Upper Mantle Seismic Structure of the Turkana Depression: Evidence from Body-Wave Travel-Time Tomography

Abstract:

The Turkana Depression, a topographically subdued, broadly rifted zone between the East African and Ethiopian plateaus, disrupts the N-S narrow rift morphology that characterizes most of the East African Rift. The two uplifted plateaus are often attributed to mantle plumes, however, gaps in seismic coverage have hindered attempts to resolve the single-vs-multiple plume debate instigating several hypotheses for Turkana’s unusual breadth and low-lying nature. Turkana may be situated between two mantle upwellings, or part of one large superplume province, with low elevations explained by highly stretched crust rather than a lack of dynamic support. Rifting style, if broad or focused to narrow zones of magmatic intrusion, is also uncertain. To differentiate between end-member hypotheses we produce the first P and S body-wave tomographic models of Turkana's upper mantle, made possible, for the first time, by the NSF-NERC funded Turkana Rift Arrays Investigating Lithospheric Structure (TRAILS) project, which fills this critical data gap in East Africa.

At lithospheric depths, results reveal a lack of localized, rift-like, low wavespeeds more consistent with broad strain rather than the focused, magma-assisted rifting to the north and south, dominating the Main Ethiopian and Kenya Rifts. Low wavespeeds below active volcanoes and P to S wave relative delay-time ratios indicate melt, primarily south of Lake Turkana and along the Main Ethiopian Rift at the transition to localized rifting. The N-S continuity of the slow wavespeeds is disrupted by a NW-SE trending band of fast wavespeeds in southern Ethiopia, potentially interpreted as Pan-African lithospheric structure influencing Turkana rifting. We illuminate the sublithospheric architecture of mantle upwellings, including a prominent 300km wide slow anomaly persisting to the mantle transition zone – a potential continuation of the African Superplume. This is confined by sub-vertical fast wavespeeds in eastern Turkana, perhaps depicting the superplume edge. There is evidence of slow wavespeeds in the mid-mantle that may be capable of providing dynamic support from upwelling plume material. The superposition of multiple rifting phases that significantly thinned the crust, may instead explain Turkana’s low elevations relative to the two plateaus it separates.