Rifted margins classification and forcing parameters

Abstract

Rifted margins are the result of the successful process of thinning and breakup of the continental lithosphere leading to the formation of new oceanic lithosphere. Observations on rifted margins are now integrating an increasing amount of multi-channel seismic data and drilling of several Continent-Ocean Transitions. Based on large scale geometries and domains observed on high-quality multi-channel seismic data, this article proposes a classification reflecting the mechanical behavior of the crust from localized to diffuse deformation (strong/coupled to weak/decoupled mechanical behaviors) and magmatic intensity leading to breakup from magma-rich to magma-poor margins. We illustrate a simple classification based on mechanical behavior and magmatic production with examples of rifted margins. We propose a non-exhaustive list of forcing parameters that can control the initial rifting conditions but also their evolution through time. Therefore, rifted margins are not divided into opposing types, but described as a combination and continuum that can evolve through time and space.

Figure 1

Classification of rifted margins (a) and some examples ranked by rifting duration (b). The proposed classification is organized along two axes, the rifting axis that characterizes the overall rheology of the crust and the breakup axis that characterizes the quantity of magma involved during the rupture of the lithosphere. The four presented examples, Somalia (6), South Gabon (12), Namibia (1) and Coral Sea (3), are highlighted in a thick black frame. The other sections are either from published work: Great Bight (7), East India (8)^,, French Guyana (9), Iberia (11), Møre (14); or from Total internal studies: Southernmost Angola (5, Namibe Basin), Western South China Sea (6), Gulf of Mexico (10, Mexico), Mozambique (12) and East Agulhas (13, South Africa). All lines are located in Fig. 2.

Figure 2

Location of the examples and case studies. The examples shown in Fig. 1 are located on the world map with the oceanic floor age from Müller et al.. The main case studies are emphasized with stars. The main transform margins and continental flood basalts provinces/volcanic rifted margins ( modified from Bryan and Ferrari) are also shown.

Figure 3

Somalia case study. The section is characterized by a short necking domain accommodated on a couple of high-angle normal faults. The coupled domain is longer with faults extending down to the mantle and cutting the interpreted Moho reflection in several cases. The most external domain is characterized by low-angle faults and detachments exhuming the mantle and leaving rafts of the pre-rift material on the exhumation surface. The emplacement of oceanic magmatism is progressive over this exhumed mantle. The data are courtesy of TGS.

Figure 4

South Gabon case study. The section is characterized by a very long necking domain with a primary necking and a long low angle wedge toward the distal domain. The thinning of the crust is accommodated on low angle normal faults and core-complexes. An important continental to deltaic sedimentation (10–12 km) is associated to this long thinning of the crust with seaward prograding sequences. The coupling domain is marked by younger high-angle faults creating important topography at the base of the salt and cutting through the previous large sediment package and the lower crustal bodies. Local basins with mantle exhumation may form prior to the generation of the oceanic crust. The data are courtesy of ION and CGG-MCNV, all rights reserved.

Figure 5

South Namibia case study. The section is characterized by thick syn-rift wedges of Seaward Dipping Reflectors (SDR)^,,. The thinning of the crust seems to be accommodated on low angle normal faults and core-complexes but strongly intruded by multiple magmatic features (dikes, sills). It ends up with a very long necking domain with a primary necking and a long low-angle wedge towards the distal domain. A large basin with both Seaward and Landward Dipping Reflectors develops in continuity with the primary necking. Their relationship with faults and potential core-complexes are unclear. There is no coupling domain, the most distal domains are replaced by thick SDR packages thinning toward a classic oceanic crust. The data are courtesy of ION, all rights reserved.

Figure 6

North Coral Sea case study. The section crosses a proximal domain in a continental crust already thinned (23–24 km). The final thinning of the crust is expressed on a single shallow detachment and important shear structures in the lowermost crustal layer. The necking zone is short. Breakup is rapid with short OCT leading to a classic oceanic crust in terms of thickness but with a peculiar upper layer made of interactions between magma (intrusive and effusive) and locally high sedimentation. The data are courtesy of Searcher.

Figure 7

Forcing parameters and their influence on Rifting and Breakup axes. These parameters may be either inherited or external. When external, they can change the behavior of the margin drastically at any stage of the rifting. The obliquity is a key parameter as it seems to overprint any initial rheological/magmatic conditions. NL: Necking Line, LCC: Limit of Continental Crust, LOC: Limit of Oceanic Crust, CL: Coupling Line, EL: Exhumation Line; ML: Magmatic Line.