A paleomagnetic study of the Lower Cretaceous Cupido Limestone, northeast Mexico: Evidence for local rotation within the Sierra Madre Oriental

We collected 401 oriented samples from the Lower Cretaceous Cupido Limestone in northeast Mexico at an average stratigraphic spacing of 2 m. Analysis of the remanent magnetization reveals an unstable component that is removed upon heating to 300 °C and a component that remains stable to 550 °C. Thermal demagnetization and isothermal remanent magnetization (IRM) studies indicate that the carrier of this stable remanence is magnetite. The magnetite is believed to be detrital and therefore responsible for a remanence associated with the time of deposition.

With the exception of a small number of spurious samples, all samples are normally magnetized. This is in contrast to studies elsewhere in which as many as three reversed intervals have been identified during the time of Cupido deposition.

The pole position obtained, 31.0°N, 169.8°W (α₉₅ = 3.1°, k = 6.1, N = 401), lies some 35° to 40° south of other post-Jurassic Mexico pole positions, suggesting that the sampling site has rotated counterclockwise by a similar amount relative to stable Mexico. The sampling site is located in a northeast-trending portion of the Sierra Madre Oriental that offsets the main southeast-trending ranges by 50 km. The northeast trend is interpreted as the result of a “wrapping” of the folds around the southeast corner of the old Coahuila Peninsula, which acted as a barrier to Laramide décollement deformation. The counterclockwise rotation suggested by the paleomagnetic results therefore is thought to be due to local rotation of the folds and not related to major plate motions between North America and Mexico.