The structure of oceanic core complexes controlled by the depth distribution of magma emplacement

Abstract

Extension at mid-ocean ridges can be accommodated by detachment faults, forming oceanic core complexes that develop under low rates of magma intrusion. Modelling reveals that oceanic core complexes can also form under high rates of magma intrusion, if the magma is injected into the lower ductile layer of the crust. At mid-ocean spreading centres, extension can be accommodated by slip on large, long-lived (1–2 Myr) detachment faults that expose large tracts of lower crustal gabbroic rocks and mantle peridotite. These structures are known as oceanic core complexes. The development of detachment faults is controlled by the rate at which magma is injected into the brittle lithosphere as intrusive dykes. Recent modelling studies suggested that oceanic core complexes form under low magma injection rates, when only 30–50% of total plate separation is accommodated by the injection of magma into the lithosphere1,2,3. Yet, paradoxically, field observations4,5,6,7,8,9,10,11 document oceanic core complexes that have formed under a spectrum of magma injection rates, from amagmatic to fully magmatic conditions. Here we present a numerical model of oceanic core complex formation that explicitly considers magma intrusion not only in the brittle lithospheric layer, as in earlier simulations1,2,3, but also in the underlying ductile asthenosphere. We find that the rate of magma intrusion into the brittle layer controls fault evolution, whereas the rate of intrusion below the brittle–ductile transition has no influence on fault development, but controls the volume of gabbro exhumed. Our findings suggest that oceanic core complexes can form under high magma intrusion rates if intrusion is accommodated mainly by the ductile asthenosphere, thus reconciling the disparity between prevailing models and field observations.