Rift evolution in regions of low magma input in East Africa

Muirhead, James D.; Wright, Lachlan J. M.; Scholz, Christopher A.

doi:10.1016/j.epsl.2018.11.004

Cited by 51 publications

(115 citation statements)

References 96 publications

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“…The parallelism of nodal planes of focal mechanisms with projections of border faults to lower crustal levels indicates that the deep earthquakes beneath the Marungu and Mpulungu basins correlate with slip along the border faults (e.g., Figure ). These patterns are consistent with the time‐averaged deformation patterns (Morley, ; Muirhead et al, ). The along‐strike and cross‐rift profiles and earthquake nodal planes argue against the presence of a low‐angle detachment fault (Figure ), as is found in some collapsing orogenic belts (e.g., Abers, ; Axen, ).…”

Section: Discussionsupporting

confidence: 87%

“…Our results show that steep, seismically active, planar border faults penetrate the entire crust at present. Stratal and fault geometry indicate that the border faults have been the locus of strain since early in the basin history (Muirhead et al, ) and thermomechanical models of the topography and gravity anomalies across the Tanganyika rift are consistent with steep, deep border faults (Ebinger et al, ). They form in strong lithosphere and explain the broad, deep basins and broad high flanks (e.g., Ebinger et al, ; Ellis & King, ).…”

Section: Discussionmentioning

confidence: 99%

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Depth Extent and Kinematics of Faulting in the Southern Tanganyika Rift, Africa

et al. 2019

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Unusually deep earthquakes occur beneath rift segments with and without surface expressions of magmatism in the East African Rift system. The Tanganyika rift is part of the Western rift and has no surface evidence of magmatism. The TANG14 array was deployed in the southern Tanganyika rift, where earthquakes of magnitude up to 7.4 have occurred, to probe crust and upper mantle structure and evaluate fault kinematics. Four hundred seventy-four earthquakes detected between June 2014 and September 2015 are located using a new regional velocity model. The precise locations, magnitudes, and source mechanisms of local and teleseismic earthquakes are used to determine seismogenic layer thickness, delineate active faults, evaluate regional extension direction, and evaluate kinematics of border faults. The active faults span more than 350 km with deep normal faults transecting the thick Bangweulu craton, indicating a wide plate boundary zone. The seismogenic layer thickness is 42 km, spanning the entire crust beneath the rift basins and their uplifted flanks. Earthquakes in the upper mantle are also detected. Deep earthquakes with steep nodal planes occur along subsurface projections of Tanganyika and Rukwa border faults, indicating that large offset (≥5 km) faults penetrate to the base of the crust, and are the current locus of strain. The focal mechanisms, continuous depth distribution, and correlation with mapped structures indicate that steep, deep border faults maintain a half-graben morphology over at least 12 Myr of basin evolution. Fault scaling based on our results suggests that M > 7 earthquakes along Tanganyika border faults are possible.

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Section: Discussionsupporting

confidence: 87%

Section: Discussionmentioning

confidence: 99%

Depth Extent and Kinematics of Faulting in the Southern Tanganyika Rift, Africa

et al. 2019

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“…While this level of intra‐rift strain is commonly observed in magmatic rift segments (e.g., Bilham et al, ; Ebinger & Casey, ), this is far higher than other amagmatic rifts. In the ~10 Ma, magma‐poor Lake Tanganyika Rift, the border faults accommodate 90% of extensional strain (Muirhead et al, ). Within the northern basin of Lake Malawi, Mortimer et al () inferred that activity on intra‐rift faults diminished over the lifetime of the rift.…”

Section: Discussionmentioning

confidence: 99%

“…Active intrabasin faults have previously been documented during early rifting (McCartney & Scholz, ), but they have been inferred to be subsidiary to the border faults (Muirhead et al, ). The strain distribution across the rift in the Zomba Graben show that early‐rift intrabasin faults can accommodate a similar proportion of extension to the border faults.…”

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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“…Tracklines within the lake show the locations of Project PROBE legacy seismic data, and 2017 commercial data. The rose diagrams (Muirhead et al, ) show the trends of major (lengths >10 km) preexisting lineaments within different basement terranes.…”

Section: Introductionmentioning

confidence: 99%

Spatiotemporal Variations in Upper Crustal Extension Across the Different Basement Terranes of the Lake Tanganyika Rift, East Africa

2020

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Preexisting crustal heterogeneities are shown to influence rift process at a variety of scales.However, our understanding of how crustal inheritance influences rift-scale spatiotemporal kinematics of faulting in magma-poor rift environments is still very limited. Studies of active continental rifts can provide high-fidelity assessments of extensional processes and structures that are not possible through examination of ancient rifts that have undergone subsequent deformation events or are buried deeply beneath passive margins. We examine the influence of crustal inheritance on active rifting through balancing and restoring a series of regional cross sections across the Lake Tanganyika Rift in the Western Branch of the East African Rift System. The cross sections are produced using legacy seismic reflection data, reprocessed through prestack depth migration. This type example of a young, magma-poor continental rift transects several different basement terranes, including an Archean/Paleoproterozoic craton, and Proterozoic mobile belts. The Lake Tanganyika Rift exhibits two classic bell-shaped profiles of extension along strike, reaching a maximum of 7.15 km. A spatiotemporal integration of the extension data, and comparison with the various basement terranes the rift transects, reveals that extension in cratonic blocks is more widely distributed compared to mobile belt terranes, where strain rapidly localizes onto border faults by later rift stages. These results reveal how crustal inheritance exerts a fundamental control on the evolution of extension localization, ultimately impacting the geometry and structural architecture of rift basins.

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Rift evolution in regions of low magma input in East Africa

Cited by 51 publications

References 96 publications

Depth Extent and Kinematics of Faulting in the Southern Tanganyika Rift, Africa

Depth Extent and Kinematics of Faulting in the Southern Tanganyika Rift, Africa

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

Spatiotemporal Variations in Upper Crustal Extension Across the Different Basement Terranes of the Lake Tanganyika Rift, East Africa

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