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.

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.

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