Southward migration of continental volcanic activity in the Sierra de Las Cruces, Mexico: palaeomagnetic and radiometric evidence Maria Luisa Osete a, *, Vicente-Carlos Ruiz-Martinez a, Cecilia Caballero b, Carmen Galindo c, Jaime Urrutia-Fucugauchi b, Donald H. Tarling d a Departamento de Fisica de la Tierra, Facultad de Ciencias Fisicas, Universidad Complutense de Madrid, 28040 Madrid, Spain b Laboratorio de Paleomagnetismo y Geofisica Nuclear, Instituto de Geofisica. Universidad Nacional Aut6noma de Mexico, Mexico D.F. 04510, Mexico C Deparfamenfo de Petrologia y Geoquimica, Facultad de Ciencias Geol6gicas, Universidad Complutense de Madrid, 28040 Madrid, Spain d Department of Geological Sciences, University of Plymouth, Plymouth PL4 8AA, UK Abstract New Palaeomagnetic data for 30 sites (271 samples) and K-Ar data from ive units in the Sierra de Las Cruces, western Basin of Mexico, provide constraints on the spatial-temporal evolution of arc magmatism in the central Trans-Mexican Volcanic Belt. The normal and reversed directions show a polarity pattern with a consistent spatial zonation perpendicular to the NNW-SSE trend of the range. The magnetostratigraphy and K-Ar dates indicate that volcanic activity in the Sierra de Las Cruces migrated southeastward at a mean rate of 1.6 cm/a, between 3.6 and 1.8 Ma, and that the rate of migration may have been higher, up to 4 cm/a, during the Gauss Chron. Normal and reversed directions pass the reversal test at a 95% conidence level. The mean Plio-Quaternary palaeomagnetic direction for Sierra de Las Cruces is D=350.7\ I=30.6° (N=25, k=30.7, (95=5.3°). The declination deviates to the west of the expected direction, which suggests that small counterclockwise rotations could take place during formation of the Sierra de Las Cruces volcanics. Keywords: continental volcanism; K-Ar dating; palaeomagnetism; Trans-Mexican Volcanic Belt; volcanic activity migration 1. Introduction The Trans-Mexican Volcanic Belt ( TMVB) is an E-W-elongated structure that crosses central Mexico from the Paciic Ocean to the Gulf of Mexico between the 19th and 21st parallels ( Fig. l a inset ) . The TMVB is superimposed on the * Corresponding author. Fax: + 34-91-3944-398. E-mail address:mlosete@eucmax.sim.ucm.es (M.L. Osete) NNW-SSE structural grain of Mexico, crossing Oligocene-Miocene ignimbrites and associated rocks of Sierra Madre Occidental, the Mesozoic fold belt of the Sierra Madre Oriental, overthrust terranes of the Cordillera and Palaeozoic basement rocks. On the basis of geochronological, petrologi cal and structural data, the TMVB is often divided into three sectors: the western sector from the Paciic coast to the Colima graben, the central sector extending from the Michoacan volcanic zone towards either the Valley of Mexico and