Passive rifting of thick lithosphere in the southern East African Rift: Evidence from mantle transition zone discontinuity topography

Abstract

To investigate the mechanisms for the initiation and early‐stage evolution of the nonvolcanic southernmost segments of the East African Rift System (EARS), we installed and operated 35 broadband seismic stations across the Malawi and Luangwa rift zones over a 2 year period from mid‐2012 to mid‐2014. Stacking of over 1900 high‐quality receiver functions provides the first regional‐scale image of the 410 and 660 km seismic discontinuities bounding the mantle transition zone (MTZ) within the vicinity of the rift zones. When a 1‐D standard Earth model is used for time‐depth conversion, a normal MTZ thickness of 250 km is found beneath most of the study area. In addition, the apparent depths of both discontinuities are shallower than normal with a maximum apparent uplift of 20 km, suggesting widespread upper mantle high‐velocity anomalies. These findings suggest that it is unlikely for a low‐velocity province to reside within the upper mantle or MTZ beneath the nonvolcanic southern EARS. They also support the existence of relatively thick and strong lithosphere corresponding to the widest section of the Malawi rift zone, an observation that is consistent with strain localization models and fault polarity and geometry observations. We postulate that the Malawi rift is driven primarily by passive extension within the lithosphere attributed to the divergent rotation of the Rovuma microplate relative to the Nubian plate, and that contributions of thermal upwelling from the lower mantle are insignificant in the initiation and early‐stage development of rift zones in southern Africa.

Relationships Between Lithospheric Structures and Rifting in the East African Rift System: A Rayleigh Wave Tomography Study

“…Unlike differences observed in the Eastern Rift Branch, which might be attributed to anisotropy, these differences between the two studies are found in a region with a significant addition of new data from the Seismic Arrays for African Rift Initiation (Gao et al, ) and SEGMeNT arrays (Shillington et al, ). Therefore, we believe the differences observed in the our model give new evidence for thick lithosphere extending beneath the southern Malawi Rift is likely a result of improved resolution, and note the corroboration with recent potential field models (Sarafian et al, ) and transition zone receiver functions (Reed et al, ) suggesting the presence of the thick cratonic lithosphere in these regions.…”

Receiver Function Imaging of Mantle Transition Zone Discontinuities Beneath the Tanzania Craton and Adjacent Segments of the East African Rift System

“…However, a lack of such upwelling in the rest of the Eastern Branch and the entire Western Branch makes such a rifting mechanism unlikely for the Eastern and Western Branches. The absence of significant thermal upwelling from the lower mantle beneath other segments of the EARS has also been inferred for the Afar Depression (Reed, Gao, et al, ), the Malawi Rift of the EARS (Reed, Liu, et al, ), and the Okavango Rift (Yu, Liu, et al, ) and is suggested by geodynamic modeling (Quere & Forte, ; Stamps et al, ; ).…”

Lithospheric Structure of the Malawi Rift: Implications for Magma‐Poor Rifting Processes

“…We propose that this thick lithosphere represents the NE continuation of the Niassa Craton—a little cratonic block in eastern Zambia and that has been identified using magnetotelluric imaging (Sarafian et al, ). In addition, Reed et al () found an apparent 20‐km shallowing of the mantle transition zone that they attribute to high‐velocity anomalies in the upper mantle. This result from Reed et al () also supports the existence of relatively thick and strong lithosphere in the south‐central MR.…”

“…A number of previous studies have been conducted to investigate the crustal and upper mantle structures beneath the MR using controlled-source seismic (lake-bottom seismometers), body and surface wave tomography, and receiver function stacking (Accardo et al, 2017;O'Donnell et al, 2013;Reed et al, 2016). O'Donnell et al (2013) used Rayleigh wave phase velocity to invert for a 3-D shear wave velocity model and observed pronounced velocity lows beneath the northern and southern ends of the Lake Malawi but not beneath the central portion of the lake.…”

“…Tectonics Reed et al (2016) used receiver function stacking to image the 410 and 660-km discontinuity and observed a normal mantle transition zone beneath the entire MR indicating the absence of any thermal anomaly due to a deep hot asthenospheric upwelling at the mantle transition zone. However, in the CMR, Reed et al (2016) observed an apparently uplifted zone of 20 km within both the 410 and 660-km discontinuities, which they interpreted to be due to the presence of a cold and strong cratonic lithosphere. A surface wave tomography study by Fishwick (2010) found an E-W belt of thick lithosphere (~180-210 km) beneath the CMR and SMR.…”