Efectos de plegamiento y deformación interna en el registro paleomagnético de secuencias carbonatadas

Abstract

We report an analysis and discussion of the effects of folding and internal deformation on the paleomagnetic record of a Mid-Cretaceous carbonate formation from southem Mexico. Sampling was carried out in stratigraphic order and cover folded structures of varying amplitudes from a few centimeters to hundreds of meters, and two interbedded breccia units. The most abundant biomicrite limestones often show effects of partial to complete dolomitization and have low intensities of NRM. Thermal and altemating field demagnetization show that NRM is mainly univectorial, with only small secondary components. The two conglomerate tests are positive, suggesting that no severe remagnetization affected the carbonate sequence after deposition of the breccia units. The several fold tests suggest on the other hand that for the larger folds the remartence was affected during folding. Directions cross each other during partía! simple unfolding. Remanences from the westem flanks of the anticlines and from the eastem flanks of the sinclines consistently show larger angular dispersions and a mean direction close to the expected Mid-Cretaceous direction for the area. Remanences from the flanks of the smaller ántticlines and sinclines show a more complex behaviour suggesting a more complete remagnetization or stronger strain effects. We interpret the results in terms of the passive markers rotation model and suggest that the assymmetric strain effects are the product of flexural folding with intemal deformation.