Seismicity and seismotectonics of Madagascar revealed by the 2011–2013 deployment of the island-wide MACOMO broadband seismic array

Rakotondraibe, Tsiriandrimanana; Nyblade, A.; Wysession, M. E.; Durrheim, Raymond; Rambolamanana, G.; Aleqabi, G. I.; Shore, P.; Pratt, Martin; Andriampenomanana, Fenitra; Rümpker, Georg; Rindraharisaona, E. J.

doi:10.1016/j.tecto.2020.228547

Cited by 9 publications

(25 citation statements)

References 51 publications

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“…The correlation between lower-crustal earthquakes and the low-Pn-velocity areas might be related to magmatic activity. Focal mechanism studies in central Madagascar show predominantly normal faulting (Rindraharisaona et al 2013;Rakotondraibe et al 2020), consistent with crustal dyking.…”

Section: Pn Velocity Variationsmentioning

confidence: 60%

“…While the horizontally propagating Pn waves Map showing the lateral variations of (a) Pn velocity in the uppermost mantle, from this study, and (b) shear wave velocity at a depth of 50 km, from Pratt et al (2017). Also plotted in (a) are earthquake epicentres from Rakotondraibe et al (2020): green dots are events with depth <20 km, black dots are for depths ≥20 km, and grey dots are events located with high depth uncertainty (SEZ > 5.35 km). There is a good correlation between event locations and low-Pn-velocity zones.…”

Section: Pn Anisotropy Variationsmentioning

confidence: 89%

“…7a plots the seismicity in Madagascar from the Rakotondraibe et al (2020) study, on top of our Pn velocity model. Seismotectonic studies in Madagascar revealed that earthquakes, especially in the central region, can occur in the lower crust (Bertil & Regnoult 1998;Rindraharisaona et al 2013;Rakotondraibe et al 2020). The correlation between lower-crustal earthquakes and the low-Pn-velocity areas might be related to magmatic activity.…”

Section: Pn Velocity Variationsmentioning

confidence: 97%

“…1). Seismic events were located using the earthquake location program HYPOELLIPSE (Lahr 1999) in which the crustal model from Andriampenomanana et al (2017) was used (Rakotondraibe et al 2020).…”

Section: Datamentioning

confidence: 99%

“…Anisotropic Pn tomography of Madagascar 291 island (e.g. Rindraharisaona et al 2013;Rakotondraibe et al 2020) and its plate kinematics (e.g. Saria et al 2013;Stamps et al 2014Stamps et al , 2015Stamps et al , 2018, greatly improving our understanding of the structure and geodynamics of Madagascar.…”

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confidence: 99%

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Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

Andriampenomanana

Nyblade

Wysession

et al. 2020

Geophysical Journal International

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Summary The lithosphere of Madagascar records a long series of tectonic processes. Structures initially inherited from the Pan-African Orogeny are overprinted by a series of extensional tectonic and magmatic events that began with the breakup of Gondwana and continued through to the present. Here, we present a Pn-tomography study in which Pn travel times are inverted to investigate the lateral variation of the seismic velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show that the Pn velocities within the uppermost mantle vary by ± 0.30 km/s about a mean of 8.10 km/s. Low-Pn-velocity zones (<8.00 km/s) are observed beneath the Cenozoic alkaline volcanic provinces in the northern and central regions. They correspond to thermally perturbed zones, where temperatures are estimated to be elevated by ∼100-300 K. Moderately low Pn velocities are found near the southern volcanic province and along an E-W belt in central Madagascar. This belt is located at the edge of a broader low S-velocity anomaly in the mantle imaged in a recent surface wave tomographic study. High-Pn-velocity zones (>8.20 km/s) coincide with stable and less seismically active regions. The pattern of Pn anisotropy is very complex, with small-scale variations in both the amplitude and the fast-axis direction, and generally reflects the complicated tectonic history of Madagascar. Pn anisotropy and shear wave (SKS) splitting measurements show good correlations in the southern parts of Madagascar, indicating coherency in the vertical distribution of lithospheric deformation along Pan-African shear zone as well as coupling between the crust and mantle when the shear zones were active. In most other regions, discrepancies between Pn anisotropy and SKS measurements suggest that the seismic anisotropy in the uppermost mantle beneath Madagascar differs from the vertically-integrated upper mantle anisotropy, implying a present-day vertical partitioning of the deformation. Pn anisotropy directions lack the coherent pattern expected for an incipient plate boundary within Madagascar proposed in some kinematic models of the region.

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Section: Pn Velocity Variationsmentioning

confidence: 60%

Section: Pn Anisotropy Variationsmentioning

confidence: 89%

Section: Pn Velocity Variationsmentioning

confidence: 97%

Section: Datamentioning

confidence: 99%

mentioning

confidence: 99%

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Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

Andriampenomanana

Nyblade

Wysession

et al. 2020

Geophysical Journal International

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The slow downwearing of Madagascar: Inferring patterns and controls on long‐term basin‐averaged erosion rates from in situ¹⁰Be at the catchment and regional level

Brosens

Cox

Campforts

et al. 2023

Earth Surf Processes Landf

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Cosmogenic nuclide analysis of river sediment provides insight into catchment‐wide erosion rates and dynamics. Here, we investigate spatial patterns and controls on 10Be‐inferred erosion rates in Madagascar, a moderately seismically active microcontinent surrounded by passive margins with locally steep topography and a climate that varies from humid tropical to semiarid. We use a compiled dataset of 99 detrital 10Be measurements, 63 of which are new, covering more than 30% of the country and a wide range of topographic, bioclimatic and geologic characteristics. Overall, 10Be erosion rates are low (2.4–51.1 mm kyr−1), with clear differences between regions. The lowest rates are measured on the central highlands ( ∼ 8 mm kyr−1), in the Alaotra–Ankay graben ( ∼ 11 mm kyr−1) and in the large north‐central catchments ( ∼ 11 mm kyr−1). Higher rates are found on the steep eastern escarpment ( ∼ 20 mm kyr−1), in the northwest ( ∼ 31 mm kyr−1) and in the southwest ( ∼ 29 mm kyr−1). A stepwise linear regression model identified elevation as the main factor associated with variations in 10Be erosion rates (lower rates for higher catchments). Random within‐between statistical models (REWB), on the other hand, indicated that the differences between different regions can be explained by differences in river concavity, seismic events and gully (lavaka) densities, whereas additional variation within regions is only linked to seismicity. We find no correlation between catchment or river steepness and 10Be‐inferred erosion rates. Our results indicate that in Madagascar, long‐term erosion rates are overall low and that simple topography‐based models do not explain variations in rates of landscape change inferred from 10Be concentrations in river sediment. We demonstrate that identifying different regions aids in interpreting spatial patterns of erosion rates and that REWB models can be a powerful tool in deciphering environmental controls on 10Be erosion rates.

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Lavaka (erosional gullies) provide productive patch environments for flora and farming in Madagascar's grassy highlands

Cox,

Carrère,

Rakotondrazafy

et al. 2023

Plants People Planet

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Societal Impact StatementLavaka, large gullies in Madagascar, can cause problems for farmers and for infrastructure, but these features also have beneficial aspects that have generally been overlooked. They provide plant refugia, and commonly host species that would otherwise not thrive on the grassy uplands. Farmers sometimes make use of lavaka to plant crops that require more protection or soil moisture. Colonial narratives blame lavaka erosion on poor land management but ignore their pre‐human‐settlement existence on the landscape and the ecological functions they serve. The knowledge and wisdom of Malagasy farmers are key to better understanding the complex roles that lavaka plays in the landscape.SummaryThis paper combines a review of lavaka—erosional gullies in Madagascar's grass‐covered highlands—with new observations of their importance as ecological patch environments for a wide range of plants not seen elsewhere within the grassy biome. Lavaka play a role as a natural refugia for flora that would otherwise not thrive on the exposed and infertile hillslopes, and local farmers exploit their topography and sediment deposits to grow a variety of crops. We provide a classification scheme for lavaka based on the extent to which they have been colonised by vegetation. Our analysis is based on observations made over many field seasons in the highlands as well as interviews with Malagasy farmers living with lavaka on their land. We emphasise the importance of local knowledge and environmental wisdom in the analysis of landscape evolution, and we conclude that lavaka, while certainly an erosional problem in many places, can nonetheless provide valuable ecosystem services and agricultural opportunities that should not be overlooked.

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Seismicity and seismotectonics of Madagascar revealed by the 2011–2013 deployment of the island-wide MACOMO broadband seismic array

Cited by 9 publications

References 51 publications

Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

The slow downwearing of Madagascar: Inferring patterns and controls on long‐term basin‐averaged erosion rates from in situ¹⁰Be at the catchment and regional level

Lavaka (erosional gullies) provide productive patch environments for flora and farming in Madagascar's grassy highlands

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Seismicity and seismotectonics of Madagascar revealed by the 2011–2013 deployment of the island-wide MACOMO broadband seismic array

Cited by 9 publications

References 51 publications

Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

The slow downwearing of Madagascar: Inferring patterns and controls on long‐term basin‐averaged erosion rates from in situ10Be at the catchment and regional level

Lavaka (erosional gullies) provide productive patch environments for flora and farming in Madagascar's grassy highlands

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Resources

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The slow downwearing of Madagascar: Inferring patterns and controls on long‐term basin‐averaged erosion rates from in situ¹⁰Be at the catchment and regional level