Campa y Coney, 1983

12
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 11, 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'iqueest 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 dCveloppementde 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-stratigraphicterrane 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 1981). 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

Transcript of Campa y Coney, 1983

Page 1: Campa y Coney, 1983

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

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CAMPA AND CONEY

LEGEND

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

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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 para-autochthonous displaced fragment of North America, the Caborca terrane in the State of Sonora.

Chihuahua terrane The Chihuahua terrane is underlain by unmoved

cratonic North American Precambrian basement. This basement outcrops in only scattered localities in northeastern Sonora, but it has been penetrated in wells in the State of Chihuahua (unpublished well reports, Petroleos Mexicanos, Gerencia de Exploracion; see also

I Lopez 1979, Figs. 4-8). The Precambrian basement is overlain depositionally by a cratonic assemblage of up to 1 3000 m of Paleozoic sandstones, shales, and limestones (Malpica and de la Torre 1980) with a fauna and lithologies very similar to those of well known sequences in Arizona and New Mexico of the southwestern United States (Peirce 1976). Toward the southeastern edge of the terrane, upper Paleozoic detritial sequences seem to reflect proximity to the southwestward extremity of the Appalachian-Ouachita -Marathon orogenic belt (Bridges 1964).

The southeastern boundary of the terrane is an assumed deep-seated fault along the northwestern frontal zone of the accreted late Paleozoic Ouachita- Marathon orogen. The southwestern boundary is a major tectonic discontinuity separating disparate Precambrian- age belts and Paleozoic - early Mesozoic stratigraphy. The discontinuity has been termed the Mojave-Sonora megashear (Silver and Anderson 1974; Anderson and Silver 1979). We simply term it the Mojave-Sonora discontinuity. The terrane is covered by latest Jurassic and younger Mesozoic rocks, which are part of the great transgression out of the Gulf of Mexico.

Caborca terrane The stratigraphic column for the Caborca terrane is

based on isolated outcrops west of the city of Caborca, Sonora, which yield a Precambrian basement (Darnon et a1. 1962; Anderson and Silver 1979) overlain by a very thick miogeoclinal sequence of late Precambrian through Paleozoic age (Cooper and Arellano 1946; Malpica and de la Torre 1980), considered to be very similar to the Cordilleran miogeoclinal sequence of southwestern Nevada and southern California (Weber et al. 1979; Anderson et al. 1979). Overlying the Paleozoic rocks are Upper Triassic marine and continental deposits of the Barranca Formation and Liassic to Upper Jurassic clastic and volcanociastic rocks (Anderson and Silver 1979). The Caborca terrane is certainly composite since southeast of Hermosillo, Sonora there are outcrops of lower Paleozoic sequences of deep-marine affinity (Peiffer 1979) and there are exposures of Lower Mesozoic rocks of uncertain affinity in northeast Baja California (Gastil and Miller 1981).

The close similarity of the pre-Late Jurassic sequences to those of southwestern Nevada and southern California and the marked contrast to sequences of the same age in nearby Chihuahua terrane have led to the suggestion that the Caborca terrane is a displaced fragment of North American Precambrian basement and its Paleozoic miogeoclinal cover plus various Paleozoic allochthons and younger overlap assemblages, all brought southeastward up to 800 km along the Middle Jurassic Mojave-Sonora megashear (Silver and Ander- son 1974; Anderson and Silver 1979).

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CAMPA AND CONEY

J u r a s s i c

U . T r i a s s i c

Caborca

D e v . - M i s s .

O r d .

C a m b .

U . P r e c a m b .

P r e c a m b

Chihuahua

C e n o . &

A p t .

N e o c . .

T i t o n .

P e r m .

P e n n . - M i s s

D e v . S i I . O r d .

C a m b

P r e c a m b

y T - l c o n g ~ o m e r a t e O 0 0 - l i m e s t o n e I;.:.] v o l c a n i c hl-l [-Isandstone . . . d o l o m i t e ~ l m e t a m o r p h i c . . _ . I . . .

FIG. 2. Tectono-stratigraphic columns for northwestern Mexico. Also shows lithologic symbols for Figs. 3 and 4. Abbreviations: Precam. = Precambrian basement; U. Precamb. = Upper Precambrian sedimentary rocks; Camb. = Cambrian; Ord. = Ordovician; Sil. = Silurian; Dev. = Devonian; Miss. = Mississippian; Penn. = Pennsylvanian; Perm. = Permian; U.

I Trias. = Upper Triassic; L., M., U. Jura. = Lower, Middle, and Upper Jurassic; L. Cret. = Lower Cretaceous; U. Cret. = Upper Cretaceous; Titon. = Tithonian; Neoc. = Neocornian; Apt. = Aptian; Alb. = Albian; Ceno. = Cenomanian; Turon. = Turonian; Sant. = Santonian; Camp. = Campanian; Tert. = Tertiary; Paleo. = Paleocene; Plio. = Pliocene. Sources:

, citations in text, unpublished data from Pemez and Instituto Mexicano del Petroleo files, and fieldwork by the authors.

I Zone 11: eastern Mexico of the newly opened Gulf of Mexico. However, in Much of eastern Mexico is overlain by upper scattered exposures and from well data there is evidence

Mesozoic and Cenozoic superjacent terranes, which are beneath the'cover of the basement terranes of concern I part of the great post-Middle Jurassic transgression out here (Fig. 3). The largest are the Coahuila and Maya

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CAN. J. EARTH SCI. VOL. 20, 1983

Coahuila Maya

C a m p

S a n t .

~ e n ~ o ~ . ~ ' " . A l b .

A p t .

N e o c .

T i t o n .

J u r a .

P e r m i a n

P a l e o . - P L i o .

U . C r e t .

L . C r e t .

U . J u r a . i

M . ~ u r a .

L . J u r a . P e r m .

P e n n . M i s s .

t D e v .

?

Sierra Madre

U . C r e t .

T u r o n .

L . C r e t .

U . J u r a . M . J u r a .

U . T r i a s . -

L . J u r a .

P e r m .

P e n n . M i s s . D e v . S i l .

C a m b . - O r d

P r e c a m b .

V e r t i c a l a r r o w b e s i d e c o l u m n

s h o w s r a n g e o f b a s e m e n t t e r r a n e

R o c k s a b o v e a r e s u p e r j a c e n t ( o v e r l a p ) t e r r a n e

FIG. 3. Tectono-stratigraphic columns for eastern Mexico. See caption to Fig. 2 for details.

terranes, which are both certainly composite but appear to be late Paleozoic accretions to North America related to closure of the proto-Atlantic Ocean as Africa and South America impinged on North America, which produced the later stages of the Appalachian-Ouachita -Marathon orogeny in latest Paleozoic time (Graham et al. 1975; Flawn et al. 1961). A third basement terrane is the Sierra Madre. This poorly exposed and poorly understood basement terrane is probably composite and is here portrayed as including part, at least, of the Altiplano region of central Mexico in the States of Durango, Zacatacas, and San Luis Potosi. Part of the

Sierra Madre basement terrane may be a displaced fragment of North American basement similar to the Chihuahua terrane.

The terranes of eastern Mexico are overlapped first by a Late Triassic to Middle Jurassic early rift assemblage of continental redbeds and evaporites, then by a post-Middle Jurassic through Late Cretaceous marine transgression assemblage. Both of these sequences are part of the history of the opening of the Gulf of Mexico. The Sierra Madre and Maya terranes are also both locally overlain by an Early to Middle Jurassic (?) continental redbed and volcanic sequence, which we

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AND CONEY 1045

interpret to be part of a Jurassic continental magmatic arc. Accretion of the Coahuila and Maya terranes onto the North American continent occurfed during the latest Paleozoic to Middle Triassic. The Maya and Sierra Madre terranes, however, were still mobile with respect to North America until Late Jurassic time, since Upper Jurassic limestones are the first units to overlap all three terranes. The mobility of the Maya and Sierra Madre terranes is related to movement along the Torreon- Monterrey discontinuity. The discontinuity is appar- ently the southeastern extension of the Mojave-Sonora discontinuity. Movement along the feature thus occurred after the Middle Triassic and before the Late Jurassic.

Coahuila terrane The known lithology of the Coahuila terrane consists

of two parts. One is a much deformed and generally mildly metamorphosed upper Paleozoic (mostly Permian) flysch with andesitic volcanics cut by scattered granodioritic plutons, which usually yield latest Paleozoic radiometric ages (Bose 1921 ; Flawn and Diaz 1959; Flawn et al . 1961; Denison et al . 1971). A second part is the so-called "frontal zone" of the Ouachita- Marathon orogen, which includes rocks of Cambrian through late Paleozoic age. These sequences are thrust northwestward over cratonic North America in the Ouachita and Marathon Mountains and we assume they are likewise thrust northwestward over the Chihuahua terrane in Mexico. Unconformably above these basement terranes lie continental redbeds and evaporites of Late Triassic to Middle Jurassic age (Irnlay 1943; Humphrey 1956).

The upper Paleozoic rocks are here interpreted to be the "hinterland" or interior zone of the Appalachian- Ouachita-Marathon orogenic belt and probably represent magmatic arc and fore-arc assemblages accreted against North America during late Paleozoic closure of the proto-Atlantic Ocean. The frontal zone may have been distal, deep ocean-floor deposits that lay south of North America. The Mesozoic redbed and evaporitic sequences are considered equivalent to similar facies found along the Atlantic and Gulf coastal regions to the north in the United States and are here assumed to be related to early rifting that later led to the opening of the Gulf of Mexico. The southern boundary of the Coahuila terrane is the Torreon-Monterrey discontinuity.

Maya terrane Where exposed the Maya terrane is apparently more

heterogeneous than, but somewhat similar to, and probably a displaced part of, the Coahuila terrane. A large meta-plutonic complex, at least in part of Permo-Triassic age, is exposed in Chiapas (Damon et al. 1981; Webber and Ojeda Rivera 1957), and there are

highly deformed and metamorphosed Paleozoic flysch sequences (Hernandez 1973). In southernmost Chiapas and neighboring Guatemala there are Devonian to Permian sediments and possible older early Paleozoic and Precambrian rocks whose affinities with North America are unknown (Clemons et al. 1974). Jurassic redbeds and volcanic rocks and Middle-Upper Jurassic to Cretaceous transgressive marine sequences cover the basement terrane.

The boundary between the Maya and Coahuila terranes is not known, but it is possibly the southeastern extension of the Torreon-Monterrey discontinuity where it enters the Gulf of Mexico somewhere between Matamoros and Vera Cruz.

Sierra Madre The Sierra Madre Oriental is mainly a sequence of

folded and imbricately thrust-faulted upper Mesozoic limestones, shales, and sandstones of the superjacent Gulf of Mexico transgressive sequence deformed during the Late Cretaceous - early Tertiary Laramide orogeny (de Cserna et al. 1977; Tardy 1980; de Cserna 1956). In several anticlinoria, however, an older basement terrane is revealed (Carrillo 1961, 1965; Ramirez 1978; de Cserna et al . 1977). Near Ciudad Victoria the crystalline basement is a metamorphic complex of "Grenville" age (Fries et al. 1962b). Structurally above it is a sedimentary sequence that ranges from Cambro- Ordovician (?) to Pennsylvanian in age and culminates in a Permian flysch (Carrillo 1961; Malpica and de la Torre 1980). This sequence is perhaps not unlike what one might expect as a southeasternmost occurrence of the North American craton in Chihuahua terrane or perhaps the frontal zone of the Coahuila terrane here displaced far to the southeast. Farther south near the trans-Mexico volcanic axis a Lower Jurassic marine sedimentary sequence with a fauna of "Pacific" aspect is exposed structurally above Paleozoic rocks of Mississip- pian and Permian age, which in turn lie above Precambrian rocks. How far and to what degree the Paleozoic and Precambrian rocks extend beneath the remainder of the Sierra Madre terrane are presently unknown. It seems likely the Sierra Madre terrane as portrayed in Fig. 1 is composite.

At scattered localities between Torreon and Ciudad Victoria along the northern margin of the terrane south of the Torreon-Monterrey discontinuity there are exposures of red conglomerates, siltstones, sandstones, and silty shales below Upper Jurassic limestones (Zuluaga Formation) (C6rdoba 1963). The red sediments frequently have a considerable volcanic content, and there are occasional volcanic rocks as well. Usually evaporitic shales and siltstones intervene between the redbeds and the Zuluaga limestones. The rocks are often slightly metamorphosed, retaining a faint

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cleavage. Some of these rocks have been considered Mexico volcanic axis, unconformably cover the Paleozoic basement. Recent fieldwork suggests most are basement terranes as an overlap assemblage in many eauivalent to the Lower to Middle Jurassic Nazas areas. arma at ion (Pantoja-Alor 1963) exposed west of Alisitos terrane Torreon, which we interpret to be part of the Lower to This terrane is mainly a very thick Lower to Middle Middle Jurassic magmatic arc (Damon et al. 1981) Cretaceous (Aptian-Albianl submarine to partly known from southern Arizona and Senora, where it lies continental andesitic volcanic and associated volcano-

upon what we the Chihuahua clastic sequence that forms much of the western part of terrane. If the correlation is correct, these rocks are northern Baja California (Allison 1955; Gastil et al. displaced far to the southeast from their equivalents to 1978) The telrane possibly includes older rocks that the north. Since the basement of these rocks is not may extend into latest Triassic or Jurassic time. The

are shown in a separate beside the terrane is inmded by Cretaceous plutons and is in part Sierra Madre Oriental terrane column in Fig. 3. metamorphosed. The terrane no doubt has correlatives

Zone 111: western Mexico This vast region makes up almost one half of the

Republic of Mexico and is formed by a composite group of suspect terranes here considered to have accreted to North America in later Mesozoic to early Tertiary time. The principal basement terranes (Fig. 4) are a series of submarine volcanic and sedimentary rocks of magmatic arc aspect, and are at least of Late Jurassic to late middle Cretaceous age and possibly reach back to the Late Triassic and (or) Jurassic. Scattered through these terranes are smaller terranes with older ages and distinct facies. All of the above terranes are difficult to organize into any paleogeographic reconstruction with the re- mainder of Mexico until late Mesozoic or even early Cenozoic time. Any portrayal of this complex region is complicated by the fact that much of it is covered by middle Tertiary and younger superjacent terranes such as the Sierra Madre Occidental and the trans-Mexico volcanic axis.

The main late Mesozoic arc terranes are the Alisitos terrane of Baja California, the composite Guerrero terrane of southwestern Mexico, and the Juarez terrane in the State of Oaxaca. The smaller terranes are Mixteca and Oaxaca; both include Paleozoic rocks and the Oaxaca also includes Precambrian rocks. The Xolapa and Sonabari terranes are meta-plutonic complexes of largely unknown age and origin. Important deep-seated faults are known to bound these smaller terranes in several places, and we know of no evidence that permits the conclusion that any of them necessarily forms a basement to the larger submarine magmatic arc terranes.

The terranes of western Mexico were apparently accreted onto and consolidated with the North American continent during what might be termed the "greater" Laramide orogeny, which took place from Late Cretaceous to early Tertiary time. Folding, faulting, and some metamorphism at this time are widespread throughout this complex and varied region. In any event, poorly dated continental sediments considered to be of Late Eocene to Oligocene age and better dated middle Tertiary volcanic sequences, mostly in the Sierra Madre Occidental and beneath the younger trans-

across the Gulf of California in southern Sonora south of Guaymas and in Sinoloa (shown in Fig. 1 as the Guerrero terrane). It may also reappear in southernmost Baja California south of La Paz.

Guerrero terrane This vast terrane is best known in the Sierra Madre del

Sur south of the trans-Mexico volcanic axis. It is certainly composite, and at the time of this writing can be subdivided into at least three separate sequences. The relationships one to another of these three subdivisions is still not clear. All three subterranes are at least Late Jurassic to mid-Cretaceous in age and are composed of submarine volcanic and sedimentary squences, but there are Upper Triassic rocks known near Zacatecas (McGehee 1976). The stratigraphy in each is different, as are grade of metamorphism and deformational style. The three subterranes are: (1) Teloloapan-Ixtapan, (2) Zihuatanejo, and (3) Huetamo (see Fig. 4).

The Telolapan-Ixtapan terrane (Campa et al . 1974) is a sequence of andesitic volcanic and volcanoclastic sediments interstratified with limestone, shale, and sandstone. The sediments have yielded fossils of Late Jurassic and Early Cretaceous ages. No older basement is presently known. The assemblage has been affected by low-grade regional metamorphism and is quite severely deformed. Along the eastern margin of the terrane in the State of Guerrero the volcanic and sedimentary assemblage is thrust eastward over shelf carbonates of Cretaceous age that are part of the Mixteca terrane platform (Campa et al . 1976).

The Zihuatanejo terrane is best known along the south coast of Michoacan and in Colima. It is made up of mainly andesitic volcanic rocks, interbedded limestones with Albian fauna, and some shale, sandstone, and conglomerate. There are also locally continental redbeds with dinosaur footprints. The assemblage is deformed, but shows no significant metamorphism (Campa et a l . , in press). A recent Pemex well in the State of Colima has penetrated over 3000 m of andesitic volcanic rocks and limestones.

The Huetamo terrane is best known in Michoacan where a sequence of Upper Jurassic marine volcanoclas-

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CAMPA AND CONEY

Alisi tos Guerrero

I I Z i h u a t a n e j o H u e t a m o T e l o l o a p a n

Juarez

T e r t .

? A l b

Mixteca

C r e t . - v" ... Ib' -&..* >I .'

Apt. - - - - - - . I, V . "

4 I I I I

U . Jura.

Y

U" .= Titon. ,v>:

' Tert.

Apt. Alb.

Titon.

7

Oaxaca

A l b .

Neoc.

Perm.

Penn. Miss.

Camb.-Ord

P r e c a m b .

I FIG. 4. Tectono-stratigraphic columns for western Mexico. See caption to Fig. 2 for details.

tic sediments is overlain by a Neocomian flysch-like sequence of sands and shales. The flysch passes upward into Albian limestones interbedded with tuff and red- beds. Upwards the abundance of redbeds increases and the entire upper part of the assemblage is continental redbeds and ignimbritic volcanics that are probably Late Cretaceous in age. The terrane is deformed in moderately tight upright folds but is not metamor- phosed. There is no known basement older than the marine sediments of Late Jurassic age.

I Juarez terrane The Juarez terrane is the most easterly of the Upper 1 Mesozoic submarine volcanic and sedimentary assem-

blages. It is, in fact, found within 200 krn of the Gulf of Mexico where eastward verging thrust faulting in Sierra

de Juarez has carried it over the Maya terrane and the Gulf coastal plain. The Juarez terrane is also distinctive in that it lies "inboard," that is, east, of the Oaxaca Precambrian terrane. Deformation is very severe, but a provisional column is made up of Upper Jurassic calcareous shale and sandstones, Neocomian thin- bedded cherty limestones, and pillowed andesitic volcanic rocks (Charleston 1980; Carfantan 1981). There are also ultramafic occurrences. Along the western margin of the terrane the base of the assemblage appears to be made up of a very thick east-dipping mylonitic gneiss of unknown age.

Mixteca terrane The Mixteca terrane consists of a tectonically

juxtaposed two-part metamorphic basement with an

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1048 CAN. 1. EARTH

intervening ultramafic body. Radiometric age determina- tions from metamorphic rocks of the lower package yield early Paleozoic apparent ages (Ortega 1978). The metamorphic rocks are overlain in places by deformed Pennsylvanian terrigenous sediments (Calderbn Garcia 1956; Silva 1970), and in other places Lower to Middle Jurassic marine rocks overlie the metamorphic rocks. These rocks are in turn overlain by Neocomian shales and limestones, Aptian-Albian-Cenomainian lime- stones, and finally a flysch-like Upper Cretaceous sequence (Calderbn Garcia 1956).

Oaxaca terrane The Oaxaca terrane consists of a granulite and

anorthositic crystalline metamorphic basement above which sits depositionally an uppermost Cambrian to Ordovician terrigenous deposit that is overlain by Mississippian, Pennsylvanian, and Permian sediments (Pantoja-Alor and Robinson 1967). The basement has been isotopically dated as "Grenville" in age (Fries et al. 1962a; Ortega et al. 1977). The early Paleozoic fossils found above the basement complex are apparently most similar to South American forms rather than North American (Robinson and Pantoja-Alor 1968). Above the Paleozoic rocks are redbeds and Aptian-Albian limestones.

Xolapa terrane The Xolapa terrane occurs as a long narrow belt along

the southern coast of the States of Oaxaca and Guerrero. It is defined by its complex metamorphic-plutonic aspect and much development of migmatite. Where its northern boundary is known it is in contact with terranes north of it by major deep-seated faults. Isotopic ages from the terrane range from Jurassic to Tertiary (de Cserna 1965; Guerrero et al. 1978; Campa et al. , in press).

Vizcaino terrane The Vizcaino terrane, which is certainly composite, is

used here to encompass a very complex juxtaposition of oceanic rocks found on the western coast of Baja California on the Vizcaino peninsula and on ~ e d r o s Island. The terrane may underlie much of southern Baja California. Lithologies include ophiolitic rocks, mklanges, and submarine volcanic sequences of arc aspect. Ages range from Triassic through Jurassic and into Early Cretaceous (Rangin 1976, 1978; Minch et al. 1976; Mina 1965). The assemblage is Franciscan-like in aspect, and thus is similar to coastal California. The assemblage is overlain by a Great Valley-like basin sequence.

Other terranes Two other small terranes are found within the

Guerrero terrane along the western margin of the Sierra Madre Occidental in southern Sonora and Sinoloa. The Sonabari terrane is a meta-plutonic complex and the San

SCI. VOL. 20, 1983

Jose de las Rusias terrane (Malpica and de la Torre 1980) is a Carboniferous sedimentary sequence. The origins and relationships of these terranes are unknown'to the authors at this time and are not further discussed until work in progress is completed.

Mineral resource distribution in Mexico and the tectono-stratigraphic terranes

The distribution of Mexico's mineral resources has been described largely as a function of present-day major physiographic-geologic provinces (Salas 1975) and more recently as a function of paleo-Benioff zone morphology (Damon et al. 1.981). The former study was largely descriptive and was accompanied by a useful map (Salas 1975). The latter study carried a genetic implication based largely on age distribution of mineral deposit zonations identified by groupings of deposit types (Damon et al. 198 1). We present a preliminary alternative portrayal, which is also descriptive (Fig. 5). We show that at least two major commodity distributions seem to be a function of the distribution of the basement terranes presented in this report. We offer no genetic explanation, but we feel the relationships are interesting and worth further study. They are consistent with the conclusions of studies elsewhere (Albers 1981; Berg 1981).

Our data base for mineral resource distribution is the metallogenetic map of Mexico compiled by Salas (1975). His map shows the principal producing mines of Mexico since colonial times, a period of over 400 years. About 400 localities are shown on the map. We have simply superimposed the terrane map of our Fig. 1 on the metallogenetic map of Mexico. Then we counted occurrences, regardless of size, of two major commodity groups: (1) the precious metals, gold and silver, with or without lead, zinc, and cobalt, and (2) deposits of lead and zinc, with or without associated gold, silver, and cobalt.

When we performed this exercise we found the following:

(1) Over 70% of Mexico's important gold and silver mines are located within the Mesozoic accreted terranes of Alisitos, Guerrero, and Juarez. If one adds in the deposits of the Altiplano region of central Mexico, which lie just east of the Guerrero terrane in a region of presently unknown basement, the percentage goes to over 84%.

(2) Only 8% of Mexico's gold and silver comes from terranes floored by North American Precambrian basement such as the Chihuahua terrane, and only 7% comes from the Maya-Coahuila terranes.

(3) Over 61% of Mexico's productive mines for lead and zinc are in terranes floored by North American Precambrian basement such as the Chihuahua terrane.

(4) Only 17% of the lead and zinc occurrences are in

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CAMPA AND CONEY 1049

L_J Atlsltam. Ouerrero, nnd ~ u a r

a ChChihuahua terrano

FIG. 5. Selected mineral resource distributions in Mexico. Distribution of gold-silver and lead-zinc in Mexico with respect to atmeted terranes of Alisitos, Guerrem, and J m z and cratonic Ncnth America terrane of Chihuahua. Mineral localities after Sda. (1975). Terranes after Fig. 1 of this report.

the Mesozoic accreted terranes and only 14% are found within the Maya-Coahuila terranes.

(5) Of the known massive sulfide volcanogenetic deposits, all are in the Mesozoic submarine volcanic accretionary terranes .

The distribution patterns reported above are in- teresting when one considers that with the exception of the massive sulfides the vast majority of the deposits concerned are Late Cretaceous or Cenozoic in age (Salas 1975; Darnon et al. 198 l ) , and are thus younger than the basement terranes upon which they are found. They are also younger than the accretion of the various suspect terranes of North America. We would certainly not wish to convey the impression that the distribution patterns we discern necessarily nullify the genetic models based on Benioff zone morphology. We would simply point out that the distribution pattern we observe strongly suggests a significant element of basement control on resource distributions in Mexico. This we feel is provocative and warrants further analysis both for future resource discovery strategies and for deeper insight into mineral deposit genesis in Mexico.

Acknowledgments The authors are grateful to Petroleos Mexicanos, the

Instituto Mexicano del Petroleo, the University of Guerrero, and the National Science Foundation for support, particularly field work leading to this report. This work is also part of a larger international effort supported by the United States Geological Survey (USGS), the Canadian Geological Survey (GSC), and Petroleos Mexicanos, which involves Norm Silberling and David L. Jones (USGS), James Monger (GSC), Maria Fernanda Campa (Pemex), and Peter J. Coney (University of Arizona). The objective of this in progress project is to compile a tectono-stratigraphic terrane map of the North American Cordillera. Continuing cooperation in Mexico with Paul Damon and discussions with Spencer Titley (University of Arizona) have been very useful, particularly in our understanding of Mexican mineral deposits. Discussions with Henry Berg and John Albers (USGS) and with W. R. Dickinson (University of Arizona) have been encouraging and very helpful.

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