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Late-Stage Volcanism in the Galápagos: A Distinct Mechanism from Hawaiian Rejuvenated Volcanism

Karen Harpp, Dennis Geist

Abstract

Whereas rejuvenated volcanism at Hawai'i has been clearly defined geologically, geochemically, and temporally, comparable late-stage behavior at other archipelagos deviates from Hawai'i's behavior. Unlike Hawai'i, the oldest Galápagos islands do not exhibit a clear, significant hiatus or erosional period, widely variable eruption rates, or distinct shield-building and post-shield phases. For instance, volcanism at Santa Cruz began ~1.6 Ma and continued to ~200 ka, when it focused along an E-W fissure system. The southern flank lavas erupted <200 ka, with morphologically youngest lavas on the NNW flank. Faults crosscutting the southern flank formed 200-33 ka. San Cristóbal was constructed between 2.1 Ma and ~300 ka, is dominated by ENE-trending faults and fissures, and includes a young volcanic field along its northern coast, active from 174 to 5 ka. Floreana's alkaline lavas erupted from 1.6 to 0.03 Ma, with no evidence of an eruptive hiatus. Gravity surveys prove that neither San Cristóbal nor Santa Cruz ever had large calderas like the western shields. The youngest lavas on all three islands exhibit heterogeneous compositions that overlap those of older lavas. At Santa Cruz and San Cristobal, older volcanism is weakly alkalic, shifting to alkalic and MORB-like tholeiites in younger lavas.  Most eastern Galápagos volcanoes were formed near the Galápagos Spreading Center, located <100 km from the plume >3 to 1 Ma. Formation of these volcanoes under the GSC's influence, coupled with emplacement on thin, near-ridge lithosphere may have influenced their morphology and evolution, resulting in late-stage activity distinct from Hawaiian rejuvenated volcanism.