Deep Structure of the North Natal Valley (Mozambique) Using Combined Wide‐Angle and Reflection Seismic Data

Abstract

The North Natal Valley (NNV) and the Mozambique Coastal Plain (MCP) are key areas for the understanding of the SW Indian Ocean history since the Gondwana break‐up. Nevertheless, the deep structures and the nature of the NNV and MCP remain discussed in the absence of deep geophysical data. In 2016, the NNV, MCP and Limpopo margin (LM) have been investigated along seven wide‐angle and MCS profiles. The combined wide‐angle and reflection seismic interpretation along the N‐S MZ7 profile reveals an upper sedimentary sequence characterized by low velocities generally not exceeding 3 km/s, with thicknesses varying from 0.150 km in the central part to ∼2.8 km in the south. The underlying sequence is formed of a 2.53.0 km thick volcano‐sedimentary sequence which presents important lateral and with depth changes and presence of high velocity lenses, indicating inter‐bedded volcanic sills and recurrent magmatic episodes. The south of the NNV including the Naude Ridge (NR) presents a disturbed sedimentary cover with structural highs and southward‐dipping reflectors and sub‐basins. The crust, reaching 35–40 km onshore below the MCP, gently thins below the continental shelf to a regular thickness of ∼29 km below the NNV. Crustal velocities reveal low velocity gradients, with atypical high velocities. South the ND, the crust thins to 15 km. We interpret the velocity architecture combined with the evidences of volcanism at shallower depths as indicating an intensively intruded continental crust. Contrary to what is proposed in most geodynamic models, the Mozambique Coastal plain and the Natal Valley are both of continental nature, with an abrupt necking zone located south of NR. The Antarctica plate was therefore situated at the eastern limit of these two domains before the Gondwana breakup.

Crustal structure of the East African Limpopo margin, a strike-slip rifted corridor along the continental Mozambique Coastal Plain and North Natal Valley

“…2). However, the tectonostratigraphic study of Li et al (2021) suggests a volcano-sedimentary basement here that formed during the intra-continental rifting period. In this case, the MR would not extend further north, but instead positive gravity anomalies would reflect the presence of large sedimentary features such as contourites, which are commonly observed in the area (Moulin et al, 2020;Thiéblemont et al, 2019).…”

“…The oldest reliable age for basalts in the area concerns the Lower Cretaceous Movene igneous that disconformably cover Karoo sediments to the east of the Lebombos monocline (Flores, 1973;Watkeys, 2002). As already stressed in companion papers (e.g., Moulin et al, 2020;Leprêtre et al, 2021), the crustal nature in the area is the subject of a vigorous debate. The MCP gravity signature was previously in-Figure 1.…”

“…With new deep seismic data acquired within the scope of the PAMELA project, companion papers of this study argue for continental crust below the MCP and NNV. Despite its high magmatic content, Moulin et al (2020) and Leprêtre et al (2021) show the continuity of a thick crust with a comparable velocity structure to surrounding cratons and other continental structures worldwide.…”

“…MZ3 crosses four out of the seven profiles acquired during the MOZ3-5 cruise: MZ2 and MZ6 (Schnürle et al, 2018) acquired over the NNV and MZ4 and MZ5 that also cross the LM (Watremez et al, 2021). Two other profiles acquired in the NNV, MZ1 and MZ7, are presented by Moulin et al (2020) and Leprêtre et al (2021). Ifremer's SolidQC software was initially used for processing MCS data.…”

“…2). Velocity models built from these data challenge the tight-fit scenario as they unravel the continuity of a thick and highly intruded continental crust beneath the MCP and NNV (Moulin et al, 2020;Leprêtre et al, 2021;Watremez et al, 2021). Conversely, these results strengthen previous studies that interpreted continental crust in the area (Domingues et al, 2016;Hanyu et al, 2017) and brings strong support for a "loose" fit plate reconstruction without plate overlap (Thompson et al, 2019).…”