Using the Extrusive Volcanic Features of Mt. Marsabit, Kenya to Identify Regional Tectonic Stress

Abstract

Extrusive volcanic features like vents, craters, and cones can produce alignments and other linear structures that indicate the orientations of feeder dikes and regional tectonic stresses. These dikes form parallel to the maximum compressional stress (σ1) and perpendicular to minimum compressive stress (σ3), and/or exploit preexisting planes of weakness. Volcanic constructs fed by these magmatic intrusions are therefore indicators of tectonic stress directions and subsurface structural fabrics, which can be deduced through detailed mapping and assessment of the spacing, shapes, and linear arrays of these volcanic features. Mt Marsabit (2.32°N, 37.97°E) is a massive 6,300 km2 off-axis volcano located in Northern Kenya on the eastern shoulder of the Kenyan Rift, 170 km east from the center of the East African Rift System (EARS). Initial construction began in the Miocene, with the peak of volcanic activity occurring in the Pliocene. A multitude of maar craters, scoria cones, and tuff cones developed in the Quaternary, primarily along the northern and eastern slopes. These extrusive volcanic features were mapped in the late eighties but have not been revisited using modern technology. Here we present findings from our analyses of the morphologies and alignments of 242 of extrusive volcanic features found on Marsabit. We then interpret the subsurface feeder system to off-axis volcanism in this sector of the EARS. Methodologies modified from Paulsen and Wilson (2010) and Muirhead et al. (2015) are used to map volcanic craters and cones using Google Earth. The orientations, shapes, and positions of volcanic features observed over the 8,000 km2 region indicate the presence of NE-SW orientated feeder dikes, which trend oblique to the general N-S trends observed in nearby sectors of the EARS such as faults found immediately south of Lake Turkana and the Elgeyo escarpment of the Kenyan rift. The strong NE-SW orientation of volcanic lineaments on Marsabit suggests that either a local NW-SE extension direction or a NE-SW orientated crustal fabric controls the geometry of the underlying plumbing system to this off-axis volcano. From this data we have created a series of rose diagrams to indicate the possible angles and locations of subsurface dikes, illustrating the regional tectonic stress field in Marsabit. We then compare these rose diagrams to other features found in the EARS.