Effect of Variations in Magma Supply on the Crustal Structure of Mid-Ocean Ridges: Insights from the Western Galápagos Spreading Center

摘要

We report results from a seismic refraction experiment across three sections of the Western Galápagos Spreading Center (WGSC) with contrasting axial morphology. Tomography models show the presence of an axial low velocity zone at the three study areas. After correcting for thermal effects, we estimate the melt content within these regions. At each of the three sites, the largest melt reservoir is located at or just below the Moho, which is 5.25 km deep at site GALA-1 within an axial-valley morphological domain, 6 km deep at GALA-2 in a morphological transitional domain, and 7.5 km deep at GALA-3 within the axial-high domain. The tomography model does not require melt above the Moho at GALA-1, nor at GALA-2, where we find little evidence for crustal melt between the Moho and a melt lens previously imaged at 2.8 km depth. In contrast, at GALA-3 the low velocity anomaly requires the presence of a few percent melt throughout the crust, with two distinct crustal reservoirs: one at the level of the seismically imaged melt lens reflector (1.6 km deep), and a deeper one at 3–4 km depth. The differences in axial melt content and distribution between the three sites are consequences of variations in magma supply, with lower magma supply resulting in less frequent upward migration of melt from the main Moho reservoir to crustal levels. At higher melt supply, transfer of melt to the crust above the main Moho reservoir becomes more frequent, resulting in the formation of distinct crustal melt reservoirs.