Late Mesozoic and Cenozoic thermotectonic history of the Mexican Pacific margin (18 to 25 degrees N), new insight from apatite and zircon fission-track analysis of coastal and offshore plutonic rocks

Apatite and zircon fission-track dating was used to constrain cooling histo ries on granitic samples taken from the offshore Acapulco trench batholith and onshore Manzanillo and Puerto Vallarta batholiths located along the sou thwestern active margin of Mexico, and from the La Pat batholith of souther n Baja California. The apatite data indicate that many of the samples in th e Manzanillo, Acapulco trench batholiths cooled rapidly below 60 degrees C shortly after emplacement between 70 and 55 Ma. The La Pat batholith and th e sample NM-20-08 of the Acapulco trench batholith experienced an older and slower cooling across the apatite partial annealing zone. The zircon fissi on-track ages obtained from two samples of the puerto Vallarta batholith in dicate a rapid cooling from 250 to 110 degrees C between 54 and 50 Ma. This non-coeval cooling of batholiths at two different depths is probably due t o uplift and erosion associated with the Laramide event in southwestern Mex ico. The three samples from the Puerto Vallarta batholith suggest a younger evolution across low temperatures with two flat stages located near the ba se of the apatite partial annealing zone and above it. The first flat stage suggests that the denudation or rock uplift rate was reduced at a depth co rresponding to the base of the apatite partial annealing zone. The younger single apatite ages of the Puerto Vallarta batholith, and the last common c ooling of the other batholiths indicate the mild thermal influences of both the Plio-Pleistocene Trans-Mexican Volcanic Belt and the opening of the Gu lf of California. (C) 1999 Elsevier Science B.V. All rights reserved.