Seismic structure of the Carnegie ridge and the nature of the Galápagos hotspot

Abstract

S U M M A R YThe Galápagos volcanic province (GVP) includes several aseismic ridges resulting from the interaction between the Galápagos hotspot (GHS) and the Cocos-Nazca spreading centre (CNSC). The most prominent are the Cocos, Carnegie and Malpelo ridges. In this work, we investigate the seismic structure of the Carnegie ridge along two profiles acquired during the South American Lithospheric Transects Across Volcanic Ridges (SALIERI) 2001 experiment. Maximum crustal thickness is ∼19 km in the central Carnegie profile, located at ∼85 • W over a 19-20 Myr old oceanic crust, and only ∼13 km in the eastern Carnegie profile, located at ∼82 • W over a 11-12 Myr old oceanic crust. The crustal velocity models are subsequently compared with those obtained in a previous work along three other profiles over the Cocos and Malpelo ridges, two of which are located at the conjugate positions of the Carnegie ones. Oceanic layer 2 thickness is quite uniform along the five profiles regardless of the total crustal thickness variations, hence crustal thickening is mainly accommodated by layer 3. Lower crustal velocities are systematically lower where the crust is thicker, thus contrary to what would be expected from melting of a hotter than normal mantle. The velocity-derived crustal density models account for the gravity and depth anomalies considering uniform and normal mantle densities (3300 kg m −3 ), which confirms that velocity models are consistent with gravity and topography data, and indicates that the ridges are isostatically compensated at the base of the crust. Finally, a two-dimensional (2-D) steady-state mantle melting model is developed and used to illustrate that the crust of the ridges does not seem to be the product of anomalous mantle temperatures, even if hydrous melting coupled with vigorous subsolidus upwelling is considered in the model. In contrast, we show that upwelling of a normal temperature but fertile mantle source that may result from recycling of oceanic crust prior to melting, accounts more easily for the estimated seismic structure as well as for isotopic, trace element and major element patterns of the GVP basalts.

Geophysical characterization of mantle melting anomalies: A crustal view

“…The degree of dependence of the solution obtained on the initial model can be assessed by conducting a number of inversions with a variety of randomly generated initial models and noisy data sets (e.g. Sallarès et al, 2005). If all models have the same probability and the initial models cover the full region of non-null probability within the parameter space, the a posteriori covariance of the solutions obtained can be interpreted as a statistical measure of the solution uncertainty (Tarantola, 1987).…”

“…This is the case for the models showing low-velocity lower crust in the Greenland margin (Korenaga et al, 2000) and the GVP (Sallarès et al, 2005). In these cases, alternatives or modifications to the thermal anomaly model need to be considered in explaining the geophysical observations.…”

“…There, the crustal density distribution of the adjacent oceanic basins is different from that of the thickened ridge segments, as has been shown in the case of the GVP (Sallarès et al, 2005). When possible, it is therefore preferable to include reliable information on the lateral crustal density variations by jointly inverting, or combining, seismic and gravity data (e.g.…”

“…Deep damp melting has been proposed to be the main source of magmatism, for instance, in GVP (Cushman et al, 2004) and Iceland (Maclennan et al, 2001). Sallarès et al (2005) modified the quantitative approach of Korenaga et al (2002) to simulate mantle melting for a 2-D, steady-state, triangular melting regime resulting from mantle corner flow (e.g. Plank and Langmuir 1992).…”

“…Galápagos (Sallarès et al, 2005). What seems evident currently is that causes of hotspot magmatism are probably manifold.…”