Influence of regional tectonics and pre-existing structures on the formation of elliptical calderas in the Kenyan Rift

Abstract

Calderas are formed by the collapse of large magma reservoirs and are commonly elliptical in map view. The orientation of elliptical calderas is often used as an indicator of the local stress regime; but, in some rift settings, pre-existing structural trends may also influence the orientation. We investigated whether either of these two mechanisms controls the orientation of calderas in the Kenyan Rift. Satellite-based mapping was used to identify the rift border faults, intra-rift faults and orientation of the calderas to measure the stress orientations and pre-existing structural trends and to determine the extensional regime at each volcano. We found that extension in northern Kenya is orthogonal, whereas that in southern Kenya is oblique. Elliptical calderas in northern Kenya are orientated NW-SE, aligned with pre-existing structures and perpendicular to recent rift faults. In southern Kenya, the calderas are aligned NE-SW and lie oblique to recent rift faults, but are aligned with pre-existing structures. We conclude that, in oblique continental rifts, pre-existing structures control the development of elongated magma reservoirs. Our results highlight the structural control of magmatism at different crustal levels, where pre-existing structures control the storage and orientation of deeper magma reservoirs and the local stress regime controls intra-rift faulting and shallow magmatism.

Historical Volcanism and the State of Stress in the East African Rift System

“…Two of them, Fantale, and The Barrier, have roughly orthogonal geometries with caldera axes elongated within ±15 • of the spreading direction ( Table 2). This supports the arguments of and Robertson et al (2015) that although the current stress regime dominates recent eruptive fissures and dykes, it is not the dominant control on caldera orientation or crustal magma storage. The group 2 calderas are smaller and contain persistent or recently active lava lakes in pit craters.…”

“…The geometric relationship between plate motion, plate boundary orientation and the resulting structures can be defined according to the model of Tuckwell et al (1996), who classified geometrical models of mid-ocean ridge spreading, three of which (orthogonal, oblique, transtension) are observed in nature. Robertson et al (2015) used a similar system to describe the geometry of rift extension, using the Kenyan Rift as an example. The three models can be described using two angles: α is the angle between the rift azimuth and the plate motion direction (S HMIN ) and φ is the angle between the fault or dyke azimuth and the plate motion direction.…”

“…Inherited structures have been proposed to explain the eccentricity of calderas in the MER and Kenya Rifts Robertson et al, 2015). Robertson et al (2015) argued that NW, trans-rift fault structures in the basement of the Kenya Rift led to elongate reservoirs beneath the southern population of elliptical calderas, including Longonot.…”

“…Robertson et al (2015) argued that NW, trans-rift fault structures in the basement of the Kenya Rift led to elongate reservoirs beneath the southern population of elliptical calderas, including Longonot. suggested that E-W inherited structures in the MER controlled the E-W elongated Kone, Fantale, and Gedemsa calderas.…”

Magmatic rifting and active volcanism: introduction

“…Guth (2015) proposes a simple method for adjusting these records, which enables the geological estimates of erupted volumes to be reconciled with more recent seismic estimates. Robertson et al (2015) examine the interaction between tectonics and volcanism in Kenya, focusing in particular on the elliptical shape of calderas formed by the collapse of large magma reservoirs during or following eruptions. The orientation of elliptical calderas has often been used to indicate the local stress regime.…”