The Valle de Bravo Volcanic Field: Geology and geomorphometric parameters of a Quaternary monogenetic field at the front of the Mexican Volcanic Belt
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Gerardo J. Aguirre-Díaz, María del Carmen Jaimes-Viera, Jorge Nieto-Obregón, 2006. "The Valle de Bravo Volcanic Field: Geology and geomorphometric parameters of a Quaternary monogenetic field at the front of the Mexican Volcanic Belt", Neogene-Quaternary Continental Margin Volcanism: A perspective from Me´xico, Claus Siebe, José Luis MacíasGerardo, J. Aguirre-Díaz
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The Valle de Bravo Volcanic Field is one of four monogenetic volcanic fields identified in the central sector of the Mexican Volcanic Belt. Michoacán-Guanajuato, Jilotepec, and Chichinautzin are the other three. The Valle de Bravo Volcanic Field is located at the southern front of the belt where it covers the western flank of the Pleistocene-Holocene Nevado de Toluca volcano. It covers an area of 3703 km2 and includes at least 120 cinder cones, one shield volcano, a few lava domes, and two lava dome complexes. It overlies a rough paleotopography of Mesozoic metamorphic rocks (schists, metalimestones, and pillow lavas), Paleocene-Eocene granitic rocks, and Eocene-Oligocene ignimbrites. Based upon morphometric parameters that were calibrated with reported isotopic ages, four groups of cinder cones were identified, older than 40 ka, 40–25 ka, 25–10 ka, and younger than 10 ka. Lava domes occur sporadically as high domes, low domes, and coulees, with ages between Pliocene to Pleistocene. We also observed several mafic lava flows that lack a cone source, suggesting that they erupted from fissures. A geomorphologic analysis of the cinder cones indicates a relatively young age for most of them, since craters are still evident and flanks are little eroded. Many lava flows still show levees and some of them are little vegetated and lack soil, which is significant for this densely forested and humid area. An analysis of the distribution of the cinder cones shows that most vents follow a NE alignment. By contrast, the domes tend to be aligned in a NW trend. This suggests that emplacement of cinder cones follows the maximum horizontal compressional stress direction, parallel to the Cocos–North America plate convergence (NE), whereas the lava domes are better developed along the minimum horizontal stress direction, perpendicular to convergence (i.e., NW).
The Valle de Bravo Volcanic Field is one of four monogenetic volcanic fields identified in the central sector of the Mexican Volcanic Belt; Michoacán-Guanajuato (Hasenaka and Carmichael, 1985; Connor, 1987), Jilotepec (Leyva-Suárez et al., 2004; Aguirre-Díaz et al., 2004), and Chichinautzin (Martín del Pozzo, 1982; Siebe et al., 2004, 2005) are the other three (Fig. 1). It is located at the southern front of the central sector of the Mexican Volcanic Belt, ∼80 km to the south-southwest of Mexico City's main square, and just to the southwest of Nevado de Toluca stratovolcano (Fig. 1). The Valle de Bravo Volcanic Field covers a wide area of ∼3703 km2, and its boundaries are within 100°29′40″W to 99°50′50″W longitude and 18°48′35″ to 19°28′45″ latitude. For practical reasons, we use UTM (NAD27 Mexico) coordinates throughout this report, and the area studied lies within 14Q 340,000–410,000 E and 2,080,000–2,150,000 N (Fig. 2). The Valle de Bravo Volcanic Field lies mostly in the southern portion of the state of México, but parts of the field are in the easternmost state of Michoacán.
The Valle de Bravo Volcanic Field can be regarded as part of the Zitácuaro–Valle de Bravo area studied by Blatter et al. (2001) and Blatter and Carmichael (1998, 2001). Aguirre-Díaz et al. (2003, 2004) defined the Valle de Bravo Volcanic Field as a Pliocene-Quaternary monogenetic field of cinder cones and domes. Here, we prefer to keep the volcanic regions of Valle de Bravo (this work) separated from that of Zitácuaro (Blatter et al., 2001; Blatter and Carmichael, 1998), because the former is basically composed of cinder cones and andesitic-dacitic monogenetic domes (single or discrete domes), with ages up to the Pliocene, and the latter includes long-lived silicic complexes, such as the Zitácuaro caldera (Capra et al., 1997), and other large silicic dome complexes, besides some cinder cones and discrete domes.
There have been several geologic studies done previously in this area by other authors that date back to the 1970s, which mainly focused on the nearby Nevado de Toluca stratovolcano and some cinder cones of the westernmost portion of the Chichinautzin monogenetic volcanic field (e.g., Bloomfield, 1974, 1975; Bloomfield and Valastro, 1974, 1977; Bloomfield et al., 1977; Macías et al., 1997), the Mesozoic basement rocks (e.g., Campa et al., 1976; Elías-Herrera et al., 2000), or included the area in regional mapping projects (e.g., Pasquaré et al., 1991; Garduño-Monroy and Gutiérrez-Negrín, 1992; García-Palomo et al., 2000, 2002). Other studies have devoted their attention to the petrologic character of the lavas of the Valle de Bravo Volcanic Field and have provided new 40Ar-39Ar ages of volcanic rocks of this area, including ignimbrites as old as 35 Ma and lavas as young as 5 ka (Blatter and Carmichael, 1998, 2001; Blatter et al., 2001).
The purpose of this work is to describe the geology and stratigraphy of the Valle de Bravo Volcanic Field and to define it on the basis of a geologic map, i.e., the distribution in space and time of the cinder cones, lava domes, and related products of the Valle de Bravo Volcanic Field (Fig. 2). By using geomorphologic parameters of the volcanic structures that formed this field, we determined the different ages of these structures, principally cinder cones, and defined the age range of them. Then, based upon the distribution of the cinder cones and lava domes of the Valle de Bravo Volcanic Field, we infer the direction of the paleostress regime for this region contemporaneous with the formation of the volcanic field, and suggest the presence of buried normal faults that could have served as conduits for the products that erupted to form the Valle de Bravo Volcanic Field.
GEOMORPHOLOGIC PARAMETERS OF THE VALLE DE BRAVO VOLCANIC FIELD
Studies of cinder cone volcanic fields of relatively young age (Quaternary) generally include the geomorphology and morphometric parameters of their volcanic structures in order to determine volumes, number and type of volcanic structures, and relative ages between these structures on the basis of their erosional stage (e.g., Bloomfield, 1975; Settle, 1979; Wood, 1980; Martín del Pozzo, 1982; Hasenaka and Carmichael, 1985; Connor, 1987, 1990). These parameters are easily obtained from direct observations in the field, from aerial photographs, and from topographic quadrangles. However, these parameters need to be calibrated with absolute age determinations; in the case of the Valle de Bravo Volcanic Field we used published ages summarized in Table 1.
The Valle de Bravo Volcanic Field includes 120 cinder cones (Table 2 202; Fig. 3), seven isolated or discrete lava domes (Table 3; Fig. 3), one shield volcano (San Agustín), and several lava plateaus (Figs. 2 and 3). Geomorphologic parameters were studied on all except for the lava plateaus.
Following the normal procedure of geomorphologic studies on cinder volcanic fields (e.g., Porter, 1972; Settle, 1979; Wood, 1980; Connor, 1987), we have used the morphometric parameters of base diameter (db), crater diameter (dc), and cone height (h) to obtain the youth index, which is the ratio of h/db (Hooper, 1995), volumes, and slope angle of the cones. The diameters used are those obtained from the average between the longest and the shortest axes of an ellipse, since most of the cones do not have perfect circular basal shapes. For the more eroded cones without visible craters, a value of zero was assigned for crater diameter, as has been done in other cases (Wood, 1980; Hasenaka and Carmichael, 1985; Connor, 1987). The results of these measurements are shown in Table 4, which includes the youth index of cinder cones within the area (Table 1). Other parameters, such as cone slope and cone volume, were calculated using the following formulas:
We used Hooper's youth index (Hooper, 1995) to estimate the morphometric age of the Valle de Bravo Volcanic Field cinder cones. This approach is valid since the parameters used by Hooper were obtained from cinder cones of the Michoacán-Guanajuato Volcanic Field, which has a similar age range and similar weather conditions to those of the Valle de Bravo Volcanic Field. We assume that these weather conditions were similar also in the recent geologic past (Pliocene-Quaternary), since both volcanic fields occur at the southern front of the Mexican Volcanic Belt, within a relatively close distance (less than 200 km away), and with about the same elevation range (1500–3000 m above sea level), and, therefore, have similar erosion rates. Hooper (1995) defined two mathematical models to determine the youth index, the linear and the nonlinear. In the linear model, steps of 8 yr were used in the computation, and in the nonlinear model, 45.1 yr steps were used. These models were calibrated to absolute ages of the cinder cones of the Michoacán-Guanajuato Volcanic Field (Hooper, 1995). Both models were applied to the morphometric data of the Valle de Bravo Volcanic Field (Fig. 4).
Four groups of cones are deduced on the basis of Hooper's youth index (Table 4), from oldest to youngest these are: older than 40 ka, 40–25 ka, 25–10 ka, and younger than 10 ka. As the age of the cone increases, the slope of its flanks decreases, as does the ratio between the cone's height and the basal diameter. Thus, young cones (younger than 10 ka) have an average slope of ∼27° that decreases as the cone ages to slopes of ∼13° in the cones older than 40 ka, whereas the h/d b ratio decreases with age from 0.213 to 0.106 (Table 5).
From Figure 4, it can be observed that the linear model of Hooper's youth index works fine for the youngest and oldest cinder cones, but shows some problems for the intermediate-age cones (25–40 ka), but in general it does not show discrepancies. This intermediate-age discrepancy is not observed in the nonlinear model, where all but a single value (point F, Fig. 4) show an agreement between the youth index and the estimated age.
The morphometric characteristics of each group are described in the following sections.
The Older than 40 ka Group
The older than 40 ka group represents 48.4% of the total number of cones in the Valle de Bravo Volcanic Field. These cones have a height range of 0.6–0.30 km, and an average height of 0.12 km; the diameter range is 0.60–2.7 km, with an average diameter of 1.25 km; the slope varies between 5° and 18°, with 13° in average; the d c/d b ratio resulted in 0.063–0.364 and an average of 0.058; and the h/d b ratio has values of 0.055–0.143 and an average of 0.106 (Table 6). These cones represent the largest volume of the Valle de Bravo Volcanic Field, at 9.46 km3. This group is mainly distributed in the central and northern parts of the volcanic field (Fig. 2), with some cones scattered around to the south. The typical morphology of these cones is that of low hills with eroded craters, either with wide craters due to the increase in crater diameter by erosion (Figs. 5 and 6), or lack of craters (the oldest) and round tops.
The 40–25 ka Group
This group includes Pleistocene (undated) cones. They only represent 5% of the total, with six structures. The basal diameter ranges between 0.650 and 1.4 km, with an average of 1.179 km; the height varies between 0.1 and 0.25 km, with an average of 0.182 km; the slopes are between 17° and 21°, with an average of 16° (Table 7). The h/d b ratio is 0.148–0.157, with an average of 0.154; the d c/d b ratio is 0–0.216, with an average of 0.036. The volume estimated for this group is 0.746 km3. These cones are morphologically degraded but still show a conic shape and generally radial drainage forming deep grooves along their slopes (Fig. 5). Since this group includes very few and scattered structures, it was not possible to define if the cones are aligned, or if they tend to group in a particular site.
The 25–10 ka Group
This group includes 22 cones with morphometric ages between 10 and 25 ka. It represents 18% of the total number of cones in the Valle de Bravo Volcanic Field. This group has the following parameters (Table 8): heights between 0.4 and 0.46 km, with an average of 0.18 km; the basal diameter range is 0.45–2.25 km, with an average of 1.15 km; the slopes are 18°–24°, with an average of 19°; the h/d b ratio range is 0.085–0.178, with an average of 0.159; and the d c/d b ratio range is 0.154–0.296, with an average of 0.072. The estimated volume for this group is 3.17 km3. Morphologically, the cones of this group are similar to the youngest group, since this group still has steep slopes and preserved craters (Figs. 5 and 6). However, these cones are slightly more degraded than the youngest group and commonly show a radial drainage cutting the flanks. Cones of this group are more common in the central and southern portions of the Valle de Bravo Volcanic Field (Fig. 2).
The Younger than 10 ka Group
This group includes the youngest cones. 29% of the total number of the cones is Holocene, according to the calculated morphometric parameters. This group includes cones with heights between 0.08 and 0.58 km, average of 0.244; base diameter range of 0.25–2.175, average of 1.14 km; slopes of 20°–47°, average of 27°; h/d b of 0.18–0.28, average of 0.21; d c/d b of 0.11–0.57, average of 0.12; and an estimated volume of 4.99 km3 (Table 9). These cones have the steepest slopes, some even with concave up shapes (Figs. 5 and 6), and show little erosion on their craters and flanks.
The Valle de Bravo Volcanic Field includes 21 discrete domes (Figs. 2 and 3). There are also three large polygenetic dome complexes that are not part of the Valle de Bravo Volcanic Field, but that are included in the geologic map of the study area (Fig. 2). These are the Zitácuaro volcanic complex, which has evidence of several volcanic phases from 12 Ma until Holocene time (Capra et al., 1997), and the little known Villa de Allende and Santiago del Monte dome complexes, both at the northern part of the Valle de Bravo Volcanic Field (Fig. 2).
The discrete domes were grouped in three types according to their morphology, following the classification of Blake (1990); these are, Peléan type, coulée type, and low type (Fig. 7). The morphometric parameters of the domes are shown in Table 10.
The Peléan-type domes are the most numerous, with 10 of the 21 being of this type, representing 42% of the total. These types of domes are characterized as high mounds with steep flanks and crumble breccias at the flanks; some may or may not have spines at their tops, and in general, they have pyramidal or conic forms; some are so pronounced that their heights are larger than their basal diameters (Blake, 1990; Figs. 7A and 8A). In the Valle de Bravo Volcanic Field, this group of domes has a basal diameter range of 1.0–5.3 km, with an average of 2.13 km; and their heights range between 0.14 and 1.0 km, with an average of 0.36 km. An example from the Valle de Bravo Volcanic Field is shown in Figure 8A. These domes occur throughout the Valle de Bravo Volcanic Field, but most of them occur in the eastern-central part of the field. They are formed by porphyritic dacites and andesites.
The coulée-type domes include six structures in the Valle de Bravo Volcanic Field, representing 25% of the total. These domes are formed when a viscous lava flow pours out from the vent asymmetrically, forming a lobe or tongue-shaped flow, reaching a relatively short distance from the conduit (Figs. 7B and 8B). They generally present ribs or curved pressure ridges on their tops showing the flow's movement, and some may also present levées along their sides. The coulée-type domes in the Valle de Bravo Volcanic Field have these characteristics: short and thick lava flows with levées and pressure ridges formed during the flow movement. Most of them (four domes) are located at the eastern portion of the field, near the western flanks of Nevado de Toluca and San Antonio stratovolcanoes (Fig. 2). García-Palomo et al. (2000) mentioned these domes and assigned to them a Pliocene age and a dacitic composition. At the western portion of the Valle de Bravo Volcanic Field, there are two other domes of this group that are mentioned in the study of Blatter et al. (2001), who assigned also a Pliocene age and a dacitic composition to them. Base diameters (flow length) range between 0.92 and 2.32 km, with an average of 1.41 km; their height is 0.16–0.40 km, with an average of 0.24 km (Table 10).
The group of low domes includes five structures in the Valle de Bravo Volcanic Field, representing only 21% of the total. This type of dome has well-rounded smooth and low hill profiles (Figs. 7C and 8C) that contrast with the conic and outstanding shapes of the first group. These structures are wider than taller. They have base diameters of 1.35–1.80 km, with an average of 1.58 km; and heights of 0.12–0.20 km, with an average of 0.14 km (Table 10). They are mainly located at the northern-central part of the Valle de Bravo Volcanic Field, although they tend to be quite separated from each other (Fig. 2).
DISTRIBUTION AND STRUCTURAL CONTROL OF CINDER CONES AND DOMES
The cinder cones and domes of the Valle de Bravo Volcanic Field show a scattered distribution at first view (Fig. 3). However, a statistical analysis of the distribution of the cinder cones indicates that most vents follow specific alignments, with orientations plotted in structural rose diagrams (Fig. 9). The rose diagrams for vent alignments were made by considering any three or more aligned vents; thus, alignments of only two vents were not considered as aligned. Because of this, one vent could be part of more than one alignment; i.e., a vent could be part of two or more alignments. From the rose diagram for the cone alignments, it is clear that most of them follow a NE direction (Fig. 9A). In contrast, most of the domes tend to be aligned in a NW trend (Fig. 9B). According to Nakamura (1977) and Connor et al. (1992), this suggests that emplacement of cinder cones followed the maximum horizontal compressional stress direction, which is parallel to the Cocos–North America plate convergence direction, i.e., NE (DeMets et al., 1990), whereas the lava domes developed better along the minimum horizontal stress direction, perpendicular to the plate convergence direction, i.e., NW. These observations may be interpreted in several ways. One possible interpretation considers the effects caused by the different rheology of the magmas during ascent that formed cinder cones and associated lavas on one hand, and of those that formed the lava domes on the other hand, which are relatively more evolved and thus more viscous. Hypothetically, the more viscous magmas had more time to evolve, because their ascent was slower than those of the relatively more mafic magmas that produced cinder cones, and vice versa. The magmas associated with the cinder cones apparently were favored by the presence of normal faults, which allowed them to ascend more rapidly. Based on the alignments of the cones, these faults should have a NE orientation. However, there is no clear evidence of NE-oriented normal faults within the Valle de Bravo Volcanic Field. So, if these faults exist, they must be buried by the Valle de Bravo Volcanic Field rocks. Connor and Conway (2000) explained that sometimes the feeding dikes related to vents of monogenetic volcanic fields are few or not visible in relatively young volcanic fields, but in relatively old and eroded volcanic fields the dikes become visible, and in some cases the number of dikes could exceed the number of vents. This implies that the feeding dikes of the vents of the Valle de Bravo Volcanic Field, which may be in turn associated to the regional stress and thus to faults, are not exposed yet in the Valle de Bravo Volcanic Field, but it is inferred that they exist at depth. Connor and Conway (2000) also mentioned that in some cinder cone volcanic fields, the topography caused by faulting (fault scarps) may be suppressed because the stress is accommodated in the crust by dilation due to dike injection rather than by fault slip.
In a more regional view, the NE direction of these assumed faults follows the same trend as the Tenochtitlán fault system, reported by De-Cserna et al. (1989) and more recently studied by García-Palomo et al. (2002). Thus, it is possible that these vents are structurally controlled by this fault system. Similarly, the domes may be controlled by a regional fault system perpendicular to the NE-oriented Tenochtitlán system, i.e., NW oriented. This could well be the Taxco–San Miguel de Allende fault system, originally reported by Demant (1978) and confirmed by several other authors later (e.g., Pasquaré et al., 1988; Johnson and Harrison, 1990; Aguirre-Díaz et al., 2005). The Taxco–San Miguel de Allende fault system has a regional NNW to NW trend and has been related to a major crustal discontinuity in central México (Molina-Garza and Urrutia-Fucugauchi, 1993; Aguirre-Díaz et al., 2005).
However, many other factors could have influenced the rheology and evolution of the magmas that formed the Valle de Bravo Volcanic Field, for instance, the initial composition of the magmas at the source, the original water content, temperature and pressure, and changes of these factors with time, which interact at subduction zone settings. Blatter and Carmichael (1998, 2001) have described some of these factors and how they affected the composition of the rocks of the Valle de Bravo Volcanic Field. In this study, we present the geologic map of the Valle de Bravo Volcanic Field, we show and emphasize the distinct distribution of the cinder cones with respect to the domes, and we indicate that the Valle de Bravo Volcanic Field's cinder cones can be subdivided in at least four groups according to their age. Our future studies will focus on the changes with time of the composition and mineralogy of these groups, considering the extruded volume (output rate) and possible changes in space and time of this volcanism.
The Valle de Bravo Volcanic Field is one of four major monogenetic fields in the central sector of the Mexican Volcanic Belt, located at the southern front of this province.
It is Quaternary in age, with cones as young as Holocene. It was contemporaneous in part with the activity of the Nevado de Toluca stratovolcano, which lies next to the volcanic field.
It includes 120 cinder cones, 21 discrete lava domes, and 1 shield volcano, covering an area of 3703 km2.
A statistical analysis of the vent alignments indicates that cinder cones favor a NE alignment, whereas the domes favor a NW alignment. The former is parallel to the direction of the actual convergence between Cocos and North America plates and follows the trend of the Tenochtitlán regional fault system, and the latter is perpendicular to this direction and is closer to the trend of the Taxco–San Miguel de Allende regional fault system.
We are grateful for the careful reviews and comments on this work by Dawnika Blatter and Charles Connor, whose comments resulted in a much improved final version of the manuscript. We acknowledge financial support from the Dirección General de Asuntos del Personal Académico of the Universidad Nacional Autónoma de México, grant PAPIIT-IN120999, and from Consejo Nacional de Ciencia y Tecnología (CONA-CYT), grant 33084T, both to Gerardo J. Aguirre-Díaz.
Figures & Tables
- cinder cones
- landform evolution
- lava flows
- Mexico state
- Michoacan-Guanajuato volcanic field
- Nevado de Toluca
- Trans-Mexican volcanic belt
- volcanic features
- Chichinautzin volcanic field
- Valle de Bravo volcanic field
- Jilotepec volcanic field