Vent distribution and structural inheritance in an embryonic rift: The example of the Chyulu Hills off-rift magmatic range (South Kenya)

Abstract

In the East African Rift System (EARS) tectonic inheritance rules the location, the shape and the development of rift segments. Such lithospheric/crustal-scale inherited structures may be exploited by magma favouring its ascent to shallow crustal level and, possibly, to the surface producing volcanism. In the eastern branch of the EARS in southeast Kenya, the Pleistocene Chyulu Hills volcanic field (CHVF) is a volcanic range formed east of the main rift along a NW-SE trending Proterozoic crustal-scale shear zone (the Emali-Voi shear zone). The volcanic range consists of hundreds (at least 514) of scoria cones of basaltic composition, often coalescent and well aligned along the trace of the crustal shear zone. Morphostructural and spatial analyses of vents and fractures in the CHVF show that vent density and clustering change moving south-eastward along the ridge mimicking the age pattern of volcanism with the youngest activity in the southeast of the CHVF. Notably, the parameters defining the vent self-similar clustering indicate a decreasing thickness of the upper brittle crust along the CHVF from about 12 km in the north-west to about 7 km in the southeast, in agreement with the crustal structure of the area as imaged by seismic and geophysical data sets. Here we propose an evolutionary model of the CHVF occurring at spatial and time scales quite different from that of magma-assisted rifting processes, where CHVF is an example of an embryonic rift stage with a focussed magma injection into the continental lithosphere localized along a weakness discontinuity represented by an inherited crustal shear zone.