Partitioning of magmatic and tectonic extension from hours to millions of years at the Southeast Indian Ridge, 37°S

Ocean floor formed at intermediate spreading ridges typically consists of volcanic effusion products (80-90%) and regularly-spaced normal fault scarps (10-20%) that shape elongated abyssal hills. This fabric forms over millions of years as the divergence of two tectonic plates induces discrete events of magmatic intrusion and fault slip at the ridge axis, which can last from several seconds to several months. Little is known, however, on how the repetition of such events ultimately shapes the partitioning of tectonic and magmatic strain that is encoded in the morphology of the seafloor. To address this, we quantify the amount of fault slip and magmatically-accommodated extension during the early days of the April 2024 rifting event that took place on the Southeast Indian Ridge at 37°S, and was documented by the OHA-GEODAMS seismo-geodetic observatory (Royer et al. EGU26-GD5.1).

Using elastic dislocation modelling in a Bayesian framework, we find that the rifting event accounted for 2–4 m of horizontal extension, of which ∼85% involved the emplacement of a magmatic fracture that propagated along the axis within less than 2 hours. We attribute the remainder of the extension to dominantly aseismic slip on axial valley bounding faults. This "instantaneous" fraction of magmatic extension is strikingly similar to that revealed by bathymetric analyses (M∼90%), which quantify deformation averaged over hundreds of thousands of years. We therefore propose that the long-term "M-fraction" that characterizes intermediate-spread seafloor could be determined at the scale of individual rifting events, possibly by static stress transfers between a propagating dike and adjacent faults. At the Southeast Indian Ridge, such events likely recur every ∼50 years and are separated by periods of seismic quiescence, as mid-ocean ridge normal faults may primarily grow when triggered by magmatic activity.