The stress pattern of Iceland

Moritz Ziegler^1,2, Oliver Heidbach¹, Mojtaba Rajabi³, Gylfi Páll Hersir⁴, Kristján Ágústsson⁴, Sigurveig Árnadóttir⁴, Arno Zang¹

¹Helmholtz Centre Potsdam, German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany, mziegler@gfz-potsdam.de ; oliver.heidbach@gfz-potsdam.de ; arno.zang@gfz-potsdam.de

²University of Potsdam, Institute of Earth and Environmental Science, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany

³Australian School of Petroleum, The University of Adelaide, Adelaide, SA 5005, Australia, mojtaba.rajabi@adelaide.edu.au

⁴Iceland GeoSurvey (ÍSOR), Grensásvegur 9, 108 Reykjavík, Iceland, gylfi.pall.hersir@isor.is ; sigurveig.arnadottir@isor.is ; kristjan.agustsson@isor.is

This webpage is a summary of: Ziegler, Moritz, Mojtaba Rajabi, Oliver Heidbach, Gylfi Páll Hersir, Kristján Ágústsson, Sigurveig Árnadóttir, Arno Zang, The stress pattern of Iceland, Tectonophysics, 674, 101-113, 2016.

The regional stress pattern along divergent plate boundaries has not been studied extensively yet because of the inaccessibility of submerged mid-ocean ridges. Few and scattered earthquake focal mechanism solutions are the only sources of stress orientation in these areas in the World Stress Map (WSM) database (Heidbach et al., 2008; 2010). Iceland is one of the few places on Earth with an onshore divergent plate boundary (e.g., Einarsson, 1991, Bird, 2003). It is in a unique geological and tectonic setting, where an oceanic ridge (the Mid-Atlantic Ridge) traverses a (purported) mantle plume (e.g., Lawver & Müller, 1994, Allen et al., 2002). The rift zones in and around Iceland are dominated by volcanic systems of different sizes and activities which are postulated to have a plumbing system extended by the hotspot compared to a “normal” divergent plate boundary (Allen et al., 2002).

Iceland received particular attention in the first compilation of present-day crustal stress by Hast (1969). Since then, several researchers investigated the stress in different parts of Iceland. In total, the compilation of stress data records in the World Stress Map (WSM) database 2008 contains 38 data of the contemporary S_Hmax orientation and the stress regime (Heidbach et al., 2008; 2010). This small data set is not sufficient to map the stress field because it is expected to be highly variable as a result of extensive interactions between tectonic and volcanic processes.

Here we describe a new comprehensive compilation of the contemporary S_Hmax orientation for Iceland with 495 data records (Figure 1; database available as material supplementary to Ziegler et al. (2016)). The majority of S_Hmax orientation data records in Iceland are derived from earthquake focal mechanism solutions (35%) and geological fault slip inversions (26%). In particular, we analysed 37 km of borehole acoustic image logs from 57 geothermal wells to interpret present-day stress indicators, i.e. borehole breakouts (BOs) and drilling induced fractures (DIFs). These contribute 20% of the data. In addition, a few S_Hmax orientations originate from focal mechanism inversions and the alignment of fissure eruptions. All data are quality ranked according to the WSM quality ranking system which assigns qualities of:

• A (reliability of orientation ± 15°),
• B (± 15–20°),
• C (± 20–25°),
• D (± 25–40°), and
• E (>± 40°)

(Zoback, 1992, Sperner et al., 2003, Heidbach et al., 2010).

Figure 1: The first comprehensive stress map of Iceland with 307 data records with qualities A-D (World Stress Map quality criteria; Sperner et al., 2003; Heidbach et al., 2010). Lines indicate the orientation of maximum horizontal stress S_Hmax with line length proportional to quality. Symbols in the middle of the lines indicate the measurement type. Colour coding indicates the stress regime with red indicating normal faulting, green indicating strike slip faulting, blue indicating thrust/reverse faulting, and black for unknown. The plate boundaries according to Bird (2003) and Einarsson (2008) are indicated in grey.

Our data compilation shows that the S_Hmax orientations derived from different depths and stress indicators are consistent with one another. Present-day S_Hmax orientations in Iceland fall into four distinct subsets (Figure 2):

Our two key findings are:

• There is no significant depth-dependent variation in the orientation of S_Hmax (± 25°), and
• four distinct contemporary stress provinces are present in Iceland. These provinces correspond to the large-scale regional tectonic setting.

Figure 2: Simplified tectonic map of Iceland showing mean orientations of S_Hmax in the four stress provinces estimated from the quality A-D data (black lines). The standard deviation from quality A-C is indicated by dark grey triangles. Light grey triangles indicate the standard deviation of quality A-D data. The plate boundaries are from Einarsson (2008) and Bird (2003). The plate motion (mm/yr) is indicated by black arrows relative to a fixed North American plate (Geirsson et al., 2006). The plates and proposed hot spot location are indicated by grey shading (from Wolfe et al., 1997). Tectonic features are labelled according to Einarsson (2008): RR: Reykjanes Ridge, RPR: Reykjanes Peninsula Ridge, WVZ: Western Volcanic Zone, SISZ: South Iceland Seismic Zone, EVZ: Eastern Volcanic Zone, NVZ: Northern Volcanic Zone, HFZ: Húsavík-Flatey-Zone, GOR: Grimsey-Oblique-Ridge, DZ: Dalvík Zone, KR: Kolbeinsey Ridge.

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