Publication: Palaeomagnetism of Late Miocene to Quaternary volcanics
from the eastern segment of the Trans-Mexican Volcanic Belt
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Publication Date
2000
Authors
Vegas, Ramón
Núñez Aguilar, Juan Ignacio
Urrutia Fucugauchi, Jaime
Tarling, Donald H.
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Elsevier, B.V.
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Abstract
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 Rı´o Frı´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.