At Site 493, DSDP Leg 66, dioritic basement was reached below lower Miocene (NN1 Zone, 22-24 Ma) terrigenous sediments. Petrographical, mineralogical (including microprobe analyses), and chemical features of the dioritic rocks reveal their magmatic affinity with the calc-alkaline series. Furthermore, their radiometric age (35.3 m.y.) links the basement to the Sierra Madre Occidental in Mexico and to mid-Tertiary volcanic arcs in Central America. The presence of Oligocene diorite 50 km from the trench axis confirms the truncation of the south Mexico margin, which we explain as the result of a 650 to 800 km left-lateral displacement of Central America relative to North America. Truncation must have occurred in the late Miocene, after the diorite intrusion and prior to the present subduction.

Supplement to: Bellon, Herve; Maury, R C; Stephan, J F (1982): Dioritic basement, Site 493: Petrology, geochemistry, and geodynamics. In: Watkins, JS; Casey Moore, J; et al. (eds.), Initial Reports of the Deep Sea Drilling Project (U.S. Govt. Printing Office), 66, 723-730