Looking for Magma in Magma-Poor Rifted Margins

Continental breakup mechanisms are recorded within the Continent-Ocean Transitions (COT) of distal passive margins. The variability of basement architectures imaged by seismic data suggest that different, often complex, tectono-magmatic interactions occur. Rifted margins are commonly defined either as magma-rich or magma-poor archetypes based on the recognition of a number of morphological features.

Magma-rich rifted margins generally record an early onset of decompression melting and melt extraction relative to crustal thinning. Reflection seismic data reveals the common presence of Seaward Deeping Reflectors (SDR) at the COT related to the emplacement of extrusive basaltic lava flows as deduced from the drilling constraints available. Other geophysical data sets evidence the coincident occurrence at depth of high velocity bodies, interpreted as the intrusive magmatic counterpart of SDR.

In contrast, magma-poor rifted margins show a late onset of melt extraction relative to crustal separation allowing for mantle exhumation. In these margins, the unambiguous identification of the location and amount of magmatic products related to rifting and breakup has often be problematic, explaining why they have long been referred to as non-volcanic.

In this contribution, we will show several examples of magma-poor rifted margins (Bay of Biscay, Australia-Antarctica, SE India, and Gulf of Aden) and analogies with ultra-slow spreading systems to illustrate evidence of magmatism in COT and the involvement of melt production during continental breakup. Reflection seismic observations coupled to geophysical quantitative analyses suggest that onset of magmatic production is often progressive controlling the final width of the COT. The observed structural style of COT appears largely controlled by a syn-kinematic interplay between hydration (i.e serpentinization) and magmatic processes occurring in the exhumed mantle. The oceanward increasing melt production coincides with a change in the rooting depth of faults interpreted as an evolution of the basement rheology.

Even if the amount of melt is difficult to evaluate and its extraction delayed relative to crustal separation, melt production is also a key parameter eventually controlling the mechanisms of continental breakup at magma-poor rifted margins.