Numerical Modelling of the Heat Flow from Multiple Dyke Injection Events in Afar, Ethiopia

The mechanism of magma transport through cold lithosphere and crust, and the distance over which it travels, controls the presence and type of resultant surface volcanism as well as the rate of spreading of the crust. As a result, in the Afar province in the north east of Ethiopia the East African rift is slowly spreading via the process of dyke injection and normal faulting to form a new ocean. The region is an ideal setting to study the processes of multiple and shallow dyke injection, and how these can lead to the splitting of the crust. In this study, numerical modelling through the finite difference method has been applied to the geological setting of an actively spreading rift, and a solution for the two-dimensional heat flow equation is computed according to the explicit method. The initial temperature profile is set up by the crustal geotherm, and a constant temperature boundary condition for the domain is assumed. We also assume that the physical and chemical properties of the crust can be simplified to be homogeneous over the intruded depth, so that the thermal diffusivity of the crust can be treated as being independent of depth and temperature. Dykes of a basaltic composition are intruded sequentially at a constant rate. Each one is instantaneously intruded into the crust at a constant temperature, and the model computes how heat is dissipated over time throughout the depth of the crust. The resultant temperature profile is used to calculate a profile of melt fraction and from this the distribution and quantities of melt within the crust are predicted. The model is used to investigate how the size and frequency of dyke intrusions affect the distribution of melt within the East African Rift. Changes in the initial conditions such as increased temperature, altered geothermal gradient or intrusion frequency and spacing can also affect the resultant melt fraction and ultimately the magma composition. The results could be compared with previous tomographic and petrological studies conducted in the region providing constraints on the degree of fractionation and composition of the melt present in the crust. This thermal modelling of the crust beneath Afar develops our understanding of the nature and frequency of magma intrusions in the East African Rift System, and the conditions for the creation and evolution of the magmas that are seen at the surface.