BASIN AND RANGE-LIKE STRUCTURE IN THE CENTRAL DEPRESSION OF CHIAPAS, MEXICO

Chiapas’ Central Depression, bound to the northeast by the Fold and Transcurrent Fault Belt Province and to the southwest by the Chiapas Massif, has usually been regarded as a wide syncline, whose major axes largely coincide with the Angostura Dam and the Rio Grijalva proper.

Recent field and satellite image studies show that superimposed to a broad synclinoidal land form, a series of narrow, elongated horsts and grabens occur, thus forming a basin and range-like structure. In many places, Early Neogene andesitic volcanic or hypabyssal bodies are located spatially close to faults.

The Central Depression, like most of Chiapas’ geologic provinces, shows a NW-SE trend. In La Concordia-Socaltenango-A.A. Corzo Area (15° 45’-16° 15’ N Lat. and 12° 20’- 93°00’ W Long.), a typical part of it, five horsts (I-V from SW to NE) are detected. They separate four trench-like grabens; the horsts’ width decreases NE-ward. The ?Paleozoic Chiapas Massif (granite, and schists) makes up Horst I. Horst II includes the Middle Jurassic, ~2000 m thick, continental phyllarenitic Todos Santos Formation, and a ?Paleozoic metamorphic complex. The Albian-Turonian, ~1500 m thick, Sierra Madre Limestone makes up Horst III and part of the Horst IV; the latter also includes the Campanian-Maestrichtian, ~500 m carbonate Angostura Formation, which entirely constitutes Horst V. The Cretaceous units may be tightly folded in some parts.

The grabens (A-D also from SW to NE) are largely occupied by Quaternary deposits, and/or by peneplained, fine grained, steeply dipping Todos Santos strata. The Angostura Dam (Mexico’s largest) occupies Grabens B and C, which are narrower than A and D. Graben A, the widest, is somewhat arcuate length-wise, instead of rectilinear as the others. The largest volcanic stack occurs west of this graben’s northern part. Faults normal to the grabens and horsts, break them up in several blocks of different sizes.

The age of this basin and range-like structure is at least Early Neogene, as disclosed by the associated volcanics; it indicates a tensional stress regime, whose regional significance is not as yet well understood. The prevailing tectonic regime there is largely compressional, due to Cocos Plate subduction.