Crustal thickness along the western Galápagos Spreading Center and the compensation of the Galápagos hotspot swell

Abstract

Wide-angle refraction and multichannel reflection seismic data show that oceanic crust along the Gala ¤pagos Spreading Center (GSC) between 97 ‡W and 91 ‡25PW thickens by 2.3 km as the Gala ¤pagos plume is approached from the west. This crustal thickening can account for V52% of the 700 m amplitude of the Gala ¤pagos swell. After correcting for changes in crustal thickness, the residual mantle Bouguer gravity anomaly associated with the Gala ¤pagos swell shows a minimum of 325 mGal near 92 ‡15PW, the area where the GSC is intersected by the WolfD arwin volcanic lineament (WDL). The remaining depth and gravity anomalies indicate an eastward reduction of mantle density, estimated to be most prominent above a compensation depth of 50^100 km. Melting calculations assuming adiabatic, passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of V30 ‡C. The associated thermal expansion and increase in melt depletion reduce mantle densities, but to a degree that is insufficient to explain the geophysical observations. The largest density anomalies appear at the intersection of the GSC and the WDL. Our results therefore require the existence of compositionally buoyant mantle beneath the GSC near the Gala ¤pagos plume. Possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the Gala ¤pagos Islands that is later transported to the GSC. Our estimate for the buoyancy flux of the Gala ¤pagos plume (700 kg s 31 ) is lower than previous estimates, while the total crustal production rate of the Gala ¤pagos plume (5.5 m 3 s 31 ) is comparable to that of the Icelandic and Hawaiian plumes. ß

Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observations

“…From the available seismic data, there is no evidence for locally thinner crust at second-order offsets along the GSC ( Fig. 7e; Canales et al 2002). Rather, crustal thickness increases gradually toward the hotspot, consistent with inferences from gravity data.…”

“…7h) and axial MBA values gradually decrease by .60 mGals ( Fig. 7f; Canales et al 2002) suggesting thicker crust and/or warmer temperatures toward the Galapagos hotspot. These regional gradients are interrupted at some second-order discontinuities (95830 ′ W and 96830 ′ W).…”

“…Gomez & Briais 2000); how discontinuities evolve and grow, in particular the relationship between second-and third-order offsets at fast and intermediate ridges (e.g. Carbotte & Macdonald 1992;Cormier et al 1996;Canales et al 2002); and what drives the migration of these discontinuities (e.g. Macdonald et al 1984Schouten et al 1987;Tucholke & Schouten 1988;Lonsdale 1989Lonsdale , 1994Cormier 1997).…”

Thin crust as evidence for depleted mantle supporting the Marion Rise

“…Notes: a) Marion -Zhang et al 14 ; b) Iceland& Azores -Wang et al 5 ; Galapagos -Canales et al 6 ; Marion -Zhang et al 14 . c) Maximum and minimum elevations are measured from shallowest point on axis, or midpoint of Iceland plateau to the rift valley axis point where average depth stabilizes or blocking transform.…”

Patterns in Galápagos Magmatism Arising from the Upper Mantle Dynamics of Plume‐Ridge Interaction

“…One enigma, for example, has arisen from geophysical evidence for a small crustal thickness anomaly (2-3 km), combined with evidence from lava petrology that, together, lead to estimates of small excess temperatures of only 20-30 °C beneath the Western Galápagos Spreading Center (GSC) [Canales et al, 2002;Cushman et al, 2004]. These estimates are low compared to those of the excess temperature of the Galápagos plume itself (30-150 °C [Villagomez et al, 2007] 130±60 °C [Hooft et al, 2003]) or other mantle plumes (100-200 °C) [Wolfe et al, 1997;Allen et al, 1999;Wolfe et al, 2009;Putirka, 2012].…”