Upper Mantle P and S Wave Velocity Structure of the Kalahari Craton and Surrounding Proterozoic Terranes, Southern Africa

Abstract

New broadband seismic data from Botswana and South Africa have been combined with existing data from the region to develop improved P and S wave velocity models for investigating the upper mantle structure of southern Africa. Higher craton‐like velocities are imaged beneath the Rehoboth Province and parts of the northern Okwa Terrane and the Magondi Belt, indicating that the northern edge of the greater Kalahari Craton lithosphere lies along the northern boundary of these terranes. Lower off‐craton velocities are imaged beneath the Damara‐Ghanzi‐Chobe Belt, and may in part reflect thinning of the lithosphere beneath the incipient Okavango Rift. Lower velocities are also imaged to the north and northwest of the Bushveld Complex beneath parts of the Okwa Terrane, Magondi Belt, and Limpopo Belt, indicating that cratonic upper mantle in some areas beneath these terranes may have been modified by the 2.05‐Ga Bushveld and/or 1.1‐Ga Umkondo magmatic events.

Crustal and Upper Mantle Shear Wave Velocity Structure of Botswana: The 3 April 2017 Central Botswana Earthquake Linked to the East African Rift System

“…Some studies have suggested the presence of the Maltahohe microcraton below the Rehoboth Province (Begg et al, ; Wright & Hall, ). This seems confirmed by the high and wave velocity anomalies of the study by Ortiz et al () that was attributed to a thick cratonic lithosphere. A major part of the Rehoboth Province was formed during the Proterozoic (2.2–1.9 Ga) around an Archean nucleus (van Schijndel et al, ).…”

“…The strongest negative mantle anomaly of our model is found in the northeast of Botswana, beneath the northeastern tip of the Okavango Rift Zone (ER in Figures c and d, B–B and D–D ). The Okavango Rift Zone is an incipient rift and is often interpreted as the southern terminus of the EARS (e.g., Bufford et al, ; Ortiz et al, ; Kinabo et al, ; Leseane et al, ; Modisi et al, ; Reeves, ; Scholz et al, ; Yu, Gao, et al, ; Yu, Liu, et al, ). Our model is in agreement with this interpretation because the location of the low‐velocity anomaly matches the continuation of the southwestern branch of the EARS with Lake Kariba as its last surface expression (Figure a).…”

Topography of the 410 and 660 km Discontinuities Beneath the Cenozoic Okavango Rift Zone and Adjacent Precambrian Provinces

“…This observation, together with results from other studies using the SAFARI data, including minor (≤ ~5 km) crustal thinning (Fadel et al, 2018; Yu, Liu, Reed, et al, 2015), absence of rifting‐related mantle flow field as revealed by shear wave splitting analysis (Yu, Gao, et al, 2015), and small (~ −1%) velocity anomalies in the upper mantle directly beneath the rift zone compared to other continental rifts (Yu et al, 2017), is inconsistent with the hypothesis that active mantle upwelling plays a major role in rift initiation. Minor mantle velocity anomalies associated with the ORZ are also suggested by recent body (Ortiz et al, 2019) and surface wave (Fadel et al, 2020) seismic tomography studies utilizing the same data set that is used for MTZ discontinuity imaging in this study.…”

“…The recent crustal and upper mantle shear wave velocity model of Botswana from Rayleigh wave inversion (Fadel et al, 2020) indicates that fluids or melt in the lower crust and uppermost mantle beneath the ORZ is connected to the continuation of the southwestern branch of the EARS. A lack of elevated mantle conductivity has been revealed from inversion of MT data (Khoza et al, 2013), and tomography studies (Ortiz et al, 2019; Yu et al, 2017) have suggested that a localized low‐velocity anomaly is constrained in the upper asthenosphere beneath the ORZ without reaching the MTZ. In addition, the existence of a hot mantle upwelling would be expected to generate complex or radial patterns of azimuthal anisotropy, which is inconsistent with the uniformly absolute plate motion (APM)‐parallel fast orientations (Figure 6) beneath the ORZ and its surrounding areas (Yu, Gao, et al, 2015).…”

“…The thick lithosphere beneath the Kalahari and Congo cratons is commonly revealed from surface‐wave (e.g., Chevrot & Zhao, 2007; Fadel et al, 2020; Li & Burke, 2006) and body wave tomography (James et al, 2001; Ortiz et al, 2019; Youssof et al, 2015; Yu et al, 2017), RF (Wittlinger & Farra, 2007), and MT studies (e.g., Evans et al, 2011; Khoza et al, 2013; Miensopust et al, 2011; Muller et al, 2009). Most body wave tomography investigations report a much thicker lithosphere (Figure 8) beneath the Kaapvaal and Zimbabwe cratons extending to depths of at least 250 km and locally reaches 300–350 km (e.g., James et al, 2001; Youssof et al, 2015), which is comparable to the average value of 300 km from inversion of RFs (Wittlinger & Farra, 2007).…”