Abstract
Detachment faulting related to oceanic core complexes (OCCs) has been
suggested to be a manifestation of slow seafloor spreading. Although
numerical models suggest OCCs form under low magma supply, the specific
interaction between magmatism and tectonic faulting remains elusive.
This paper examines seismic observations detailing the spatiotemporal
interactions between magmatism, high-angle faulting, and detachment
faulting at a slow-spreading mid-ocean ridge in the West Philippine
Basin. We identified a magma-rich spreading phase at 36 Ma, indicated by
a magmatic top basement and normal oceanic crust with
shallow-penetrating high-angle faults. An axial valley reveals an
along-strike transition from normal to highly tectonized oceanic crust
over a distance of 70 km. Two older OCCs with concave-down fault
geometries and a younger OCC with steep-dipping faulting suggest
sequential detachments with the same polarity. Our findings suggest: (1)
slow seafloor spreading is cyclical, alternating between high-angle
faulting with a relatively high magma supply and detachment faulting
with limited magma supply; (2) sequential development of younger
detachments in the footwall of its predecessor leads to an asymmetric
split in the newly accreted crust; and (3) the life cycle of OCC ends
with high-angle faults that overprint the detachment and act as magma
pathways, sealing the OCC. Our study captures the dynamic interaction
between high-angle and detachment faults and their concurrent and
subsequent relationship to magmatic systems. This reveals that strain
distribution along strike is critical to OCC formation, thus enriching
our understanding beyond conventional considerations such as spreading
rates and melt budgets at mid-ocean ridges.