campa y coney, 1983
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Tectono-stratigraphic terranes and mineral resource distributions in ~ e x i c o '
MARIA FERNANDA CAMPA Petroleos Mexicanos, Mexico, D. F .
AND PETER J. CONEY
Department of Geosciences, University of Arizona, Tucson, AZ 85721, U.S.A. Received March 1 1, 1982 Accepted May 10, 1982
About 80% of the southern part of the North American Cordillera within the Republic of Mexico is made up of suspect terranes. These terranes are suspect because their paleogeographic setting with respect to cratonic North America at various times through much of Phanerozoic time is uncertain. Much of northeastern and southeastern Mexico is underlain by basement accreted during late Paleozoic time, an extension of the Appalachian-Ouachita orogeny. This orogen has been considerably modified by Jurassic strike-slip translations related to the opening of the Gulf of Mexico. Western and southwestern Mexico is largely made up of several distinct but coeval latest Jurassic to Late Cretaceous submarine magmatic arc terranes with unknown basement that appear to have accreted against the disrupted North American margin by early Tertiary time. Only northeastern Sonora and the State of Chihuahua appear to be floored by unmoved North American cratonic basement. The combined effect of Mesozoic accretions and translations essentially eliminates the overlap of South America upon Mexico that is drived from late Paleozoic - early Mesozoic reconstructions of the closed Atlantic Ocean. This new vision of accretionary and translational tectonics in Mexico has profound implications for the study of tectogenesis in the southern Cordillera as well as for the interpretation of Mexico's vast natural resources. Preliminary analysis indicates that Mexico's gold-silver and lead-zinc deposits are directly or indirectly related to the terrane distributions discussed.
Environ 80% de la partie sud de la Cordill2re de 1'AmCrique du Nord a I'intCrieur de la RCpublique du Mexique est formke de terrains ma1 dCfinis. Ces terrains sont mal dCfinis parce que leur contexte palCogCographique en rapport avec le craton de 1' AmCrique du Nord pour les diffkrents temps gCologiques surtout pour le PhanCrozo'ique est indCterminC. De grandes regions du nord-est et du sud-est du Mexique reposent sur un socle rCsultant d'une accrCtion au cours du PalCozoique su@rieur, une extension de I'orogCnkse Appalache-Ouachita. Cette orogCnkse fut considkrablement affectke par des translations le long de dCcrochements au Jurassique accompagnant l'ouverture du golfe du Mexique. L'ouest et sud-ouest du Mexique est principalement constitut de terrains de nature diffkrente mais contemporains de la fin du Jurassique suptrieur jusqu'au CrCtacC sugrieur, formant des arcs d'origine magmatique sous-marine, dont le socle est inconnu mais semble rCsulter d'une accrCtion contre la marge rupturCe de 1'AmCrique du Nord durant le Tertiaire infkrieur. Seulement le nord-est de Sonora et 1'Etat du Chihuahua semblent prksenter un socle correspondant au craton non-dCplacC de 1'AmCrique du Nord. L'effet combink des accrktions et des translations du MCsozo'ique Ccartent l'hypothkse d'un chevauchement de 1'AmCrique du Sud sur le Mexique invoquCe dans les reconstitutions de la fermeture de 1'ocCan Atlantique durant la pCriode du PalCozoique supCrieur jusqu'au MCsozoique infkrieur. Cette nouvelle approche de tectonique d'accrCtions et de translations pour le Mexique peut contribuer au dCveloppement de la tectogknkse du sud de la Cordillkre et tgalement a l'interprttation concernant les vastes ressources naturelles du Mexique. Des Ctudes prkliminaires indiquent que les gites d'or-argent et de plomb-zinc du Mexique sont directement et indirectement reliCs 21 la distribution de terrains discutks dans le prCsent article. Can. J . Earth Sci. 20, 1040-1051 (1983) [Traduit par le journal]
Introduction Ben-Avraham et al. 1981). The approach has also Tectono-stratigraphic terrane analysis (Jones and yielded preliminary insights into the distribution and
Silberling 1979) has proven fruitful in studies of the genesis of Cordilleran mineral resources (Albers 1981; North American Cordillera. It has resulted in a series of Berg 1981). new maps (Coney et al. 1980; Berg et al. 1978; Jones et Tectono-stratigraphic terrane analysis is in its infancy al. 1981) that have shed much light on Cordilleran in Mexico. We report here the results of preliminary tectonic evolution and has focused attention on major work now in progress (Carnpa and Coney 198 1). We are issues in the interpretation of the tectonic evolution of certain future work will require revision of what we continental margin mountain systems (Coney 1981; portray here, but we are confident that the broad outline
we propose is in general valid. These preliminary results 'This paper was presented at a sympos~um entitled provide a different vision of the tectonics of Mexico and
"Metallogeny and Tectonics of the North American at the Same time provide insight into the tectonic Cordillera" held at the GAC/MAC/CGU Joint Annual Meeting evolution of the southern part of the North American in Calgary, Alberta, May 13, 1981. Cordillera. Finally, we suggest that some mineral
CAMPA AND CONEY
FIG. 1. Tectono-stratigraphic terranes of Mexico. Basement terrane boundaries are shaded black lines. Superjacent (overlap) terranes shown by V pattern.
resource distributions in Mexico appear to be markedly controlled by the terrane distributions. This is a relationship that has not previously been perceived.
The tectonics of Mexico has an inherent complexity not shared by most of the Cordillera to the north: here, Paleozoic Appalachian and Mesozoic Atlantic Ocean and Gulf of Mexico features mingle with those of the North American Cordillera. The paleogeographic implications of this complex union have always been a mystery, and include the earliest configuration of the North American late Precambrian - early Paleozoic continental margin in this region as well later Paleozoic and Mesozoic interactions. Added complexities are the relationships between the southern Mexico - Caribbean - Central America region and northwestern South America. Most of these problems derive at the outset from geometric reconstmctions of the closed Atlantic Ocean in Permo-Triassic time (Coney 1978). The well known overlap of South America upon all of the Caribbean Sea and Central America and including up to 50% of the Mexican Republic leads to one of two conclusions: (1) rocks exposed in the overlapped region
did not exist during Permo-Triassic time, or (2) they were somewhere else. When these geometric facts are taken into consideration along with the nature of much of the geology of this region, the implied mobility of tectonic elements places severe doubts on classic interpretations of Mexican tectonic history. When these geometric relationships are combined with our terrane analysis they indicate, for example, that only about 20% of Mexico can be unequivocally underlain by unmoved autochthonous North American Precambrian basement. The remainder of the Republic is thus "suspect" (Coney et al. 1980; Coney 1981) in its paleogeographic affinities during long periods of Phanerozoic time.
Description of terranes A preliminary tectono-stratigraphic terrane map of
Mexico is shown as Fig. 1. The map shows the distribution of major basement terranes, which in most cases are areas assumed to be underlain by a basement litho-tectonic assemblage defined as internally homoge- neous within the boundaries of the terrane. Some terranes are designated as "composite," and include
1042 CAN. J. EARTH SCI. VOL. 20, 1983
internal complexities the details of which are still to be worked out. The proposed internal homogeneity of terranes is represented by a stratigraphic record (in other words, a geologic history) that ties the terrane together as a tectono-stratigraphic entity in space and time. Boundaries between terranes are major discontinuities in stratigraphy that we believe mark abrupt or cryptic changes in age and (or) lithology that cannot be easily explained as due to a facies change or an unconfonnity. Many of the boundaries are known faults and all are suspected to be.
Most of Mexico is in fact covered at the surface by what are termed "superjacent" terranes or overlap assmblages. Only some of these are shown in Fig. 1. These younger overlap assemblages cross terrane boundaries, indicating paleogeographic unity over a region larger than that represented by the basement terranes. Examples would be the mid-Tertiary volcanic plateau of the Sierra Madre Occidental and the late Tertiary trans-Mexico volcanic axis, both of which are shown in Fig. 1.
The basement terranes of Mexico may be grouped into zonations that divide the Republic into three major tectono-stratigraphic subdivisions: (1) a northwestern zone, which is a direct continuation southward into Mexico of autochthonous North American cratonic Precambrian basement and its Paleozoic-Mesozoic cover; (2) an eastern zone, surrounding the Gulf of Mexico, of mainly late Paleozoic age, which though heterogeneous has a common origin as material accreted to North America during the latest Paleozoic Appalachian-Ouachita-Marathon orogeny; and (3) a western zone, making up Mexico's wider Pacific margin, which is characterized by a heterogeneous assemblage of mainly submarine volcanic and sedimentary rocks of late Mesozoic age, with presently unknown basement. Also found here are scattered smaller terranes, which include in part older rocks and whose paleogeographic affinities with North America are presently unknown or at best very speculative.
Zone I: northwestern Mexico Zone I (Fig. 2) is made up of two terranes: the
autochthonous North American cratonic terrane of Chihuahua and what may be a pa