Timing and Volume of Magma Emplacement During Rifting, Breakup, and Initial Spreading: from Simple Endmember Models to Overlooked Complexities

A major achievement in the study of rifted margins was the establishment of the “magma-poor” vs. “magma-rich” archetypes distinguishing between margins with exhumed mantle and margins whose continental crust is heavily intruded and overlain by extrusive magmatic flows (e.g., seaward dipping reflections (SDRs)). However, this binary approach, mostly dictated by the magmatic budget/mantle potential temperature, cannot account for observations made at high-resolution, wide-angle seismic data. These data show markedly variable along and across strike volumes of magmatic products from the intra-segment scale (< 100 km) to the margin scale (> 100 km). These observations highlight that the binary magma-rich vs. magma-poor classification is only a first order simplification and other factors that so far have been overlooked control the timing and volume of magma emplacement during rifting, breakup and initial seafloor spreading.

Two main limitations exist when describing magmatic systems at rifted margins: 1) distinguishing among inherited continental crust, newly created magmatic crust and serpentinized mantle in seismic data is challenging due to their comparable geophysical properties (density and seismic velocity); and 2) modelling magmatic systems is limited by the poor knowledge of their initial conditions (mantle potential temperature and inherited compositional variations in the subcontinental mantle). The study of magma-rich margins is challenging as different factors may control the timing and volume of magma emplacement, and hence control their crustal shape. These factors include: (1) the initial conditions mentioned above; (2) the mode of lithosphere extension (e.g., pure shear vs. depth-dependent lithosphere thinning); and (3) external rift-independent factors (e.g., strain rates or elevated temperature linked to mantle plumes). Thus, new observational approaches are needed to describe the tectono-magmatic evolution of margins and unravel the spatio-temporal evolution of magmatic processes at the transition from rifting to seafloor spreading.

In our presentation, we first present along and across strike seismic observations that show evidence for variability in the timing and volume of the first magmatic addition with respect to the onset of steady-state seafloor spreading. These observations allow us to explore and discuss the importance of strain rate and initial conditions and provide insights into the dominant processes controlling the tectono-magmatic evolution during rifting, breakup, and initial spreading. Finally, we propose a simple approach that focuses on the mapping of first order interfaces. This approach allows us to determine the crustal shape and the nature of top basement, both of which are diagnostic for extensional and/or magmatic processes. We combine this approach with a simple geometric/kinematic/isostatic model, which allows us to calculate the relative timing and volume of magma emplacement and its subsequent isostatic equilibration.