Concurrence of folding and remagnetization events in the Monterrey Salient (NE Mexico)

Carbonate formations from many locations reveal magnetizations that are secondary. Attaching a numerical age to these remagnetizations would provide information on orogenic processes. To contribute to the effort of assigning an age to remagnetization events, Lower Cretaceous formations from the Monterrey Salient in northeast Mexico were sampled. Individual fold tests reveal eight site-pair synfolding remagnetizations (set A) of Eocene age. The remanence is a chemical remanent magnetization (CRM) carried by magnetite. The magnetization of set B is possibly due to an early carbonate alteration that produced a remanence significantly older than folding. A complexity of set B is that the site pairs reveal k-versus-unfolding percentage diagrams with maximum unfolding peaks at 100 to 125%. An explanation of multiple deformation phases with an earlier horizontal folding phase around a non-plunging horizontal axis and a later vertical axis rotation is suggested for the past 100% peaks of the B sites. The latter deformation, rotation about a vertical axes, is revealed by a declination-strike test in set A sites and site means of the full collection that deviate counterclockwise from the reference declination. By combining synfolding results with published 40Ar/39Ar illite ages of folding (Fitz-Diaz et al., 2016), remagnetization ages in set A sites of 48–52Ma are obtained. The magnetization in set B sites is likely older, though not well constrained. Interestingly, prior results from the central Sierra Madre Oriental, to the south, show two remagnetization events, Late Cretaceous and Early Eocene in age, in a succession from W to E (Nemkin et al., 2015). The age of folding and remagnetization acquisition in set A sites along the Monterrey Salient are concurrent with the frontal, remagnetized folds in the central Sierra Madre Oriental. Thus, the timing of major remagnetization in the Monterrey Salient occurred in the Eocene.