Crustal structure beneath the Malawi and Luangwa Rift Zones and adjacent areas from ambient noise tomography

Wang, Tuo; Feng, Jikun; Liu, Kelly H.; Gao, Stephen S.

doi:10.1016/j.gr.2018.10.018

Cited by 34 publications

(43 citation statements)

References 63 publications

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“…Magma‐assisted rifting, where magmatic fluids and volatiles enable extension at lower stresses than the available tectonic forces, is thought to play an important role in facilitating rifting in thick continental lithosphere (Buck, ; Ebinger et al, ). There is no evidence of active magmatic activity or more than a few kilometers of crustal thinning in the Zomba Graben (Reed et al, ; Wang et al, ). Furthermore, the geochemistry of hot springs in southern Malawi does not suggest a magmatic influence (Dulanya et al, ), and the nearest active volcano, Rungwe, is located ~700 km to the north (Figure ).…”

Section: Discussionmentioning

confidence: 99%

Active Fault Scarps in Southern Malawi and Their Implications for the Distribution of Strain in Incipient Continental Rifts

et al. 2020

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The distribution of deformation during the early stages of continental rifting is an important constraint on our understanding of continental breakup. Incipient rifting in East Africa has been considered to be dominated by slip along rift border faults, with a subsequent transition to focused extension on axial segments in thinned crust and/or with active magmatism. Here, we study high-resolution satellite data of the Zomba Graben in southern Malawi, an amagmatic rift whose topography is dominated by the west-dipping Zomba fault. We document evidence for five subparallel fault scarps between 13 and 51 km long spaced~10-15 km apart. The scarps consist of up to five segments between 4 and 18 km long, separated by minima in scarp height and river knickpoints. The maximum height of each fault scarp ranges from 9.5 ± 4.2 m to 35.3 ± 14.6 m, with the highest scarp measured on the intrabasin Chingale Step fault. We estimate that the scarps were formed by multiple earthquakes of up to M w 7.1 and represent a previously unrecognized seismic hazard. Our calculations show that 55 ± 24% of extensional strain is accommodated across intrabasin faults within the~50 km wide rift. This demonstrates that a significant proportion of displacement can occur on intrabasin faults during early-stage rifting, even in thick continental lithosphere with no evidence for magmatic fluids.Plain Language Summary When continents begin to stretch, earthquakes occur on faults that incrementally accumulate slip to eventually form a rift valley. To estimate the hazard posed by these earthquakes, it is important to understand where these faults are located and how much stretching they each accommodate. We analyze faults in the Zomba Graben, a young rift in southern Malawi, using high-resolution satellite data. Steep scarps indicate that recent earthquakes have occurred at both the edges and in the middle of the rift valley. Pronounced activity in the rift middle is thought to require a thinned rigid outer layer of the Earth or mechanically unfavorable faults at the rift edge. Neither of these factors have been observed in southern Malawi. We suggest that the distribution of active faults, and hence of earthquakes, is likely controlled by weaknesses in the middle and lower parts of the Earth's crust.

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Section: Discussionmentioning

confidence: 99%

Active Fault Scarps in Southern Malawi and Their Implications for the Distribution of Strain in Incipient Continental Rifts

et al. 2020

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“…The black lines A‐A′, B‐B′, and C‐C′ show the trace of the lithospheric‐scale 2‐D forward gravity models presented in Figure . The white circles with black infill represent the passive seismic stations of the Seismic Arrays for African Rift Initiation experiment (Gao et al, ; Wang et al, ). Color‐coded squares represent locations of passive seismic stations of the SEGMeNT experiment (Borrego et al, ; Table ).…”

Section: Methodsmentioning

confidence: 99%

“…Magnetotelluric and seismic techniques have been widely used to estimate the Moho and lithosphereasthenosphere boundary (LAB) depths (e.g., Borrego et al, 2018;Sarafian et al, 2018;Wang et al, 2019). However, the magnetotelluric and seismic methods are often used to investigate the lithospheric structure along profiles or in small regions because of the expense and time needed for data collection, thus limiting their application for investigating large regions like the entire MR.…”

Section: Methodsmentioning

confidence: 99%

“…We use 2-D forward modeling of the gravity data along profiles A-A′, B-B′, and C-C′ (see Figure 3 for locations of the profiles) using the 2-D GYMSYS Oasis Montaj software (Talwani et al, 1959) to determine the lithospheric structure beneath the MR. Profiles A-A′ and C-C′ follow the passive seismic stations of the Seismic Arrays for African Rift Initiation experiment (Gao et al, 2013;Wang et al, 2019), and B-B′ is an east-west profile that cuts across the widest part of the MR and includes portions of the Luangwa Rift in Zambia. Gravity models have the limitation of nonunique solutions; however, we construct geological models by using realistic constraints on rock densities and lateral variations of rock units (e.g., Fletcher et al, 2018;Leseane et al, 2015;Mahatsente et al, 1999;Mickus et al, 2007;Simiyu & Keller, 2001).…”

Section: -D Forward Gravity Modelingmentioning

confidence: 99%

“…Gravity models have the limitation of nonunique solutions; however, we construct geological models by using realistic constraints on rock densities and lateral variations of rock units (e.g., Fletcher et al, 2018;Leseane et al, 2015;Mahatsente et al, 1999;Mickus et al, 2007;Simiyu & Keller, 2001). Because these constraints are not readily available in all parts of the MR and surrounding areas for the 2-D forward modeling of the gravity data, we used as initial constraints, estimates of crustal thicknesses obtained from the inversion of Rayleigh wave phase velocity maps constructed from ambient noise tomography recorded by broadband (Seismic Arrays for African Rift Initiation) seismic data sets, which range between 36-38 km for the A-A′ profile and 38-40 km for the B-B′ and C-C′ profiles (Wang et al, 2019). Similarly, we use seismically derived depth to LAB ranging between 150-170 km for profile A-A′ and 170-210 km for profiles B-B′ and C-C′ (Fishwick, 2010) as initial constraints of the 2-D forward modeling of the gravity data.…”

Section: -D Forward Gravity Modelingmentioning

confidence: 99%

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

et al. 2019

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Our understanding of how magma‐poor rifts accommodate strain remains limited largely due to sparse geophysical observations from these rift systems. To better understand the magma‐poor rifting processes, we investigate the lithospheric structure of the Malawi Rift, a segment of the magma‐poor western branch of the East African Rift System. We analyze Bouguer gravity anomalies from the World Gravity Model 2012 using the two‐dimensional (2‐D) radially averaged power‐density spectrum technique and 2‐D forward modeling to estimate the crustal and lithospheric thickness beneath the rift. We find: (1) relatively thin crust (38–40 km) beneath the northern Malawi Rift segment and relatively thick crust (41–45 km) beneath the central and southern segments; (2) thinner lithosphere beneath the surface expression of the entire rift with the thinnest lithosphere (115–125 km) occurring beneath its northern segment; and (3) an approximately E‐W trending belt of thicker lithosphere (180–210 km) beneath the rift's central segment. We then use the lithospheric structure to constrain three‐dimensional numerical models of lithosphere‐asthenosphere interactions, which indicate ~3‐cm/year asthenospheric upwelling beneath the thinner lithosphere. We interpret that magma‐poor rifting is characterized by coupling of crust‐lithospheric mantle extension beneath the rift's isolated magmatic zones and decoupling in the rift's magma‐poor segments. We propose that coupled extension beneath rift's isolated magmatic zones is assisted by lithospheric weakening due to melts from asthenospheric upwelling whereas decoupled extension beneath rift's magma‐poor segments is assisted by concentration of fluids possibly fed from deeper asthenospheric melt that is yet to breach the surface.

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The Malawi Active Fault Database: an onshore-offshore database for regional assessment of seismic hazard and tectonic evolution

Williams

Wedmore

Scholz

et al. 2021

Preprint

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Crustal structure beneath the Malawi and Luangwa Rift Zones and adjacent areas from ambient noise tomography

Cited by 34 publications

References 63 publications

Active Fault Scarps in Southern Malawi and Their Implications for the Distribution of Strain in Incipient Continental Rifts

Active Fault Scarps in Southern Malawi and Their Implications for the Distribution of Strain in Incipient Continental Rifts

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

The Malawi Active Fault Database: an onshore-offshore database for regional assessment of seismic hazard and tectonic evolution

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