The Mozambique Ridge (MOZR) is one of the basement high structures located in the Southwest Indian Ocean, parallel to the southeast African continental margin. It was formed as a result of the tectono-magmatic evolution of the Gondwana breakup. The origin of the MOZR has been highly debated, with models suggesting either continental or oceanic origin. With new free-air gravity anomaly and multichannel seismic (MCS) reflection data, we present results of 2D density modeling along two seismic profiles acquired by R/V XiangyangHong 10 at the northern Mozambique Ridge (N-MOZR) between 26°S and 28°S. We observed high free-air gravity anomaly and strong positive magnetic anomaly related to the emplaced seaward dipping reflectors (SDR) and high density lower crustal body (HDLCB), and high Bouguer gravity anomaly associated with the thinning of the continental crust underneath the N-MOZR over a distance of ~82 km. This suggests a thinned and intruded continental crust bound by the Mozambique Fracture Zone (MFZ) that is characterized by gravity low and negative magnetic anomaly. This fracture zone marks the continent-ocean boundary (COB) while the N-MOZR is the transform margin high, i.e., marks the continent-ocean Transition (COT) of the Southern Mozambique margin, following the definition of transform margins. We suggest that the N-MOZR was formed by continental extension and subsequent breakup of the MFZ, accompanied by massive volcanism during the southward movement of the Antarctica block. The presence of SDR, HDLCB, and relatively thick oceanic crust indicates the volcanic nature of this transform margin.
Figure 1. Tectonic overview of the Mozambique passive margin and Mozambique Ridge (MOZR). a. Bathymetric map (data from GEBCO_2014, https://www.gebco.net/data_and_products/gridded_bathymetry_data/), with 1 000-m contours in black, showing the dominant geological features and magnetic anomalies. The solid yellow lines in a along the northern MOZR (N-MOZR) mark the location of our seismic and gravity profiles. b. Magnetic anomaly (data from EMAG2, http://geomag.org/models/emag2.html). The black lines in b indicate the fracture zones along the Mozambique Basin, with one of them crossing the N-MOZR. c. Free-air gravity anomaly map (data from WGM2012, http://bgi.omp.obs-mip.fr/data-products/Grids-and-models/wgm2012). The NW-SE black lines in c indicate the magnetic anomalies M12 to M0 identified by Goodlad et al. (1982); the W-E black lines in c indicate the magnetic anomalies M10N and M4, and XR is the extinct spreading center proposed by Marks and Tikku (2002); the white triangles in c are Deep Sea Drillhg Project (DSDP) Sites 248 and 249 (Simpson, 1974); the white stars in c are the rock samples DR1, DR2, DR3, DRQ (Ben-Avraham et al., 1995; Mougenot et al., 1991); the white circles in c are wells located along the Mozambique Coastal Plain; the three red circles in c show the wells that recovered the Early Cretaceous basalts; and the solid red line in c refers to the Ariel Graben (AG). (d). Bouguer gravity anomaly map (data from WGM2012: http://bgi.omp.obs-mip.fr/data-products/Grids-and-models/wgm2012). the Abbreviations: S-01: Seismic01; S-02: Seismic02; MozB: Mozambique Mobile Belt; ZC: Zimbabwe Craton; LB: Limpopo Belt; KC: Kaapval Craton; MCP: Mozambique coastal plain; MC: Mozambique Channel; DFZ: Davie Fracture Zone; NNV and SNV: Northern and Southern Natal Valley; MFZ: Mozambique Fracture Zone; MOZR: Mozambique Ridge; N-MOZR: northern Mozambique Ridge; C-MOZR: central Mozambique Ridge; S-MOZR: southern Mozambique Ridge; MB: Mozambique Basin; MP: Madagascar Plateau; MAD: Madagascar.
Figure 2. Uninterpreted seismic01 profile from NW-SE, showing the northern Natal Valley, northern Mozambique Ridge, Mozambique Fracture Zone (MFZ) and Mozambique Basin.
Figure 3. Enlarged section of profile seismic01 showing the high-amplitude SDR in the northern Mozambique Rudge (N-MOER) (a) and its simple seismic interpretation (b). T1, T2 and T3 are stratigraphic unconformities observed in the N-MOZR, and t1 and t2 are unconformities observed in the Mozambique Basin (Gao et al., 2020).
Figure 4. Seismic profile crossing the Mozambique Fracture Zone, showing the shear zone below the lava flow that separates the low amplitude reflectors from the northern Mozambique Ridge to high amplitude reflectors in the Mozambique Basin.
Figure 5. Seismic profile crossing the Mozambique Basin. Shows the major fault thought to be a result of tectonic reactivation, diffractions related to deformation phases and the presence of Moho reflection at 9 s TWT.
Figure 6. Seismic interpretation of profile seismic01 across the NNV, N-MOZR, and MB (a); magnetic (blue line) and bouguer gravity (green line) anomaly profiles along the seismic01 line (b); observed and modeled free-air gravity anomaly (c); i and 2D density model for profile seismic01 with density values for each block in unit of g/cm3 (d). In a, TWT is two-way travel time; T1, T2 and T3 are stratigraphic unconformities in the N-MOZR and t1 and t2 are unconformities in the MB (Gao et al., 2020). For b, data are from EMAG2 (http://geomag.org/models/emag2.html; Maus et al., 2009) and WGM2012 (http://bgi.omp.obs-mip.fr/data-products/Grids-and-models/wgm2012; Bonvalot et al., 2012). In d, the solid red lines indicate the Seaward dipping reflectors, the red V-shap symbol indicates the presence of an intrusion, COB: continent-ocean boundary.
Figure 7. Seismic interpretation of profile seismic02 at the eastern tip of N-MOZR (a); magnetic (blue line) and Bouguer gravity (green line) anomaly profiles along the seismic02 line (b); observed and modeled free-air gravity anomaly (c); and 2D density model of profile seismic02 with density values for each block in g/cm3 (d). In a, TWT: two-way travel time. For b, data are from EMAG2 (http://geomag.org/models/emag2.html; Maus et al., 2009) and WGM2012 (http://bgi.omp.obs-mip.fr/data-products/Grids-and-models/wgm2012; Bonvalot et al., 2012). In d, the solid red lines indicate lava flow. COB: continent-ocean boundary.