Abstract
Oceanic crust is continuously created at mid-oceanic ridges and seafloor spreading represents one of the main processes of plate tectonics. However, if oceanic crust architecture, composition and formation at present-day oceanic ridges are largely described, the processes governing the birth of a spreading center remain enigmatic. Understanding the transition between inherited continental and new oceanic domains is a prerequisite to constrain one of the last major unsolved problems of plate tectonics, namely the formation of a stable divergent plate boundary. In this paper, we present newly released high-resolution seismic reflection profiles that image the complete transition from unambiguous continental to oceanic crusts in the Gulf of Guinea. Based on these high-resolution seismic sections we show that onset of oceanic seafloor spreading is associated with the formation of a hybrid crust in which thinned continental crust and/or exhumed mantle is sandwiched between magmatic intrusive and extrusive bodies. This crust results from a polyphase evolution showing a gradual transition from tectonic-driven to magmatic-driven processes. The results presented in this paper provide a characterization of the domain in which lithospheric breakup occurs and enable to define the processes controlling formation of a new plate boundary.
Highlights
At rifted margins, a complex, heterogeneous continental crust faces a new, commonly layered, 6 to 7 km thick magmatic oceanic crust, entirely formed at a spreading center
The first oceanic crust is mapped as a sharp boundary referred to as Continent-Ocean Boundary (COB) using different criteria such as the location of the oldest magnetic anomaly[9], identification of Mid Oceanic Ridge (MOR) type magmatic rocks[10], step ups or downs of top basement, landward limit of typical oceanic Moho[12], transition from wedge-type stratigraphic patterns to passive infill and the occurrence of a so called “Breakup Unconformity”[13]
It remains debated[15] if breakup is a fast, catastrophic event resulting in a sharp boundary, or, alternatively, if it is a gradual process resulting in a transitional domain, i.e. a diffuse plate boundary
Summary
The Gulf of Guinea results from the separation between the African and South American plates. We interpret it as a second major detachment system, cross-cutting the previous one This interpretation is based on the fact that the pre-tectonic sediments (Unit A) overlying the tilted blocks of continental crust are not any more present oceanward above the basement. This observation suggests that the juxtaposed basement is younger than the deposition of the sedimentary unit A. We interpret the over-tilted architecture of the latest continental block (Fig. 2c) as resulting from a new detachment fault cutting the first detachment fault and leading to mantle exhumation Such a particular sedimentary architecture allowing the recognition of multiple detachment faults has been extensively detailed at the Australia-Antarctica margins by ref.[7]. These two parallel reflections define a 2 sec (TWT) thick basement, interpreted as the steady state oceanic crust
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