EASTWARD YOUNGING OF AGES ACROSS THE PENINSULAR RANGES BATHOLITH AND PLUTONS : GEOCHRONOLOGICAL CONSTRAINTS FOR THE TECTONIC HISTORY OF THE PENINSULAR RANGES BATHOLITH AND TECTONIC IMPLICATIONS FOR WESTERN MÉXICO

The Mesozoic Peninsular Ranges batholith (PRB) has long been known for its remarkable longitudinal regularity but transverse asymmetry in age and composition. As part of an ongoing geochronological study of this active margin, new age determinations (387 ⁴⁰Ar/³⁹Ar step-heating and 18 apatite fission-track dates) on various plutons of the Peninsular Ranges batholith (between the 28^oN and 34^oN parallels) are presented here. The plutons were emplaced between 140 ~ 80 Ma and show a distinct systematic age pattern that decreases regionally (and within the individual plutons) away from the coast toward the northeast.

 By a compilation of existing age data (PRB and western México) in combination with the new age data, new chrontours were constructed. It can be seen that the systematic eastward younging of dates is not restricted to the northern PRB, but can be extended from southern California as far south as Jalisco, central México, toward the east, and Chiapas toward the southeast. Furthermore, restoration of the Alta and Baja California to its pre-drift position with respect to the rest of México (with the Los Cabos batholith near Puerto Vallarta), confirms that new ⁴⁰Ar/³⁹Ar and K/Ar biotite chrontours cross into mainland México, from Sonora to Jalisco without disruption. Thus, this decrease in ages across the zone, and within individual plutons, is therefore attributed to regional eastward migration of granitic intrusion foci combined with superimposed east-side-up tilting of crustal-scale sized blocks containing the plutons. This suggests that starting during Early Mesozoic along the Pacific coast, and extending at least to Eocene time in Mainland México, the locus of magmatism migrated from west to east across this part of southwestern North America. Moreover, this also indicates that a long and linear (NNW-SSE) subduction zone, analogous to the modern setting of the South American Andes, formed the basic framework of western México during this time.

This interpretation supports geological arguments that the PRB Province has been part of the North America craton since the Early Cretaceous and has undergone only limited northward tectonic displacement, and that by simply closing the Gulf of California to its pre-Miocene opening, as indicated by the geological evidence, the Peninsula is restored to its original position.