The principal morphotectonic provinces of Mexico are: (1) Sierra Madre del Sur composed of middle Paleozoic metamorphics, (2) Sierra Madre Oriental made up of folded Mesozoic carbonates resting on folded Paleozoic sediments overlying Precambrian crystallines, (3) Gulf Coastal Plain and Yucatan Peninsula consisting of Tertiary marine sediments affected locally by salt tectonics and resting on folded Mesozoic sediments and Paleozoic metamorphics, (4) Sierra Madre Occidental consisting of flat-lying Tertiary lavas and pyroclastics which rest on folded Mesozoic sediments and Paleozoic metamorphics, (5) Trans-Mexico Volcanic Belt of late Tertiary and Quaternary age, (6) Sonoran Basin and Range Province comprising folded and faulted Paleozoic and Mesozoic sediments and volcanic , and (7) Baja California Peninsula composed of Cretaceous granitic batholiths and Mesozoic and Tertiary clastics and volcanics.

Present structure and resultant physiography developed from the consolidation of three orthogeosynclines into as many structural belts, two of which bordered the southern peninsular extension of the Precambrian hedreocraton of North America and underwent regional metamorphism and granitic emplacement during the middle Paleozoic orogeny at the end of the Paleozoic and block-faulting with accompanying volcanic activity during early Mesozoic time.

The third structural belt of Mesozoic-Tertiary age developed from an orthogeosyncline which covered the entire country from south to north and was affected by regional metamorphism and granitic emplacement in its western part toward the end of the Cretaceous and by orogeny mainly in its eastern part during early Tertiary time. This orogeny, which formed the Sierra Madre Oriental, was followed in the western two-thirds of the country by block-faulting and extensive volcanism during the remainder of the Tertiary, whereas east of the present Sierra Madre front deltaic deposits filled the molasse basins grading eastward into finer clastics of the Gulf Coast. During Pleistocene and Recent time a chain of basaltic volcanoes developed along a belt crossing the country from east to west at th latitude of Mexico City.

To date, commercial oil production has only been established in the Coastal Plain of the Gulf of Mexico; in the northern and southern districts, production is obtained from Tertiary clastics filling the molasse basins, whereas in the central district it comes from carbonates of the late Jurassic and Cretaceous miogeosyncline.

End_of_Article - Last_Page 412------------