Seismic Constraints on the Formation of the Galápagos and Iceland Platforms
Abstract
The Galápagos and Iceland hotspots have formed broad shallow platforms on which sit the oceanic islands. The platforms are typically surrounded by a steep escarpment and a flexural moat. There are several competing hypotheses for the formation of the steep escarpments, including faulting, inflation of the platform by lower crustal flow, and volcanic accretion. Recent seafloor surveys along the edge of the Galápagos platform reveal that its escarpment is terraced and sinuous, characteristics that are inconsistent with faulting. The platform is inferred to form by episodic eruption of large lava flows that coalesce to form an escarpment by levee formation. Less is known about the morphology of the edge of the Iceland platform (also known as the insular shelf). It has been proposed that the Iceland platform formed by inflation as a result of lower crustal flow along the ridge away from the plume center. Here we use seismic refraction data to constrain the internal structure and thickness of the edges of the Galápagos and Iceland platforms as well as the process that governs the formation of their escarpments. The Galápagos refraction line is 250 km long and extends from the center of the platform across its leading edge and onto oceanic crust. The data were recorded on four ocean-bottom hydrophones and two broadband three-component land seismometers. The Iceland refraction line is located on the northern Iceland shelf at 66.5°N and extends 700 km east from the Kolbeinsey Ridge to the Aegir Ridge. The westernmost 350 km of the profile crosses the Iceland platform and its escarpment and extends onto the adjacent seafloor of the Iceland Plateau and included 20 ocean-bottom seismometers. Arrival times of crustal refractions, mantle head waves, and Moho reflections indicate a striking similarity in the structure of the Galápagos and Iceland platforms and their edges. At the platform edges crustal thickness increases rapidly (the thickness doubles over a distance of ~50 km), and low P-wave velocities are found throughout the crust (mostly <6.5 km/s). These low velocities are consistent with high porosities, which support the formation of the platform edge by repeated stacking of lava flows. In contrast, beneath the interior platform of both hotspots, the thickness of the lower crust is about two-thirds of the total crustal thickness. Under the center of the platform the ratio of intrusive to extrusive lavas, as inferred from seismic velocities, is similar to that of oceanic crust. We conclude that construction of the platform edges is dominated by volcanic accretion, whereas construction of the interior of the platforms is dominated by intrusive magmatism. This interpretation suggests that if platform volcanoes are the source of terraced lava flows, then individual flows have lateral extents of many tens of kilometers.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V52A..02H
- Keywords:
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- 3037 MARINE GEOLOGY AND GEOPHYSICS / Oceanic hotspots and intraplate volcanism;
- 3075 MARINE GEOLOGY AND GEOPHYSICS / Submarine tectonics and volcanism;
- 7220 SEISMOLOGY / Oceanic crust;
- 8137 TECTONOPHYSICS / Hotspots;
- large igneous provinces;
- and flood basalt volcanism