No abstract

 First 'rift-scale' analysis of silicic magmagenesis in Ethiopia  Limited assimilation of high-18 O Pan-African basement in Quaternary magmas  Fractional crystallization is variable and amplified in regions of low magma flux  We predict ~45 Ma of magmatic intrusion has removed fusible components from crust  Greater crustal assimilation for Oligocene magmas supports secular variations

He cosmogenic rifts magmatic accretion extension a b s t r a c tCrustal extension at mature continental rifts and oceanic ridges occurs by a combination of normal faulting and magma injection, which interact to create rift morphology. Quantifying the relative roles of faulting and melt intrusion in accommodating extension at magmatic rifts remains difficult and requires studies at sufficient spatial and temporal scales to resolve the interaction between these processes. In this study we provide new chronological constraints based on cosmogenic exposure dating for the $ 100 kyr topographic evolution of a young and active magmatic rift segment in Afar, Ethiopia. We combine structural investigations, field mapping, geochemical analysis and cosmogenic 3 He exposure dating of lava surfaces in order to investigate the interplay between volcanic activity and fault growth in the northern part of the axial depression, where the rift segment intersects a large stratovolcano. Our results allow us to determine the roles of the various magma reservoirs feeding this rift system and their interactions during accretion over the past 100 kyr. New age data for key lava units allow several magmatic cycles to be distinguished. Each cycle lasts 20-40 ka resulting in periods of high and low magma supply rate. The variations in magma supply rate at the segment extremity strongly affect the development of the rift depression, with the availability of melt controlling the morphological impact of faulting. Melts from different magma reservoirs feeding the segment are chemically distinct and geochemical analysis of lavas from the rift floor allows their respective contributions to maintaining magmatic accretion to be estimated. We propose that melts from the magma reservoir at the northern end of the segment contribute around one-third of the length of this portion of the segment, whereas the mid-segment reservoir is responsible for the remaining two-thirds of the segment accretion.

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.