Browsing by Author "Rajaonarison, Tahiry A."

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    A Geodynamic Investigation of Continental Rifting and Mantle Rheology: Madagascar and East African Rift case studies
    Rajaonarison, Tahiry A. (Virginia Tech, 2021-02-18)
    Continental rifting is an important geodynamic process during which the Earth's outer-most rigid shell undergoes continuous stretching resulting in continental break-up and theformation of new oceanic basins. The East African Rift System, which has two continentalsegments comprising largely of the East African Rift (EAR) to the West and the easternmostsegment Madagascar, is the largest narrow rift on Earth. However, the driving mechanismsof continental rifting remain poorly understood due to a lack of numerical infrastructure tosimulate rifting, the lack of knowledge of the underlying mantle dynamics, and poor knowl-edge of mantle rheology. Here, we use state-of-art computational modeling of the upper660 km of the Earth to: 1) provide a better understanding of mantle flow patterns and themantle rheology beneath Madagascar, 2) to elucidate the main driving forces of observedpresent-day∼E-W opening in the EAR, and 3) to investigate the role of multiple plumesor a superplume in driving surface deformation in the EAR. In chapter 1, we simulate EdgeDriven convection (EDC), constrained by a lithospheric thickness model beneath Madagas-car. The mantle flow associated with the EDC is used to calculate induced olivine aggregates'Lattice Preferred Orientation (LPO), known as seismic anisotropy. The predicted LPO isthen used to calculate synthetic seismic anisotropy, which were compared with observationsacross the island. Through a series of comparisons, we found that asthenospheric flow result-ing from undulations in lithospheric thickness variations is the dominant source of the seismicanisotropy, but fossilized structures from an ancient shear zone may play a role in southern Madagascar. Our results suggest that the rheological conditions needed for the formationof seismic anisotropy, dislocation creep, dominates the upper asthenosphere beneath Mada-gascar and likely other continental regions. In chapter 2, we use a 3D numerical model ofthe lithosphere-asthenosphere system to simulate instantaneous lithospheric deformation inthe EAR and surroundings. We test the hypothesis that the∼E-W extension of the EAR isdriven by large scale forces arising from topography and internal density gradients, known aslithospheric buoyancy forces. We calculate surface deformation solely driven by lithosphericbuoyancy forces and compare them with surface velocity observations. The lithosphericbuoyancy forces are implemented by imposing observed topography at the model surfaceand lateral density variations in the crust and mantle down to a compensation depth of 100km. Our results indicate that the large-scale∼E-W extension across East Africa is driven bylithospheric buoyancy forces, but not along-rift surface motions in deforming zones. In chap-ter 3, we test the hypothesis that the anomalous northward rift-parallel deformation observedin the deforming zones of the EAR is driven by viscous coupling between the lithosphereand deep upwelling mantle material, known as a superplume, flowing northward. We testtwo end-member plume models including a multiple plumes model simulated using high res-olution shear wave tomography-derived thermal anomaly and a superplume model (Africansuperplume) simulated by imposing a northward mantle-wind on the multiple plumes model.Our results suggest that the horizontal tractions from northward mantle flow associated withthe African Superplume is needed to explain observations of rift-parallel surface motions indeforming zones from GNSS/GPS data and northward oriented seismic anisotropy beneaththe EAR. Overall, this work yields a better understanding of the geodynamics of Africa.
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    A Geodynamic Investigation of Plume-Lithosphere Interactions Beneath the East African Rift
    Rajaonarison, Tahiry A.; Stamps, D. Sarah; Naliboff, John; Nyblade, Andrew; Njinju, Emmanuel A. (American Geophysical Union, 2023-04)
    The force balance that drives and maintains continental rifting to breakup is poorly understood. The East African Rift (EAR) provides an ideal natural laboratory to elucidate the relative role of plate driving forces as only lithospheric buoyancy forces and horizontal mantle tractions act on the system. Here, we employ high-resolution 3D thermomechanical models to test whether: (a) the anomalous, rift-parallel surface deformation observed by Global Navigation Satellite System (GNSS) data in the EAR are driven by viscous coupling to northward mantle flow associated with the African Superplume, and (b) the African Superplume is the dominant source mechanism of anomalous rift-parallel seismic anisotropy beneath the EAR. We calculate Lattice Preferred Orientations (LPO) and surface deformation from two types of mantle flow: (a) a scenario with multiple plumes constrained by shear wave tomography and (b) a single superplume model with northward boundary condition to simulate large-scale flow. Comparison of calculated LPO with observed seismic anisotropy, and surface velocities with GNSS and plate kinematics reveal that there is a better fit with the superplume mantle flow model, rather than the tomography-based (multiple plumes) model. We also find a relatively better fit spatially between observed seismic anisotropy and calculated LPO with the superplume model beneath northern and central EAR, where the superplume is proposed to be shallowest. Our results suggest that the viscous coupling of the lithosphere to northward mantle flow associated with the African Superplume drives most of the rift-parallel deformation and is the dominant source of the first-order pattern of the observed seismic anisotropy in the EAR.
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    Instantaneous 3D tomography-based convection beneath the Rungwe Volcanic Province, East Africa: implications for melt generation
    Njinju, Emmanuel A.; Stamps, D. Sarah; Atekwana, Estella A.; Rooney, Tyrone O.; Rajaonarison, Tahiry A. (Oxford University Press, 2023-10)
    Within the Western Branch of the East African Rift (EAR), volcanism is highly localized, which is distinct from the voluminous magmatism seen throughout the Eastern Branch of the EAR. A possible mechanism for the source of melt beneath the EAR is decompression melting in response to lithospheric stretching. However, the presence of pre-rift magmatism in both branches of the EAR suggest an important role of plume-lithosphere interactions, which validates the presence of voluminous magmatism in the Eastern Branch, but not the localized magmatism in the Western Branch. We hypothesize that the interaction of a thermally heterogeneous asthenosphere (plume material) with the base of the lithosphere enables localization of deep melt sources beneath the Western Branch where there are sharp variations in lithospheric thickness. To test our hypothesis, we investigate sublithospheric mantle flow beneath the Rungwe Volcanic Province (RVP), which is the southernmost volcanic center in the Western Branch. We use seismically constrained lithospheric thickness and sublithospheric mantle structure to develop an instantaneous 3D thermomechanical model of tomography-based convection (TBC) with melt generation beneath the RVP using ASPECT. Shear wave velocity anomalies suggest excess temperatures reach ∼250 K beneath the RVP. We use the excess temperatures to constrain parameters for melt generation beneath the RVP and find that melt generation occurs at a maximum depth of ∼140 km. The TBC models reveal mantle flow patterns not evident in lithospheric modulated convection (LMC) that do not incorporate upper mantle constraints. The LMC model indicates lateral mantle flow at the base of the lithosphere over a longer interval than the TBC model, which suggests that mantle tractions from LMC might be overestimated. The TBC model provides higher melt fractions with a slightly displaced melting region when compared to LMC models. Our results suggest that upwellings from a thermally heterogeneous asthenosphere distribute and localize deep melt sources beneath the Western Branch in locations where there are sharp variations in lithospheric thickness. Even in the presence of a uniform lithospheric thickness in our TBC models, we still find a characteristic upwelling and melt localization beneath the RVP, which suggest that sublithospheric heterogeneities exert a dominant control on upper mantle flow and melt localization than lithospheric thickness variations. Our TBC models demonstrate the need to incorporate upper mantle constraints in mantle convection models and have global implications in that small-scale convection models without upper mantle constraints should be interpreted with caution.
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    Lithospheric Structure of the Malawi Rift: Implications for Magma‐Poor Rifting Processes
    Njinju, Emmanuel A.; Atekawana, Estella A.; Stamps, D. Sarah; Abdelsalam, Mohamed G.; Atekwana, Eliot A.; Mickus, Kevin L.; Fishwick, Stewart; Kolawole, Folarin; Rajaonarison, Tahiry A.; Nyalugwe, Victor N. (AGU, 2019-11-11)
    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‐Wtrending 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 lithosphereasthenosphere 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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    Numerical Modeling of Mantle Flow Beneath Madagascar to Constrain Upper Mantle Rheology Beneath Continental Regions
    Rajaonarison, Tahiry A.; Stamps, D. Sarah; Fishwick, Stewart; Brune, Sascha; Glerum, Anne; Hu, Jiashun (AGU, 2019-12-27)
    Over the past few decades, azimuthal seismic anisotropy measurements have been widely used proxy to study past and present-day deformation of the lithosphere and to characterize convection in the mantle. Beneath continental regions, distinguishing between shallow and deep sources of anisotropy remains difficult due to poor depth constraints of measurements and a lack of regional-scale geodynamic modeling. Here, we constrain the sources of seismic anisotropy beneath Madagascar where a complex pattern cannot be explained by a single process such as absolute plate motion, global mantle flow, or geology.We test the hypotheses that either Edge-Driven Convection (EDC) or mantle flow derived from mantle wind interactions with lithospheric topography is the dominant source of anisotropy beneath Madagascar.We, therefore, simulate two sets of mantle convection models using regional-scale 3-D computational modeling.We then calculate Lattice Preferred Orientation that develops along pathlines of the mantle flow models and use them to calculate synthetic splitting parameters. Comparison of predicted with observed seismic anisotropy shows a good fit in northern and southern Madagascar for the EDC model, but the mantle wind case only fits well in northern Madagascar. This result suggests the dominant control of the measured anisotropy may be from EDC, but the role of localized fossil anisotropy in narrow shear zones cannot be ruled out in southern Madagascar. Our results suggest that the asthenosphere beneath northern and southern Madagascar is dominated by dislocation creep. Dislocation creep rheology may be dominant in the upper asthenosphere beneath other regions of continental lithosphere.
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    Role of Lithospheric Buoyancy Forces in Driving Deformation in East Africa From 3D Geodynamic Modeling
    Rajaonarison, Tahiry A.; Stamps, D. Sarah; Naliboff, John (2021-03-28)
    Despite decades of investigation, the origin of forces driving continental rifting remains highly debated. Deciphering their relative contributions is challenging due to the nonlinear and depth-dependent nature of lithospheric rheology. Recent geodynamic studies of the East African Rift (EAR) report contradicting results regarding the relative contribution of horizontal mantle tractions and lithospheric buoyancy forces. Here, we use high-resolution 3D regional numerical modeling of the EAR to isolate the contribution lithospheric buoyancy forces to observed deformation. Modeled surface velocities closely match kinematic models of the Somalian Plate, Victoria Block, and Rovuma Block motions, but provide poor fit to along-rift surface motions in deforming zones. These results suggest that lithospheric buoyancy forces primarily drive present-day similar to E-W extension across the EAR, but intrarift deformation may result from viscous coupling to horizontal asthenospheric flow.