Quantifying Continental Margin Deformation North and South of the Opening of the Gulf of California—Evidence for Subduction Erosion?

A striking contrast exists in the geology of the continental margin north and south of the opening of the Gulf of California. For 1200km north of the Gulf opening, a reasonably well-preserved forearc region is associated with strongly magnetic 130-100 Ma plutonic rocks, volcanics and ophiolitic basement. This package of rocks is also coupled with a weakly magnetic eastern belt of 100-90 Ma tonalite and trondhjemite. In contrast, the southern continental margin bound by the Acapulco Trench lacks both a forearc basin and the 130-100 Ma plutonic belt. Instead, Late Cretaceous and Early to Middle Cenozoic granitoids crop out both along the coast and in the offshore region east of the Acapulco Trench, extending 1300 km south to 16°N. The geologic contrast between the northern and southern segments has been attributed to Cenozoic subduction erosion of the latter. The transition region between these two regimes occurs ~50 km northwest of the Gulf opening at 23°N along the Pacific coast of Baja California. North of Todos Santos, Late Cretaceous and younger strata depositionally overlie the western margin of the 130-100 Ma plutonic belt. The plutonic rocks are well-expressed in aeromagnetic images, even where buried. South of Todos Santos, 100-90 m.y. granitoids crop out along the coast and underlie the offshore Todos Santos basin; the strongly magnetic basement and undeformed forearc rocks are absent. Near Todos Santos, 94 Ma orthogneiss intercalated with upper amphibolite facies (garnet ± andalusite) Late Triassic(?) wallrocks are structurally juxtaposed above epidote amphibolite calc-schist of the mid-Cretaceous accretionary complex, suggesting significant forearc deformation. Previous 40Ar/39Ar data gathered south of Todos Santos reveal 5-10 m.y. of discordance between hornblende and biotite in the 100-90 m.y. granitoids. The age discordance could be related to either transient heating from 83-68 Ma granitoids located to the east or subduction erosion deformation similar to that described along the Acapulco margin. To determine the cause of discordance, we are integrating monazite (U-Th)/He thermochronology and geochemical analysis with K-feldspar multi-domain diffusion modeling and other low-temperature thermochronology to assess the magnitude of denudation. An increase in the magnitude of denudation towards the continental margin would indicate that subduction erosion is continuous along the southern continental margin for 1300 km. Alternatively, if the discordance were caused by thermal heating, the magnitude of denudation would be more homogeneous. To further understand the contrast in batholith denudation across the north-south boundary, these data will be compared with thermal history results from the westernmost batholith exposures in the La Paz region, ~100 km to the north, where the forearc is still preserved.