Combined Paleomagnetic and Petromagnetic Study of the Upper Cretaceous Volcanic Sequence in Western Mexico: Implications for Tectonics and Magnetostratigraphy of the Jalisco Block

Abstract

Results of detailed paleomagnetic studies of the Upper Cretaceous Autlan volcanic sequence from the Sierra Cacoma area (Jalisco block, western Mexico) are reported. The 67.4 ± 1.2 Ma whole-rock K-Ar date for these lavas and the magnetic polarity stratigraphy indicate that flows were emplaced after the Cretaceous Normal Superchron and just prior to the Cretaceous-Tertiary boundary. Fifteen sites (~150 oriented samples) were collected along two vertical profiles. Rock-magnetic experiments permit identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements versus temperature in most cases yield reasonably reversible curves with Curie points close to magnetite. Judging from the ratios of hysteresis parameters, all samples fall in the pseudo-single domain grain-size region, probably indicating a mixture of multi-domain and a significant amount of single-domain grains. Reliable paleomagnetic directions were obtained for 14 sites, corresponding to 2 normal and 12 reverse polarity sites. Lowermost flows in the sequence yield reverse polarity magnetization, and probably formed during Chron 31r of the reference geomagnetic polarity time scale. The remaining flows, including those belonging to radiometrically dated sites, correspond to Chron 30n. These tentative magnetic correlations suggest that the entire volcanic sequence was emplaced during a time span of about 2 Ma. The mean paleo-direction obtained from 14 sites is Inc = 44.2°, Dec = 320.6°, k = 45, a95 = 6.0°. The mean inclination is in reasonably good agreement with the expected value for 65 and 70 Ma, as derived from reference poles for the North American craton. Declination is significantly different from expected (D = 345.8° for 65 Ma and D = 346.2° for 70 Ma), which suggests a counterclockwise tectonic rotation of about 25° for this region within the Jalisco block.