Vicente Carlos Ruiz-Martínez

Palaeomagnetism of Late Miocene to Quaternary volcanics from the eastern segment of the Trans-Mexican Volcanic Belt

A systematic palaeomagnetic study in the eastern part of the Trans-Mexican Volcanic Belt includes 39 Miocene, Pliocene and Quaternary volcanic rocks in the southeastern Mexico Basin (Sierra Nevada and Sierra de Ri´o Fri´o), the Altiplano area, and the Palma Sola Massif. A total of 430 samples have been selectively demagnetized using mostly alternating field demagnetizing methods, supplemented by thermal analyses. Most characteristic remanences are carried by low-Ti titanomagnetites, with occasional titanohematites or slightly maghemitized low-Ti titanomagnetites, of similar direction. Seven sites were discarded because they presented intermediate directions, hydrothermal alteration or were remagnetized by lightning strikes. The mean directions of 32 sites, together with 24 sites from Sierra de las Cruces in the western Mexico basin, indicate rocks older than 2 Ma are rotated some 10° counterclockwise with respect to Quaternary rocks, whereas there is no rotational difference between Miocene and Pliocene rocks. Statistical analyses between different regrouped populations confirm that the rotational pattern is due to the age of the volcanics rocks but not to their spatial distribution. The Quaternary mean direction from the three Mexico Basin ranges is consistent with the geographical reference pole. In contrast, the Pliocene mean direction from volcanic rocks of the Altiplano area and the Sierra de Las Cruces is slightly rotated some 10° westwards with respect to the reference direction from North America. No significant rotations have been observed in the eastern TMVB (from the western Mexico Basin to the border of the Altiplano), between late Miocene and late Pliocene times. It suggests that a very small, counterclockwise vertical-axis rotation may have been taken place in this segment of the TMVB between late Pliocene and Quaternary times. Comparisons of these results with a summary of the available palaeomagnetic data in the area indicate that the previously reported Quaternary rotations are of questionable reliability, and that the large counterclockwise rotations, reported in Cretaceous to Miocene rocks, probably took place before the late Miocene. These new palaeomagnetic data support the idea that the eastern TMVB since the late Miocene, has been a zone of extension with a little, left-lateral shear component.

Palaeomagnetic and AMS study of the Tarfaya coastal basin, Morocco: an early Turonian palaeopole for the African plate

Abstract An early Turonian (c. 93 Ma) anoxic, cyclic marine deposition is registered in the unfolded outcrops from the Tarfaya coastal basin, where very high sedimentation rates enable the investigation of past geomagnetic field record at high temporal resolution. One hundred and fourteen samples have been sampled along a 10.5 m vertical profile (c. 200–500 ka) of orbital-scale forced sedimentation. Rock magnetic investigations reveal mineralogy principally controlled by diamagnetic and paramagnetic behaviour, along with very low concentrations of low-coercivity ferromagnetic material which is probably magnetite. A well-defined magnetic fabric can be seen with the minimum susceptibility axis perpendicular to the foliation plane, and magnetic lineation compatible with NW African palaeostress since sedimentation times and/or the palaeocurrent associated with upwelling system deposition. Magnetic signature has the potential for performing reliability checks of reversed tiny wiggles, which were found in four samples not considered for the tectonic analysis. Alternating field demagnetization shows a single, stable, low-coercivity directional component. The new palaeopole (N=88; PLat=64.3°, PLon=256.3°, A95=2.5°; K=38.7), obtained after moderate (f=0.8) inclination flattening correction, is the first early Turonian palaeopole for the NW African Craton. It can contribute to the 90 Ma-centred sliding window of the different proposed synthetic Apparent Polar Wander Paths.

A detailed paleomagnetic and rock-magnetic investigation around Cretaceous-Paleogene boundary: the Autlan (Western Mexico) volcanic sequence revisited

We present a detailed rock-magnetic and paleomagnetic survey from Autlan volcanic succession in western Mexico. The principal aim of this study is to extend paleomagnetic data from Autlan lavas in order to confirm vertical-axis rotation observed in reconnaissance study and to evaluate long-term variation of the geomagnetic field strength based on existing and global data. The mean inclination (44.7°) is in agreement with the expected inclination for 60 and 70 Ma, as derived from available reference poles for the North American craton. The declination (333.6°), however, is significantly different from those expected, which suggests a statistically significant counterclockwise tectonic rotation ranging between 10° ± 6° and 14° ± 7°. As a measure of paleosecular variation (PSV), we obtained a geomagnetic field dispersion of 9.6° (upper and lower limits: 7.2°–11.9°) in perfect agreement with the previously published PSV compilation of selected Cretaceous data from lavas. The mean virtual dipole moments available for Autlan lavas are about 65% of the present geomagnetic axial dipole but are in reasonably good agreement with other comparable quality determinations between 5 and 90 Ma. This reinforces the hypothesis that low geomagnetic field strengths persisted for the entire Jurassic extending into the Upper Cretaceous.