U-Pb geochronology of Cenozoic plutons in the Pinotepa Nacional-Salina Cruz region and patterns in the migration of magmatism along the SW continental margin of Mexico

The coastal zone of the State of Oaxaca in southern Mexico is occupied by an extensive record of Cenozoic plutons that represent the testimony of arc-magmatism related to the subduction of the Farallon plate. Magmatism moved northwards to the Trans-Mexican Volcanic Belt after margin truncation due the southeastward displacement of the Chortis block and subsequent subduction erosion. We report the results of LA-ICP-MS U-Pb geochronology of 24 samples of Cenozoic plutons and nine samples of host rocks from the Oaxaca coastal zone, as well as representative petrologic, geochemical and thermobarometric analyses. Based on new U-Pb dating, four episodes of arc magmatic activity were recognized within the 50-15 Ma interval with a general southeast decreasing rate of 21.7 km/myr. Consideration of the obtained U-Pb ages and previous K-Ar, Ar-Ar and U-Th/He cooling ages indicates rapid cooling in the Oligocene-early Miocene during exhumation followed by margin truncation. Dating of plutons along the coastal zone has revealed previously unknown Carboniferous-Permian plutons. The gradual cessation of magmatism that accompanied the truncation of continental margin was followed by progressive jumping in the locus of arc-magmatism to the eastern Trans-Mexican Volcanic Belt. This was due to the flattening of the subducted slab behind the North America-Caribbean-Farallon triple junction that accompanied the displacement of the Chortis block. A tear structure associated with the change in the angle of the subducted plate migrated gradually to the present-day position in the Isthmus of Tehuantepec. This geological/tectonic interpretation helps explain the distribution of plutons in this central western part of Mexico.

Automated summary

The coastal zone of the State of Oaxaca in southern Mexico contains a rich record of Cenozoic plutons that are evidence of arc-magmatism related to the subduction of the Farallon plate. Through dating and analysis, it was found that the magmatism gradually moved northwards to the Trans-Mexican Volcanic Belt and eventually ceased. The geological interpretation provides an explanation for the distribution of plutons in the central western part of Mexico.