Abstract
Dike intrusions produce faulting at the surface along with seismic swarms and possible eruptions. Understanding the geometry and kinematics of dike-induced fractures can provide relevant information on what controls magma emplacement and the associated hazards. Here, we focus on the Harrat Lunayyir volcanic field (western Saudi Arabia), where in 2009 a dike intrusion formed a NNW-SSE oriented, ten-kilometer-long and up to one-meter deep graben. This widens from ~2 km in the SSE to ~5 km in the NNW, showing a well-defined border normal fault to the west but a diffused fracture zone to the east. We conducted a fixed-wing drone survey to create high resolution (3.4 cm) ortho-rectified images and DEMs of the western fault and of a portion of the eastern fracture zone to determine the fracture geometry and kinematics. We then integrated these results with field observations and InSAR data from the 2009 intrusion. Both fault zones contain smaller segments (hundreds of meters long) consisting of normal faults and extension fractures, showing two dominant orientation patterns: NNW-SSE (N330° ± 10°) and NW-SE (N300° ± 10°). The NNW-ESE oriented segments are sub-parallel to the inferred 2009 dike strike (N340°), to pre-historical Harrat Lunayyir eruptive fissures (N330°) and to the overall Red Sea axis (N330°). This suggests that these segments reflect the present-day off-rift stress field close to the Red Sea shoulder. However, the NW-SE oriented segments are oblique to this pattern and exhibit en–echelon structures, suggesting different processes such as: (1) a transfer (or soft-linkage) between dike-parallel fault segments, (2) a topographic control on the fault propagation and (3) a possible reactivation of inherited regional faults. Vertical fault offsets obtained by the drone survey along the western fault vary with the local lithology and these data are not consistent everywhere with the offsets derived from the 2009 InSAR measurements. Field evidence within lava flows also shows the occurrence of previous slipping event(s) on the fault before the 2009 intrusion. Collectively, we suggest that the mismatch between the drone and InSAR datasets is related to the different spatial and temporal resolutions offered by the two techniques.
Highlights
Magmatic divergent plate boundaries are affected episodically by dike intrusions
To estimate the opening direction to characterize the kinematics of the faults and the open fractures, we identified some matching margins on the fracture sides, mostly from the ortho-photos and at a few locations in the field
To examine whether our fracture orientation dataset reflects larger scale structures seen in Harrat Lunayyir and the surrounding region, we compared it with orientations of: (1) all the fractures mapped from InSAR data of the 2009 intrusion (Figures 1c, 9C; Baer and Hamiel, 2010; Jónsson, 2012); (2) eruptive fissures; (3) aeromagnetic lineations (Figure 9E; Zahran et al, 2017) identified in and around the Harrat Lunayyir volcanic field (Figure 1b); (4) regional Oligo-Miocene dikes (Figures 1a, 9F; USGS map 1963; Bosworth et al, 2005; Bosworth and Stockli, 2016); and (5) regional faults and lineaments mapped in the northern Arabian shield (Figures 1a, 9G; Beziat and Bache, 1985)
Summary
Magmatic divergent plate boundaries are affected episodically by dike intrusions. These intrusions are usually parallel to the plate boundary and orthogonal to the main regional minimum compressive stress (σ3) occurring both inside or outside (off-axis) of the rift axis. Understanding the structure and kinematics of rifts, and directly observing rifting events and episodes are of great importance for both volcanic and seismic hazard assessments and for determining how and where magma is propagating
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