Crustal structure of the Nile Delta: Interpretation of seismic-constrained gravity inversion

Abstract:

Interpretations of the tectonic setting of the Nile Delta are challenged by its thick sedimentary cover that conceals the underlying structures. We constructed a crustal thickness model for the Nile Delta utilizing a satellite gravity inversion constrained by seismic data and geological information followed by 3D forward density modelling along a profile passing through the Delta. Results indicate that the Nile Delta is formed of two distinctive crustal units (1) the Southern Delta block (SDB), and (2) the Northern Delta basin (NDB) separated by a flexure zone of listiric faults, widely described as hinge zone and/or hinge line. The SDB is characterized by E-W trending low gravity (~ -40 mGal) anomaly indicative of thick crust (depth to Moho ~ 35 to 38 km and crustal thickness ~ 32 to 35 Km) and manifesting continental crust characteristics. The SDB is bounded from the south by the Cairo-Suez normal fault arrays and from the north by the flexure zone. The NDB with its thick sedimentary cover is characterized by high gravity anomaly (> 40 mGal) that we attribute to crustal thinning and stretching and a shallow mantle. Starting from the flexure zone, the depth to Moho is ~ at 35 Km and decreases northward beneath the Nile fan reaching 25 Km at the southern borders of the Herodotus Basin; crustal thickness ranges between ~ 25 Km at the flexure zone to ~ 10- 12 Km at the southern borders of the Herodotus Basin. No oceanic crust is present under the NDB nor under the Nile fan. The results of the 3D forward modeling are in agreement with the interpretations of the Nile Delta crustal model. Findings indicate that the NDB is located over a transition zone between the North African continental crust and the Mediterranean oceanic crust, a typical non-volcanic passive margin transition zone. Our model is consistent with the assumption that the hinge zone was formed by the northward gravitational sliding of thick sediment accumulations (at least 5-7 Km in NDB compared to 1.5 Km in SDB) during and after sedimentation along northward dipping east-west set of growth and listiric normal faults. The recognition of the transition zone crust of the continental passive margin has profound implications for heat flow and thermal maturation of hydrocarbons of the Nile Delta petroleum and gas provinces.