Paleomagnetism of the Todos Santos and La Silla Formations, Chiapas: Implications for the opening of the Gulf of Mexico

摘要

We report paleomagnetic data for the Lower to Middle Jurassic La Silla and Todos Santos formations of southern Mexico, in west-central Chiapas and the Tehuantepec Isthmus region. Volcanic rocks and red beds of these formations were deposited prior to or during the early stages of Gulf of Mexico opening. Dual-polarity characteristic magnetizations reside primarily in hematite and pass intraformational conglomerate, regional tilt, and reversal tests; they are thus interpreted as primary magnetizations. Our sampling sites are concentrated in three localities; around La Angostura Lake, 17 accepted sites yield a tilt corrected mean of declination (Dec) = 325?°, inclination (Inc) = 4.6?° (k = 11.9, ?±95 = 10.8?°); in the Mat?-as Romero region, the mean is Dec = 312.9?°, Inc = 3.2?° (based on only seven sites); and in the Custepec area, Jurassic andesitic dikes intruding rocks of the Permian Chiapas Massif yield a corrected mean of Dec = 335.0?°, Inc = 5.0?° (six sites). The mean directions are discordant with respect to expected North America reference directions, and indicate a counterclockwise rotation of 35?° to 40?°. Inclinations indicate deposition or emplacement at near equatorial paleolatitudes (2.1?°N ?± 3.4?°). This paleolatitude estimate is statistically indistinguishable from those previously observed in the La Boca Formation of the Huizachal Group in northeast Mexico. The localities we sampled in southern Mexico are separated by a??150 km, suggesting that the paleomagnetic signature of these rocks reflects regional-scale rather than local deformation. These Jurassic paleomagnetic directions support a rotational origin for the Gulf of Mexico. The data are also consistent with an Early to Middle Jurassic reconstruction that places the Chiapas Massif offshore the Tamaulipas state in the western Gulf of Mexico, and an Euler rotation pole for the Maya Block for this time period in the eastern gulf. The apparent polar wander path defined by paleomagnetic poles for the Chiapas Massif and Jurassic rocks reported here suggests that relative motion between North America and the Maya Block occurred between Late Permian and Early Jurassic time, during a protracted rifting phase, and then in the Late Jurassic in association with seafloor formation in the Gulf.