SOFTENING OF THE SUBCONTINENTAL LITHOSPHERIC MANTLE BY ASTHENOSPHERE MELTS AND THE CONTINENTAL EXTENSION/ OCEANIC SPREADING TRANSITION

Ophiolitic peridotites exposed along the Western Alps and the Northern Apennines represent the oceanic lithosphere of the Ligurian Tethys, which separated the European and African (Adria) plates during Late Jurassic (160-150 Ma). Petrological and geochemical evidence shows that in many instances the peridotites were originally subcontinental mantle which was exhumed from spinel-facies depths to shallow levels, intruded by MORB-type gabbroic rocks, and directly covered by MORB basaltic flows and Upper Jurassic oceanic sediments. Oceanic spreading was preceded by continental extension of the European-Adria plate, leading to progressive thinning of the continental lithosphere and exhumation of the subcontinental mantle. This was accompanied by nearadiabatic upwelling of the underlying asthenosphere which underwent decompressional partial melting. Resulting fractional melts migrated through and reacted with the overlying lithospheric mantle. The Alpine-Apennine ophiolitic peridotites record an early composite history of melt percolation, melt/peridotite interaction, and melt impregnation. The interstitial crystallization of the migrating melts produced significant refertilization of the lithospheric mantle, i.e. addition of basaltic components in the form of gabbroic microgranular aggregates. After this initial stage, melt fractions were more efficiently mixed and aggregated to form MORB-like magmas, which intruded as gabbroic dykes and eventually extruded as variably evolved MORB-type lava flows. An important effect of the upward migration of asthenospheric melts was a significant heating of the extending subcontinental lithosphere. Geothermometric estimates show that during melt percolation temperatures were increased from T = 900-1100 oC (temperatures of residence in the lithospheric mantle) to T ³ 1250 oC. Therefore, the thermochemical erosion of the lithospheric mantle was a fundamental step in the evolution of the Ligurian-Piedmontese Basin. The temperature increase resulted in a softening of the lithosphere. 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. We examine the thermal, rheological, and geodynamical consequences of the temperature increase caused by impregnation of the extending continental lithosphere by means of simple quantitative models. Assuming that percolation and impregnation in the initial stages occurred by diffuse porous flow, we show that approximate analytical solutions of the porous flow and temperature equations suggest that impregnation was a relatively fast process (of the order of a few Ma), and that the temperature of the impregnated volume reached approximately asthenospheric values (as shown also by thermometric estimates). We then model the time-dependent thermal evolution of a thinned and impregnated continental lithosphere by solving numerically the transient heat transfer equation. The volumetri extent of the impregnation is parameterized as the ratio between the thickness of the lithospheric mantle and the thickness of the impregnated zone. The pre-impregnation lithospheric geotherm is estimated as a function of thinning, and it is assumed that a volume of variable thickness (measured from the lithosphere/asthenosphere boundary) is instantaneously brought at near-asthenospheric temperature and then begins cooling (this approach was chosen on the basis of the previous inference that impregnation is a geologigally short process).The subsequent thermal evolution of the lithosphere is examined, both as a function of time and of the thickness of the impregnated zone. The rheological consequences of the thermal evolution are assessed by estimating rheological profiles (strength envelopes) for various values of the thickness of the impregnated zone. The structure of the lithosphere is taken as consisting of a two-layer crust and a lithospheric mantle. The upper crust is represented by wet quartz rheology, the mantle by dry peridotite. Two cases are considered for the lower crust: felsic granulite and mafic granulite. The resulting total strength of the lithosphere is estimated by integration of the rheological profiles. The decrease in total lithospheric strength following impregnation is a strong non-linear function of time and of the thickness of the impregnated volume. As an example, the time evolution is presented for the case in which the lithosphere is thinned to half its original thickness, and the impregnation affects the whole thickness of the lithospheric mantle. The initial strength (between 3.5x1012 and 4.5x1012 N m-1, according to the composition of the lower crust) is reduced by approximately one order of magnitude within 2-3 Ma of the time of the impregnation, and recovers asymptotically its initial value within 20-30 Ma. An examination of the dependence of total strength on thickness of impregnated volume at a given time (chosen in the present example as 0.25 Ma) shows that the consequences of impregnation are negligible (reduction in strength < 20%) if the impregnation affects only half of (or less than) the thickness of lithospheric mantle. However, the reduction in strength becomes very significant as more of the lithospheric mantle is impregnated, reaching ~ 80% if the impregnation affects the whole lithospheric mantle up to the Moho. These values naturally depend on boundary and initial conditions, and on the assumed rheology. Nevertheless, the conclusion that rapid impregnation – if it affects most of the lithospheric mantle – results in decreases of total lithosphere strength of up to one order of magnitude within a few Ma of the upward percolation of asthenospheric melts is reasonably model-independent. Moreover, we have estimated only the bulk softening caused by a temperature increase. The presence of a diffuse melt phase is likely to have other rheological consequences (e.g., intrinsic decrease in the creep strength of the aggregate, recrystallization, concentration of deformation) which would further weaken the lithospheric mantle. The transition from continental extension to oceanic spreading in the Mesozoic Ligurian Tethys was certainly favoured, and possibly controlled, by the thermomechanical erosion of the lithosphere caused by the upward percolation of asthenospheric melts and the consequent significant transient softening that led to total lithosphere failure. As a working hypothesis, we propose that this thermomechanical erosion (or the one, dynamically different but rheologically similar, cause by the arrival of a plume at the lower boundary of the lithosphere) may be the controlling factor leading to the continental extension/oceanic spreading transition.

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