The Effect of Obliquity of Slip in Normal Faults on Distribution of Open Fractures

Abstract

Close to surface, cohesive rocks fail in extension, which results in open fractures that can be several tens of meters wide, so-called massively dilatant faults. These open fractures make fault slip analysis in rifts challenging, as kinematic markers are absent. Faults in rifts often have oblique slip kinematics; however, how the amount of obliquity is expressed in the surface structure of massively dilatant faults remains enigmatic. Furthermore, the structures of oblique dilatant faults at depth is largely unconstrained. To understand the subsurface structures we need to understand how different obliquities of slip influence the surface structures and the corresponding structures at depth. We present analog models of oblique massively dilatant faults using different cohesive materials in a sandbox with adjustable basement fault slip obliquity from 0 • to 90 •. Experiments with different mean stress and material cohesion were run. Using photogrammetric 3D models, we document the final stage of the experiments and investigate selected faults by excavation. We show that fault geometry and dilatancy changes systematically with angle of obliquity. Connected open fractures occur along the entire fault to a depth of 6-8 cm, and as isolated patches down to the base of the experiments. Using the scaling relationship of our models implies that transition from mode-1 to shear fracturing occurs at depths of 250-450 m in nature. Our experiments show the failure mode transition is a complex zone and open voids may still exist at depths of at least 1 km. We apply our results to the dilatant faults in Iceland. We show that the relationship between angle of obliquity and average graben width determined on faults on Iceland matches experimental results. Similarly, fracture orientation with respect to fault obliquity as observed on Iceland and in our experiments is quantitatively comparable. Our results allow evaluation of the structure of massively dilatant faults at depth, where these are not accessible for direct study. Our finding of a complex failure mode transition zone has consequences for our understanding of fracture formation, but also influences our interpretation of fluid flow in rift systems such as magma ascent or flux of hydrothermal waters.

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

“…21). The straight lines 10 plotted in the figure represent the relationship between vertical offset and horizontal opening corresponding to a simple dilatant normal fault at depth with dips ranging between 60°and 70°, being within the dip range as commonly inferred for Iceland (Angelier et al, 1997;Grant and Kattenhorn, 2004;Gudmundsson, 1992;von Hagke et al, 2019;Trippanera et al, 2015). We further define the parameter R as ratio of opening width and vertical offset as Eq.…”

“…3a). Fissures can be 15 early stages of MDF or represent the lateral ends of MDF, which show en échelon fracturing when formed at oblique slip (Acocella et al, 2003;Grant and Kattenhorn, 2004;Gudmundsson, 1987a;von Hagke et al, 2019). Viewed in our DEMs (Fig.…”

“…MDF guide the flux of water, magma or hydrocarbons and are therefore of interest for geo-hazard assessment, hydrocarbon 10 exploration, geothermal energy or geodynamics (Crider and Peacock, 2004;Faulkner et al, 2010;Ferrill and Morris, 2003;Grant and Kattenhorn, 2004;Gudmundsson, 1987a;Kettermann et al, 2015Kettermann et al, , 2016Rowland et al, 2007). During the past decades, MDF have been studied in the field (Bubeck et al, 2018;Gudmundsson, 1987bGudmundsson, , 1987aHjartardóttir et al, 2012;Sonnette et al, 2010;Tibaldi et al, 2016;Trippanera et al, 2015), and using analog and numerical models (Abe et al, 2011;van Gent et al, 2010;Grant and Kattenhorn, 2004;von Hagke et al, 2019;Holland et al, 2006;Kettermann et al, 2015Kettermann et al, , 2016Kettermann et al, , 15 2019Martel and Langley, 2006;Smart and Ferrill, 2018). In these studies, the surface geometries have been described including tilted blocks (i.e.…”

The nature and age of basement host rocks and fissure fills in the Lancaster field fractured reservoir, West of Shetland

“…Geological observations in actively rifting regions such as Iceland and analogue modelling studies (e.g. van Gent et al 2010;von Hagke et al 2019) have shown that highly dilated and interconnected fissure systems can form in mechanically strong basement rocks, such as charnockite, granite and basalt, during active extensional faulting as stresses become tensile in the uppermost crust. Thus subvertical open fissures in the upper few hundred metres closest to the surface pass down at greater depths into normal fault shear fractures dipping at c. 60°at depth (Fig.…”

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

“…Iceland is a volcanic island in the Atlantic Ocean on the Mid-Atlantic Ridge separating the Eurasian and North American plates. It is linked to a deep mantle plume (Einarsson, 1991;Lawver and Muller, 1994;Vink, 1984;Wolfe et al, 1997), with associated melt production forming the Icelandic shelf (Brandsdóttir et al, 2015) with a local crustal thickness of at least 25 km (Allen et al, 2002). It is located between the Reykjanes Ridge segment in the SW and the Kolbeinsey Ridge in the north (Fig.…”