Controls on structural and magmatic variability along rifted margins: From observations to interpretations

Recent advances in the development of long offset, high-resolution seismic imaging methods, combined with drill-hole data and direct field observations resulted in a paradigm shift in interpreting the structural and magmatic evolution of rifted margins. In particular, the discovery of hyper-extended domains associated with variable amounts of late magmatic additions results in a fundamental question: what controls the pronounced structural and magmatic variability observed along rifted margins (Figure 1 )? In order to identify and understand the processes that may account for this variability, a new approach has been developed that . enables the mapping and interpretation of the architectural variations observed along Atlantic type rifted margins. The key is to combine geological and geophysical observations. To first order, the architecture of the crust can be linked with direct constrains on the depositional environments and sedimentary facies, the nature and kinematics of basement structures, subsidence evolution and the volume and timing of magmatic additions observed at different parts of the margin.

The first part of the presentation reviews, using multi channel seismic sections and outcrop observations, key architectural features, such as tilted blocks, necking zones, hyperextended and exhumed mantle domains and various types of magmatic additions observed in Atlantic type rifted margins. The focus will be on first order geometrical relationships between these building blocks as well as on their stacking order as observed in onshore remnants. A key result is that, on a first order, rift systems can be subdivided in mappable rift domains that can be identified using geological and geophysical criteria. The mapping and characterization of these· domains highlights the along-strike structural, magmatic and stratigraphic variability of the margin. A first order screening of conjugate margins also confirms the asymmetry of distal parts of margins and their segmentation.

A second part discusses the interaction between extensional structures and magmatic processes at rifted margins. A key observation is that major crustal/lithospheric thinning predates in most margins an excess magmatic event that is commonly related to breakup. This observation suggests that the link between magmatic/asthenospheric and structural/lithospheric processes is more complex and cannot be predicted with a simple depth-dependent rift model. Indeed, the evolution of rift systems reflect the interplay between their inheritance (innate/“genetic code”) and the physical processes at play (acquired/external factors) that may explain the observed changes in time and space across a rift system. Understanding when, where and how magma is first produced and how it interacts with the extensional systems is a key to understand final breakup as well as to explain the structural and magmatic variability observed along Atlantic-type systems. Observations suggest that both inheritance and rift-induced processes play a significant role in the development of rift systems. Thus, it is not only important to determine the “genetic code” of a rift system, but also to understand how it interacts and evolves during rifting. The discussion of how these different processes may interact in time and space and how they may control the final architecture of rifted margins is the major subject of the talk.