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 represent 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.

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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. two-pyroxene andesite, stand less than 3,000 m above sea level, 1976; Demant, 1978). Silicic pyroclastic rocks and intercalated and comprise volumes, less than 70 km3 (Demant and others, basaltic flows of the Sierra Madre Occidental just north of Guada- 1976; Thorpe and Francis, 1975; Nelson, 1976; Luhr and Nel- lajara have yielded K-Ar ages of 4.5 to 9.5 m.y. (Watkins and son, 1980). From the Colima volcanoes eastward, the major others, 1971). Ignimbrites of this older province extend along the: volcanic edifices possess summit elevations in the range 4,000 northern edge of the TMVB as far as Pachuca, about 100 km to 6,000 m, are more voluminous (generally >200 km3), and northeast of Mexico City (Demant, 1978). East of the Valley of are constructed with a high proportion of amphibole-bearing Mexico, the axis of Quaternary volcanism transgresses folded andesite and dacite (Bloomfield and Valastro, 1977; Demant marine sedimentary strata of the Sierra Madre Oriental and and others, 1975; Nixon, 1979). This portion of the arc averages Pliocene-Miocene plateau lavas belonging to an eastern alkaline 100 to 200 km in width and lies behind an arc-trench gap of province (Demant and Robin, 1975; Robin and Tournon, 1978). more than 300 km at its eastern extremity. The peculiar geometry of the Mexican arc in comparison to The boundary between these subprovinces is occupied by the other circum-Pacific andesite provinces has generated a remarkable Colima Graben, a region of high-angle faulting oriented north- array of concepts concerning its origin. Previous studies have south and intersecting the northwest-southeast structural trends of related volcanism to (1) a continental prolongation of the Clarion the Guadalajara area. The southern end of this feature is dominated fracture zone (Mooser and Maldonado-Koerdell, 1961); (2) an by Volcan Colima, the most active volcano in Mexico, and the extension of the San Andreas fault system from the Gulf of Califor- extinct(?) Nevado de Colima. On the northern and western margins nia (Gastil and Jensky, 1973); (3) an ancient geosuture subjected to of the Colima volcanoes, Holocene scoria cones of strongly under- left-lateral transcurrent displacement and later reactivated in mid- saturated analcime-bearing basanites and minettes (Luhr and Car- dle Tertiary time (Mooser, 1972); and (4) a phenomenon related to michael, 1979) nestle incongruously among the many calc-alkaline subduction at the Middle America Trench (Gunn and Mooser, cinder cones in the region. The comenditic dome complex of Sierra 1970; Mooser, 1972; Demant and Robin, 1975; Robin and Nicolas, La Primavera lies farther north, just west of the city of Guadalajara 1978; Menard, 1978). These hypotheses and many more have been (Mahood, 1977, 1978). The Colima Graben, therefore, represents a summarized by Demant (1978). region where alkaline and calc-alkaline volcanism have developed A source of confusion arising from the earlier work concerns contemporaneously at the "volcanic front" of a convergent plate the age connotation of the popular term, "Trans-Mexican Volcanic margin. Belt," which varies from author to author even though the same Quaternary volcanic activity between the eastern terminus geographic entity is tacitly implied. For example, Mooser (1972), of the TMVB and the Guatemalan volcanic chain is restricted to conducting investigations in the Valley of Mexico, considered that two isolated regions (Fig. 1). Historically active volcanism at

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Figure 2. Plot of shallow (0 to 33 km) earthquakes of magnitude (mb) >4 that occurred during the period 1963 to 1974. Arrows at continental margin indicate slip vectors of focal-mechanism solutions for shallow (0 to 76 km) underthrusting events taken from Dean and Drake (1978) and Molnar and Sykes (1969). Arrows on oceanic crust approximate Cocos-NOAM and Rivera-NOAM motion. Magnetic lineations are given in m.y. B.P. The trench contour is 2,200 fathoms (4 km). C = Cocos plate; R = Rivera plate; P = Pacific plate; EPR = East Pacific Rise; TFZ = Tamayo fracture zone; RFZ = Rivera fracture zone; OFZ = Orozco fracture zone; TR = Tehuantepec Ridge; IS = Isthmus fault; M = Motagua fault system; CP = Cuilco-Chixoy-Polochic fault system; CT = Cayman trough. All other symbols are the same as Figure 1, except the smaller cones which are represented by filled triangles.

San Martin Tuxtla has produced alkaline lavas of sodic affinity, these changes include the contrasting age of ocean crust across the including picritic basalts, basanitoids, and hawaiites (Pichler and Tehuantepec Ridge (Truchan and Larson, 1973), the change in dip Weyl, 1976; Thorpe, 1977), rocks quite distinct from the potassic of the subducted plate across this lineament (discussed below), and suites of the Colima Graben. Farther south, in the Chiapanecan arc the absolute motions of the Caribbean and NOAM plates relative (Damon and Montesinos, 1978), calc-alkaline volcanism of Quater- to Cocos convergence. nary age is present but restricted to El Chichon (Fig. 1) and a small The truncated nature of the Mexican continental margin indi- center farther south; it does not become extensive until the Mexico- cates that a sliver of continental lithosphere has been removed (de Guatemala border. Cserna, 1961, 1965, 1976), but the timing, mechanism, direction of transport, and nature of this missing fragment have not yet been TRENCH AND CONTINENTAL MARGIN resolved (Malfait and Dinkelman, 1972; Kesler, 1973; Karig and others, 1978). From the late Miocene to Holocene age of the trench Several detailed studies have been made along the northern fill and from the morphology of the trench slope, Karig and others part of the Middle America Trench (Fisher, 1961; Shorand Fisher, (1978) concluded that accretion at the trench probably began in the 1961; Ross and Shor, 1965; Ross, 1971; Karig and others, 1978). Miocene and postdated translation of marginal terranes. The con- The continental margin can be subdivided into two morphologic temporaneity of volcanism within the TMVB suggests that, cer- provinces that are separated by a sharp inflection in the trench axis tainly by Quaternary time, subduction played a dominant role where the Tehuantepec Ridge intersects the continental margin. along the Mexican coast. Northwest of this junction, the continental shelf is quite narrow, and the trench is U-shaped in cross section, reaching depths of SEISMICITY about 5 km below sea level. The trench extends northward as far as Islas Tres Marias, where it ends abruptly against a southeast- Seismic activity along the Middle America arc is intense (Kel- trending fault scarp that likely separates oceanic crust in the south leher and others, 1973) and exhibits many of the characteristics from a thinned continental crust to the north (Shor and Fisher, associated with subduction processes. Previous investigations have 1961). Southeast of its junction with the Tehuantepec Ridge, the studied the geometry of the plate boundaries and the sense of trench is characterized by a broader continental shelf, a V-shaped motion at earthquake hypocenters, and they have examined rela- profile, and water depths in excess of 6 km. Factors that influence tionships between these features and continental margin volcanism

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OF Z

111 W 10 6 W 101 W 9 6 W

Figure 3. Map of intermediate earthquakes of magnitude (mb) >4 for the period 1963 to 1974. Diamonds = 100- to 150-km focal depths; squares = 150- to 200-km focal depths; and open circles = foci >200 km in depth. Contours of 50 km and 100 km mark the depth to the top of the inclined seismic zone, and the dot-dash line delineates the geographic limit of seismicity within the slab. The trench axis is black below 2,200 fathoms (4 km). All other symbols are the same as Figures 1 and 2.

(Sykes, 1967; Molnar and Sykes, 1969; Isacks and Molnar, 1969; oriented perpendicular to the axis of the trench and constructed Stoiber and Carr, 1973; Carr, 1976; Dean and Drake, 1978). The through Pico de Orizaba in the east, Nevado de Toluca, and the same data employed by these earlier workers were obtained for this region between Paricutin and Volcan Colima in the west (Fig. 1). study; the Earthquake Data File of the United States Geological All projections were corrected for earth curvature, and events Survey was accessed for the period 1963 to 1974. The locations of farther than ~ 120 km from the line of section were excluded. Addi- epicenters of magnitude (mb) 4 are plotted in Figures 2 and 3. tional control was provided by six closely spaced seismic sections of Earthquakes with hypocenters 0 to 33 km in depth are concen- the Mexican arc produced by Molnar and Sykes (1969), using relo- trated along the East Pacific Rise and associated transform faults, cated hypocenters from the same data base. and along the inner trench slope. Strong seismicity related to right- A number of features in Figure 3 are notable: lateral displacement along the Rivera fracture zone (Molnar, 1973) 1. The inclined seismic zone extends to a depth of less than 150 extends beyond the interridge segment, eastward, to intersect the km and is extremely short (< 250 km) in comparison to arc systems trench at approximately 104°30'W, suggesting the presence of a of the eastern Pacific and South America that involve older oceanic plate boundary. Seismic activity between this point and longitude crust (Isacks and others, 1968). 101° W is as intense as that associated with subduction of the Rivera 2. The majority of the volcanoes making up the TMVB are plate. located more than 50 km beyond the terminus of the inclined seis- Focal-mechanism solutions for shallow-focus earthquakes mic zone. ( < 76 km) at the continental margin indicate a northeasterly direc- 3. Subduction of the Rivera plate is not associated with Benioff tion of underthrusting of oceanic lithosphere (Molnar and Sykes, zone activity. 1969); Dean and Drake, 1978). The azimuths of slip vectors for the 4. A significant gap exists in the distribution of intermediate- Mexican arc vary from N41°E to N34°E, using the Cocos-NOAM depth earthquakes between longitudes 99° 30'W and 96° W. pole position of Minster and others (1974). An interesting relation- The dip of the Benioff zone decreases eastward along the arc, ship noted by Dean and Drake concerns the attitudes of fault plane:? from about 30° in the vicinity of Volcan Colima (situated 100 km along the arc. Northwest of the Gulf of Tehuantepec, the plunge of above the seismic plane) to perhaps as little as 20° beneath Toluca slip vectors averages 15°, while that for vectors to the south is and near San Andrés Tuxtla. At El Chichón, the Benioff zone approximately 21°. This discontinuity occurs across the landward subtends an angle of about 30°, and this angle increases southeast- extension of the Tehuantepec Ridge and coincides with distinct ward to about 40° below Central American volcanoes (Stoiber and morphological changes noted in the trench. Carr, 1973; Carr, 1976). The discontinuity in the dip of the seismic The distribution of earthquake foci at intermediate depths zone across the Tehuantepec Ridge is in the same sense as that ( > 100 km) are shown in Figure 3. Contours representing the depth observed for the plunge of slip vectors in underthrust solutions to the top of the Benioff zone are drawn at 50-km intervals, and the (Dean and Drake, 1978) and suggests that a rupture exists in the termination of the inclined seismic zone is indicated. The location downgoing plate at this locality. of the contours was controlled by three cross sections (not shown) A second major discontinuity is apparent in the western part of

the arc. Here, contours of depth to Benioff zone terminate abruptly EVOLUTION OF OCEAN LITHOSPHERE AND ITS at the Colima Graben, which marks a 100-km offset in the align- BEARING ON THE MODERN ARC ment of major volcanoes. This offset overlies the boundary between Rivera and Cocos subduction (discussed below). The structure and interpretation of the ocean crust off the The "thickness" of the seismic zone beneath the Mexican arc is Mexican coast is somewhat controversial (Atwater, 1970; Larson, about 50 km, but this measurement is considered to be more indica- 1972; Truchan and Larson, 1973; Molnar, 1973; Lynn and Lewis, tive of the uncertainties in hypocentral location than a measure of 1976; Menard, 1978). However, there does appear to be general true thickness of the Benioff zone (note the scatter of epicenters agreement that the Cocos plate represents a remnant of the larger toward the 50-km contour at longitude 101° W in Fig. 3). The more Farallon plate (Atwater, 1970) which was undergoing subduction precise (relocated) hypocenters of Carr (1976) imply a thickness of prior to 55 m.y. B.P. After that time, Menard (1978) envisaged that about 15 km for the Benioff zone beneath Guatemala, and focal- fragmentation of the Farallon plate produced two small, triangular mechanism solutions for these latter events generally reveal down- plates subjected to a regime of pivoting subduction. During the dip extension in the subducted slab (Isacks and Molnar, 1969; Dean pivoting process, the migrating triple junction of the southern and Drake, 1978). Unfortunately, no such data exist for the Mexi- (Guadelupe) plate formed the pole of rotation for ridge segments can seismic zone. and transforms to the south. The Cocos plate began to form about The most curious features of Figure 3 relate to the distribution 12 to 17 m.y. B.P. as a result of subduction of the northern portion and frequency of the intermediate-depth earthquakes along the arc. of the Guadelupe plate. Pivoting continued, but over the last several First, seismicity is absent between approximately 99°30'W, and million years, the Cocos-Pacific pole of relative motion migrated 96° W, except for five earthquakes centered at 97°30'W which scat- from the triple point to its present location at latitude 41°N, longi- ter between 100 and 140 km in depth. These latter events fall well tude 108° W (Minster and others, 1974). In an earlier model, Lynn below the main body of seismic activity and may be related to and Lewis (1976) concluded that the curved trace of major trans- strains developed near the lower boundary of the downgoing slab. forms and marked fanning of magnetic anomalies over the past 10 The top of the Benioff zone in this region is situated at about 50 km, m.y. (Fig. 4) could be accommodated by a clockwise rotation of the and shallow seismicity at the inner wall of the trench is the highest ridge coupled with an encroaching Cocos-Pacific pole. Discrimina- in the arc. Secondly, the most intense seismicity at intermediate tion between these alternate hypotheses requires a more accurate depths is located just east of the Rivera fracture zone and is asso- knowledge of the trends of transforms and magnetic anomalies west ciated with the subduction of extremely young oceanic lithosphere. of the East Pacific Rise and precise estimates of the longitudinal This situation appears paradoxical in view of the theoretically pre- variation of spreading rates. Both propositions embody the same dicted relationships between the age of oceanic lithosphere and its basic arrangement of magnetic lineations and transforms east of the seismic signature during subduction (Griggs, 1972; McKenzie, active ridge and recognize that ocean-floor evolution has been 1969). However, the history of the ocean floor in this region pro- complicated by ridge jumps between the Rivera and Siqueiros frac- vides a rational explanation for the distribution of seismicity along ture zones. the arc. A tectonic reconstruction of the age of ocean lithosphere cur-

Figure 4. Simplified reconstruction of 1 1 ow 10 5 W 95 W the age and structure of ocean lithosphere presently involved in subduction at the Middle America trench. Trend and age of magnetic lineations in the ocean crust are taken from Larson (1972), Lynn and Lewis (1976), and Karig and others (1978). Trans- 20 N forms and magnetic lineations beneath the Mexican continent were projected to allow for variations in Benioff zone dips along the arc. Ridge segments labeled I through IV are referred to in the text. Trench contours

are given in fathoms. The Rivera plate is 1 5N shaded light grey. TP = Cocos-Rivera- NOAM triple junction; PB = transform boundary between the Rivera and Cocos plates predicted from Figure 5; PR = trend of a proto-Rivera fracture zone if one was indeed present in the subducted plate; M = Mathematician Ridge; CI = Clipperton 1 ON Ridge; other symbols are those of Figures 1 and 2.

11 OW 1 0 5 W 100 W 9 5 W

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.rently involved in subduction at the Middle America Trench is Figs. 4 and 5), it is evident that the boundary between the Cocos shown in Figure 4. The trends of ridge crests and magnetic lin- and Rivera plates is essentially one of left-lateral strike slip at a rate eations are based on data presented by Larson (1972) for the Rivera of ~4 cm/yr. Uncertainties in Molnar's slip vectors could perhaps plate and on a generalized diagram of the northern part of the accommodate the proposition that the Rivera plate has recently Cocos plate taken from Lynn and Lewis (1976, Fig. 3B). Fossil been accreted to NOAM (Larson, 1972; Menard, 1978). In this case, transforms in the eastern part of the Cocos plate trend northeasterly more complex arguments are required to explain the seismicity and and represent the extensions of seismically active east-west trans- contemporaneity of volcanism along the arc. Even if such a recent forms located between ridge segments I and IV of the East Pacific accretion had taken place, it would not significantly change the Rise. For example, the Tehuantepec Ridge forms the eastern pro- direction and sense of relative motion between the Cocos and any longation of an unnamed fracture zone at 10.5° N and separates subducted portion that remained of the Rivera plate; only the mag- older crust of the Guatemala basin from young crust to the north. nitude of this motion would be affected. An extension of the Siqueiros fracture zone may form the southern The Cocos- Rivera vector may be compared with a focal- boundary of the Guatemala basin. mechanism solution for a strike-slip event that occurred in the vici- In areas affected by ridge jumps, the apparent age of subducted nity of the triple junction at a depth of 45 km within the downgoing lithosphere is deceptive. Ridge segments I and III both contain slab (Fig. 5B, Dean and Drake, 1978, event 54). The azimuthal fossil ridge crests embedded in the Pacific plate about 600 km west difference between the selected fault plane and the predicted Cocos- of the presently active spreading centers. The Clipperton Ridge Rivera transform probably lies within the error of this poor quality jumped eastward about 8 m.y. B.P., and resumed spreading in crust (C) event; the sense of motion is as expected. already 4.5 m.y. old (Anderson and Davis, 1973). Prior to this Two other features constrain the location of this triple junc- event, the age offset across the fracture zone between ridge seg- tion: a zone of shallow seismicity extending into the trench from the ments II and III was approximately 8 m.y. According to Lynn and Rivera fracture zone (Fig. 2), and an observation made by Fisher Lewis (1976, Fig. 3B), the age offset across the Tehuantepec Ridge (1961) concerning the intersection of the trench axis at this point between the same ridge segments is about 10 m.y. The latter value with a submarine mountain range, interpreted here as representing apparently relies on the accuracy of magnetic lineations in the Gua- the trace of a transform in the ocean crust. The location of a temala basin, because the position and trend of the 12-m.y. age transform-ridge-transform triple junction to the southwest is less contour agree closely with Anomaly 5A (about 12 m.y. B.P., constrained and was placed at the southern limit of the zone of according to the revised magnetic-polarity time scale of LaBrecque shallow seismicity in this, region (Fig. 2). Emanating from the triple and others, 1977), determined by Karig and others (1978) to lie on point (TP) in Figure 4 are the presently active Cocos-Rivera boun- the seaward side of the trench at 100°30'W. The Mathematician dary (predicted from Fig. 5) and the expected trace of an ancient or Ridge ceased to be an active spreading center about 4 m.y. B.P. and "proto-Rivera" transform in the ocean floor. The orientation of records an age offset of 8 m.y. between ridge segments I and II these lineaments is quite similar, and if in fact the Rivera fracture immediately prior to the ridge jump. Assuming these age offsets can zone did have an eastern extension prior to subduction, like other be extrapolated back in time, and assuming that spreading fracture zones to the south, it could accommodate a portion, if not remained approximately constant between about 12 and 30 m.y. all, of current Cocos-Rivera motion. B.P., magnetic lineations in the subducted slab can be recon- Using the above assumptions, four distinct provinces can be structed. recognized within the subducted lithosphere, each bounded by Within this evolutionary framework, fossil transforms and faults trending subparallel to the direction of plate convergence. magnetic anomalies were projected onto the Mexican continent, allowing for lateral variations in the angle of subduction inferred from the inclination of the Benioff zone. If the pivoting subduction model of Menard (1978) were implemented, the projected trans-

forms would curve slightly convex southward, and magnetic line- / Cocos- ations would remain approximately perpendicular to them, but this would not significantly change the general topology. In the region where the Cocos and Rivera plates interact, the factors affecting this tectonic reconstruction are more complex. The relative motion of the Rivera with respect to the Pacific plate (Fig. 5 A) is taken to be 6 cm/yr from spreading rates derived for the past 0 2 4 4 m.y. by Larson (1972); NOAM-Pacific motion of 5.6 cm/yr is 1 i i given by Minster and others (1974). Both vector orientations, how- cm/yr ever, represent averages of slip directions given by Molnar (1973) for focal-mechanism solutions derived for Pacific-NOAM motion near the Tamayo fracture zone and for Rivera-Pacific motion along Figure 5. Vector diagrams for deducing relative motions the Rivera fracture zone. It was these solutions that Molnar used to between the Rivera, Cocos, Pacific, and NOAM plates. A: Rivera- substantiate Atwater's (1970) suggestion that the Rivera constituted Pacific-NOAM relative motions, using data from Larson (1972), an independent plate. This produces a resultant vector for Rivera- Molnar (1973), and Minster and others (1974). B: Relative motion NOAM convergence of ~2 cm/yr, trending northeasterly. When between Cocos-NOAM (Minster and others, 1974) and Rivera- this vector is combined with a vector for Cocos-NOAM motion NOAM (from Fig. 5A) at the trench-trench-transform triple point (Fig. 5B) calculated from the pole of Minster and others (1974) for a (TP) shown in Figure 4. ES4 = focal-mechanism solution for strike- point located at the trench-trench-transform triple junction (TP in slip event 54 of Dean and Drake (1978).

When the age of the ocean lithosphere and the distribution of eastern arc. Available chemical data indicate that andesitic rocks in intermediate-depth earthquakes within the downgoing slab are the central (Toluca-Iztacc'ihuatl) region are typically more magne- examined, some systematic relationships are revealed (Figs. 3, 4). sian (higher Mg/ Mg+ Fe2+) than rocks of similar silica content in the The most intense seismicity at these depths occurs in ridge segments western arc, and they are quite different from many Cascade andes- I and III of the Cocos plate, where the subducted lithosphere is ites (Whitford and Bloomfield, 1976; Nixon, 1980). older than approximately 20 m.y. For the same slab length mea- Alkaline volcanic activity at San Andrés Tuxtla is enigmatic, sured perpendicular to the trench, the frequency of events in seg- with no obvious relationship to subduction of the Tehuantepec ment I increases markedly toward the east and corresponds to the Ridge and its likely prolongation in the slab as a tear fault. How- direction of increasing age of ocean floor. The eastern limit of this ever, compositionally similar volcanism is found farther north seismic activity ends abruptly at a fossil transform which marks the along the Gulf Coast (Robin and Tournon, 1978). Thorpe (1977) boundary between crustal provinces of differing age in the sub- suggested that this alkaline volcanism was related to fracturing ducted slab. This correlation is quite remarkable in view of the around the margins of the Gulf of Mexico, and a possible relation- completely independent manner in which each of these diagrams ship does exist between the Tuxtla region and the Isthmus fault was constructed. Seismic activity does not reappear until the east- (Fig. 3). This fault zone is seismically active and may be associated ern part of ridge segment II, where it seems to be controlled by an with extensional tectonism bordering the Cocos-NOAM-Caribbean age contour of -20 m.y. B.P. The gap in seismicity noted earlier, triple junction. Muehlberger and Ritchie (1975) selected the therefore, corresponds to a wedge of extremely young oceanic Polochic-Chixoy-Cuilco fault system as the present NOAM- lithosphere and implies that a thin, hot oceanic slab younger than Caribbean plate boundary, noted its bifurcation into a region of about 20 m.y. can be subducted aseismically at these rates of con- complex high-angle faulting extending into the Isthmus of Tehuan- vergence (6 to 8.5 cm/yr). Hence, aseismic subduction of the young tepec, and chose a fault trending NE-SW at the Guatemala-Mexico Rivera plate at a much slower convergence rate of 2 cm/yr (and frontier as the continuation of this boundary toward the trench. presumably a lower strain rate) is reasonable. If this interpretation The Guatemala earthquake of February 4, 1976, however, rather is correct, then clearly the inclined seismic zone does not necessarily dramatically demonstrated that the Motagua fault system is also place constraints on the extent or presence of a downgoing slab. active and that the juncture between the Caribbean and NOAM The observed length of the seismic zone beneath the Mexican arc plates likely encompasses all of these fault zones. In the past, this suggests that the time constant for thermal relaxation in the des- plate boundary has been represented by a series of en echelon, cending slab is about 4 m.y., as subduction at the continental mar- curving fault zones extending from the northern terminus of the gin has probably been continuous since the Miocene (Karig and Guatemalan volcanoes to Honduras (Muehlberger and Ritchie, others, 1978). 1975; Plafker, 1976; Malfait and Dinkelman, 1972). These earlier Although seismicity along the arc may be related to the age and workers have suggested that the western part of the Caribbean plate structure of the subducted slab, there is no reason why calc-alkaline is being pinned by Cocos subduction and is undergoing extension as volcanism so far removed from the trench should be related to the the main mass of the Caribbean plate moves eastward. It is appar- same tectonic framework, unless it, too, is closely associated with ent in Figure 4 that this region, with its localized volcanic activity, the subduction process. In fact, some remarkable correlations exist coincides with the subduction of a segment of older oceanic lithos- between the structure and nature of the TMVB and the downgoing phere. The diffuse triple junction is bounded in the north by the lithosphere. Isthmus fault and in the south by the northeast-southwest-trending lineament pointed out by Muehlberger and Ritchie (1975). The At the Cocos-Rivera juncture, the TMVB is characterized by: seaward extension of this latter structure is marked by a line of (1) a pronounced offset or transverse discontinuity in arc volcanism; earthquakes extending into the trench (Kelleher and others, 1973) (2) a restricted zone of concurrent alkaline and calc-alkaline vol- and may coincide with a fossil transform at the southern margin of canic activity; and (3) a region of east-west extension effected by the Guatemala basin (Figs. 3, 4). high-angle faulting along the Colima Graben. All of these features may be related to a hinge-fault mechanism operating within the downgoing slab and transmitting stresses to SEGMENTATION AT SUBDUCTION ZONES: the base of the continental lithosphere. The geometry of the arc- DISCUSSION AND CONCLUSIONS trench gap suggests that the subducted portion of the Rivera plate is inclined less steeply than the 30° dip inferred for the Cocos plate A detailed analysis of the Mexican arc suggests that late Qua- just east of Colima. The lack of a zone of intermediate-depth earth- ternary volcanism of the Trans-Mexican Volcanic Belt is related to quakes along this boundary promotes the likelihood of a hinge fault subduction at the Middle America Trench and confirms the seg- which could be caused by differences in buoyancy between young mented nature of this continental margin. The subducted slab is and old oceanic lithosphere (Molnar and Atwater, 1978; Menard, broken into three separate segments bounded by hinge faults which 1978). Since the volcanic products of the TMVB are very young and are related to structural lineaments formed in the ocean floor. Lat- the triple junction is stable in the sense of McKenzie and Parker eral variations within the TMVB relate to these segments as follows: (1967), movement of the Cocos-Rivera boundary with time is (1) a western arc associated with aseismic subduction of the Rivera unimportant. plate; (2) a central and eastern arc related to subduction of a shal- The sea floor east of ridge segments I and II exhibits a pro- lowly dipping segment of the Cocos plate, extending from the Riv- nounced difference in intermediate-depth seismicity across the era fracture zone to the Tehuantepec Ridge; and (3) a transition prolongation of the Orozco fracture zone, yet there is no evidence zone, the Colima Graben, where alkaline volcanism overlies a hinge to suggest a hinge fault at this location. Segments I and II, there- fault at the Cocos-Rivera boundary. fore, are considered as a continuous structural entity that is being Weakly developed calc-alkaline volcanism extending from the subducted beneath amphibole-bearing andesites of the central and Isthmus of Tehuantepec to the Guatemala-Mexico border is related

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to the subduction of an older segment of ocean lithosphere. This ACKNOWLEDGMENTS region represents a' diffuse triple junction between the NOAM, Caribbean, and Cocos plates. Alkaline volcanism at San Andrés This study was supported, in part, by a graduate fellowship at Tuxtla may be related to extensional tectonism in the vicinity of the the University of British Columbia and by NRC Grant 67-8841 Isthmus fault, which marks the northern limit of this triple junction. awarded to R. L. Armstrong. I thank Dan Au of the Department of The tectonic elements of the Mexican arc described above bear Geophysics and Astronomy, University of British Columbia, who little relationship to those proposed previously (Stoiber and Carr, provided computer programs for manipulating the seismic data. I 1973; Carr and others, 1974). Implicit to earlier segmentation mod- also thank R. L. Armstrong, M. J. Carr, R. L. Chase, Z. de Cserna, els of convergent plate margins is the concept that tear faults prop- G. R. Gastil, and R. P. Phillips for keen and constructive criticism agated at the trench divide the downgoing slab into segments of the manuscript. typically 100 to 300 km in width. Each segment is capable of mov- ing independently in response to subduction; consequently, deep REFERENCES CITED faults may develop in the overlying lithosphere parallel to subducted segment boundaries. Anderson, R. N., and Davis, E. E., 1973, A topographic interpretation of the Mathematician Ridge, Clipperton Ridge, East Pacific Rise system: In the TMVB, elongate clusters of cinder cones oriented Nature, v. 241, p. 191-193. northeast-southwest were regarded as the surficial expression of Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonic segment boundaries in the subducted slab because of their coinci- evolution of western North America: Geological Society of America dence with the direction of plate convergence. In fact, Holocene Bulletin, v. 81, p. 3513-3536. 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