Browsing by Author "Fishwick, Stewart"

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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.