b 1 WAr ages from the Guerrero transect date four thermotectonic events affecting the

Abstract. Because of questions regarding the validity of proposed accreted ter-ranes, the tectonostratigraphic evolution of the southern margin of the North America plate in Mexico remains controversial. Here we bring new data to this de-bate. We report whole rock K/Ar isotopic ages for 19 hypabyssal igneous and low grade metamorphic rock samples collected on a 300 km long, W-E geological transect in southern Mexico. Analysis of these new data, using structural and stratigraphic information from the transect and frequency plots of isotopic ages reported else-where in Mexico and the U.S. Cordillera, shows that four thermotectonic events af-fected the transect, most of Mexico and the U.S. Cordillera. These events are: I. (301 Ma +8 Ma), metamorphism/uplift due to the Alleghenian Orogeny; II. (221 Ma +5 Ma), metamorphism/uplift due to the Sonoman Orogeny; 111. (94.4 k2.4- 82.8 +2.2 Ma), metamorphism/uplift, thin-skinned tectonics and magmatism due to Sevier Orogeny; and IV. (50.9 k1.3 Ma- 28.8 "0.9 Ma), magmatism and normal faulting due to the waning Laramide Orogeny and onset of Basin and Range extension. The map pattern of regions affected by these four events is independent of proposed ter-rane boundaries in Mexico; and shows that most of the present real estate of Mexico has been a coherent part of the North America plate since at least Pennsylvanian time. 3 1