Magmatic Strain Accommodation in the Turkana Basin

Abstract:

The East African Rift System (EARS) is the largest active continental rift system in the world.
The observed lithospheric thinning has been related to asthenospheric intrusions in the upper
mantle. Magmatic and extensional strain in the EARS are commonly accommodated in the
upper crust by faulting and dike intrusions.

The Turkana Basin, located in the Kenya Rift along the eastern branch of the EARS, is an
endorheic basin encompassing 131,000 km² between the Ethiopian and Kenyan domes. The
predominant north-south orientation of structural and volcanic features suggests an overall east-
west oriented minimum compressional stress in the Turkana Basin. However, some localized
examples diverge from this generalized trend.

We present data from Mt Marsabit, Kenya, a massive basaltic shield volcano located
approximately 100 km east of the southern tip of Lake Turkana. We modified methodologies
described by Paulson and Wilson (2010) and Muirhead et al. (2015) to map and analyze over
240 linear arrays and cone morphologies found on the dormant volcano. Analyses indicate the
presence of northeast-southwest oriented subsurface feeder dikes, which differ to the north-
south oriented fault and volcanic lineaments in the Turkana Basin forming under the regional E-
W extension direction. The obliquely oriented dikes in the Mt Marsabit region may have resulted
from local off-axis stress rotations or could be explained by subsurface feeder dikes exploiting
pre-existing weaknesses within the lithosphere.