Volcanic passive margins

• Published in: Comptes rendus. Geoscience Vol. 337; no. 16; pp. 1395 - 1408
• Main Author: Geoffroy, Laurent
• Format: Journal Article
• Language: English; French
• Published: Paris Elsevier 01.12.2005
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Marine; wavelength; flexure; models; extension; passive continental margins; Volcanic passive margins; lithosphere; normal faults; melting; volcanic features; crust; rupture; convection; accretion; Dykes; depth; propagation; deformation; mantle; segmentation; anticlines; tectonics; Small-scale mantle convection
• ISSN: 1778-7025; 1631-0713; 1778-7025
• DOI: 10.1016/j.crte.2005.10.006

Compared to non-volcanic ones, volcanic passive margins mark continental break-up over a hotter mantle, probably subject to small-scale convection. They present distinctive genetic and structural features. High-rate extension of the lithosphere is associated with catastrophic mantle melting responsible for the accretion of a thick igneous crust. Distinctive structural features of volcanic margins are syn-magmatic and continentward-dipping crustal faults accommodating the seaward flexure of the igneous crust. Volcanic margins present along-axis a magmatic and tectonic segmentation with wavelength similar to adjacent slow-spreading ridges. Their 3D organisation suggests a connection between loci of mantle melting at depths and zones of strain concentration within the lithosphere. Break-up would start and propagate from localized thermally-softened lithospheric zones. These soft points could be localized over small-scale convection cells found at the bottom of the lithosphere, where adiabatic mantle melting would specifically occur. The particular structure of the brittle crust at volcanic passive margins could be interpreted by active and sudden oceanward flow of both the unstable hot mantle and the ductile part of the lithosphere during the break-up stage.Original Abstract: Les marges volcaniques marquent une rupture lithospherique au-dessus d'un manteau anormalement chaud, probablement convectif a petite echelle. Elles presentent des traits evolutifs et structuraux distincts des marges non volcaniques. La deformation en extension, tres rapide, s'accompagne d'une fusion catastrophique du manteau et de l'accretion d'une croute magmatique epaisse. Les failles qui accommodent l'extension sont syn-magmatiques et a pendage vers le continent. Elles sont associees au developpement d'anticlinaux en roll-over, d'echelle crustale. Les marges volcaniques presentent une segmentation axiale, de longueur d'onde analogue a celle des dorsales lentes. L'organisation tridimensionnelle de ces marges suggere une relation entre zones de fusion mantellique en profondeur et sites de concentration de l'extension lithospherique. La rupture continentale s'initierait et se propagerait a partir de colonnes lithospheriques amollies thermiquement. Ces < points mous rheologiques > seraient situes au droit des points hauts de cellules de convection a petite echelle, ou se produirait specifiquement la fusion mantellique par decompression adiabatique. Lors de la rupture complete de la lithosphere, un fluage tres rapide vers la zone de rupture du couple asthenosphere/lithosphere ductile pourrait expliquer la structure originale de la croute fragile des marges volcaniques.