The ability to date deformation directly within internal zones of mountain belts is very important to the understanding of the overall evolution of orogenic zones. The Rb-Sr isotopic dating of greenschist facies syn kinematically crystallised white micas as presented by Freeman (1995) has wide applicability.
We have now applied this technique to a major orogen forming structure in the Western Alps. The Basal Briançonnais thrust (B.B.T.) is one of the largest structures in the Western Alps, the structure is continuous over 150km through the Western Alps. The structure carries the basement of the internal zones over the relict Valais basin in a top to the West overshear (foreland propagating deformation).
In the area around the Col du Petit St. Bernard, Franco-Italian Alps; the geology consists of highly deformed calc-schists in the footwall of the B.B.T., and a series of basement (sandstones) and cover (marl and quartzites) sheets in the hanging wall. The footwall (Petit St. Bernard unit) is highly foliated, with an increase in the degree of foliation as the B.B.T. is approached. Rocks which contain a fabric related to the B.B.T. in the footwall of the B.B.T. are L-S tectonites. These rocks record a very strong WNW-ESE lineation with a top to the WNW overshear (up foliation). Syn deformationally recrystallised white micas which define the lineation in the B.B.T. shear zone all record Rb-Sr white mica ages of ~31 Ma (31.1±0.5, 30.9±1.9, 32.9±1, 27.1±3.9 Ma). Therefore the ductile deformation which caused crystallisation and recrystallisation of white micas in the B.B.T. shear zone ceased at ~ 31 Ma.
The B.B.T. cuts structures within the hanging wall which relate to a deformational event with different structural
characteristics (Top NNW overshear). The white mica fabric which relates to this deformation has a distinctly different chemical composition from the fabric relating to the B.B.T.. The NNW overshear fabric has a higher celadonite content that implies they formed at higher pressures, or lower temperatures than the B.B.T.. The mica fabrics also record
significant variations in Ti, Ba, and Na, with minor variations in Fe and Mg. Preliminary dating of the hanging wall white mica fabrics record significantly older ages than the deformation on the B.B.T.. A pre-Mesozoic mica population is present in the hanging wall. This implies that the hanging wall has not undergone pervasive deformational recrystallisation or a thermal event in excess of 500-550 °C, which would reset the white micas during the alpine event. Further dating of the hanging wall white mica fabrics is in progress and should provide important information on the formation of deformational fabrics prior to the B.B.T. formation.
For the succesful application of this technique, samples which are dominated by the mica population of interest must be analysed. To discern between the different mica
populations microchemical analysis must occur prior to
isotopic dating. Individual deformation fabrics which can be related to specific shear zones form with characteristic
compositions. If the deformation occurs below the closure temperature when the mineral is dated it will yield the time since crystallisation, e.g. when looking at deformational
fabrics it shows the time since ductile deformation ceased. The B.B.T. displays the same kinematics but with an
opposite sence of shear as the Entrelor thrust described by Freeman (1995) (The Entrelor thrust is a back thrust). The age of the B.B.T. is 31 Ma compared to the Entrelor thrust (in the hinterland of the B.B.T.) which is 34 Ma. This
suggests that deformation was propagating from the hinterland towards the foreland in the internal parts of the orogenic belt. This technique allows for a knowledge of deformational sequences within orogenic wedges to be established.
Freeman, S., Terra Abstracts, Terra Nova 7, 42 (1995).