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 i n the southeastern Mexico Basin (Sierra Nevada and Sierra de Rio Frio), the Altiplano area, and the Palma Sola Massif. A total of 430 samples have bee n selectively demagnetized using mostly alternating field demagnetizing met hods, supplemented by thermal analyses. Most characteristic remanences are carried by low-Ti titanomagnetites, with occasional titanohematites or slig htly maghemitized low-Ti titanomagnetites, of similar direction. Seven site s were discarded because they presented intermediate directions, hydrotherm al alteration or were remagnetized by lightning strikes. The mean direction s of 32 sites, together with 24 sites from Sierra de las Cruces in the west ern Mexico basin, indicate rocks older than 2 Ma are rotated some 10 degree s counterclockwise with respect to Quaternary rocks, whereas there is no ro tational difference between Miocene and Pliocene rocks. Statistical analyse s between different regrouped populations confirm that the rotational patte rn is due to the age of the volcanics rocks but not to their spatial distri bution. 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 degrees westwards with respect to t he reference direction from North America. No significant rotations have be en observed in the eastern TMVB (from the western Mexico Basin to the borde r of the Altiplano), between late Miocene and late Pliocene times. It sugge sts that a very small, counterclockwise vertical-axis rotation may have bee n taken place in this segment of the TMVB between late Pliocene and Quatern ary times. Comparisons of these results with a summary of the available pal aeomagnetic data in the area indicate that the previously reported Quaterna ry rotations are of questionable reliability, and that the large counterclo ckwise rotations, reported in Cretaceous to Miocene rocks, probably took pl ace 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. (C) 2000 Elsevier Science B.V . All rights reserved.