Lava emplacement at the East-Pacific Rise, 16°N
Abstract
The PARISUB cruise was led in 2010 using the R/V L'Atalante, the Autonomous Underwater Vehicle AsterX and the manned submersible Nautile (Ifremer). The goal is to investigate the processes that occur during the interaction between the Mathematicians hotspot and the East-Pacific Rise at 16°N. The present spreading axis has an elevation of at least 400m above the average depth of the North Pacific ridge, indicating a high magmatic production. Lava previously sampled in this area are enriched in incompatible elements and isotopically, revealing so the contribution of the plume. Multibeam bathymetry, gravity, magnetism and near-bottom data (high-resolution bathymetry, gravity, magnetism) are used to measure tectonic structures, to individualize volcanic flows in relation with axial density variations and magnetic micro-anomalies. We present the results of the first high-resolution mapping of the ridge crest using AUV. The structure of the axial summit graben strongly varies along-axis. It consists either in one unique, well-depicted narrow graben, or in other places in two narrow and parallel grabens. In other locations (at 15°46'N for ex.), the axial graben is much wider and the tectonic deformation is widely distributed. Changes in axial graben morphology coincides with slight changes (<5°) in orientation and variations of the depth of the underlying magma chamber. Most of the flows originate at the axial summit graben. Most often lava has drained fully or partially such that the point sources of the flow can be localized. Despite an expected high effusion rate, lobate flows predominate over sheet flows. Pillow flows are also well-represented. All these morphological characteristics as well as the estimated volumes of volcanic flows are here combined to reconstitute the volcano-tectonic history of the spreading segment. We identify remarkable smooth lava flows areas which surfaces range 0.2 to at least 1.5 km2, with a thickness of few meters to 17 meters. These flows are primary composed of jumbled flows with occurrences of sheet flows, and minor occurrence of pillows on flow boundaries. Levees are observed at flow boundaries, which are characterized by lobate shape of the edges. Flows display a flat or slightly depressed surface, likely due the lava drained off from underneath when continuing to propagate. These flows emplaced on sub-horizontal floor such that existing theoretical models for lava emplacement are not applicable since they require significant slopes. Here we propose a theoretical model of lava flow emplacement on sub-horizontal basement. The objective is to propose a coherent set of dynamic equations which relate geometrical, mechanical and rheological parameters of lava flows from their emission to their arrest on sea floor. This model is similar to the emplacement of a simple 2D lava dome in which magma is first emitted from a dike. Lava flows run according to its viscosity and to the flux of lava. It is considered as a Bingham material characterized by a Newtonian viscosity and a stress threshold under which no strain is observed. Our model shows that lava emplace within few hours to a couple of days, as it is often observed onland for lava that display comparable morphologies.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V53D2648D
- Keywords:
-
- 3035 MARINE GEOLOGY AND GEOPHYSICS / Midocean ridge processes;
- 3037 MARINE GEOLOGY AND GEOPHYSICS / Oceanic hotspots and intraplate volcanism;
- 5475 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Tectonics;
- 8178 TECTONOPHYSICS / Tectonics and magmatism