Evaluating the Importance of High Temperatures and Fluids Within the Eastern Rift, Using Ambient Noise Tomography and Seismicity

The processes of continental rifting leading to rupture and seafloor spreading are, to first order, explained by stretching and thinning of continental lithosphere, and isostatic compensation for the thinning. In magmatic rift zones, mass transfer and heating from intrusions and extrusions add additional complexity, with magma fundamentally changing the density and velocity structure of the crust and mantle. The depth at which melt resides in the crust coupled with volume and composition is commonly characteristic of the stage of rifting for magmatic rifts. The < 6 My old Eastern rift in East Africa has experienced less than 20% crustal extension and yet magma chambers occur in the upper 15 km, and erupt silicic lavas. The rift development, therefore, may be more dependent on the volume of magma available, and less to age. -factors break down when thinning is underestimated by the addition of magmatic intrusion. Here we produce new absolute shear velocity maps using ambient noise tomography on the CRAFTI network in southern Kenya and northern Tanzania to determine the crustal velocity structure for a cratonic rift zone. We observe several low velocity zones that are likely associated with melt. We then incorporate local seismicity results which map to the low velocity regions which we interpret as melt. Using the focal mechanisms we can then differentiate active intrusions from cooling hot material. The low velocity zones with significant seismicity are predominantly within the mid crust at 10 - 15 km depths, which are similar to those of the more developed Main Ethiopian Rift. The developed mid-crustal magmatic plumbing system coupled with evolved melts suggests the Eastern rift is a more developed rift than age and thinning would suggest. Our research therefore suggests that the time of onset of rifting is less important than the level of the melt in the crust for rifting.