Emergence/Subsidence Histories Along the Carnegie and Cocos Ridges and Their Bearing Upon Biological Speciation in the Galápagos

Abstract

Using plate motion reconstructions for the Nazca, Cocos, and South American plates in relation to the Galápagos hotspot, we find that age‐depth dependence of bathymetry, Galápagos plume dynamic topography, crustal relaxation, and magmatic production permit reasonable estimates for the subsidence of former islands along the Carnegie and Cocos Ridges. Our dynamic topography estimates are partially based upon geodynamic theory (fluid mechanics and elasticity) and also upon detailed bathymetric observations and analysis. For three different target areas over Carnegie Ridge and two over Cocos Ridge, we infer subaerial landmasses over the past ~20 Ma. We provide rough estimates of the surface areas of these paleo‐archipelagos, which were comparable to that of the present‐day Galápagos archipelago. We construct a history of emerged (paleo‐island) topography along the conjugate pair Carnegie‐Cocos hotspot tracks and infer that significant subaerial landscapes existed for at least the past ~20 Ma. The genetic ancestors of endemic faunal species in Galápagos (e.g., marine iguanas) migrated away from South America, genetically diverging from their land counterparts much earlier than the age of the present Galápagos Islands (~3 Ma). At 16.5 Ma, the total emerged landmass on the Carnegie Ridge alone may have been ~2.5 times that of the current Galápagos archipelago. (The conjugate landmass along the Cocos Ridge has been subducted.) We propose that this large landmass, at least one‐fourth the size of Iceland, be called Darwinia in recognition of its importance in the evolution of some Galápagos endemic flora and fauna made famous by Darwin's explorations.

The Galápagos Islands: biogeographic patterns and geology

“…Seafloor studies in the Galápagos region have identified many flat‐topped seamounts. These include a former archipelago on the Cocos Ridge at 8 Ma that was at least 200–300 km across, and another at 12 Ma that was 300 km across (Orellana‐Rovirosa & Richards, 2018). At 16.5 Ma there is evidence of a massive island on the Carnegie Ridge; this was 170 km across, with an area ~ 2.8 times larger than the present day Galápagos archipelago and an elevation over 500 m.…”

Insights Into the Origins and Compositions of Mantle Plumes: A Comparison of Galápagos and Hawai'i

“…Even though the GSC is migrating northeast, away from the plume, southward ridge jumps and rift propagation events have kept the plume and the ridge closely associated throughout the past 15–20 My (Mittelstaedt et al, 2012, 2014; Wilson & Hey, 1995; see also Meschede & Barckhausen, 2000; Werner et al, 2003). During several periods, most recently ~5–8 Ma, the GSC overlay the plume, resulting in deposition of plume‐generated lavas on both the Cocos and Nazca plates and formation of the Cocos and Carnegie aseismic ridges (Figure 1c; Orellana‐Rovirosa & Richards, 2018; D. S. Wilson & Hey, 1995). Since ~5 My, the GSC and plume have maintained a separation of ~145–215 km (Mittelstaedt et al, 2012).…”

Elastic Flexure of Young, Overlapping Basaltic Lava Flows Offshore the Galápagos and Hawaiian Islands: Observations, Modeling, and Thermal/Chronological Analysis

“…Here α is the flexural parameter, where x b , the distance from the line load to the maximum amplitude of the forebulge, can be written as , where D is the flexural rigidity and ρ s is the density of the underlying lava flow (modified from Turcotte & Schubert, 1982 to address a change in the hydrostatic restoring force). We assume all flows erupted subaerially, because this area of the Galápagos is eastward of the currently active hotspot (beneath Fernandina Island) and has likely suffered up to several hundred meters of thermal subsidence since they were erupted (see, e.g., Orellana‐Rovirosa & Richards, 2018). Therefore, the contribution to the hydrostatic restoring force is only a result of the displacement of the underlying lava flow, as the inflationary flow replaces air, not water.…”