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The Relationship Between Quaternary Volcanism in Central Mexico and the Seismicity and Structure of Subducted Ocean Lithosphere

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The Relationship Between Quaternary Volcanism in Central Mexico and the Seismicity and Structure of Subducted Ocean Lithosphere The relationship between Quaternary volcanism in central Mexico and the seismicity and structure of subducted ocean lithosphere GRAHAM T. NIXON Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia V6TI W5, Canada ABSTRACT Several aspects of this study have a bearing on the segmented nature of converging margins in general: Late Quaternary volcanism in central Mexico is related to the 1. The tectonic evolution of the ocean floor may determine the subduction of young ocean lithosphere at the Middle America. nature of segmentation at the site of subduction. Trench. Along-arc variations in seismicity, volcano structure, and 2. The complete record of volcanism in the TMVB over the composition of volcanic products bear a remarkable correlation past million years can be related to the present plate configuration. with the age and structural framework of the downgoing slab. 3. Alkaline and calc-alkaline volcanism have developed con- Morphological and pétrographie characteristics of major temporaneously at a converging plate margin. volcanoes within the Trans-Mexican Volcanic Belt (TMVB) serve 4. Lineaments in volcanic arcs may reflect the structural com- to distinguish two calc-alkaline subprovinces: plexity of the crust rather than segment boundaries in the sub- 1. A western arc, averaging 60 km in width, associated with ducted slab. aseismic subduction of the Rivera plate. The main cones of this region are dominated by two-pyroxene andesites, comprise volumes INTRODUCTION «S70 km3, and stand less than 3,000 m above sea level. 2. A broad central and eastern arc related to subduction of a Studies of convergent plate margins during the past decade gently inclined segment of the Cocos plate bounded by the Rivera have assembled a wealth of geological and geophysical evidence transform and the Tehuantepec Ridge. Major volcanic edifices pos- that permits a tectonic subdivision of subduction zones (Stoiber sess summit elevations in the range 4,000 to 6,000 m, have appro- and Carr, 1973; Carrand others, 1974; Stauder, 1973, 1975; Bara- priately larger volumes (typically >200 km3) and are constructed zangi and Isacks, 1976). According to models developed by Stoiber with a high proportion of amphibole-bearing lavas. and Carr (1973), the descending lithosphere is broken by tear faults, The boundary between these subprovinces is marked by a propagated at the trench, that divide the slab into discrete segments, north-south-oriented structural depression, the Colima Graben, typically less than 300 km across. Each segment of oceanic litho- and it coincides with a 100-km offset in the "volcanic front." Exten- sphere descends into the mantle with a different strike and dip, sional tectonism in the Colima Graben, accompanied by mixed producing offsets in features such as the inclined seismic zone, calc-alkaline and alkaline volcanism of potassic affinity, is likely trench axis, and alignment of cones along the "volcanic front." The related to a hinge-type transform fault which marks the Cocos- boundaries between segments are recognized by transverse features Rivera plate juncture in the downgoing slab. such as mapped fault zones (commonly with strike-slip displace- A third segment of ocean floor is presently interacting with ment), elongate clusters of cinder cones or loci of large volcanic continental lithosphere south of the Gulf of Tehuantepec, where eruptions, and concentrations of shallow earthquakes. Where all Quaternary volcanism is weakly developed within a tectonically these criteria are used in combination, the weight of evidence gener- complex region that marks the diffuse Cocos-NOAM-Caribbean ally favors segmentation, although the actual number of segments triple junction. The northern limit of this triple junction is defined in any particular arc may be disputed. As the model is extended into by the seismically active Isthmus fault, which may be related to areas of more complex plate interaction—for example, near the alkaline volcanic activity at San Andrés Tuxtla. triple junction of the Cocos, Caribbean, and North American A tectonic reconstruction based on the evolution of oceanic (NOAM) plates (Carr, 1976—or if it is applied to arcs where only a crust reveals that the distribution of intermediate-depth earth- limited number of the criteria that purport to distinguish segments quakes along the arc is directly dependent upon the age of the are present—as in the Cascade volcanic chain (Hughes and others, subducted slab. Ocean lithosphere younger than approximately 20 1980)—the relationships between the subducted slab and tectonic m.y. is subducted aseismically at convergence rates approaching 9 fabric of the overriding plate become more questionable. cm/yr. The length of the inclined seismic zone indicates that the Regional lineaments in the TMVB formed by Holocene cinder time constant for thermal relaxation in the slab is approximately 4 cones and sites of historic eruptions served as the basis for a seg- m.y. The TMVB overlies the aseismic extension of this young ocean mentation model for the Mexican arc (Stoiber and Carr, 1973; Carr lithosphere. and others, 1974). The arc was subdivided into six segments, about This article is included in a set of papers presented at a symposium on "Subduction of oceanic plates," held in November 1979. Geological Society of America Bulletin, v. 93, p. 514-523, 5 figs., June 1982. 514 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/6/514/3434432/i0016-7606-93-6-514.pdf by guest on 25 September 2021 Figure 1. Generalized tectonic map of Mexico modified from de Cserna (1961). Trans-Mexican Volcanic Belt: vertical ruling = western arc; stipple = calc-alkaline- alkaline province of the Colima Graben; V-pattern = central and eastern arc. Filled triangles denote major calc-alkaline cones of the "volcanic front"; open triangles repre- sent selected smaller cones; filled circles indicate caldera complexes. 1 = San Juan; 2 = Sanganguey; 3 = Ceboruco; 4 = Tequila; 5 = Sierra La Primavera; 6 = Nevado de Colima; 7 = Volcan Colima; 8 = Paricutin; 9 = Nevado de Toluca; 10 = Popocatépetl; 11 = Iztacc'ihuatl; 12 = La Malinche; 13 = Los Humeros; 14 = Pico de Orizaba; 15 = San Andrés Tuxtla; 16 = El Chichón; 17 = Tacana. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/6/514/3434432/i0016-7606-93-6-514.pdf by guest on 25 September 2021 516 G. T. NIXON 150 to 200 km in width, whose boundaries were assumed to lie volcanism in the TMVB began about 30 m.y. ago, but that volcanic parallel to the northeasterly direction of underthrusting at the activity was not widespread until mid-Miocene time. An Oligocene trench. Study of earthquake focal mechanisms of the Central Amer- to Holocene age was accepted in later studies of the same region ican arc by Dean and Drake (1978) did not substantiate the pro- (Negendank, 1972, 1973; Bloomfield, 1975; Bloomfield and Valas- posed segmented nature of the Mexican continental margin. tro, 1977; Richter and Negendank, 1976). At the eastern extremity I will review the general structure and compositional variabil- of the volcanic belt, Robin (1976) recognized a "primitive" TMVB, ity of volcanism in central Mexico and relate these features to the comprising Miocene andesites, and a later phase of "Neovolcanic" seismicity and structure of the young ocean lithosphere presently activity, commencing approximately 2.5 m.y. B.P. (Robin and Nic- being consumed at the Middle America Trench. It will be shown olas, 1978; Caritagrel and Robin, 1978). In fact, Cantagrel and that the tectonic evolution and age of the ocean lithosphere may Robin (1978) state that the east-west trend of contemporary play an important role in determining both the nature of segmenta- calc-alkaline volcanism has changed little since the mid-Miocene. tion and the seismic signature of the subducted slab. Although this claim may indeed be correct, it certainly requires substantiating by further K-Ar geochronometry in central and western Mexico. New K-Ar dates obtained on andesitic lavas asso- THE TRANS-MEXICAN VOLCANIC BELT ciated with the earliest stages of cone-building at Iztacc'ihuatl, near Mexico City, and at Volcán Tequila, in the western part of the arc, The locus of andesitic volcanism in central Mexico extends in a yield ages of approximately 1 m.y. (G. T. Nixon, J. E. Harakel, R. west-east direction for more than 1,000 km, from the Pacific Coast L. Armstrong, and A. Demant, unpub. data). This study, there- to the margins of the High Mexican Plateau overlooking the Gulf fore, is concerned specifically with volcanic rocks of the TMVB of Mexico. Inspection of the Tectonic Map of Mexico (Fig. 1; de younger than ~ 1 m.y. old. Cserna, 1961) reveals the complex nature and extreme diversity of Chemical and petrographic data show that the TMVB may be "basement" terranes underlying the volcanic belt as it transects the divided into two distinct calc-alkaline provinces (Fig. 1): (1) a structural grain of the Mexican continent (de Cserna, 1965, 1976; western arc, averaging 60 km in width and extending from the Demant and Robin, 1975). In the west, the TMVB is underlain by Pacific coast to the Colima Graben, and (2) a central and east- the ignimbrite province of the Sierra Madre Occidental which ern arc, stretching from the Colima volcanoes through the extends northward along the western Cordillera of Mexico to the areally extensive cinder cones and lava flows of Michoacan to United States border. Where the two provinces intersect, gently the locally more restricted volcanism associated with major dipping volcanic formations of the Sierra Madre Occidental are cut volcanic lineaments oriented north-south in the Sierra Ne- by longitudinal graben structures associated with Quaternary vol- vada (Iztacciihuatl-Popocatépetl) and Orizaba-Cofre de Perote canic activity within thé TMVB. High-angle faults and tensional regions. fractures extend from Volcan Sanganguey, near Tepic, to the Major cones of the western TMVB are built predominantly of Chapala region, 50 km south of Guadalajara (Demant and others.
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