DATING SYNFOLDING REMAGNETIZATION EVENTS IN THE CENTRAL SIERRA MADRE ORIENTAL, MEXICO

By combining paleomagnetism and ⁴⁰Ar/³⁹Ar deformation dating, we studied the remagnetization history of the Tamaulipas Formation in the central Sierra Madre Oriental (Mexican fold-thrust belt). Specifically, we find that the history of the central Sierra Madre Oriental involved two spatially and temporally separate chemical remagnetization events. The Zimapán Basin shows a clustering of paleomagnetic directions to the northwest and down with synfolding remanence acquisition at ~77 Ma (Late Cretaceous). Folds 20/21, 22/23, 24/25, and 28/29 in the Zimapán Basin yielded a maximum clustering declination/inclination of 306.7°/35.4°, 312.1°/55.3°, 340.5°/49.6°, and 302.6°/53.7°, respectively. The Zimapán Basin yielded four folds with a synfolding magnetization (one being very late synfolding). Paleomagnetic directions in the Tampico-Misantla Basin are to the southeast and upward, preserving a younger synfolding remagnetization event that occurred at ~44 Ma (mid-Eocene). Folds 3/4, 9/10, 11/12, 16/17, and 18/19 in the Tampico-Misantla Basin yielded a maximum clustering declination/inclination of 150.4°/-35.8°, 163.7°/-47.7°, 179.3°/-40.3°, 149.0°/-35.4° and 138.8°/-45.5°, respectively. One fold (5/6) in the Tampico-Misantla Basin yielded a declination of 164.3° and a shallow down inclination of 9.3° due to incongruous results from site 6. Three folds in the eastern side of the Tampico-Misantla Basin produced synfolding results, one fold in the western side of the Tampico-Misantla Basin is very late synfolding and the other two produced postfolding magnetizations. The interpreted cause of this chemical remagnetization is the growth of nanoscale magnetite. This mineral growth is the result of interactions of the carbonates with an Fe-bearing fluid that was also responsible for local illite neomineralization. Our study offers a novel and robust method for absolute dating of synfolding remagnetizations.