Structural characteristics and tectonic division of the Zambezi Delta basin in the offshore East Africa: evidences from gravity and seismic data
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Abstract: The Zambezi Delta basin is a passive marginal basin located on the East African coast that has good oil and gas exploration potential. Due to the special geological evolutionary background of the Beira High in the Zambezi Delta basin, it has a low gravity anomaly, and the existing seismic survey lines do not cover the whole basin; therefore, it is difficult to interpret the structural characteristics of the whole basin based solely on gravity or seismic data. Based on satellite altimetry gravity anomaly data, this study infers the distribution characteristics of faults in the Zambezi Delta basin by using the normalized vertical derivative of the total horizontal derivative (NVDR-THDR) technique. Then, constrained by seismic data, the gravity anomaly at the Moho interface is extracted by using the fast forward method of the double-interface model of the gravity anomaly, and this anomaly is then removed from the Bouguer gravity anomaly to obtain the sedimentary layer gravity anomaly. The thickness of the sedimentary strata is obtained by inversing the sedimentary basement depth of the whole basin. Then, uplifts and depressions are divided based on a sedimentary layer thickness of 3 km. This research demonstrates that the Zambezi Delta basin mainly features nearly SN-trending and NE-trending faults and that these faults exhibit east‒west partitioning. The nearly SN-trending strike-slip faults controlled the sedimentary development of the basin, and the NE-trending tensile faults may have acted as migration channels for oil, gas and magma. The “overcompensation” effect of the Moho interface gravity anomaly on the gravity anomaly of the sedimentary layer is caused by the depression of the Moho interface beneath the Beira High, which results in a low gravity anomaly value for the Beira High. The pattern of uplifts and depressions trends NE and has the structural characteristics of east‒west blocks.
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Key words:
- Zambezi Delta basin /
- satellite altimetry gravity anomaly /
- Beira High /
- fault division /
- uplift and depression pattern
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Figure 8. NVDR-THDR of the Bouguer gravity anomaly and the location of the seismic profile in the study area.
Figure 9. Comparison of the NVDR-THDR of Bouguer gravity anomaly and the seismic profile.
Figure 12. Comparison of the gravity anomaly of the sedimentary layer and the Jurassic bottom interface.
Figure 13. Comparison of the geological profile (Mueller et al., 2016) and the gravity anomaly.
Figure 16. The NVDR-THDR of the Bouguer gravity anomaly and the distribution of faults in the study area.
Figure 20. Comparison of the distribution characteristics of uplifts and depressions with the results of previous studies in the study area.
Figure 21. Oil and gas well distribution characteristics and tectonic framework map of the study area.
Table 1. Statistical table of the main strata densities (refer to Leinweber et al., 2013)
Main layer Layer Max thickness/km Main density(103 kg · m−3) Average density(103 kg · m−3) Water layer water layer 2.6 1.03 1.03 Cenozoic sediment layer sediment layer 1 0.9 1.95 2.30 sediment layer 2 0.8 2.18 sediment layer 3 1.8 2.39–2.46 sediment layer 4 1.8 2.40 Mesozoic sediment layer sediment layer 5 2.4 2.51–2.58 2.60 sediment layer 6 1.3 2.62 sediment layer 7 0.9 2.60 sediment layer 8 2.6 2.64 Crust upper crust onshore 4.7 2.69 2.90 middle crust onshore 32.0 2.91 lower crust 8.0 2.90–3.05 Mantle upper mantle - 3.27 3.27 
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Table 2. Statistical table of the inferred results for the main faults
Number Identifying characteristics Trend Position Nature Length/km DFZ NVDR-THDR continuous ridge value nearly SN east side of basin strike-slip – F3 NVDR-THDR continuous ridge value NE north side of basin tension 557 F4 NVDR-THDR continuous ridge value and local dislocation NE eastern part of the basin, which is centered tension 259 F5 NVDR-THDR continuous ridge value nearly SN mid-basin strike-slip 164 F6 NVDR-THDR continuous ridge value nearly SN northwestern basin tension 392 F7 NVDR-THDR continuous ridge value NE north side of Beira High tension 614 F8 NVDR-THDR continuous ridge value NE south side of Beira High tension 305 F9 NVDR-THDR continuous ridge value nearly SN west side of basin strike-slip 265
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