MECHANISMS OF THE FAULTS ASSOCIATED WITH THE LARAMIDE FOLDS IN THE MESA CENTRAL, MEXICO

Six types of faults are observed in the Laramide deformed rocks in San Luis Potosí province of the Mesa Central, México. A common type is low angle reverse faults. They are encountered everywhere in the study region. High angle layer-parallel reverse faults occurred in the high angle normal limbs of fold. We interpret that they formed in the early period of folding and became steeper over time. There are two types of low angle normal faults. Some low angle layer-parallel normal faults are observed in the inverted limbs of folds. They were reverse faults when the stratigraphic sequence was normal. Other low angle normal faults are not parallel to the beddings and should be the results of reactivation of preexisting fractures after folding. Generally, the high angle normal faults are not parallel to the beddings. They are explained as the new-formed faults under extensional regime after the formation of folds. Finally, the observed oblique layer-parallel faults are consistent with the plunging of fold hinges. That is to say, the slickenlines on the faults are perpendicular to the fold hinges. These results indicate that different types of faults could be formed in continuous folding. Some faults such as high angle reverse faults and low angle normal faults cannot be explained by Anderson´s theory.