Active transform faults in the Gulf of Guinea: insights from geophysical data and implications for seismic hazard assessment

Abstract

The seismotectonics of Western Africa show the occurrence of major earthquakes (e.g., 1636 southwestern Ghana, 1855 offshore Monrovia, 1939 offshore Accra, and 1983 Gaoual-Guinea) and prominent offshore transform faults. However, there is no analysis that links the continental active tectonics with the oceanic fault zones of the Gulf of Guinea. We study the active tectonics by firstly mapping the main transform faults using a combination of bathymetric, gravimetric, and magnetic data. The data analysis associates regional seismicity (historical and instrumental) with focal mechanisms as extracted from the recently published seismotectonic map of Africa. We identify active transform faults, the Chain (CFZ), Romanche (RFZ), Saint Paul (SPFZ), and Arkhangelskiy (AFZ) fault zones. We also calculate strain rates on these faults from Late Cretaceous (–85 Ma) to present time using paleomagnetic data and infer slip rates from the seismic moment data. The strain rates show a first stable trend around 2 cm/year and then accelerate to 4 cm/year in the last 10 million years. The comparison of Late Quaternary strain rates with geodetic strain rates shows an accumulation of seismic energy that could lead to the initiation of an Mw 7–7.5 earthquakes on the Saint Paul transform fault. Our seismotectonic analysis clearly links oceanic and continental tectonics, with about a 20°–30° anticlockwise fault trend rotation for CFZ, RFZ, and SPFZ. The potential for the occurrence of large earthquakes in the Gulf of Guinea should be taken into account for a realistic regional seismic and tsunami hazard of the Gulf of Guinea.

Seismogenic Fault Reactivation in Western Central Africa: Insights From Regional Stress Analysis

“…In addition to the observation of similar structural trends between oceanic fracture zones and onshore fault and fracture systems, our analysis shows that the stresses acting on the offshore oceanic fracture zones are comparable with the stresses acting along the onshore areas of the continental margin (Figures 7a, 7b and 7e); and that the onshore fault systems have a high slip tendency in this contemporary stress field (Figures 8 and 9). Some of the oceanic fracture zones are active, thus representing intraplate faults that are possibly activated by far‐field strain transfer from transform faults along the spreading ridges (Figure 2a; Meghraoui et al., 2019). Based on the results of stress analysis in this study, we propose that zones of higher stress magnitudes along distal offshore oceanic fracture zones extend further into the continent and may be driving stress loading on pre‐stressed, favorably oriented fault systems onshore, along the continental margin.…”

“…A few studies argue for the lateral continuation of the oceanic fracture zones onto the continent of West Africa and causative relationship with onshore earthquakes. This is based on the alignment of on‐shore magnetic lineaments in Nigeria with the trends of the offshore fracture zones (Ajakaiye et al., 1986), on the colocation and alignment of rifted transform margins such as the Ghanaian and Ivorian coastline with the Romanche and St Paul fracture zones respectively (Figure 2a; Antobreh et al., 2009), and on the recent (<10 million years) acceleration of strain rates on oceanic transform faults post‐continental break‐up in the Late Cretaceous (Meghraoui et al., 2019). However, questions remain on the link between the current stress regime acting on the margin of western African continent and the mechanisms and triggers of seismic reactivation of preexisting structures.…”

Ali Mehmet Celâl Şengör: A geologist who unravels the histories of continents and oceans

“…Their calculations indicate that the strain rates were around 2 cm/year and then accelerated to 4 cm/year in the last 10 million years. Meghraoui et al (2019) conclude that a comparison of Late Quaternary strain rates with geodetic strain rates suggests an accumulation of seismic energy that can lead to the initiation of Mw 7-7.5 earthquakes on the Saint Paul transform fault. Yang et al (2019) present new field, petrographic, and geochemical data on Neoarchean S-type granites in southeastern Manitoba in the Western Superior Province of Canada to discuss the tectonic significance of these granites.…”

“…On the basis of zircon data from southwestern England, southern Portugal, and Germany, the authors propose that the Variscan rocks in Europe had a similar geodynamic evolution. Meghraoui et al (2019) use bathymetric, gravimetric, and magnetic data to assess the seismic hazards of active transform faults in the Gulf of Guinea, western Africa. The authors identify several active transform faults, including the Chain, Romanche, Saint Paul, and Arkhangelskiy faults.…”

The Intraplate Stress Field of West Africa

“…To date, much of what is known about Western Africa's stress field is inferred from the largest earthquakes along the transform passive margins (Kutu, 2013;Blundell, 1976;Langer et al, 1985;Nkodia et al, 2022a;Suleiman et al, 1993). As a result, the preferred mechanism invokes inherited structural weaknesses (i.e., local paleo-tectonics) and the transmission of far-field stresses from the ridges (i.e., regional contemporary-tectonics) (Nkodia et al, 2022a;Meghraoui et al, 2019). How applicable these explanations are for other earthquakes within the continental interiors, especially for smaller events, still remains uncertain.…”