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‐W trending 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 lithosphere‐asthenosphere 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.
Melt intrusions into the lithospheric mantle and crust during extensional tectonics play a key role in weakening the lithosphere during magma-assisting rifting. Magma-assisted continental rifting involves magmatic intrusions that are sourced from melt generated in the sublithospheric mantle beneath the rift axis, which developed when mantle potential temperatures are higher than average (i.e., McKenzie & Bickle, 1988). The source of melt generation in the sublithospheric mantle beneath rifts has been proposed to originate from thermal perturbations due to plumes (e.g.,
We used aeromagnetic and satellite gravity data to investigate lithospheric structure beneath the Cretaceous Chilwa Alkaline Province (CAP) in southern Malawi and adjacent Mozambique. The CAP consists of granites, syenites, nepheline syenites, basanites, phonolite dikes, and minor carbonatite bodies. The intrusions were emplaced in the Precambrian (Mesoproterozoic-Neoproterozoic) terranes of the Southern Irumide and Mozambique orogenic belts. Aeromagnetic data show the CAP as overlapping circular anomalies typical of nested igneous ring complexes formed through caldera collapse mechanism. We used gravity data to infer that: (1) the CAP was sourced from~30-km wide igneous bodies now preserved in the upper crust at~5-km depth.(2) the CAP is underlain by up to~45-km thick crust (due to mafic magmatic underplating) and a lithosphere as thin as~90 km. These data suggest that mafic magmatic underplating and lithospheric thinning occurred during a Cretaceous rifting event. We propose, based on these results and taking into account previous petrographic, geochemical, geochronological, and isotopic studies, that the silica undersaturated magmatic phase of the CAP was due to flux melting of the asthenosphere. This was followed by silica-rich magmatic phase due to decompression melting of the asthenosphere as a result of lithospheric thinning. Lithospheric thinning and ascendance of the asthenospheric melt might have been facilitated by the presence of late Neoproterozoic to early Cambrian suture zone. The heat provided by the mafic magmatic underplating resulted in partial melting of the lower crust to form the silica-saturated CAP intrusions from mixed magma sources.
Summary The West and Central African Rift System (WCARS) is the only stable continental geological structure on Earth that is formed by large-scale topographic massifs (swells). However, knowledge of the origin and evolution of the WCARS remains limited mainly due to the scarcity of high-resolution geophysical observations. To better understand the crustal structure beneath the entire WCARS, we use the XGM2016 global gravitational model and the ETOPO1 global topographic-bathymetric model to determine a gravimetric Moho (crust-mantle boundary) model constrained by seismic Moho depth estimates at 41 seismological stations distributed irregularly within the study area. The result reveals a regional Moho deepening to ∼40 km beneath the Hoggar, Aïr and Tibesti Massifs. The largest Moho deepening to ∼46 km is detected beneath the Archean Congo Craton, while the Moho depth under the Adamawa Plateau reaches 42 km. The Moho geometry beneath the Chad Basin, the Chad Lineament and the Termit Basin is relatively even with the Moho depth mostly within 24-26 km. A significant Moho deepening as well as large Moho depth variations within 32-45 km beneath the Saharan Metacraton and the Congo Craton (especially under its northern margin) reflect the metacratonization processes that occurred during the Neoproterozoic. The Niger Delta and the Benue Trough are characterized by a very thin continental crust with the Moho depth varying from ∼20 km in the south along the Atlantic coastline to ∼24 km in the north-eastern branch of the Cretaceous Benue Trough around the Garoua-Yola Rift.
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.
Summary 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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