The structure of the crust and uppermost mantle beneath Madagascar

Abstract

S U M M A R YThe lithosphere of Madagascar was initially amalgamated during the Pan-African events in the Neoproterozoic. It has subsequently been reshaped by extensional processes associated with the separation from Africa and India in the Jurassic and Cretaceous, respectively, and been subjected to several magmatic events in the late Cretaceous and the Cenozoic. In this study, the crust and uppermost mantle have been investigated to gain insights into the present-day structure and tectonic evolution of Madagascar. We analysed receiver functions, computed from data recorded on 37 broad-band seismic stations, using the H-κ stacking method and a joint inversion with Rayleigh-wave phase-velocity measurements. The thickness of the Malagasy crust ranges between 18 and 46 km. It is generally thick beneath the spine of mountains in the centre part (up to 46 km thick) and decreases in thickness towards the edges of the island. The shallowest Moho is found beneath the western sedimentary basins (18 km thick), which formed during both the Permo-Triassic Karro rifting in Gondwana and the Jurassic rifting of Madagascar from eastern Africa. The crust below the sedimentary basin thickens towards the north and east, reflecting the progressive development of the basins. In contrast, in the east there was no major rifting episode. Instead, the slight thinning of the crust along the east coast (31-36 km thick) may have been caused by crustal uplift and erosion when Madagascar moved over the Marion hotspot and India broke away from it. The parameters describing the crustal structure of Archean and Proterozoic terranes, including average thickness (40 km versus 35 km), Poisson's ratio (0.25 versus 0.26), average shear-wave velocity (both 3.7 km s -1 ), and thickness of mafic lower crust (7 km versus 4 km), show weak evidence of secular variation. The uppermost mantle beneath Madagascar is generally characterized by shear-wave velocities typical of stable lithosphere (∼4.5 km s -1 ). However, markedly slow shear-wave velocities (4.2-4.3 km s -1 ) are observed beneath the northern tip, central part and southwestern region of the island where the major Cenozoic volcanic provinces are located, implying the lithosphere has been significantly modified in these places.

Upper Mantle Earth Structure in Africa From Full‐Wave Ambient Noise Tomography

“…Further to the south along the Malawi and Luangwa Rift, we detect a prominent high wave speed anomaly (+5–7%) beneath the southern Lake Malawi region at upper mantle depths (~100–200 km), as imaged by Sarafian et al () and Adams et al (). This portion of our model domain is well covered due to seismic data collected throughout Madagascar (e.g., Andriampenomanana et al, ; Pratt et al, ).…”

Crustal Radial Anisotropy and Linkage to Geodynamic Processes: A Study Based on Seismic Ambient Noise in Southern Madagascar

“…The second rifting stage is characterized by the East Africa/ Madagascar rift locus migration toward the west (Geiger et al, 2004), the continuous rifting, and the breakup from Africa along a strike-slip boundary with passive margin formation. Coexisting NW directed, low-angle normal faults effectively thinned the crust below the western Madagascar island (Andriampenomanana et al, 2017). The breakup was accompanied by the deposition of thick sedimentary sequences with a maximum thickness of up to 10 km (Pratt et al, 2017;Rindraharisaona et al, 2017).…”

Ambient noise Rayleigh wave tomography across the Madagascar island

“…The iterative inversion at each grid point was performed using a starting model that consists of a constant gradient shear velocity crust over an AK135 (Kennett et al 1995) half space upper mantle with crustal parameters defined within the values obtained by previous studies in the area (e.g. Andriampenomanana et al 2017;Pratt et al 2017). Several trials using smoothing values in the 0-10 range were conducted before constraining the smoothness to 0.8 for the inversions.…”

“…However, in the recent past, the lithospheric and upper-mantle velocity structure of the Madagascar platform and neighbouring regions have been moderately investigated using various seismic tomographic techniques (e.g. Barruol & Fontaine 2013;Rindraharisaona et al 2013;Reiss et al 2016;Andriampenomanana et al 2017;Mazzullo et al 2017;Rindraharisaona et al 2017). All but one of these studies (Paul & Eakin 2017) have generally alluded to a thin crust (∼13-25 km) along the eastern and western boundaries of the island and a thick crust (∼32-45 km) beneath the centre.…”

“…This is suggestive that possible connections between these zones lie at greater depth and supports the idea of a possible upwelling asthenosphere beneath the island. Andriampenomanana et al (2017), from joint inversion of receiver functions and Rayleigh-wave phase-velocity measurements, observed a slight thinning of the crust along the eastern margin of Madagascar which they interpreted as a result of crustal uplift and erosion when Madagascar moved over the Marion hotspot and India broke away from it. Amidst this new-found enthusiasm for seismological studies in Madagascar, the number of studies investigating the lithosphere in the region is still limited and most suffer from limitations in geographical extent and/or resolution.…”