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Heat Flow, Subsidence and Crustal Structure of the Gulf of Lions (NW Mediterranean): A Quantitative Discussion of the Classic Passive Margin Model
A variety of geophysical data is used to demonstrate that classical passive margin models, based on lithospheric attenuation, inadequately account for the evolution of the Provencal Basin, an Oligo-Miocene rifted margin. Two distinct problems are addressed: 1) The origin of thermal asymmetry, which is indicated by the increase in heat flow from the Gulf of Lions (50-70 mW/m2) to the Sardinian margin (70-90 mW/m2); 2) The history of subsidence of the western margin, documented in detail through well data and seismics.
These issues are discussed with reference to predictions obtained from a thermal model that accounts for two types of geodynamic evolution during rifting: the uniform extension model (UEM), in which crust and mantle are uniformly attenuated during rifting, and the depth-dependent model (DDM), in which the mantle is more attenuated than the crust.
We show that the UEM adjusted to fit the observed crustal thickness accounts reasonably well for the moderate heat flow in the Gulf of Lions, and for the tectonic subsidence at the end of the rifting. Any supplementary heat input during rifting (as results from the DDM) is not justified by the data. In contrast, the DDM more adequately accounts for the higher heat flow observed to the east.
Significant differences between observations and predictions are observed however: 1) the tectonic subsidence observed today in the lower part of the margin of the Gulf of Lions is 1000 m higher than predicted by the UEM; 2) the tectonic subsidence of the Sardinian margin should be 400-500 m lower than in the Gulf of Lions, when the DDM is adjusted to fit the higher heat flow. Although only incomplete data are available in the Sardinian margin, this difference of tectonic subsidence is probably not observed. Consequently, the thermal asymmetry is probably not due solely to an asymmetrical rifting, but could be related to renewed tectonism to the east; 3) the observed crustal thinning (1.85) and the crustal extension (1.2) are significantly different in the Gulf of Lions. These differences are discussed with reference to the recent north-south compression, to a possible affinity of the Provencal with marginal-type basins, and to alternative mechanisms of rifting.
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