WHOLESALE MELTING OF THE SOUTHERN MIXTECO TERRANE AND ORIGIN OF THE XOLAPA COMPLEX

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.