Neogene - Quaternary Rifting of the Southern Malawi Rift and Linkage to the Late Carboniferous – Early Jurassic Shire Rift

Ojo, Oyewande O.; Thomson, Stuart N.; Laó‐Dávila, Daniel A.

doi:10.1130/abs/2021am-368982

Cited by 3 publications

(6 citation statements)

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“…The fault slip rates we obtain are also comparable to preliminary estimates from the offset of late Quaternary reflector in the Central and South basins of Lake Malawi (Fig. 4; Wright et al, 2019) and apatite fission track modelling of footwall uplift in southern Malawi (Ojo et al, 2022a). This suggests that the systems-based approach is an appropriate method to estimate faults slip rates in Malawi where no other constraints are currently available.…”

Section: Assessment Of Fault Slip Rate Estimates In the Mssmsupporting

confidence: 85%

“…Hanging wall flexural modelling in the basins south of Lake Malawi indicates negligible flexural extension (<1 %) due to the much lower throws (<1 km) on the region's border faults (Fig. A4, Table A2; Bloomfield, 1965;Kolawole et al, 2022;Ojo et al, 2022b), and so c hwf is set to 1 for these basins.…”

Section: Slip Ratesmentioning

confidence: 99%

“…In the onshore basins in southern Malawi, hanging wall sediment thickness is constrained by a combination of basement-penetrating boreholes aeromagnetic data and electrical resistivity surveys (Table A2; Fig. A4; Bloomfield, 1965;Bloomfield and Garson, 1965;Habgood et al, 1973; Kolawole et al, 2022;Ojo et al, 2022b;Walshaw, 1965). Given a profile of hanging wall deflection, it is possible to derive the resulting flexural extensional strain (ε) within a half-graben as follows (Billings and Kattenhorn, 2005;Muirhead et al, 2016):…”

Section: A1 Hanging Wall Flexure In Malawimentioning

confidence: 99%

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Geologic and geodetic constraints on the magnitude and frequency of earthquakes along Malawi's active faults: the Malawi Seismogenic Source Model (MSSM)

Williams

Wedmore

Fagereng

et al. 2022

Nat. Hazards Earth Syst. Sci.

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Abstract. Active fault data are commonly used in seismic hazard assessments, but there are challenges in deriving the slip rate, geometry, and frequency of earthquakes along active faults. Herein, we present the open-access geospatial Malawi Seismogenic Source Model (MSSM; https://doi.org/10.5281/zenodo.5599616), which describes the seismogenic properties of faults that formed during ongoing east African rifting in Malawi. We first use empirically derived constraints to geometrically classify active faults into section, fault, and multifault seismogenic sources. For sources in the North Basin of Lake Malawi, slip rates can be derived from the vertical offset of a seismic reflector that dated lake cores indicate is 75 ka. Elsewhere, slip rates are constrained from advancing a systems-based approach that partitions geodetically derived rift extension rates in Malawi between seismogenic sources using a priori constraints on a regional strain distribution and a hanging wall flexural extension in magma-poor continental rifts. Slip rates are then combined with source geometry and empirical scaling relationships to estimate earthquake magnitudes and recurrence intervals, and their uncertainty is described from the variability in logic tree outcomes used in these calculations. Sources in the MSSM are 5–269 km long, which implies that large-magnitude (Mw 7–8) earthquakes may occur in Malawi. However, low slip rates (0.05–2 mm yr−1) mean that the frequency of such events will be low (recurrence intervals of ∼103–104 years). We also find that, for 9 out of 11 faults in Lake Malawi's North Basin, differences in the slip rates, when estimated independently from the geodetic data and the offset seismic reflector, are not statistically significant. The MSSM represents an important resource for investigating Malawi's increasing seismic risk and provides a framework for incorporating active fault data into seismic hazard assessment elsewhere in the East African Rift and other tectonically active regions.

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Section: Assessment Of Fault Slip Rate Estimates In the Mssmsupporting

confidence: 85%

Section: Slip Ratesmentioning

confidence: 99%

Section: A1 Hanging Wall Flexure In Malawimentioning

confidence: 99%

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Geologic and geodetic constraints on the magnitude and frequency of earthquakes along Malawi's active faults: the Malawi Seismogenic Source Model (MSSM)

Williams

Wedmore

Fagereng

et al. 2022

Nat. Hazards Earth Syst. Sci.

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“…Although the Malawi Rift largely developed during the Cenozoic evolution of the EARS, it is proposed to have propagated southwards based on: 1) the extensive outcrops of Cenozoic syn-rift sequences in the northern basins of the rift which are absent in the south (Delvaux, 2001), and southward thinning of the syn-rift sedimentary cover (Specht and Rosendahl, 1989;Flannery and Rosendahl, 1990), and 2) southward decrease in the throw of border faults with corresponding changes in the seismic depositional facies of the syn-rift sequences (Scholz et al, 2020). However, it is also plausible that the initiation of Cenozoic rifting across the entire length of the Malawi Rift could have been synchronous and that rifting occurs at different rates along the rift length (Ojo et al, 2021). Overall, crustal stretching rates decrease from north (4.5 mm yr −1 ) to south 1.3 mm yr −1 (Stamps et al, 2008;.…”

Section: Geologic and Tectonic Setting Of The Malawi Riftmentioning

confidence: 99%

“…Furthermore, we estimate strain rates along the BMF as a function of cumulative strain (cumulative displacement) against time (initiation of rift faulting at 23 Ma; Van Der Beek et al, 1998;Mortimer et al, 2016;Ojo et al, 2021) using the relationship strain rate = displacement/time. The BMF has lower strain rates (0.025 mm/yr) compared to the most prominent and larger offset border faults along the Malawi Rift which include the Livingstone Fault (0.32 mm/yr), Usisya Fault (0.31 mm/yr), and the South Basin Border Fault (0.20 mm/yr).…”

Section: Implications For Strain Localization During Early-stage Rift...mentioning

confidence: 99%

Under-Displaced Normal Faults: Strain Accommodation Along an Early-Stage Rift-Bounding Fault in the Southern Malawi Rift

et al. 2022

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One of the fundamental problems in continental rift segmentation and propagation is how strain is accommodated along large rift-bounding faults (border faults) since the segmentation of propagating border faults control the expression of rift zones, syn-rift depo-centers, and long-term basin evolution. In the Southern Malawi Rift, where previous studies on the early-stage rifting only assessed border fault structure from surficial and topographic expression, we integrate surface and subsurface data to investigate border fault segmentation, linkage, and growth as proxies for strain accommodation along the Bilila-Mtakataka Fault (BMF) System. We used 30 m-resolution topographic relief maps, electrical resistivity tomography (ERT), and high-resolution aeromagnetic data to characterize the detailed fault geometry and provide a more robust estimate of along-fault displacement distribution. Our results reveal a discrepancy between sub-aerial segmentation of the BMF geometry (six segments), scarp height (five segments) reflecting the most recent episodes of fault offset, and cumulative throw (three composite segments) reflecting the long-term fault offset. We also observe that although the BMF exhibits continuity of sub-aerial scarps along its length, the throw distribution shows a higher estimate at the Northern-to-Central segment relay zone (423 m absolute, 364 m moving median) compared to the Central-to-Southern segment relay zone (371 m absolute, 297 m moving median). The ERT profiles across the relay zones suggest a shallower basement and a possible canyon-mouth alluvial fan stratigraphy at the Central-to-Southern segment relay zone, contrasting the deeper basement and “simpler” electrical stratigraphy at the Northern-to-Central relay. The results suggest a more complex long-term evolution of the BMF than was assumed in previous studies. A comparison of BMF’s maximum displacement-vs-length with those of other Malawi Rift border faults and global normal fault populations suggest that although the BMF has possibly reached its maximum length, it remains largely under-displaced as its 580–837 m maximum displacement is significantly lower than that of faults of equivalent length. We suggest that the BMF may continue to accrue significant strain as tectonic extension progresses in the Southern Malawi Rift, thus posing a major seismic hazard in the region.

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Under-Displaced Normal Faults: Strain Accommodation along an Early-Stage Rift-Bounding Fault in the Southern Malawi Rift

Ojo

Ohenhen

Kolawole

et al. 2022

Preprint

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One of the fundamental problems in continental rift segmentation and propagation is how strain is accommodated along large rift-bounding faults (border faults) since the segmentation of propagating border faults control the expression of rift zones, syn-rift depo-centers, and long-term basin evolution. In the Southern Malawi Rift, where previous studies on the earlystage rifting only assessed border fault structure from surficial and topographic expression, we integrate surface and subsurface data to investigate border fault segmentation, linkage, and growth as proxies for strain accommodation along the Bilila-Mtakataka Fault (BMF) System. We used 30 m-resolution topographic relief maps, electrical resistivity tomography (ERT), and high-resolution aeromagnetic data to characterize the detailed fault geometry and provide a more robust estimate of along-fault displacement distribution. Our results reveal a discrepancy between sub-aerial segmentation of the BMF geometry (six segments), scarp height (five segments) reflecting the most recent episodes of fault offset, and cumulative throw (three composite segments) reflecting the long-term fault offset. We also observe that although the BMF exhibits continuity of sub-aerial scarps along its length, the throw distribution shows a higher estimate at the Northern-to-Central segment relay zone (423 m absolute, 364 m moving median) compared to the Central-to-Southern segment relay zone (371 m absolute, 297 m moving median). The ERT profiles across the relay zones suggest a shallower basement and a possible canyon-mouth alluvial fan stratigraphy at the Central-to-Southern segment relay zone, contrasting the deeper basement and "simpler" electrical stratigraphy at the Northern-to-Central relay. The results suggest a more complex long-term evolution of the BMF than was assumed in previous studies. A comparison of BMF's maximum displacement-vs-length with those of other Malawi Rift border faults and global normal fault populations suggest that although the BMF has possibly reached its maximum length, it remains largely under-displaced as its 580-837 m maximum displacement is significantly lower than that of faults of equivalent length. We suggest that the BMF may continue to accrue significant strain as tectonic extension progresses in the Southern Malawi Rift, thus posing a major seismic hazard in the region.

show abstract

Neogene - Quaternary Rifting of the Southern Malawi Rift and Linkage to the Late Carboniferous – Early Jurassic Shire Rift

Cited by 3 publications

References 0 publications

Geologic and geodetic constraints on the magnitude and frequency of earthquakes along Malawi's active faults: the Malawi Seismogenic Source Model (MSSM)

Geologic and geodetic constraints on the magnitude and frequency of earthquakes along Malawi's active faults: the Malawi Seismogenic Source Model (MSSM)

Under-Displaced Normal Faults: Strain Accommodation Along an Early-Stage Rift-Bounding Fault in the Southern Malawi Rift

Under-Displaced Normal Faults: Strain Accommodation along an Early-Stage Rift-Bounding Fault in the Southern Malawi Rift

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