Magmatism at Incipient Rift, Galapagos Triple Junction: Tapping the Off‐Axis East Pacific Rise Melting Regime

Abstract Incipient Rift (IR) is the latest in a sequence of short‐lived rifts that form ridge‐ridge‐ridge triple junctions with the East Pacific Rise (EPR) in the Galapagos triple junction region. IR extends ∼65 km southeastward from its intersection with the EPR at 2°40′N. IR originated ∼0.4 Ma; its opening rate is ∼15 km/Myr. Seafloor photographs document active fissuring and volcanism on IR up to 60 km east of the EPR. Eruptive fissures cut sedimented seafloor and lavas overlie sediment. Geochemical data suggest IR lavas >7 km from the EPR have a parental magma composition produced by smaller extents and higher pressures of melting compared to lavas near the EPR. IR eruptions may tap melt accumulating at the lithosphere‐asthenosphere boundary (LAB) as part of the off‐axis EPR melting regime beneath this young oceanic lithosphere. Melt overpressure at the LAB combined with IR extension would explain observations.