Fault and graben growth along active magmatic divergent plate boundaries in Iceland and Ethiopia

Abstract

Recent studies highlight the importance of annual‐scale dike‐induced rifting episodes in developing normal faults and graben along the active axis of magmatic divergent plate boundaries (MDPB). However, the longer‐term (102–105 years) role of diking on the cumulative surface deformation and evolution of MDPB is not yet well understood. To better understand the longer‐term normal faults and graben along the axis of MDPB, we analyze fissure swarms in Iceland and Ethiopia. We first focus on the simplest case of immature fissure swarms, with single dike‐fed eruptive fissures; these consist of a <1 km wide graben bordered by normal faults with displacement up to a few meters, consistent with theoretical models and geodetic data. A similar structural pattern is found, with asymmetric and multiple graben, within wider mature fissure swarms, formed by several dike‐fed eruptive fissures. We then consider the lateral termination of normal faults along these grabens to detect their upward or downward propagation. Most faults terminate as open fractures on flat surface, suggesting downward fault propagation; this is consistent with recent experiments showing dike‐induced normal faults propagating downward from the surface. However, some normal faults also terminate as open fractures on monoclines, which resemble fault propagation folds; this suggests upward propagation of reactivated buried faults, promoted by diking. These results suggest that fault growth and graben development, as well as the longer‐term evolution of the axis of MDPB, may be explained only through dike emplacement and that any amagmatic faulting is not necessary.

High-Resolution Digital Elevation Modeling from TLS and UAV Campaign Reveals Structural Complexity at the 2014/2015 Holuhraun Eruption Site, Iceland

“…Faults associated with dike intrusions and rifted margins may develop in both oblique and normal spreading conditions. However, there is still debate about the vertical propagation direction of faults (Opheim and Gudmundsson, 1989;Gudmundsson, 1992;Grant and Kattenhorn, 2004;Trippanera et al, 2015b), thus demonstrating the need for detailed structural analysis in extensional regions.…”

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

“…Geometries of MDF have been described for several sites in Iceland with respect to fracture length, opening, throw, obliquity and segment linkage (Acocella et al, 2000;Bonali et al, 2019a;Bubeck et al, 2017;Gudmundsson, 1987bGudmundsson, , 1987avon Hagke 15 et al, 2019;Tentler and Mazzoli, 2005;Trippanera et al, 2015). Relationships of length and opening are complex (Hatton et al, 1994), have largest openings at the fault center (Gudmundsson, 1987a) and smaller opening and throw at shorter lengths (Tentler and Mazzoli, 2005).…”

“…Correlations of opening and throw are weak, but show different distributions at fault tips and centers, possibly caused by different stages of fault growth (Gudmundsson, 1987a;Tentler and Mazzoli, 2005). However, a direct relation of larger vertical offsets and larger dilatancy has been suggested (Acocella et al, 2003;Trippanera et al, 2015). 20…”

“…Extensional faults in cohesive rocks can develop massive dilatancy (several tens of meters) at shallow levels in the crust (Abe 5 et al, 2011;Acocella et al, 2003;van Gent et al, 2010;Gudmundsson, 1987aGudmundsson, , 1987bHolland et al, 2006;Kettermann et al, 2015;Opheim and Gudmundsson, 1989;Rowland et al, 2007;Trippanera et al, 2015). These massively dilatant faults (MDF) are common in rift zones such as the Icelandic Rift, the East African Rift or on volcanoes such as Hawaii or the Campi Flegrei, Italy (Acocella et al, 2003;Gudmundsson, 1987b;Martel and Langley, 2006;Rowland et al, 2007;Vitale and Isaia, 2014).…”

“…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.…”