The tectonic boundary between the Grenville-age Oaxacan and Paleozoic Acatl¿n crystalline complexes in southern Mexico, named the Caltepec fault zone (CFZ), is characterized for the first time as a dextral transpressional, NNW trending and ENE dipping ductile fault zone of Early Permian age. The complexes are welded by a syntectonic magmatic epidote-bearing granite along the entire length of the CFZ. From east to west, the 2--6 km wide CFZ consists of disrupted and retrograded banded gneisses of the Oaxacan complex, quartz-feldspar mylonite, and the syntectonic magmatic epidote-bearing Cozahuico granite (CZG) with huge xenoliths (up to several kilometers long and up to 600 m wide) of the Proterozoic gneisses, thrust westward over metasedimentary tectonites of the Acatl¿n complex. The CZG shows magmatic fabrics that represent a transition to solid-state deformation characterized by subvertical foliation, subhorizontal NNE and SSE dipping mineral stretching lineation and dextral kinematics. The megaxenoliths underwent partial melting developing banded migmatites with layers of epidote-bearing granitic neosome. The parallelism of fabrics in these anatexitic rocks and in the enclosing deformed granite suggests that ductile deformation, migmatization of xenolithic gneisses, and granite emplacement along the CFZ were coeval. The neosome yielded a U-Pb zircon concordant age of 275.6 ¿ 1 Ma probably dating the peak of the tectonothermal event. We interpret the CFZ as a major terrane boundary accommodating transpressional interaction between the Acatl¿n and Oaxaquia blocks, which were amalgamated in an oblique convergent setting by Early Permian time, as the leading edge of Gondwana impinged onto the southern margin of Laurentia along the Marathon-Ouachita suture to form Pangea.