flow. These massively dilatant faults (MDFs) arewidespread in Iceland and the East African Rift, but the details of theirgeometry are not well documented, despite their importance for fluid flow inthe subsurface, geohazard assessment and geothermal energy. We present alarge set of digital elevation models (DEMs) of the surface geometries ofMDFs with 5–15 cm resolution, acquired along the Icelandic rift zone usingunmanned aerial vehicles (UAVs). Our data present a representative set ofoutcrops 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 amuch better overview than ground-based fieldwork, bridging the gap betweenoutcrop-scale observations and remote sensing. We acquired photosets ofoverlapping images along about 20 km of MDFs and processed these usingphotogrammetry to create high-resolution DEMs and orthorectified images. Weuse 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 inthe damage zones. Ground truthing of the data was done by fieldobservations.Mapped vertical offsets show typical trends of normal fault growth bysegment coalescence. However, opening widths in map view show variations atmuch higher frequency, caused by segmentation, collapsed relays and tiltedblocks. These effects commonly cause a higher-than-expected ratio ofvertical offset and opening width for a steep normal fault at depth.Based on field observations and the relationships of opening width andvertical offset, we define three endmember morphologies of MDFs: (i) dilatantfaults with opening width and vertical offset, (ii) tilted blocks (TBs) and(iii) opening-mode (mode I) fissures. Field observation of normal faultswithout visible opening invariably shows that these have an opening filledwith recent sediment. TB-dominated normal faults tend to have the largestratio of opening width and vertical offset. Fissures have opening widths upto 15 m with throw below a 2 m threshold. Plotting opening width versusvertical offset shows that there is a continuous transition between theendmembers. We conclude that for these endmembers, the ratio betweenopening width and vertical offset R can be reliably used to predict faultstructures at depth. However, fractures associated with MDFs belong to onelarger continuum and, consequently, where different endmembers coexist, aclear identification of structures solely via the determination of R isimpossible. Downloadandlinks Article (PDF, 28354 KB) Supplement (49563 KB) How to cite Back to top top How to cite. Weismüller, C., Urai, J. L., Kettermann, M., von Hagke, C., and Reicherter, K.: Structure of massively dilatant faults in Iceland: lessons learned from high-resolution unmanned aerial vehicle data, Solid Earth, 10, 1757–1784, https://doi.org/10.5194/se-10-1757-2019, 2019. 1 Introduction Back to toptop Extensional faults in cohesive rocks can develop massive dilatancy (severaltens of meters) at shallow levels in the crust(Abeet al., 2011; Acocella et al., 2003; van Gent et al., 2010; Gudmundsson,1987a, b; Holland et al., 2006; Kettermann et al., 2015; Opheim andGudmundsson, 1989; Rowland et al., 2007; Trippanera et al., 2015). Thesemassively dilatant faults (MDFs) are common in rift zones such as theIcelandic rift and the East African Rift or in active volcanic systems suchas Hawaii(Acocellaet al., 2003; Gudmundsson, 1987b; Martel and Langley, 2006; Rowland et al.,2007). MDFs guide the flux of water, magma or hydrocarbons and are thereforeof interest for applications such as geohazard assessment, hydrocarbonexploration and geothermal energy production(Criderand Peacock, 2004; Faulkner et al., 2010; Ferrill and Morris, 2003; Grantand Kattenhorn, 2004; Gudmundsson, 1987a; Kettermann et al., 2015, 2016;Rowland et al., 2007).During the past decades, MDFs have been studied in the field(Bubecket al., 2018; Gudmundsson, 1987a, b; Hjartardóttir et al., 2012;Sonnette et al., 2010; Tibaldi et al., 2016; Trippanera et al., 2015) andusing analog and numerical models(Abeet al., 2011; van Gent et al., 2010; Grant and Kattenhorn, 2004; von Hagkeet al., 2019; Holland et al., 2006; Kettermann et al., 2015, 2016, 2019;Martel and Langley, 2006; Smart and Ferrill, 2018). The surface geometrieshave been described including dilatancy, tilted blocks (i.e., rigid, detachedblocks tilted towards the hanging wall of a fault) and extension fractures.However, many of the observations are based on local measurements consideredrepresentative of the regional structures.In this study, we investigate the structure and evolution of massivelydilatant faults in Iceland (Figs. 1, 2) byidentifying and characterizing surface geometries at the regional scale atcentimeter resolution. We achieve this by extracting data of MDFs fromhigh-resolution maps generated from unmanned aerial vehicle (UAV)-basedphotogrammetry. Mapping faults in centimeter resolution over kilometerlengths allows for bridging the gap between outcrop-scale and regionalobservations. This enables us to quantify the geometry of the studied faultsat high detail over the entire fault lengths. The ultimate goal of this workis to introduce a new classification scheme that correlates a ratio ofmeasured fault aperture and fault throw with actual underlying faultstructures that are often overprinted by sedimentation or erosion. Todescribe different types of discontinuities, we use the terminology ofPeacock et al. (2016).Figure 1(a) MDF close to the Ásbyrgi canyon in Kelduhverfi (cf. Hatton et al., 1994). Along this fault segment, opening widths reach up to20 m and vertical offsets up to approximately 15 m of displacement. Hw:hanging wall, Fw: footwall, TB: tilted block; 66∘2′14′′ N,16∘34′15′′ W. (b) MDF of the Krafla fissure swarm. The hanging wallhas been covered by a lava flow from the Krafla fires, also surrounding thetilted blocks; 65∘48′44′′ N, 16∘44′32′′ W. (c) Southernpart of the Almannagjá fault in Thingvellir. A prominent tilted block isdipping eastwards and fault segments are linked by a collapsed relay;64∘14′42′′ N, 21∘8′30′′ W. (d) Fault segment of theTheistareykir fissure swarm. The opening of the dilatant fault (footwall inthe west) with an eastward-dipping tilted block is being filled by sands andaeolian sediments; 65∘50′50.38′′ N, 17∘0′20′′ W.
