A Gravity Study of the Crustal Structure Beneath the Young Amagmatic Rukwa‐Tanganyika Rift Zone

AbstractThe Rukwa‐Tanganyika Rift Zone (RTRZ), experiencing multiphase rifting, forms the nonvolcanic southern part of the Western Branch of the East African Rift. We investigate the crustal structure beneath the RTRZ to understand how strain migrates beneath multiphase rifts. We use power spectral analysis of Bouguer gravity data from the World Gravity Map 2012 and 2‐D forward modeling to estimate the crustal thickness beneath the RTRZ. We find thin crust (∼30–36 km) beneath the Tanganyika Rift and the Rukwa Rift, separated by a region of thick crust (∼38–46 km) beneath the Ufipa and Mbozi Horsts that extends southward beneath the Bangweulu Craton (∼40–46 km) and the Rungwe Volcanic Province (∼40 km). The eastern flank of the Rukwa Rift is characterized by a broad zone of shallow Moho (∼32–34 km). Our 2‐D forward models indicate that the crust beneath the eastern part of the Ufipa Horst is significantly thinner, suggesting it is tilted to the west. We argue that extension in the RTRZ is mainly accommodated by slip along border faults of the southern Tanganyika Rift, producing the westward tilting of the Ufipa Horst. Part of the extension is accommodated by active deformation of the Ufipa Horst, particularly at its transition to the Rukwa Rift, where the crust is thinnest. We conclude that the crustal thinning facilitates strain transfer from the Tanganyika Rift to the Rukwa Rift by enabling the infiltration of metasomatic fluids into the lower crust, which increases the crustal buoyancy, lubricates, and reactivates pre‐existing structures within the Ufipa Horst.