Análisis e interpretación de fábricas tectónicas en rocas carbonatadas cálcicas.

Resumo

Two basic types of tectonic fabrics in limestone are differentiated, those of calc-tectonites and of non-tectonites. The limestone with both fabric types may display mechanical twinning, but the calctectonites also show crystallographic preferred orientation (CPO1), while non-tectonite limestones predominately show microstructures related to processes of pressure dissolution. Combined analysis of CPO1 and shape preferred orientation (SPO1) in calc-tectonites allows us to deduce the conditions and intensity of deformation. In addition, the relationship between twinning intensity and peak differential stress can be calibrated with asymptotic equations that show that differential stress decreases while the burial depth increases, in accordance with the crustal strength envelope that characterizes the regime of power law creep. The most usual values of peak differential stress within natural calc-tectonites developed at the greenschist facies conditions range between 150 and 300 MPa. The orientation and magnitude of the stress and the ellipsoid of finite strain attributable twinning can be measured by the study of the twinning orientation and its distribution along the fabrics of non-tectonite limestones, but only when the deformation is very small (<3%). The maximum differential stress attained for these rocks in natural conditions typically ranges between 8-300 MPa. The palaeostresses attainable under shallow crustal conditions are limited by the stress required to activate frictional sliding on faults (approximately given by Byerlee’s rule, in which the differential stress for frictional sliding also increases with increasing depth). Here we review the assumptions and the limitations of the different structural petrographic methods used to constrain stresses operative during carbonate rock deformation, and highlight recent progress in techniques of calcite fabric analysis.