Geology of Mexico: An Overview
Donald P. McGookey
Physiography
A considerable part of Mexico has elevations above 2000 meters.
There are four large mountainous trends: Sierra Madre Occidental, Sierra Madre
Oriental, Trans-Mexican Volcanic Belt (TMVB), and Sierra Madre del Sur.
Plate Tectonics
Mexico is a patchwork quilt of microcontinents (terrains) that accreted
to the North American continent from north to south starting in the late Paleozoic
and continuing into the early Cenozoic. In total the blocks comprise an isthmus
between the Pacific Ocean and Gulf of Mexico. They are separated from the
continent by an east-southeast left-lateral wrench system that extends from
southeast California to the east coast called the Mojave-Sonora Megashear.
Exposures of the shear zone are poor because of widespread cover by Tertiary
extrusives and younger valley-fill. The existence of this feature is not accepted by
everyone, but considerable scattered evidence argues for the interpretation.
Magmatism
As the blocks docked and moved southwest with the rest of the
continent, they over road the east side of the Pacific oceanic crust. This
subduction resulted in continuous magmatic activity during the Mesozoic and
Cenozoic that has migrated from north to south. Along the west side of the Sierra
Madre Occidental trend there are exposures of a Jurassic-Cretaceous batholith.
This mountain trend is primarily a very thick pile of Late Eocene to early
Miocene extrusives dominated by Oligocene ignimbrites. The current subduction
underlies the Sierra Madre del Sur and generates magmas responsible for the
Trans-Mexican Volcanic Belt and the Central America volcanic arc (El Chichon).
Gulf of Mexico Basin
As South America moved south during the breakup of Pangaea the
Yucatan block was pulled away from the southeast part of Texas forming a large
basin with an oceanic crust, but no evidence of a sea-floor spreading ridge. The
movement of the Yucatan block apparently happened in two stages. An early
Mesozoic opening formed the basin. This was the basin at the time of mid-
Jurassic salt deposition. An additional late Mesozoic movement is indicated by a
large central area that is salt-free.
Mining
The rich copper deposits of Arizona continue south into the state of
Sonora. If you are looking for the Treasure of Sierra Madre, I suggest you look
there. In the northeast part of the country there are a number of Mississippi-valley
type mineral deposits (fluorite, lead, zinc and barite). There is very little coal.
Oil and Gas
In the Gulf of Mexico basin there are five megagiant (over 5 Billion
BO) fields. One is in east Texas and four are in Mexico. The Golden Lane fields
along the west edge of the Gulf were discovered in the early 1900’s by drilling
seeps. These fields produce from Jurassic-Cretaceous reefs that fringe a large
carbonate bank similar to the Horseshoe Atoll of the Midland Basin. In a deep
trough west of the bank debris from the reefs formed a thick pile that resulted in
the Posa Rica fields. The Reforma areas fields of southeast Mexico are along the
east side of a salt basin. The large A. J. Bermudez field developed during the
1970’s produces from Cretaceous carbonates in a highly faulted anticline that
drapes over a salt pillow. The last megagiant field is offshore west of the Yucatan
peninsula along the east side of a large deep water salt basin. It extremely large
(over 15 billion BO) and bizarre. It resulted from Mid-Tertiary overthrusting of
Mesozoic carbonates. The main pay is a 1000 meter thick section of breccia at the
top of the Cretaceous that was originally deposited on the slope west of the
peninsula.
AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009