To investigate the controls on continental rifting in the western branch of the East Africa Rift System, we conduct shear velocity imaging of the crust and uppermost mantle beneath the weakly extended Malawi Rift and the Rungwe Volcanic Province (RVP). We use local‐scale measurements of Rayleigh wave phase velocities between 9‐ and 100‐s periods combined with constraints on basin architecture and crustal thickness to invert for shear velocity from the surface to ~135 km. Our resulting 3‐D model reveals a localized low‐velocity anomaly associated with the RVP extending from the crust and through the upper mantle, which can be explained with modestly elevated temperatures. Away from the RVP, velocities within mantle flanking the rift are fast (>4.6 ± 0.1 km/s), suggesting depleted lithospheric mantle to depths of ~100 and >135 km to the west and east of the rift, respectively. The upper mantle beneath the rift axis is characterized by thinned lithosphere with slower velocities than the surrounding plateau, suggestive of thermal and/or chemical modification by the rifting process. Slowest velocities are mildly asymmetric about the rift axis, with the lowest velocities observed beneath the rift and adjacent footwall escarpments. The underlying asthenosphere is only moderately slow (~4.25 ± 0.1 km/s), including beneath the RVP, precluding the presence of significant volumes of partial melt. The positions of localized lithospheric modification and basin‐bounding border faults correlate with the location of Proterozoic mobile belts, suggesting that these sutures provide lithospheric‐scale weakening mechanisms necessary for localizing strain and allowing extension to occur in the Malawi Rift.
The Shanxi Rift is one of the world's largest Cenozoic continental rifts. It is one of the most earthquake prone areas in China and has volcanic activities characterized by small volumes of magma. In order to study continental rifting mechanism, intraplate volcanism, and seismicity of the Shanxi Rift, we construct a high‐resolution 3‐D lithospheric shear velocity (Vs) model with a focus of the crust by jointly inverting Rayleigh wave dispersion curves and ellipticity measurements. Compared with published models, our 3‐D Vs model has improved the resolution for shallow structures. Our model reveals several significant velocity features. (1) The sedimentary layer of the Datong Basin is much thicker than those in the southern Shanxi Rift, implying that the sedimentation rate of the Datong Basin is higher, which could be attributed in part to asthenosphere upwelling beneath the Datong Basin area in the past few Ma in addition to passive rifting. (2) A low‐velocity body is observed in the middle crust beneath the Datong Volcanoes, suggesting the presence of a small amount of partial melting in the middle crust. (3) Lower velocities in the middle and lower crust beneath the Datong Basin are coincident with the shallowing of deep earthquakes from the southern to northern Shanxi Rift in the crust, suggesting that higher temperature may elevate the brittle‐to‐ductile transition zone, leading to the shallowing of focal depths of earthquakes in the Shanxi Rift from south to north.more » « less
- NSF-PAR ID:
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Medium: X
- Sponsoring Org:
- National Science Foundation
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