Crustal structure beneath the Galápagos Archipelago from ambient noise tomography and its implications for plume-lithosphere interactions

Abstract

[1] To constrain the seismic velocity structure of the crust beneath the Galápagos Archipelago, we conducted a tomographic study using high-frequency Rayleigh waves obtained from cross correlations of ambient noise. We analyzed waves with periods between 5 and 8.5 s, sensitive to shear wave velocity (V S ) structure between about 3 and 10 km depth, after accounting for the effect of water depth. Crustal velocities are up to 25% lower than those of very young crust at the East Pacific Rise and are comparable to those of Hawaii. We attribute the lower than normal velocities to the combined effect of heating and the presence of melt in the crust above the Galápagos plume as well as the construction of a highly porous volcanic platform emplaced atop preexisting oceanic crust. On average, V S between 3 and 10 km depth beneath the western archipelago is up to 15% lower than beneath the eastern archipelago. We attribute the west-to-east velocity increase to a decrease in porosity of the volcanic platform and to cooling of the crust after its passage above the Galápagos plume. The results of this study, in combination with previous work, indicate that many of the unusual aspects of the Galápagos Archipelago are the result of variations in the thickness and internal structure of the chemical and thermal lithospheres. Our findings indicate that observed variations in the flexural response to loading observed in the Galápagos cannot be explained by the current thermal state of the lithosphere. Instead, the flexural response likely represents varying elastic strength at the time of loading. We also propose that the northwest and northeast trending alignments of volcanic centers found throughout the archipelago (the Darwinian lineations) may be associated with preexisting zones of weakness in the lithosphere formed during earlier episodes of ridge jumping and ridge propagation that were later reactivated by stresses generated by plume-lithosphere interactions.

Mantle flow and multistage melting beneath the Galápagos hotspot revealed by seismic imaging

“…3e). This high-velocity lid is attributed to olivine dehydration 22,24 , which increases seismic velocities and viscosity 26 , thereby decreasing the upward flow rate. The highvelocity lid is thickest beneath southern Isabela and above the columnar low-velocity anomaly imaged at depths greater than 150 km.…”

“…1). That experiment has already provided information on the structure of the crust, the structure of the mantle to depths of ∼125 km, and the thickness of the mantle transition zone [22][23][24] . Here we combine our previous observations of Rayleigh wave phase velocities with new observations of teleseismic S wave delay times to constrain mantle structure to depths of 300 km.…”

Imaging rapidly deforming ocean island volcanoes in the western Galápagos archipelago, Ecuador

“…Using ambient noise tomographic methods with a sparse network of seismometers, Villagómez et al . [] estimate Vp and Vs for the uppermost ~10 km of the crust. Crustal velocities are ~10% lower in the volcanically active western sector of the archipelago, as compared to the older, largely inactive eastern portion of the Galápagos platform.…”

“…The lid is ~30 km thicker beneath Isabela and Fernandina, which have enriched basalts with the highest 3 He/ 4 He [e.g., Graham et al ., ; Kurz and Geist , ], suggesting a melt residuum layer at the base of the lithosphere [ Villagómez et al ., ]. Crustal velocity structure and thickness beneath the Galápagos archipelago are constrained by a WSW striking seismic refraction profile south of Isabela Island [ Toomey et al ., ], ambient noise tomography [ Villagómez et al ., ], and predictive models of gravity data [ Feighner and Richards , ]. These results indicate that crustal thickness increases sharply from ~8 km west of the platform to ~15 km on the platform [ Feighner and Richards , ; Toomey et al ., ].…”

Back‐Projection Imaging of Extended, Diffuse Sources of Volcano‐Tectonic Seismicity During June 2010 at Sierra Negra Volcano, Galápagos

“…We focus on early June 2010 when several hundred microearthquakes (0.9 < M L < 1.4) were detected along the southeastern flanks of the Sierra Negra caldera, most likely due to a small dike intrusion at~5-8 km (Davidge et al, 2017;Tepp et al, 2014; Figure 1). Sierra Negra is among the best studied of roughly 20 island-forming oceanic shield volcanoes that comprise the Galápagos archipelago~900 km off the western coast of Ecuador (e.g., Amelung et al, 2000;Chadwick Jr et al, 2006;Geist et al, 2008;Jónsson, 2009;Padrón et al, 2012;Reynolds et al, 1995;Tepp et al, 2014;Villagómez et al, 2011). Spectral magnitudes for frequencies >0.16 Hz are highest when VT earthquakes were previously detected (4-10 June), and are particularly high during the first major swarm of earthquakes on 4 June.…”