KINEMATIC MODEL FOR BASEMENT-INVOLVED BACKTHRUSTING: EXAMPLES OF THE MALARGÜE FOLD AND THRUST BELT, CORDILLERA PRINCIPAL, MENDOZA, ARGENTINA

A kinematic model is developed in order to explain basement-involved backthrusts systems associated to uplifted basement blocks. In several places of the Malargüe fold and thrust belt there are some structures that show features, as tilted long limbs of folds and widely spaced backthrusts, that cannot be explained by classical models. In our model, the backthrusts and their hangingwall blocks are rotated due to shearing in the underlying basement rocks; this shearing take place when the displacement of the uplifted block towards the foreland is obstructed. In order to implement the model we must take into account two situations. The small basement blocks above the backthrusts can rotate rigidly or it can be slightly strained, and in each case the backthrusts rotation angle will be different. For the case of strained blocks we derive an equation that calculates the backthrust initial dip and so its rotation. Once these parameters are obtained it is possible to calculate the necessary angular shear for such rotation provided the dips of both foreland-directed thrust (or main thrust) and backthrust are known. We consider three scenarios with different geometry and geological implications to analyze this angular shear: a) shearing acting at the upper level of the main-thrust's hangingwall; b) shearing of the whole hangingwall with loss of material; and c) the whole hangingwall is sheared without volume loss. The latter requires the smallest angular shear and so it could be the most suitable to explain this basement-involved structures developed in shallower crustal levels.