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J. Rosas-Elguera, Universidad de Guadalajara (Mexico)
A. Goguichaisvilli, Universidad Nacional Autónoma de México (Mexico)
A. Alva-Valdivia, Universidad Nacional Autónoma de México (Mexico)
J. Urrutia-Fucugauchi, Universidad Nacional Autónoma de México (Mexico)
R. Maciel, Universidad de Guadalajara (Mexico)
M. Alatorre, Universidad de Guadalajara (Mexico)
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Subduction of the Farallon plate beneath North America formed the Rivera and Cocos oceanic plates, the extensive magmatic arcs of the Sierra Madre Occidental (SMO) and the Trans-Mexican Volcanic Belt (TMVB). Southern Mexico consists of crustal blocks separated by a regional extensional structural system; the latter, called the Guadalajara triple junction, is defined by the Tepic-Zacoalco (TZR), Colima (CR), and Chapala (CHR) rifts. TZR and CHR separate the SMO from the Jalisco and Michoacan blocks, whereas CR is the boundary between the Jalisco and Michoacan blocks. The initial stage of the TMVB is marked by widespread Miocene basaltic volcanism, emplaced from the Nayarit state, in the west, to the longitude of Mexico City. A summary of paleomagnetic studies of Miocene volcanic succession which span from 11 to 7.5 Ma from the TMVB is presented.
The Tepic, Guadalajara, Los Altos, Queretaro, and Cotija areas were studied. The mean paleodirection obtained for Tepic area is I = 33.7°, D = 358.4°, k = 140, α95 = 3.0°, N = 17. whereas for Guadalajara is I = 31.1°, D = 354.6°, k = 124, α = 2.1°, which corresponds to the mean paleomagnetic pole position Plat = 84°, Plong = 129.8°, k = 29, α95 = 4.4°. These directions are in reasonably good agreement with the expected paleodirections for middle Miocene time. The mean paleomagnetic direction calculated for Los Altos and Queretaro is I = 32.46°, D = 341.2°, k = 7.2 and α = 11.6° which define a counterclockwise rotation of ∼ 15° about a vertical axis. Finally, paleomagnetic data of the Michoacan block indicate a counterclockwise rotation of ∼ 24° about a vertical axis. Our results suggest that no major block rotation has occurred in the Nayarit and Guadalajara areas since about 10 Ma but 15°-24° anticlockwise tectonic rotations for the Los Altos, Queretaro and Michoacan block areas is in accordance with the proposed left-lateral transtensional tectonic regime.
Several plate models suggest either dextral or sinistral oblique convergence of the Cocos plate relative to North America. Our new results help to constrain these different models. These data demostrate that deformation in the studied areas are as old as late Miocene, and are related to sinistral oblique convergence of the Cocos plate relative to North America - inducing SE relative motion of the Michoacan block. The structural trends along both CHR and CR are thereby explained. On the other hand, right-lateral transtension along the TZR is related to the westward motion of the Jalisco block because of oblique convergence of the Rivera plate.
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