The southern extension of the Mixteco terrane has been currently considered to abut the Xolapa Complex at the Chacalapa-Tierra Colorada fault. However, geologic reconnaissance along several N-S sections across those terranes shows that most ranges of the Sierra Madre del Sur between Ayutla and San Luis Acatlán in the state of Guerrero are composed of a thick sedimentary sequence that passes gradually from shales and sandstones, to schists, gneisses and migmatites of the Xolapa Complex. This sedimentary and formerly unknown unit represents a southerly facies change from continental-shallow marine rocks of the Middle Jurassic Tecocoyunca Group, to the predominantly deep water siliceous and pelitic sedimentary rocks of this new unit which, across a steep metamorphic gradient, gradually becomes indistinguishable from high grade metamorphic rocks of the Xolapa Complex. These wholly documented field relationships have the following important tectonostratigraphic implications: a) The Acatlán Complex, basement of the Mixteco terrane, is reduced to about 60 % of its present mapped distribution, b) the Chatino terrane (Xolapa Complex and its cover), as recently proposed, is indeed an autochthonous tectonostratigraphic unit, c) the Chacalapa-Tierra Colorada fault is no longer a limit between terranes, and d) an extraordinary tectonothermal event of post-Jurassic and pre-Oligocene age with the characteristics of a full orogeny melted and deformed a deep sedimentary basin of Middle Jurassic age, which probably extended beyond the present truncated limit of the Pacific continental margin in southern Mexico.