FRAGMENTATION PROCESSES OF THE CRETACEOUS GRANITES IN THE WEST-CENTRAL PORTION OF MEXICO: A GEOPHYSICAL APPRAISAL

Abstract:

A granitic belt was emplaced between the southern tip of Baja California (BC) and Acapulco in late Cretaceous time along the active subduction zone of the Pacific plate; the farther south the younger the ages of these intrusives. At Chron 6.0 (20 Ma) the trench traced a smooth curve from Vancouver, Canada to Manzanillo, Mexico situation that persisted until ~5.0 Ma. At 15 Ma the Guadalupe plate was a single plate from Guadalupe Island to south of the Clipperton and Tehuantepec fracture zones, enclosing the Cocos, Rivera, and a portion of the present Pacific plates. At Chron 4n.2 (7.90 Ma) the Guadalupe trench was still continuous but subduction had stopped west of BC inducing a left-lateral shear zone between the active and abandoned trenches. This event initiated the separation of BC from mainland Mexico, breaching the granitic belt. At that time the batholiths at the tip of BC and those in the Bahía de Banderas region were close to each other, as suggested by their similar ages, as well as similar isotopic and geochemical data. Between them, along the mouth of the Gulf of California, extension of the crust has been documented by seismological means; however, not much is known about the specific processes that gave rise to crustal weakening and eventual break up of the batholiths involved. There are indications that in the Bahía de Banderas region, some of those processes may be presently taking place, inducing the separation of portions of the batholith from mainland Mexico. In this study we use gravimetric and magnetic measurements in order to learn about the fractionation of the granitic batholith. We create 2-D models and perform 3-D inversions of these fields to analyze the Bahía de Banderas region.