Counterclockwise rotation of the Michoacan block: Implications for the tectonics of western Mexico

Subduction of the Farallon plate beneath North America resulted in formation of 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. In this study, we carried out combined radiometric and paleomagnetic analyses in the Michoacan block. Radiometric dates of 31.60 to 8.39 Ma confirm both the southern extension of the Sierra Madre Occidental and the early mafic TMVB succession into the Michoacan block. The Oligocene age agrees well with the radiometric dating reported for the southern SMO and the Tertiary volcanic fields of the Sierra Madre del Sur. Paleomagnetic data indicate a counterclockwise rotation of similar to24degrees about a vertical axis for the Michoacan block. 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 Michoacan block is as old as late Miocene, and is related to sinistral oblique convergence of the Cocos plate relative to North America-inducing the southeast 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.