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Geological Society of America Bulletin
The relationship between Quaternary volcanism in central Mexico and the
seismicity and structure of subducted ocean lithosphere
GRAHAM T. NIXON
Geological Society of America Bulletin 1982;93, no. 6;514-523
doi: 10.1130/0016-7606(1982)93<514:TRBQVI>2.0.CO;2
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The relationship between Quaternary volcanism in central Mexico
and the seismicity and structure of subducted ocean lithosphere
G R A H A M T. NIXON
Department of Geological Sciences, University of British
Vancouver, British Columbia V6TI W5, Canada
ABSTRACT
Late Quaternary volcanism in central Mexico is related to the
subduction of young ocean lithosphere at the Middle America.
Trench. Along-arc variations in seismicity, volcano structure, and
composition of volcanic products bear a remarkable correlation
with the age and structural framework of the downgoing slab.
Morphological and pétrographie characteristics of major
volcanoes within the Trans-Mexican Volcanic Belt (TMVB) serve
to distinguish two calc-alkaline subprovinces:
1. A western arc, averaging 60 km in width, associated with
aseismic subduction of the Rivera plate. The main cones of this
region are dominated by two-pyroxene andesites, comprise volumes
«S70 km 3 , and stand less than 3,000 m above sea level.
2. A broad central and eastern arc related to subduction of a
gently inclined segment of the Cocos plate bounded by the Rivera
transform and the Tehuantepec Ridge. Major volcanic edifices possess summit elevations in the range 4,000 to 6,000 m, have appropriately larger volumes (typically >200 km 3 ) and are constructed
with a high proportion of amphibole-bearing lavas.
The boundary between these subprovinces is marked by a
north-south-oriented structural depression, the Colima Graben,
and it coincides with a 100-km offset in the "volcanic front." Extensional tectonism in the Colima Graben, accompanied by mixed
calc-alkaline and alkaline volcanism of potassic affinity, is likely
related to a hinge-type transform fault which marks the CocosRivera plate juncture in the downgoing slab.
A third segment of ocean floor is presently interacting with
continental lithosphere south of the Gulf of Tehuantepec, where
Quaternary volcanism is weakly developed within a tectonically
complex region that marks the diffuse Cocos-NOAM-Caribbean
triple junction. The northern limit of this triple junction is defined
by the seismically active Isthmus fault, which may be related to
alkaline volcanic activity at San Andrés Tuxtla.
A tectonic reconstruction based on the evolution of oceanic
crust reveals that the distribution of intermediate-depth earthquakes along the arc is directly dependent upon the age of the
subducted slab. Ocean lithosphere younger than approximately 20
m.y. is subducted aseismically at convergence rates approaching 9
cm/yr. The length of the inclined seismic zone indicates that the
time constant for thermal relaxation in the slab is approximately 4
m.y. The T M V B overlies the aseismic extension of this young ocean
lithosphere.
Columbia,
Several aspects of this study have a bearing on the segmented
nature of converging margins in general:
1. The tectonic evolution of the ocean floor may determine the
nature of segmentation at the site of subduction.
2. The complete record of volcanism in the T M V B over the
past million years can be related to the present plate configuration.
3. Alkaline and calc-alkaline volcanism have developed contemporaneously at a converging plate margin.
4. Lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab.
INTRODUCTION
Studies of convergent plate margins during the past decade
have assembled a wealth of geological and geophysical evidence
that permits a tectonic subdivision of subduction zones (Stoiber
and Carr, 1973; C a r r a n d others, 1974; Stauder, 1973, 1975; Barazangi and Isacks, 1976). According to models developed by Stoiber
and Carr (1973), the descending lithosphere is broken by tear faults,
propagated at the trench, that divide the slab into discrete segments,
typically less than 300 km across. Each segment of oceanic lithosphere descends into the mantle with a different strike and dip,
producing offsets in features such as the inclined seismic zone,
trench axis, and alignment of cones along the "volcanic front." The
boundaries between segments are recognized by transverse features
such as mapped fault zones (commonly with strike-slip displacement), elongate clusters of cinder cones or loci of large volcanic
eruptions, and concentrations of shallow earthquakes. Where all
these criteria are used in combination, the weight of evidence generally favors segmentation, although the actual number of segments
in any particular arc may be disputed. As the model is extended into
areas of more complex plate interaction—for example, near the
triple junction of the Cocos, Caribbean, and North American
( N O A M ) plates (Carr, 1976—or if it is applied to arcs where only a
limited number of the criteria that purport to distinguish segments
are present—as in the Cascade volcanic chain (Hughes and others,
1980)—the relationships between the subducted slab and tectonic
fabric of the overriding plate become more questionable.
Regional lineaments in the T M V B formed by Holocene cinder
cones and sites of historic eruptions served as the basis for a segmentation model for the Mexican arc (Stoiber and Carr, 1973; Carr
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.
G e o l o g i c a l S o c i e t y of A m e r i c a Bulletin, v. 93, p. 5 1 4 - 5 2 3 , 5 figs., June 1982.
514
Figure 1. Generalized tectonic map of Mexico modified from de Cserna (1961).
Trans-Mexican Volcanic Belt: vertical ruling = western arc; stipple = calc-alkalinealkaline 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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516
G. T. NIXON
150 to 200 km in width, whose boundaries were assumed to lie
parallel to the northeasterly direction of underthrusting at the
trench. Study of earthquake focal mechanisms of the Central American arc by Dean and Drake (1978) did not substantiate the proposed segmented nature of the Mexican continental margin.
I will review the general structure and compositional variability of volcanism in central Mexico and relate these features to the
seismicity and structure of the young ocean lithosphere presently
being consumed at the Middle America Trench. It will be shown
that the tectonic evolution and age of the ocean lithosphere may
play an important role in determining both the nature of segmentation and the seismic signature of the subducted slab.
THE TRANS-MEXICAN VOLCANIC BELT
The locus of andesitic volcanism in central Mexico extends in a
west-east direction for more than 1,000 km, from the Pacific Coast
to the margins of the High Mexican Plateau overlooking the Gulf
of Mexico. Inspection of the Tectonic Map of Mexico (Fig. 1; de
Cserna, 1961) reveals the complex nature and extreme diversity of
"basement" terranes underlying the volcanic belt as it transects the
structural grain of the Mexican continent (de Cserna, 1965, 1976;
Demant and Robin, 1975). In the west, the T M V B is underlain by
the ignimbrite province of the Sierra Madre Occidental which
extends northward along the western Cordillera of Mexico to the
United States border. Where the two provinces intersect, gently
dipping volcanic formations of the Sierra Madre Occidental are cut
by longitudinal graben structures associated with Quaternary volcanic activity within thé TMVB. High-angle faults and tensional
fractures extend f r o m Volcan Sanganguey, near Tepic, to the
Chapala region, 50 km south of Guadalajara (Demant and others.
1976; Demant, 1978). Silicic pyroclastic rocks and intercalated
basaltic flows of the Sierra Madre Occidental just north of Guadalajara have yielded K-Ar ages of 4.5 to 9.5 m.y. (Watkins and
others, 1971). Ignimbrites of this older province extend along the:
northern edge of the T M V B as far as Pachuca, about 100 km
northeast of Mexico City (Demant, 1978). East of the Valley of
Mexico, the axis of Quaternary volcanism transgresses folded
marine sedimentary strata of the Sierra Madre Oriental and
Pliocene-Miocene plateau lavas belonging to an eastern alkaline
province (Demant and Robin, 1975; Robin and Tournon, 1978).
The peculiar geometry of the Mexican arc in comparison to
other circum-Pacific andesite provinces has generated a remarkable
array of concepts concerning its origin. Previous studies have
related volcanism to (1) a continental prolongation of the Clarion
fracture zone (Mooser and Maldonado-Koerdell, 1961); (2) an
extension of the San Andreas fault system from the Gulf of California (Gastil and Jensky, 1973); (3) an ancient geosuture subjected to
left-lateral transcurrent displacement and later reactivated in middle Tertiary time (Mooser, 1972); and (4) a phenomenon related to
subduction at the Middle America Trench (Gunn and Mooser,
1970; Mooser, 1972; Demant and Robin, 1975; Robin and Nicolas,
1978; Menard, 1978). These hypotheses and many more have been
summarized by Demant (1978).
A source of confusion arising f r o m the earlier work concerns
the age connotation of the popular term, "Trans-Mexican Volcanic
Belt," which varies from author to author even though the same
geographic entity is tacitly implied. For example, Mooser (1972),
conducting investigations in the Valley of Mexico, considered that
volcanism in the T M V B began about 30 m.y. ago, but that volcanic
activity was not widespread until mid-Miocene time. An Oligocene
to Holocene age was accepted in later studies of the same region
(Negendank, 1972, 1973; Bloomfield, 1975; Bloomfield and Valastro, 1977; Richter and Negendank, 1976). At the eastern extremity
of the volcanic belt, Robin (1976) recognized a "primitive" TMVB,
comprising Miocene andesites, and a later phase of "Neovolcanic"
activity, commencing approximately 2.5 m.y. B.P. (Robin and Nicolas, 1978; Caritagrel and Robin, 1978). In fact, Cantagrel and
Robin (1978) state that the east-west trend of contemporary
calc-alkaline volcanism has changed little since the mid-Miocene.
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 associated with the earliest stages of cone-building at Iztacc'ihuatl, near
Mexico City, and at Volcán Tequila, in the western part of the arc,
yield ages of approximately 1 m.y. (G. T. Nixon, J. E. Harakel, R.
L. Armstrong, and A. Demant, unpub. data). This study, therefore, is concerned specifically with volcanic rocks of the TMVB
younger than ~ 1 m.y. old.
Chemical and petrographic data show that the T M V B may be
divided into two distinct calc-alkaline provinces (Fig. 1): (1) a
western arc, averaging 60 km in width and extending f r o m the
Pacific coast to the Colima Graben, and (2) a central and eastern arc, stretching from the Colima volcanoes through the
areally extensive cinder cones and lava flows of Michoacan to
the locally more restricted volcanism associated with major
volcanic lineaments oriented north-south in the Sierra Nevada (Iztacciihuatl-Popocatépetl) and Orizaba-Cofre de Perote
regions.
Major cones of the western T M V B are built predominantly of
two-pyroxene andesite, stand less than 3,000 m above sea level,
and comprise volumes, less than 70 km 3 (Demant and others,
1976; Thorpe and Francis, 1975; Nelson, 1976; Luhr and Nelson, 1980). From the Colima volcanoes eastward, the major
volcanic edifices possess summit elevations in the range 4,000
to 6,000 m, are more voluminous (generally >200 km 3 ), and
are constructed with a high proportion of amphibole-bearing
andesite and dacite (Bloomfield and Valastro, 1977; Demant
and others, 1975; Nixon, 1979). This portion of the arc averages
100 to 200 km in width and lies behind an arc-trench gap of
more than 300 km at its eastern extremity.
The boundary between these subprovinces is occupied by the
Colima Graben, a region of high-angle faulting oriented northsouth and intersecting the northwest-southeast structural trends of
the Guadalajara area. The southern end of this feature is dominated
by Volcan Colima, the most active volcano in Mexico, and the
extinct(?) Nevado de Colima. On the northern and western margins
of the Colima volcanoes, Holocene scoria cones of strongly undersaturated analcime-bearing basanites and minettes (Luhr and Carmichael, 1979) nestle incongruously among the many calc-alkaline
cinder cones in the region. The comenditic dome complex of Sierra
La Primavera lies farther north, just west of the city of Guadalajara
(Mahood, 1977, 1978). The Colima Graben, therefore, represents a
region where alkaline and calc-alkaline volcanism have developed
contemporaneously at the "volcanic front" of a convergent plate
margin.
Quaternary volcanic activity between the eastern terminus
of the T M V B and the Guatemalan volcanic chain is restricted to
two isolated regions (Fig. 1). Historically active volcanism at
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QUATERNARY VOLCANISM AND SUBDUCTED OCEAN LITHOSPHERE
517
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,
including picritic basalts, basanitoids, and hawaiites (Pichler and
Weyl, 1976; Thorpe, 1977), rocks quite distinct from the potassic
suites of the Colima Graben. Farther south, in the Chiapanecan arc
(Damon and Montesinos, 1978), calc-alkaline volcanism of Quaternary age is present but restricted to El Chichon (Fig. 1) and a small
center farther south; it does not become extensive until the MexicoGuatemala border.
TRENCH A N D CONTINENTAL MARGIN
Several detailed studies have been made along the northern
part of the Middle America Trench (Fisher, 1961; S h o r a n d Fisher,
1961; Ross and Shor, 1965; Ross, 1971; Karig and others, 1978).
The continental margin can be subdivided into two morphologic
provinces that are separated by a sharp inflection in the trench axis
where the Tehuantepec Ridge intersects the continental margin.
Northwest of this junction, the continental shelf is quite narrow,
and the trench is U-shaped in cross section, reaching depths of
about 5 km below sea level. The trench extends northward as far as
Islas Tres Marias, where it ends abruptly against a southeasttrending fault scarp that likely separates oceanic crust in the south
from a thinned continental crust to the north (Shor and Fisher,
1961). Southeast of its junction with the Tehuantepec Ridge, the
trench is characterized by a broader continental shelf, a V-shaped
profile, and water depths in excess of 6 km. Factors that influence
these changes include the contrasting age of ocean crust across the
Tehuantepec Ridge (Truchan and Larson, 1973), the change in dip
of the subducted plate across this lineament (discussed below), and
the absolute motions of the Caribbean and NOAM plates relative
to Cocos convergence.
The truncated nature of the Mexican continental margin indicates that a sliver of continental lithosphere has been removed (de
Cserna, 1961, 1965, 1976), but the timing, mechanism, direction of
transport, and nature of this missing fragment have not yet been
resolved (Malfait and Dinkelman, 1972; Kesler, 1973; Karig and
others, 1978). From the late Miocene to Holocene age of the trench
fill and from the morphology of the trench slope, Karig and others
(1978) concluded that accretion at the trench probably began in the
Miocene and postdated translation of marginal terranes. The contemporaneity of volcanism within the TMVB suggests that, certainly by Quaternary time, subduction played a dominant role
along the Mexican coast.
SEISMICITY
Seismic activity along the Middle America arc is intense (Kelleher and others, 1973) and exhibits many of the characteristics
associated with subduction processes. Previous investigations have
studied the geometry of the plate boundaries and the sense of
motion at earthquake hypocenters, and they have examined relationships between these features and continental margin volcanism
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518
G. T. NIXON
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;
Stoiber and Carr, 1973; Carr, 1976; Dean and Drake, 1978). The
same data employed by these earlier workers were obtained for this
study; the Earthquake Data File of the United States Geological
Survey was accessed for the period 1963 to 1974. The locations of
epicenters of magnitude (mb) 4 are plotted in Figures 2 and 3.
Earthquakes with hypocenters 0 to 33 km in depth are concentrated along the East Pacific Rise and associated transform faults,
and along the inner trench slope. Strong seismicity related to rightlateral displacement along the Rivera fracture zone (Molnar, 1973)
extends beyond the interridge segment, eastward, to intersect the
trench at approximately 104°30'W, suggesting the presence of a
plate boundary. Seismic activity between this point and longitude
101° W is as intense as that associated with subduction of the Rivera
plate.
Focal-mechanism solutions for shallow-focus earthquakes
( < 76 km) at the continental margin indicate a northeasterly direction of underthrusting of oceanic lithosphere (Molnar and Sykes,
1969); Dean and Drake, 1978). The azimuths of slip vectors for the
Mexican arc vary f r o m N41°E to N34°E, using the Cocos-NOAM
pole position of Minster and others (1974). An interesting relationship noted by Dean and Drake concerns the attitudes of fault plane:?
along the arc. Northwest of the Gulf of Tehuantepec, the plunge of
slip vectors averages 15°, while that for vectors to the south is
approximately 21°. This discontinuity occurs across the landward
extension of the Tehuantepec Ridge and coincides with distinct
morphological changes noted in the trench.
The distribution of earthquake foci at intermediate depths
( > 100 km) are shown in Figure 3. Contours representing the depth
to the top of the Benioff zone are drawn at 50-km intervals, and the
termination of the inclined seismic zone is indicated. The location
of the contours was controlled by three cross sections (not shown)
oriented perpendicular to the axis of the trench and constructed
through Pico de Orizaba in the east, Nevado de Toluca, and the
region between Paricutin and Volcan Colima in the west (Fig. 1).
All projections were corrected for earth curvature, and events
farther than ~ 120 km f r o m the line of section were excluded. Additional control was provided by six closely spaced seismic sections of
the Mexican arc produced by Molnar and Sykes (1969), using relocated hypocenters from the same data base.
A number of features in Figure 3 are notable:
1. The inclined seismic zone extends to a depth of less than 150
km and is extremely short (< 250 km) in comparison to arc systems
of the eastern Pacific and South America that involve older oceanic
crust (Isacks and others, 1968).
2. The majority of the volcanoes making up the T M V B are
located more than 50 km beyond the terminus of the inclined seismic zone.
3. Subduction of the Rivera plate is not associated with Benioff
zone activity.
4. A significant gap exists in the distribution of intermediatedepth earthquakes between longitudes 99° 30'W and 96° W.
The dip of the Benioff zone decreases eastward along the arc,
from about 30° in the vicinity of Volcan Colima (situated 100 km
above the seismic plane) to perhaps as little as 20° beneath Toluca
and near San Andrés Tuxtla. At El Chichón, the Benioff zone
subtends an angle of about 30°, and this angle increases southeastward to about 40° below Central American volcanoes (Stoiber and
Carr, 1973; Carr, 1976). The discontinuity in the dip of the seismic
zone across the Tehuantepec Ridge is in the same sense as that
observed for the plunge of slip vectors in underthrust solutions
(Dean and Drake, 1978) and suggests that a rupture exists in the
downgoing plate at this locality.
A second major discontinuity is apparent in the western part of
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QUATERNARY VOLCANISM AND SUBDUCTED OCEAN LITHOSPHERE
the arc. Here, contours of depth to Benioff zone terminate abruptly
at the Colima Graben, which marks a 100-km offset in the alignment of major volcanoes. This offset overlies the boundary between
Rivera and Cocos subduction (discussed below).
The "thickness" of the seismic zone beneath the Mexican arc is
about 50 km, but this measurement is considered to be more indicative of the uncertainties in hypocentral location than a measure of
true thickness of the Benioff zone (note the scatter of epicenters
toward the 50-km contour at longitude 101° W in Fig. 3). The more
precise (relocated) hypocenters of Carr (1976) imply a thickness of
about 15 km for the Benioff zone beneath Guatemala, and focalmechanism solutions for these latter events generally reveal downdip extension in the subducted slab (Isacks and Molnar, 1969; Dean
and Drake, 1978). Unfortunately, no such data exist for the Mexican seismic zone.
The most curious features of Figure 3 relate to the distribution
and frequency of the intermediate-depth earthquakes along the arc.
First, seismicity is absent between approximately 99°30'W, and
96° W, except for five earthquakes centered at 97°30'W which scatter between 100 and 140 km in depth. These latter events fall well
below the main body of seismic activity and may be related to
strains developed near the lower boundary of the downgoing slab.
The top of the Benioff zone in this region is situated at about 50 km,
and shallow seismicity at the inner wall of the trench is the highest
in the arc. Secondly, the most intense seismicity at intermediate
depths is located just east of the Rivera fracture zone and is associated with the subduction of extremely young oceanic lithosphere.
This situation appears paradoxical in view of the theoretically predicted relationships between the age of oceanic lithosphere and its
seismic signature during subduction (Griggs, 1972; McKenzie,
1969). However, the history of the ocean floor in this region provides a rational explanation for the distribution of seismicity along
the arc.
Figure 4. Simplified reconstruction of
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). Transforms 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
shaded light grey. TP = Cocos-RiveraNOAM 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
Ridge; other symbols are those of Figures 1
and 2.
11 ow
2 0
1
519
EVOLUTION OF OCEAN LITHOSPHERE A N D ITS
BEARING ON THE MODERN ARC
The structure and interpretation of the ocean crust off the
Mexican coast is somewhat controversial (Atwater, 1970; Larson,
1972; Truchan and Larson, 1973; Molnar, 1973; Lynn and Lewis,
1976; Menard, 1978). However, there does appear to be general
agreement that the Cocos plate represents a remnant of the larger
Farallon plate (Atwater, 1970) which was undergoing subduction
prior to 55 m.y. B.P. After that time, Menard (1978) envisaged that
fragmentation of the Farallon plate produced two small, triangular
plates subjected to a regime of pivoting subduction. During the
pivoting process, the migrating triple junction of the southern
(Guadelupe) plate formed the pole of rotation for ridge segments
and transforms to the south. The Cocos plate began to form about
12 to 17 m.y. B.P. as a result of subduction of the northern portion
of the Guadelupe plate. Pivoting continued, but over the last several
million years, the Cocos-Pacific pole of relative motion migrated
from the triple point to its present location at latitude 41°N, longitude 108° W (Minster and others, 1974). In an earlier model, Lynn
and Lewis (1976) concluded that the curved trace of major transforms and marked fanning of magnetic anomalies over the past 10
m.y. (Fig. 4) could be accommodated by a clockwise rotation of the
ridge coupled with an encroaching Cocos-Pacific pole. Discrimination between these alternate hypotheses requires a more accurate
knowledge of the trends of transforms and magnetic anomalies west
of the East Pacific Rise and precise estimates of the longitudinal
variation of spreading rates. Both propositions embody the same
basic arrangement of magnetic lineations and transforms east of the
active ridge and recognize that ocean-floor evolution has been
complicated by ridge jumps between the Rivera and Siqueiros fracture zones.
A tectonic reconstruction of the age of ocean lithosphere cur-
1 0 5
9 5
W
W
N
5 N
1 ON
11
O W
1 0 5
W
1 0 0
W
9 5
W
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520
G. T. NIXON
.rently involved in subduction at the Middle America Trench is
shown in Figure 4. The trends of ridge crests and magnetic lineations are based on data presented by Larson (1972) for the Rivera
plate and on a generalized diagram of the northern part of the
Cocos plate taken from Lynn and Lewis (1976, Fig. 3B). Fossil
transforms in the eastern part of the Cocos plate trend northeasterly
and represent the extensions of seismically active east-west transforms located between ridge segments I and IV of the East Pacific
Rise. For example, the Tehuantepec Ridge forms the eastern prolongation of an unnamed fracture zone at 10.5° N and separates
older crust of the Guatemala basin f r o m young crust to the north.
An extension of the Siqueiros fracture zone may form the southern
boundary of the Guatemala basin.
In areas affected by ridge jumps, the apparent age of subducted
lithosphere is deceptive. Ridge segments I and III both contain
fossil ridge crests embedded in the Pacific plate about 600 km west
of the presently active spreading centers. The Clipperton Ridge
jumped eastward about 8 m.y. B.P., and resumed spreading in crust
already 4.5 m.y. old (Anderson and Davis, 1973). Prior to this
event, the age offset across the fracture zone between ridge segments II and III was approximately 8 m.y. According to Lynn and
Lewis (1976, Fig. 3B), the age offset across the Tehuantepec Ridge
between the same ridge segments is about 10 m.y. The latter value
apparently relies on the accuracy of magnetic lineations in the Guatemala basin, because the position and trend of the 12-m.y. age
contour agree closely with Anomaly 5A (about 12 m.y. B.P.,
according to the revised magnetic-polarity time scale of LaBrecque
and others, 1977), determined by Karig and others (1978) to lie on
the seaward side of the trench at 100°30'W. The Mathematician
Ridge ceased to be an active spreading center about 4 m.y. B.P. and
records an age offset of 8 m.y. between ridge segments I and II
immediately prior to the ridge jump. Assuming these age offsets can
be extrapolated back in time, and assuming that spreading
remained approximately constant between about 12 and 30 m.y.
B.P., magnetic lineations in the subducted slab can be reconstructed.
Within this evolutionary framework, fossil transforms and
magnetic anomalies were projected onto the Mexican continent,
allowing for lateral variations in the angle of subduction inferred
f r o m the inclination of the Benioff zone. If the pivoting subduction
model of Menard (1978) were implemented, the projected transforms would curve slightly convex southward, and magnetic lineations 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 c m / y r f r o m spreading rates derived for the past
4 m.y. by Larson (1972); NOAM-Pacific motion of 5.6 c m / y r is
given by Minster and others (1974). Both vector orientations, however, 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
the Rivera fracture zone. It was these solutions that Molnar used to
substantiate Atwater's (1970) suggestion that the Rivera constituted
an independent plate. This produces a resultant vector for RiveraN O A M convergence of ~ 2 c m / y r , trending northeasterly. When
this vector is combined with a vector for Cocos-NOAM motion
(Fig. 5B) calculated f r o m the pole of Minster and others (1974) for a
point located at the trench-trench-transform triple junction (TP in
Figs. 4 and 5), it is evident that the boundary between the Cocos
and Rivera plates is essentially one of left-lateral strike slip at a rate
of ~ 4 cm/yr. Uncertainties in Molnar's slip vectors could perhaps
accommodate the proposition that the Rivera plate has recently
been accreted to N O A M (Larson, 1972; Menard, 1978). In this case,
more complex arguments are required to explain the seismicity and
contemporaneity of volcanism along the arc. Even if such a recent
accretion had taken place, it would not significantly change the
direction and sense of relative motion between the Cocos and any
subducted portion that remained of the Rivera plate; only the magnitude of this motion would be affected.
The Cocos- Rivera vector may be compared with a focalmechanism solution for a strike-slip event that occurred in the vicinity of the triple junction at a depth of 45 km within the downgoing
slab (Fig. 5B, Dean and Drake, 1978, event 54). The azimuthal
difference between the selected fault plane and the predicted CocosRivera transform probably lies within the error of this poor quality
(C) event; the sense of motion is as expected.
Two other features constrain the location of this triple junction: a zone of shallow seismicity extending into the trench f r o m the
Rivera fracture zone (Fig. 2), and an observation made by Fisher
(1961) concerning the intersection of the trench axis at this point
with a submarine mountain range, interpreted here as representing
the trace of a transform in the ocean crust. The location of a
transform-ridge-transform triple junction to the southwest is less
constrained and was placed at the southern limit of the zone of
shallow seismicity in this, region (Fig. 2). Emanating f r o m the triple
point (TP) in Figure 4 are the presently active Cocos-Rivera boundary (predicted f r o m Fig. 5) and the expected trace of an ancient or
"proto-Rivera" transform in the ocean floor. The orientation of
these lineaments is quite similar, and if in fact the Rivera fracture
zone did have an eastern extension prior to subduction, like other
fracture zones to the south, it could accommodate a portion, if not
all, of current Cocos-Rivera motion.
Using the above assumptions, four distinct provinces can be
recognized within the subducted lithosphere, each bounded by
faults trending subparallel to the direction of plate convergence.
/
0
1
2
i
cm/yr
Cocos-
4
i
Figure 5. Vector diagrams for deducing relative motions
between the Rivera, Cocos, Pacific, and NOAM plates. A: RiveraPacific-NOAM relative motions, using data from Larson (1972),
Molnar (1973), and Minster and others (1974). B: Relative motion
between Cocos-NOAM (Minster and others, 1974) and RiveraN O A M (from Fig. 5A) at the trench-trench-transform triple point
(TP) shown in Figure 4. ES4 = focal-mechanism solution for strikeslip event 54 of Dean and Drake (1978).
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QUATERNARY VOLCANISM AND SUBDUCTED OCEAN LITHOSPHERE
When the age of the ocean lithosphere and the distribution of
intermediate-depth earthquakes within the downgoing slab are
examined, some systematic relationships are revealed (Figs. 3, 4).
The most intense seismicity at these depths occurs in ridge segments
I and III of the Cocos plate, where the subducted lithosphere is
older than approximately 20 m.y. For the same slab length measured perpendicular to the trench, the frequency of events in segment I increases markedly toward the east and corresponds to the
direction of increasing age of ocean floor. The eastern limit of this
seismic activity ends abruptly at a fossil transform which marks the
boundary between crustal provinces of differing age in the subducted slab. This correlation is quite remarkable in view of the
completely independent manner in which each of these diagrams
was constructed. Seismic activity does not reappear until the eastern part of ridge segment II, where it seems to be controlled by an
age contour of - 2 0 m.y. B.P. The gap in seismicity noted earlier,
therefore, corresponds to a wedge of extremely young oceanic
lithosphere and implies that a thin, hot oceanic slab younger than
about 20 m.y. can be subducted aseismically at these rates of convergence (6 to 8.5 cm/yr). Hence, aseismic subduction of the young
Rivera plate at a much slower convergence rate of 2 cm/yr (and
presumably a lower strain rate) is reasonable. If this interpretation
is correct, then clearly the inclined seismic zone does not necessarily
place constraints on the extent or presence of a downgoing slab.
The observed length of the seismic zone beneath the Mexican arc
suggests that the time constant for thermal relaxation in the descending slab is about 4 m.y., as subduction at the continental margin has probably been continuous since the Miocene (Karig and
others, 1978).
Although seismicity along the arc may be related to the age and
structure of the subducted slab, there is no reason why calc-alkaline
volcanism so far removed from the trench should be related to the
same tectonic framework, unless it, too, is closely associated with
the subduction process. In fact, some remarkable correlations exist
between the structure and nature of the TMVB and the downgoing
lithosphere.
At the Cocos-Rivera juncture, the TMVB is characterized by:
(1) a pronounced offset or transverse discontinuity in arc volcanism;
(2) a restricted zone of concurrent alkaline and calc-alkaline volcanic activity; and (3) a region of east-west extension effected by
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
the base of the continental lithosphere. The geometry of the arctrench gap suggests that the subducted portion of the Rivera plate is
inclined less steeply than the 30° dip inferred for the Cocos plate
just east of Colima. The lack of a zone of intermediate-depth earthquakes along this boundary promotes the likelihood of a hinge fault
which could be caused by differences in buoyancy between young
and old oceanic lithosphere (Molnar and Atwater, 1978; Menard,
1978). Since the volcanic products of the TMVB are very young and
the triple junction is stable in the sense of McKenzie and Parker
(1967), movement of the Cocos-Rivera boundary with time is
unimportant.
The sea floor east of ridge segments I and II exhibits a pronounced difference in intermediate-depth seismicity across the
prolongation of the Orozco fracture zone, yet there is no evidence
to suggest a hinge fault at this location. Segments I and II, therefore, are considered as a continuous structural entity that is being
subducted beneath amphibole-bearing andesites of the central and
521
eastern arc. Available chemical data indicate that andesitic rocks in
the central (Toluca-Iztacc'ihuatl) region are typically more magnesian (higher Mg/ Mg+ Fe 2+ ) than rocks of similar silica content in the
western arc, and they are quite different from many Cascade andesites (Whitford and Bloomfield, 1976; Nixon, 1980).
Alkaline volcanic activity at San Andrés Tuxtla is enigmatic,
with no obvious relationship to subduction of the Tehuantepec
Ridge and its likely prolongation in the slab as a tear fault. However, compositionally similar volcanism is found farther north
along the Gulf Coast (Robin and Tournon, 1978). Thorpe (1977)
suggested that this alkaline volcanism was related to fracturing
around the margins of the Gulf of Mexico, and a possible relationship does exist between the Tuxtla region and the Isthmus fault
(Fig. 3). This fault zone is seismically active and may be associated
with extensional tectonism bordering the Cocos-NOAM-Caribbean
triple junction. Muehlberger and Ritchie (1975) selected the
Polochic-Chixoy-Cuilco fault system as the present NOAMCaribbean plate boundary, noted its bifurcation into a region of
complex high-angle faulting extending into the Isthmus of Tehuantepec, and chose a fault trending NE-SW at the Guatemala-Mexico
frontier as the continuation of this boundary toward the trench.
The Guatemala earthquake of February 4, 1976, however, rather
dramatically demonstrated that the Motagua fault system is also
active and that the juncture between the Caribbean and NOAM
plates likely encompasses all of these fault zones. In the past, this
plate boundary has been represented by a series of en echelon,
curving fault zones extending from the northern terminus of the
Guatemalan volcanoes to Honduras (Muehlberger and Ritchie,
1975; Plafker, 1976; Malfait and Dinkelman, 1972). These earlier
workers have suggested that the western part of the Caribbean plate
is being pinned by Cocos subduction and is undergoing extension as
the main mass of the Caribbean plate moves eastward. It is apparent in Figure 4 that this region, with its localized volcanic activity,
coincides with the subduction of a segment of older oceanic lithosphere. The diffuse triple junction is bounded in the north by the
Isthmus fault and in the south by the northeast-southwest-trending
lineament pointed out by Muehlberger and Ritchie (1975). The
seaward extension of this latter structure is marked by a line of
earthquakes extending into the trench (Kelleher and others, 1973)
and may coincide with a fossil transform at the southern margin of
the Guatemala basin (Figs. 3, 4).
SEGMENTATION AT SUBDUCTION ZONES:
DISCUSSION A N D CONCLUSIONS
A detailed analysis of the Mexican arc suggests that late Quaternary volcanism of the Trans-Mexican Volcanic Belt is related to
subduction at the Middle America Trench and confirms the segmented nature of this continental margin. The subducted slab is
broken into three separate segments bounded by hinge faults which
are related to structural lineaments formed in the ocean floor. Lateral variations within the TMVB relate to these segments as follows:
(1) a western arc associated with aseismic subduction of the Rivera
plate; (2) a central and eastern arc related to subduction of a shallowly dipping segment of the Cocos plate, extending from the Rivera fracture zone to the Tehuantepec Ridge; and (3) a transition
zone, the Colima Graben, where alkaline volcanism overlies a hinge
fault at the Cocos-Rivera boundary.
Weakly developed calc-alkaline volcanism extending from the
Isthmus of Tehuantepec to the Guatemala-Mexico border is related
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522
G. T. NIXON
to the subduction of an older segment of ocean lithosphere. This
region represents a' diffuse triple junction between the NOAM,
Caribbean, and Cocos plates. Alkaline volcanism at San Andrés
Tuxtla may be related to extensional tectonism in the vicinity of the
Isthmus fault, which marks the northern limit of this triple junction.
The tectonic elements of the Mexican arc described above bear
little relationship to those proposed previously (Stoiber and Carr,
1973; Carr and others, 1974). Implicit to earlier segmentation models of convergent plate margins is the concept that tear faults propagated at the trench divide the downgoing slab into segments
typically 100 to 300 km in width. Each segment is capable of moving independently in response to subduction; consequently, deep
faults may develop in the overlying lithosphere parallel to subducted segment boundaries.
In the TMVB, elongate clusters of cinder cones oriented
northeast-southwest were regarded as the surficial expression of
segment boundaries in the subducted slab because of their coincidence with the direction of plate convergence. In fact, Holocene
lineaments of similar magnitude but different orientation are found
throughout the TMVB. For example, the prevalent structural direction in the western arc is northwest-southeast, whereas vents in the
Valley of Mexico are commonly aligned east-west, and in Michoacan, cinder cones are randomly oriented (Demant, 1978 and unpub.
geologic maps). When the north-south lineaments of late Pleistocene volcanoes in the Colima Graben and eastern arc are included,
the structure along the arc is seen to be quite complex and, in part,
reflects the localized trends of "basement" fracture zones. If the
northeasterly alignment of cinder cones is connected with deepseated fault zones formed in the manner suggested by Stoiber and
Carr (1973), then at least a few of these might be expected to be
seismically active and extend beyond the "volcanic front," toward
the trench. Other aspects of the segmentation concept, as originally
proposed by Stoiber and Carr (1973), do appear to have application
in Mexico. For example, the subduction of an active transform at
the Cocos-Rivera juncture is reflected in the structure of the continental lithosphere and composition of Quaternary volcanism in this
region, even though the slab is about 100 km deep.
Several conclusions of this study pertain to segmentation models in general:
1. Structural boundaries such as ancient transforms, aseismic
ridges, and possibly fossil ridge crests in the descending slab are
potential zones of weakness and may determine segment boundaries in the subducted lithosphere, at least to a first order. Such
features in the Nazca and Pacific plates currently trend subparallel
to convergence directions and may control segmentation in South
American and western Pacific arcs.
2. Deeper seismicity is dependent on the age of subducted
lithosphere, both along the arc and perpendicular to it. Lateral
variations in Benioff zone activity may be most apparent in regions
where young ocean lithosphere is being consumed.
3. The complete record of volcanism in the Trans-Mexican
Volcanic Belt over the past million years can be related to the present plate-tectonic configuration. Unlike many other Pacific arcs,
Quaternary volcanoes in Mexico overlie the aseismic extension of a
fairly young subducted slab. Compositional variation of volcanic
products along the arc is extensive, although all of these rocks may
be related to the subduction process (sensu lato).
Finally, it should be recognized that mature convergent margins have a complex tectonic history and that regional alignments
of volcanoes and fault zones may reflect this structural heritage
rather than tectonic elements of the downgoing plate.
ACKNOWLEDGMENTS
This study was supported, in part, by a graduate fellowship at
the University of British Columbia and by N R C Grant 67-8841
awarded to R. L. Armstrong. I thank Dan Au of the Department of
Geophysics and Astronomy, University of British Columbia, who
provided computer programs for manipulating the seismic data. I
also thank R. L. Armstrong, M. J. Carr, R. L. Chase, Z. de Cserna,
G. R. Gastil, and R. P. Phillips for keen and constructive criticism
of the manuscript.
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M A N U S C R I P T R E C E I V E D BY THE S O C I E T Y J U N E 2 5 ,
MANUSCRIPT ACCEPTED JUNE 25,
1981
1981
Printed in U.S.A.