Exhumed Mantle in Magma-Poor Margins: Petrology, Geodynamics and Thermal Evolution
Othmar Müntener1 and Luc L. Lavier2
1Institute of Mineralogy and Geochemistry, University of Lausanne, Lausanne, Switzerland
2Jackson School of Geosciences, University of Texas, Austin, TX
Mantle peridotites from magma-poor passive margins illustrate the distribution of the scale of upper mantle heterogeneity in extensional systems. Besides evidence for exposed lower continental crust most of the basement in OCT's is serpentinized mantle peridotite. Seeing through serpentinization to unravel the relevant mantle processes is critical to understand the thermal, petrological and rheological history of the OCT.
We summarize research on mantle processes of the conjugate Iberia-Newfoundland rift and from the Alps that show that the basement of OCT’s consists of 3 mantle domains. Thermally undisturbed, ‘cold’ subcontinental mantle formed the ocean floor next to thinned continental crust. This ‘subcontinental domain’ is separated by ductile shear zones from an ‘infiltrated (e.g. ‘hot’) domain’ dominated by plagioclase peridotite. The footwall of these mantle shear zones display complex refertilization processes and high-temperature deformation (‘thermochemical erosion’ or ‘asthenospherization’). They are juxtaposed with depleted lherzolites and dunites that reflect local zones of MORB type melt extraction (‘extraction domain’). Upwelling of partial melts through the conductive lithospheric mantle inevitably leads to freezing of the melt and to the formation of a chemical and rheological barrier. We thus propose that an actively deforming refertilization front in Alpine plagioclase peridotites and in the Iberia Newfoundland rift moved ahead of a melting front. Melt lubricated shear zones focus melt flow. Continuous uplift lead to crystallization and ultimately to the exposure of infiltrated peridotites on the seafloor. We combine petrologic data and numerical models to illustrate that these processes play a key role in the rejuvenation and erosion of the lithospheric mantle and ultimately, on the density and thermal structure beneath embryonic ocean basins.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery