Extension of the European-Adria plate resulted in continental break-up and the development of the oceanic lithosphere of the Ligurian Tethys. During continental extension, progressive thinning of the continental lithosphere was accompanied by asthenospheric upwelling which underwent decompressional partial melting; resulting fractional melts impregnated and reacted with substantial portions of the overlying lithospheric mantle (e.g., Muntener and Piccardo, 2003). This generated a significant heating of the extending subcontinental lithosphere, which induced a thermochemical erosion of the lithospheric mantle and a strong softening of the extending continental lithosphere (e.g., Piccardo et al., 2004). This decrease in lithosphere strength could have played an important role in the dynamics of extension and rifting, and favoured the transition from passive lithosphere extension to active oceanic spreading. In this work, the mechanical consequences of the impregnation of an extending continental lithosphere by asthenospheric melts is investigated by means of laboratory experimental models. Analogue experiments were performed in an artificial gravity field by using the large capacity centrifuge at the Tectonic Modelling Laboratory of the CNR-IGG and Earth-Sciences Dept. of Florence University. Similarly to previous works (e.g., Corti et al., 2003), models simulated extension of a three layer lithosphere (brittle and ductile crust, lithospheric mantle) floating above a low-viscosity fluid (representing the asthenosphere). The analogue brittle crust was simulated with K-Feldspar sand, whereas the lower crust and lithospheric mantle were modelled with mixtures of silicone and corundum sand (100:30 and 100:50 % in weight for the crust and the mantle, respectively). The asthenosphere was modelled with a low-viscosity mixture of glycerol and gypsum. In the central part of the models, the lithosphere was weakened in order to reproduce the softening of the subcontinental lithospheric mantle by impregnation of the asthenospheric melts. In particular, the mantle strength was decreased by replacing the relatively strong sand-silicone mixture with the low-viscosity fluid representing the asthenosphere. The size of the impregnated volume was parameterized as the ratio (g) between the thickness of the impregnated zone and the thickness of the lithospheric mantle (g=1/3 means that 1/3 of the mantle lithosphere upward from the lith/asth boundary is impregnated). Weakening of the continental lithosphere in the central part of the model was dependent on the factor g, with maximum reduction of ~70% for g=1 (i.e., the impregnation affects the whole lithospheric mantle up to the Moho). Experimental results display that the style and evolution of deformation are highly influenced by the amount of lithospheric weakening in central part of the models, i.e. by the parameter g. In case of g≤1/2, models are characterized by diffuse deformation, resulting in a regular array of closely-spaced faults and a Basin-and-Range style of extension. At depth, this deformation pattern is associated with no important lithospheric thinning and asthenospheric upwelling. Conversely, when g>1/2 (i.e., almost the whole lithospheric mantle is weakened by asthenospheric melts), strong strain localization is observed within the weakest areas of the model (i.e. above the impregnated mantle). Extension gives rise to the development of normal faults and a central graben structure; the deformation style can be described as narrow rifting with necking of the continental lithosphere. In this case, important thinning of the lithosphere and upwelling of the asthenosphere is observed within the necking area, resulting in an important strength reduction of the lithosphere that -with further extension- may eventually lead to continental rupture and development of a new oceanic basin. These results support that the thermomechanical erosion of the lithosphere caused by the upward percolation of asthenospheric melts and the resulting mantle softening may have represented a controlling factor leading to the continental extension/oceanic spreading transition in the Mesozoic Ligurian Tethys.
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Apr 10, 2024
Hao Zhang + 7
Open Access