We review the current knowledge of the Pleistocene Modern Chiapanecan Volcanic Arc (MCVA). This arc is related to the subduction of the Cocos plate beneath the North American plate in the State of Chiapas, southeastern Mexico. The MCVA consists of large intrusive bodies, domes, eroded volcanic landforms, and the active El Chichón, which produced the disastrous 1982 eruption, the deadliest in Mexico’s recorded history. The available geological knowledge, and new geological and aeromagnetic data on the arc, reveals a system composed of a sizeable intrusive body called the Santa Fe diorite, and small-size volcanoes such as El Chichón and Catedral, and extinct volcanoes associated with volcaniclastic deposits. A 3D-inversion of the aeromagnetic anomalies indicates that the Santa Fe diorite is a large intrusive body (27 km long, 4 km wide with a minimum volume of 1662 km3) while small volcanoes such as El Chichón have small-size magma chambers (~ 7 km3). Interestingly, our models of the causative bodies for the aeromagnetic anomalies suggest that the El Chichón volcano, as well as of other volcanic areas in the region, are not linked directly to the Santa Fe intrusive. However, new 40Ar/39Ar dates for samples from the Santa Fe intrusive (2.2 Ma), the Catedral volcano (1.6 Ma), and a mafic enclave (1.09 Ma) hosted in 1982 Chichón deposits, along with the aeromagnetic anomalies and geochemical data confirm that these extrusive and intrusive structures belong to the MCVA. The chemistry of these structures suggests that magmas generated in the upper mantle by the subduction system evolved through different processes, such as crustal contamination for the Santa Fe diorite and Catedral volcano, and crystal fractionation for El Chichón volcano.
Most of the volcanic activity in Mexico is concentrated into the Trans-Mexican Volcanic Belt (TMVB), roughly E–W trending volcanic region in central Mexico. It is generally agreed that the origin of such a belt is the subduction of the Cocos plate under the North American plate (Pardo and Suárez 1995) (Fig. 1). The latter process, however, is also responsible for the generation of other volcanic belts in southern Mexico that have been less studied than the TMVB. This situation seems paradoxical since the most recent paroxysmal explosive eruption in Mexico occurred in an apparently isolated volcano in southern Mexico, which in fact, is part of the so-called Modern Chiapanecan Volcanic Arc (MCVA; Damon and Montesinos 1978). The MCVA was subjected to more attention after the catastrophic eruption of El Chichón volcano (Chichonal) in early 1982, but the investigation into its relation to the intrusive magmatism in the region has remained largely unaddressed. In this work, we review the geologic and geophysical knowledge on the MCVA and, adding new geological and geophysical data, we attempt to present a view of the subsurface structure of the area, which is related to intrusive magmatic processes that are partially manifested on the surface.
Before 1982, El Chichón volcano had been considered as the youngest structure in what Damon and Montesinos (1978) termed as the Modern Chiapanecan Volcanic Arc. These authors defined the MCVA as a Pleistocene arc formed by a series of plutons associated with calc-alkaline andesitic to dacitic domes, necks, and stratovolcanoes. Damon and Montesinos (1978) considered volcanic and intrusive rocks in the Santa Fe mining district-Selva Negra region as of Pleistocene age, ranging from 2.17 to 2.79 Ma (K–Ar method). More recently, another study indicated that at least ten andesitic to dacitic landforms are exposed in association with NNW-SSE faults, such as the Malpaso system in central Chiapas, with ages between 1 and 0.2 Ma (Mora et al. 2007). Therefore, the age span of the intrusive and volcanic structures of the MCVA range from 2.2 to 0.22 Ma, with El Chichón (1.09 Ma to 1982) being the youngest active volcano of the MCVA. Interestingly, the SGM geologic map (1996) shows several areas with volcanoclastic deposits in northern Chiapas that may also be associated with MCVA structures, but have not been studied in detail (Fig. 2). During the 1990s, the Servicio Geológico Mexicano (SGM, Spanish acronym of the Mexican Geological Service) generated an aeromagnetic data set of Chiapas. This information was in part used to generate regional thermal models of the subduction of the Cocos plate beneath Chiapas (Manea and Manea 2008). More detailed studies have focused on particular structures, such as El Chichón, due to its recent eruption and its continuing thermal activity.
The March–April 1982 eruption of El Chichón volcano (17.36° N; 93.23° W) produced the worst volcanic catastrophe in the history of the country, estimated to have killed more than 2000 people (Báez-Jorge 1985). Furthermore, the eruption disrupted the life of the surrounding populations and caused severe social and economic hardships (e.g., Espíndola et al. 2002; Tilling 2009; De la Cruz-Reyna and Martin del Pozzo 2009). After the 1982 eruption, many researchers devoted studies to the geological, geochemical, and geophysical aspects of the volcano and its eruption (Scolamacchia and Macías 2015; and references therein). However, a review study of the regional tectonic and magmatic setting of the intrusive and volcanic structures of the MCVA had not been undertaken. One of the few geophysical studies dealing with the latter aspects is due to Jutzeler et al. (2011), who carried out a magnetometric survey and several very low-frequency electromagnetic surveys of the area, and in conjunction with aeromagnetic data from the SGM, focused on the definition of the internal structure of the volcanic edifice and its hydrothermal system. They measured magnetic susceptibilities on the surfaces of an ample range of fresh and altered material. In this work, we review the geological, stratigraphical, isotopic, and geophysical information available on the structure of the MCVA. In doing so, we revise the 1996 geologic map and the 2004 and 2005 aeromagnetic flights of the SGM of an area of ~ 180 km × 110 km that reveals intrusive subsurface bodies and those associated with small-volume volcanoes (El Chichón and Catedral volcanoes). We complement this information with data on whole-rock chemistry, 40Ar/39Ar geochronology and Sr, Nd, and Pb isotopic ratios to obtain a more detailed assessment of their relationship and origin (Appendix 1). The regional study that we carried out reveals the presence of a series of aeromagnetic domains and their trends, some associated with volcanic strata and others with sedimentary units (Appendixes 2 and 3). A more detailed and localized analysis allowed the identification of an anomaly interpreted as an intrusive body—hereafter named the Santa Fe diorite—to the east of El Chichón, for which we obtained a model of its structure by a 3D inversion of the aeromagnetic anomalies. Finally, to assess our results, we analyzed the signals around El Chichón and compared them with the model of Jutzeler et al. (2011). Our model, not significantly different from that of the latter authors, discloses the existence of four cryptodomes with deep roots beneath El Chichón, that correspond with the topographic heights and the massive lava domes cropping out at the crater rim. Here, we present a model of the subsurface structure of an area that has undergone intense magmatic activity along a period in which El Chichón volcano is its modern manifestation.
Tectonic and Geologic Background
The tectonics of southern Mexico and western Central America is dominated by the subduction of the oceanic Cocos plate beneath the Caribbean and North America continental plates forming a diffuse trench–trench-transform triple point junction (Guzmán-Speziale et al. 1989; Authemayou et al., 2011; Garduño-Monroy et al. 2015) (Fig. 1a). The North America and Caribbean plates are bounded by the left-lateral Polochic–Motagua fault system and its extension offshore into the Cayman Trough (Burkart 1983; Guzmán-Speziale 2001; DeMets 2001; Franco et al. 2012; Authemayou et al. 2011). Some authors have proposed that the Polochic fault connects to the west with the WNW-trending Tonalá fault, parallel to the Middle American trench, (Carfantan 1976; Pindell et al. 2005; Wawrzyniec et al. 2005; Ratschbacher et al. 2009; Authemayou et al. 2011; Franco et al. 2012). The Tonalá fault is an exhumed Late Miocene greenschist facies mylonite belt with left-lateral kinematic indicators (Molina-Garza et al. 2015). This complex tectonic setting includes the subduction of the ~ 2 km high Tehuantepec Ridge (TR), a narrow linear feature within the Cocos Plate that separates seafloor with different mean depths (3900 m to the NW, and 4800 m to the SE) (Truchan and Larson 1973; Couch and Woodcock 1981; Le Fevre and McNally 1985). The TR separates the 16-Ma oceanic crust to the NW from 26-Ma crust to the SE (Nixon 1982; Manea and Manea 2006, 2008). The subducted Cocos slab dips at angles of 25°–35° to the west (Pardo and Suárez 1995; Rebollar et al. 1999; Espíndola et al. 2016) and 40°–45° to the east of the TR (Rebollar et al. 1999). In Chiapas, part of the relative plate motion between North America and the Caribbean is accommodated by WNW left-lateral strike-slip faults (Guzmán-Speziale and Meneses-Rocha 2000; Meneses-Rocha 2001; García-Palomo et al. 2004) acting since Late Miocene. Volcanism inland and west of the TR appears at Los Tuxtlas Volcanic Field (LTVF) at ~ 300–350 km from the trench, while volcanism to the east is dispersed and appears up to ~ 350 km from the trench, at El Chichón and scattered along the Modern Chiapanecan Volcanic Arc (MCVA) (Damon and Montesinos 1978; Mora et al. 2007, 2012) (Fig. 1b). El Chichón is the youngest volcano at the northwestern extreme of the MCVA. The thickness of the Cocos plate under Chiapas is 39 ± 4 km dipping at an angle of about 40° (Rebollar et al. 1999; Espíndola et al. 2016). This plate geometry places El Chichón and the Santa Fe diorite at ~ 350 km from the trench and ~ 200 km above the projected Cocos plate.
Review of Regional and Local Geology
The regional geology of north-central Chiapas is shown in the 1:250,000-scale map (Villahermosa E15-8) of the Servicio Geológico Mexicano (SGM 1996; Fig. 2). The map displays the major left-lateral strike-slip faults transecting central Chiapas (e.g., La Venta-Grijalva, Tuxtla, and Malpaso) (Meneses-Rocha 2001). Figure 3 shows the local geology of El Chichón volcano synthesized from previous works (Canul and Rocha 1981; Layer et al. 2009; Macías et al. 2010; Garduño-Monroy et al. 2015) that consist of:
Evaporites interbedded with dolomitic limestones and bentonitic beds from the Lower Cretaceous (lK-evls). The top of this sequence was encountered at a depth of 2595 m in the Caimba-12 well of PEMEX, the Mexican petroleum company, and has a minimum thickness of 1000 m. These rocks are overlain by light brown to gray-massive dolomitic limestones and evaporites with a total thickness of 1500 m. These rocks crop out south of El Chichón volcano in the Caimba anticline. A thickness of 400 m of similar carbonate rocks was reached at the Unión-2 well, located SW of the town of Francisco León (Canul et al. 1983). In the regional map (Fig. 2), this succession corresponds to the Sierra Madre formation (KapssCz-Do).
Paleocene to Eocene sandstones and calcareous claystones (P-Escu) outcrop around El Chichón volcano, with an inferred minimum thickness of 2000 m. This succession corresponds to the Nanchital (TpaeLu-Ar), Soyalo (TpaLu-Ar), and El Bosque (TeLm-Ar) formations in Fig. 2. From base to top it consists of dark-gray claystone with limestone beds, gradually replaced by thinly bedded claystone and lenses or beds of sandstone.
Early Miocene Sandstones and Claystones (eMscu) consist of thick, mica-rich, light-brown sandstone and conglomerate beds containing leaves, stem fragments, and charcoal, alternating with dark-gray siltstones. This succession, up to 300 m thick, correlates with the La Laja-Depósito-Encanto formations (TomAr-Lu). The Catedral volcano and volcaniclastic fans are exposed on top of this unit mapped by the SGM as andesites (TplBvA-A) (Fig. 2).
Middle to Upper Miocene Sandstones and Conglomerates (P-Mtu) particularly limestones, sandstones, and reddish conglomerates crop out to the northeast of El Chichón.
The Santa Fe intrusive is late Pliocene (2.2–1.04 Ma), light gray with a dioritic portion exposed ~ 25 km to the east-southeast of El Chichón (Figs. 1b, and 2). The rock is phaneritic with large phenocrysts of plagioclase, K-feldspar, amphibole, and biotite. Under the microscope, this rock shows a mineral assemblage of plagioclase, orthoclase, quartz, ortho- and clinopyroxene, biotite, amphibole, and oxides. The chemical composition of this rock is dioritic (57.14 wt% silica and 7.4 wt% alkalis) (Table 1; Fig. 4a). 40Ar/39Ar ages from this rock were 2.21 ± 0.7 Ma for biotite, and 1.66 ± 0.2 Ma for whole-rock (Table 2; Fig. 4c–e). This intrusive rock corresponds to the granodiorite-diorite unit (TplGd-D) of the SGM (1996). Damon and Montesinos (1978) described volcanic and intrusive rocks in the Santa Fe mining district-Selva Negra region, with K–Ar ages ranging from 2.17 to 2.79 Ma. Recently, Jansen et al. (2017) studied the Cerro La Mina porphyry prospect and reported U–Pb ages in zircons of 1.04 Ma, as well as a 40Ar/39Ar biotite age of 689 ka (representing the cooling time of the intrusive body by the closure of biotite) (Fig. 1b).
Catedral volcano (1.64 Ma) is located ~ 11 km to the northwest of El Chichón (Fig. 1b). It is a reddish, eroded structure, extending to the northwest from its source. The deposits mimic a fan composed of a series of debris flow deposits containing metric-sized boulders and gravel immersed in a coarse sandy matrix, interbedded with pyroclastic deposits. The rock is porphyritic with plagioclase, pyroxene, amphibole, and biotite phenocrysts. In thin section, it is composed of plagioclase, ortho- and clinopyroxene, amphibole, and biotite with resorbed rims, all set in a microlithic groundmass (~ 40 vol%). This sample was analyzed to determine its whole-rock composition yielding 56.76 wt% silica, and 5.6 wt% alkalis (trachyandesite) (Table 2; Fig. 4a). The same sample yielded a whole-rock 40Ar/39Ar age of 1.64 ± 0.02 Ma (Table 3; Fig. 4c). In Fig. 2, a series of fanlike successions (TplBvA-A) that may be related to ancient volcanic centers not yet described are exposed in this part of northern Chiapas.
El Chichón volcano (1.09 Ma-Holocene) consists of a 2-km-wide Somma crater with an inner 1-km-wide crater formed during the 1982 eruption. Pre-Somma activity determined by dating of andesitic blocks in pyroclastic deposits extends back to 0.37 Ma (Layer et al. 2009). The Somma crater has an age varying from 0.276 to 209 Ma (Damon and Montesinos 1978; Duffield et al. 1984), and it was intruded by the SW dome dated at 217 ka. A series of pyroclastic and debris flow deposits constitute the outer slopes of the volcano; one of the pyroclastic deposits was dated at 55 ka. A series of trachyandesitic domes exposed to the NNW of the Somma crater have been recently recognized (Macias et al. 2010). These are the Cambac dome (187–168 ka), the Capulín domes (152 ka), and associated pyroclastic flow deposits (48–102 ka), and the NW dome (80–97 ka). At least 11 explosive eruptions have occurred at El Chichón during the last 8000 years including the 1982 eruption (Espíndola et al. 2000; Scolamacchia and Capra 2015).
Conducting this work, we collected the oldest rock dated so far; it is a dark-gray mafic enclave sampled in a 1982 pyroclastic flow deposit, with a granular texture, constituted by two populations of crystals (microphenocrysts and phenocrysts) of plagioclase, clinopyroxene, and Fe–Ti oxides. The mafic enclave is basaltic (44 wt% SiO2 and 3.27 wt% alkalis; Fig. 4a), being the most mafic rock in the El Chichón area. The 40Ar/39Ar age for this enclave was determined from an amphibole concentrate in 1.09 ± 0.21 Ma (Table 2; Fig. 4d).
Data and Method of Analysis
The geomagnetic data used in our analysis were obtained through an aeromagnetic survey carried out by the SGM, between April 2004 and February 2005 with a Geometrics G-822A magnetometer; the characteristics of this instrument, other equipment used for the survey, and the specifications of navigation and flights are given in Table 3. The data were collected with GPS navigation along N–S flight paths at elevations of 300 m above the terrain and 1000 m apart. E–W control lines every 10,000 m were also flown. No topographic reductions were necessary, as the carrier flew at constant elevations above ground level (contour type). The data were corrected for plane movement (magnetic compensation), diurnal variation and was leveled using the control lines and micro leveling. Processing of the data yielded, the total magnetic field (TMF) was obtained; the Residual Magnetic Field (RMF) was obtained by subtraction of the International Reference Geomagnetic Field (IGRF 2000, 2005) from the TMF. The RMF from an area 82 × 59 km in a mesh of 500 × 500 × 500 m was used in all the inversions. Several transformations and the modeling which are explained below were carried out with commercial software from Geosoft™ (http://www.geosoft.com).
It is assumed that removal of the TMF leaves only the signals due to the magnetic response of the bodies in the surveyed area. However, these signals are induced by the Earth’s geomagnetic field and yield in general bipolar anomalies, of cumbersome interpretation and modeling. To facilitate the interpretation, the RMF is usually transformed to values that, theoretically, would have been obtained at the pole, where the geomagnetic lines are perpendicular to the surface (Baranov and Naudy 1964). Such transformation yields the Magnetic Field Reduced to the Pole (RTP), in which the anomalies appear over the causative bodies, which simplifies the interpretation and further application of other enhancing filters. The reduction in the Pole was performed via mean values of the IGRF over the study area for the period March 2004–February 2005, with the intensity of 40.441 nT, inclination 45.98°, and declination 3.73°.
The magnetic signals obtained in the aerial surveys arise from causative bodies (surface and subsurface) of contrasting magnetic susceptibility, differing dimensions, and/or located at different depths. Thus, filtering of the signals to enhance the characteristics of the bodies, or traits that we want to infer greatly improves the interpretation. Therefore, we processed the RTP to obtain enhanced signals as follows.
We obtained 1st and 2nd Derivatives (Henderson and Zietz 1949a, b), to emphasize shorter wavelength features: The First Vertical Derivative defines near-surface features, while the Second Vertical Derivative emphasizes plan-view boundaries of deeper units, often basement rocks. The Upward Continuation extrapolates the signal as would be measured at some height; therefore, it reduces the effects of near-surface bodies as it attenuates the shorter wavelengths. The Analytical Signal in 2D is calculated from the two orthogonal derivatives of the components of the magnetic field (Nabighian 1972, 1974). Its effect is to produce a positive body-centered anomaly with sharpened contours and yields a signal that depends on the location of the bodies, but not on their magnetization directions (Blakely 1996).
The models of the causative bodies in the cases examined were obtained through inversion of the RMF using the Magnetic Vector Inversion (MVI) method developed by Ellis et al. (2012, 2013). The MVI technique is distinguished from other approaches by its use of magnetic susceptibility as the scalar proxy for the magnetic character of the model. Ellis et al. (2012, 2013) described the MVI mathematical method, along with its use in the voxel-based 3D inversion software, which is based on the Tikohonov (1963) minimum gradient regularization. The MVI inverts for the magnetization vector, rather than for susceptibility. This procedure avoids the gaps produced by the absence of knowledge on the magnitude or direction of the remnant field, which in many cases can be substantial. By using this procedure, bodies with negative susceptibility are not generated, which yields more realistic models.
The depth of the causative bodies was obtained through the Radially Averaged Power Spectrum (RAPS). This signal is the power spectrum averaged across several directions of the study area, and yields estimates of the depth of the causative bodies (Spector and Grant 1970).
Geochemical and Isotope Analyses
We analyzed three samples of the study area that correspond to an intrusive body in the Santa Fe mining district, hereafter termed Santa Fe diorite ((MCH516), a block from the Catedral volcanoclastic fan (MCH505B), and a mafic enclave from El Chichón (CH0902E). To carry out the mineral description of the samples, we prepared polished thin sections and observed them under the petrographic microscope. To perform the whole-rock geochemical, X-ray fluorescence (XRF), and Sr, Nd, and Pb isotopic analyses of these rocks we milled in the laboratory of the Geophysics Institute of UNAM, Morelia. The three whole-rock samples, phenocryst-free, were crushed, sieved and washed in deionized water for 40Ar/39Ar isotopic dating at the Geophysical Institute, University of Alaska Fairbanks. Details on the analysis procedures of the samples are given in Appendices 1 and 2.
Results of the Analyses of Aeromagnetic Data
Figure 5a shows the Residual Magnetic Field (RMF) of the study area; the magnetic field values vary from about − 300 nT to more than 700 nT at some spots. The distribution of magnetic intensities is displayed by colors with different magnetic field categories. Note the small positive anomaly associated with El Chichón at the western margin of a large negative anomaly. The large negative anomaly was associated by Jutzeler et al. (2011) to a Neogene granodiorite-diorite intrusion that slightly overlapped El Chichón anomaly, and later on, corroborated by Jansen et al. (2017) to the large plutonic body of the Santa Fe diorite; an interpretation with which we concur and follow in our terminology. In Fig. 5a, one can observe a significant normal dipolar aeromagnetic anomaly, with a NE–SW elongated magnetic high, to the south of the area, and a magnetic low to the north elongated in the E–W direction. The magnetic high of this visible anomaly is characterized by two peaks with magnetization intensities of 1.104 to 1960 nT, and polar distances of 5.8 to 15.3 km, respectively. From the center of the area to the NW, there is a small magnetic anomaly with long axis striking E–W, measuring 5350 m in the N–S direction and 6360 m in the E–W direction. It shows normal dipolar features associated with El Chichón volcano and has an intensity of magnetization of the order of 511 nT and a polar distance of 3150 m. Figure 5b shows the Reduced to the Pole Magnetic Field (RTP); note the change in the contour lines best appreciated in the shift in the positions of the summits (or bottoms) of the anomalies, which are now over the causative bodies. We used the RPMF for all the subsequent transformations.
Figure 6a shows the division of the area into zones of contrasting magnetic intensity values. The boundaries between these zones allow defining the so-called aeromagnetic domains (AMD) (López-Loera 2002; López-Loera et al. 2011). Thus, AMD’s are areas characterized by similar magnetic intensity, “texture,” or complementary magnetic polarity (López-Loera 2002). The eight AMD recognized are described in detail in Appendix 2 and Table 4. The Vertical Derivative continued 250 m upwards was applied to the RTP; this operation yields signals arising from deeper causative bodies with similar magnetic susceptibility. Figure 6b shows the results of the process and the domains that can be distinguished; their detailed description is given in Appendix 3 and Table 5. Notice that the domains found in the chart of the RTP (Fig. 6a) do not coincide entirely with those obtained from the second vertical derivative (Fig. 6b). The reason is that in the first case the signal is dominated by the contribution from shallow rocks, while in the second the contribution of the deeper bodies has been enhanced. Figure 6 shows the geologic map of the region (SGM 1996) with the domains displayed on top. It is evident that the geologic units are associated with rocks exposed at the surface, while only some of the AMDs are associated with the exposed geology (Fig. 6c). Thus, the AMDs reveal rather the magnetic basement, and some domains do not have a close association with the surface rocks. Alternatively, in Fig. 6d, the geologic map is compared with the domains (AMD) generated from the Second Vertical Derivative (Fig. 6b). Figure 6b shows, in some cases, good concordance with the exposed geological units (e.g., AMD IIb, VI). This indicates that the aeromagnetic signal is produced by the magnetized bodies at different depths and probably with varying directions of magnetization. The application of the algorithm of the First Vertical Derivative in the Z direction reveals some aeromagnetic alignments in the study area. The results are presented in Fig. 7a and b, which shows most of them trending NNE, with a second noticeable direction to the NWW.
The RMF of the study area was also transformed to obtain the Analytic Signal (AS), which was upward-continued 500 m (Fig. 8a). The AS exposes, in the central portion of the study area, a series of anomalies associated with the Santa Fe intrusive, La Minita mining district, El Chichón and Catedral volcanoes and (or) volcaniclastic/epiclastic fans (Fig. 8b). These anomalies are associated with volcanic rocks, such as those related to El Chichón and the Catedral structure located some 11 km NW from the former.
The most conspicuous feature of the RMF at this scale is a normal dipolar anomaly (Fig. 9a), whose magnetic “high” is somewhat elongated with long axis trending roughly NE–SW and to the north a magnetic “low” with the major axis in the E–W direction. There are two magnetic highs in this big anomaly; their magnetization intensities are 1104 and 1960 nT, with polar distances of 5.8 km to 15.3 km, respectively. At the NW portion of the largest anomaly high, there is a small normal dipolar anomaly trending roughly E–W. It is associated with El Chichón and shows the intensity of magnetization of the order of 511 nT, and a polar distance of 3150 m. It has dimensions of 6360 m in the E–W direction, and 5350 m perpendicularly. The Reduction in the Pole of the signal (Fig. 9b) shows, at the center, an aeromagnetic anomaly formed by a large “high” with major axis trending approximately NNE–SSW, and a “low” elongated along the E–W direction. There are two magnetic “highs” with magnetization intensities of the order of 1851 to 2081 nT, and polar distances of 10.8 to 18.4 km, respectively. The small isolated anomaly associated with the El Chichón volcano shows up to the W of the large anomaly. It consists of three small “highs” with intensities greater than 200 nT. The first vertical derivative shows an aeromagnetic configuration with linear features suggesting active tectonism in the western zone of the study area, at the location of El Chichón volcano (Fig. 7a, b). The tectonism has been less severe toward the eastern area. The analysis of the aeromagnetic lineaments shows that they are trending preferentially across the NNE–SSW and NWW–SEE directions. Figure 8a and b displays the Analytic Signal continued 500 m upwards. This transformation shows clearly the dimensions (25 km NE–SW and 14.6 km NW–SE) of a causative body located to the E and SE of El Chichón volcano. This source produces a large anomaly also observed in Fig. 9a and b. The magnetic anomaly is caused by the Santa Fe intrusive and the Minita mining district, which generates two adjacent “highs” separated probably by a fault-oriented NW–SE. Figure 9a and b shows two other anomalous zones probably associated with volcanic rocks, to the SW and NW of El Chichón anomaly, the second one related to the extinct Catedral volcano. Structurally, Catedral and Chichón volcanoes are the extremes of an NW–SE left step system in an area of geothermal interest (Arellano-Contreras and Jimenez-Salgado 2012).
Depth estimation and 3D-magnetic inversion model
Before performing the inversion, we estimated the depths of the magnetic sources associated with the anomalies on El Chichón, and the Santa Fe diorite located toward the SE of the former. To this purpose, we obtained the Radially Averaged Power Spectrum (RAPS) (Fig. 10a–c) described in the sect. 3.1. In the intrusive zone, depths of 4.5 km were estimated for deep sources, 0.64 km for intermediate sources, and 0.337 for surficial sources. For the inversion of the anomalies, we used, as a guideline for the assignment of susceptibility, the data reported by Jutzeler et al. (2011), already mentioned in the Introduction and unpublished data from Sulpizio et al. (2008), and representative values reported by Hunt et al. (1995). The data from the former authors were obtained from rocks at El Chichón and extended the values to the other bodies of the region. The inversion of the data yielded the model of Fig. 11a–c. The body with a response that fits the large semi-regional aeromagnetic anomaly of Fig. 9 (associated with the Santa Fe intrusive) has the following dimensions: 27.4 km in the N–S direction, and 14.52 km in the E–W direction. Its depth ranges between 5.7 and 7.1 km. The susceptibilities used in the model for the geological unit in the interior (in red) were 0.02 SI and 0.0053 SI for the exterior (pink); these are average values for the dioritic intrusive and the country sedimentary rocks, respectively. Our 3D model for the body under El Chichón volcano used a magnetic susceptibility of 0.040 SI from Sulpizio et al. (2008) that yields a body of irregular shape. The mentioned value of susceptibility is one of the highest susceptibility measured in the area. Regarding El Chichón, the corresponding body is elongated and somewhat bowl-shaped with three protuberances. In the E–W direction, the body is 4.4 km long, and 2.4 km in the N–S direction. The depth of both lumps is 1.58 km and 2.52 km, and the base of this body is estimated at 4.2 km.
Interpretation of Anomalies of El Chichón Volcano
Figure 12a is the RTP of a small area centered at El Chichón. The image shows an irregular monopolar anomaly, with dimensions of the order of 4850 m to the NW–SE and 4210 m to the NE–SW. The magnetic high is formed by a series of local highs with intensities between 507 nT and 771 nT. El Chichón’s crater is associated with a magnetic low of 307 nT at the saddle point between two magnetic highs of 441 nT and 771 nT, in Fig. 12a, the RTP shows NW–SE and NE–SW alignments. The form of the anomaly depicts the four anomalies described by Jutzeler et al. (2011) that the authors correlate to the SE summit, NW dome, SW dome and to the east of SE summit. Our anomaly also suggests that the edifice of the volcano and the structure beneath is bounded by zones of weakness, typically faults and (or) fractures (Fig. 12). Some of them agree with strike-slip faults found in a structural study by García-Palomo et al. (2004). The trace of the magnetic profile follows approximately the trace of the Chichón-Catedral fault, which divides El Chichón’s crater into two parts (Garduño-Monroy et al. 2015). The other alignments are probably due to faults covered by surface material or interface boundaries between geologic units. The first vertical derivative (Fig. 12b) shows the alignments in the area with more resolution. In addition to those already pointed out in the directions NW–SE and NE–SW, the existence of a long alignment in the NE–SW direction, crossing the center of El Chichón´s crater. Such alignments were also traced by CO2 and He emissions traversing the crater lake (Mazot et al. 2011), which is probably related to the San Juan Fault system (García-Palomo et al. 2004). The result of the Analytic Signal filter (Fig. 8) allows the assessment of the dimensions of the magnetic source related to El Chichón with greater precision. The anomaly trends NW–SE with altitudes of 5300 m on the northern part, and 3849 m on the south. In the NE–SW direction, it has a width ranging from 4620 m on the central portion, 4200 m on the west, and 3100 m on the east.
The application of the horizontal gradient magnitude yields U-shaped lineaments (blue lines in Fig. 12c). The alignments associated with the volcanic structure of El Chichón allow a tectonic interpretation based on the contour map of the Analytic Signal: In the first one, fractures and (or) primary faults trend NW–SE (Fig. 12d); alternatively, they trend NE–SW (Fig. 13d). These results agree with the faults determined by García-Palomo et al. (2004) who consider them as strike-slip faults. Thus, two zones of weakness intersecting within the crater area are apparent. Additionally, anomalies also correlate well with the NW and SW domes as previously observed by Jutzeler et al. (2011) suggesting that El Chichón complex is made of a variety of volcanic bodies with distinct roots/conduits but probably fed by the same magma chamber. The stress rose chart (Fig. 12d) shows that the main lineaments are oriented NW–SE in accordance with that of the known structures in the area.
The model based on the geomagnetic anomalies is an amorphous body, somewhat bowl-shaped, with several arms or apophyses. Its bottom is sunk to depths of 2.5 km and has four main protrusions associated with the existing domes at the margins of El Chichón (Fig. 13a). The body is stretched out in the E-W direction with dimensions of the order of 4.76 km and 2.76 km in the N-S direction, its top is found at depths ranging from 357 m to 1.76 km. The susceptibilities used for modeling were between 0.0247 and 0.0777 SI. One can speculate if there is an existing magma chamber at high temperature (beyond the Curie point). This conjecture is based on the large probability of the existence of a remnant of a slowly cooling magma at depth after the 1982 eruption (e.g., Bower and Woods 1998). In that case, the portion of the chamber above the Curie point would have a magnetic susceptibility value of 0. Introducing such a value into the procedure the best fit model resembles the number 8 (Fig. 14b). It has a maximum length of 2.91 km, with top and bottom 5.58 km and 8.49 km deep, respectively. It has a maximum width trending E–W for 1.72 km with a minimum of 0.46 km. The volume of this body is about 9 km3, a volume consistent with the estimated volume ejected in the 1982 eruption, that is ~ 1 km3, or about 11% of the volume of the magma chamber (Bower and Woods 1998). However, this volume corresponds to that of the magma at temperatures as low as the Curie point (~ 570 °C) being a mush composed of crystals and melt, in agreement with the zones of high coda attenuation found for seismic waves below the volcano (Zuñiga and Díaz 1994).
Interpretation and Discussion
Pleistocene–Holocene Magmatism of the MCVA
One of the most striking features observed in the regional aeromagnetic models of Chiapas is the presence of intrusive body at subsurface levels. We call this body the Santa Fe intrusive, to suggest a possible link with the small bodies reported as isolated outcrops near Santa Fe by Damon and Montesinos (1978). These authors considered volcanic and intrusive rocks in the Santa Fe mining district-Selva Negra region as of Pleistocene age, ranging from 2.17 to 2.79 Ma (as dated by the K–Ar method) and considered part of the MCVA (Fig. 1b). Recent investigations have reported younger ages for a nearby intrusion called Cerro La Minita ranging from 1.04 to 0.78 Ma (Jansen et al. 2017). These sequences were described as basaltic andesites to andesites, pyrite-bearing diorite, and pumiceous pyroclastic ash flows (Damon and Montesinos 1978). In fact, the latter authors dated the Tzontehuitz dome at 2.14 ± 0.04 Ma, La Lanza dome at 0.85 ± 0.02 Ma, Apas at 0.43 ± 0.03 Ma, and the Huitepec dome at 0.85 ± 0.03 Ma. Capaul (1987) described for the first time the MCVA as formed by relatively small volcanoes and domes, and dated the Mispia dome (Nicolás Ruiz volcano) at 0.34 and 0.84 Ma with the K–Ar method. A more comprehensive study of the MCVA indicated that at least 10 andesitic to dacitic landforms are exposed in association with NNW-SSE faults as the Malpaso shown in Figs. 2 and 7a (Mora et al. 2007). Subsequently, Mora et al. (2012) reported additional 40Ar/39Ar determinations for most of the volcanoes with new ages of Navenchauc dome (0.369 ± 0.018 and 0.277 ± 0.005 Ma), Amahuitz dome (block in a debris-avalanche deposit 0.739 ± 0.110 Ma), La Iglesia dome (0.689 ± 0.033 Ma), Mispia dome (0.975 ± 0.011 Ma), Santotón dome (0.995 ± 0.009 Ma), and Venustiano Carranza volcano (block from a pyroclastic flow deposit 0.225 ± 0.030 Ma). Therefore, the age span of these structures ranges from 2.2 to 0.22 Ma, with El Chichón (1.09 Ma to Holocene) being the youngest active volcano of the MCVA. Hence, the regional map of Chiapas (SGM, 1996), the descriptions of metallogenetic deposits of Damon and Montesinos (1978) and Jansen et al. (2017), and the mapping and description of small volume volcanoes and domes in central Chiapas (Capaul 1987; Mora et al. 2007, 2012) suggest that the 2.2 Ma Santa Fe diorite and the 1.6 Ma Catedral volcano are the results of the widespread magmatism within the MCVA. Interestingly, the SGM geologic map (1996) shows several areas with volcaniclastic deposits in northern Chiapas that may be also associated with MCVA structures as well; however, they have not been studied in detail (Fig. 2).
Magma Sources of the MCVA
In order to understand the magmas sources of the MCVA and their relationship with the magnetic anomalies, we compared previous data of the MCVA (Espíndola et al. 2000; Macías et al. 2003; Mora et al. 2007, 2012; Arce et al. 2015) with the Santa Fe diorite, the Catedral and El Chichón rocks and the Chapultenango trachybasalt in terms of whole-rock chemistry and Sr and Nd isotopic ratios. The rocks from the first two units are similar in composition to El Chichón in the total alkalies versus silica diagram (Le Bas et al. 1986), where the Catedral rock plots within the andesite field, and the Santa Fe intrusive in the trachyandesite field. Trace-element concentrations for the Santa Fe and Catedral structures also display a similar pattern than El Chichón, with a clear enrichment in incompatible elements compared to the compatible ones and marked negative anomalies in Nb, Ta and Ti, and positive anomalies in Cs, K, and Pb (Fig. 4b).
Even though all the samples are chemically similar, their Sr/Nd isotopic compositions vary considerably (Fig. 15a; Table 3). The mafic enclave shows primitive initial 87Sr/86Sr values of 0.7036 and εNd of + 5 (Table 3), being even less radiogenic than the rest of El Chichón samples. In contrast, Catedral and the Santa Fe intrusive show higher 87Sr/86Sr values (0.7059 and 0.7060, respectively) and εNd of − 1.9 and -2 (Table 3), quite similar to the country rocks of El Chichón area (Fig. 15a). Figure 15b suggests that the Catedral and Santa Fe intrusive magmas underwent contamination, probably from the local basement (Brueseke and Hart 2009). Moreover, the positive correlation between SiO2 versus 87Sr/86Sr (Fig. 15c) also suggests that these variations could not have been produced solely by crystal fractionation (Hildreth and Moorbath 1988); instead, contamination and/or assimilation and crystal fractionation acted in the generation of the Catedral and Santa Fe magmas. These chemical and isotopic comparisons, plus the new ages reported here, firmly suggest that the Santa Fe intrusive and Catedral volcano belong to the Modern Chiapanecan Volcanic Arc. The Santa Fe intrusive is one of the largest magmatic bodies in the El Chichón region, which probably produced the ore deposits in the mining district of Selva Negra.
In the regional geologic map of Fig. 2, outcrops of intrusive bodies in the Santa Fe mining district (Damon and Montesinos 1978), including the diorite and Catedral exposures, appear on the surface as isolated outcrops that are now exposed by intense erosion occurred during the past 2.2 Ma. In fact, Damon and Montesinos (1978) estimated that the rate of erosion of MCVA is in the order of 300 to 900 m/Ma. This implies that the exhumation of the intrusive bodies of the MCVA is ongoing likely driven by transtensional tectonics occurring in central and northern Chiapas (Meneses-Rocha 1985). From the aeromagnetic analyses (Fig. 9), it is evident that the biggest root of the Santa Fe diorite is not connected at depth to the Chapultenango trachybasalt (1.1 Ma) nor to the Catedral (1.6 Ma) and El Chichón volcanoes, or at least not at 500 m depth. The source of the Chapultenango trachybasalt and mafic enclaves hosted at El Chichón trachyandesitic rocks are composed of variable amounts of olivine, two-pyroxenes, plagioclase, and amphibole probably stemming from deep sources. El Chichón volcano is located 350 km from the trench and sits at ca. 200 km above the subducting slab (Rebollar et al. 1999). In this region, the continental crust has a thickness of ca. 40 km of folded Mesozoic sedimentary rocks overlying a deeper crystalline-metamorphic basement transected by regional strike-slip left-lateral faults (e.g., La Venta-Grijalva, Tuxtla and Malpaso) shown in Figs. 2 and 7a. The area has been the site of scattered volcanism of the MCVA located from Catedral to Venustiano Carranza volcanoes (Damon and Montesinos 1978; Mora et al. 2007, 2012; this work). Therefore, it seems plausible that magmas ascend relatively rapidly to the surface through fractures, many of which reach the surface and form part of the MCVA (e.g., Chichón), while others stall at depth and remain as intrusive bodies (e.g., Santa Fe diorite).
El Chichón and the Santa Fe Magnetic Sources
Many works before this study attempted to estimate the depth of the magma chamber of El Chichón (Figs. 12, 13, 14). Jiménez et al. (1999) suggested depths between 7 and 13 km from the location of the seismic hypocenters before and after the 1982 eruption, whereas Luhr et al. (1984) estimated depths in the order of 6 km from petrological considerations. Macías et al. (2003) obtained depths between 6 and 7.5 km, also from petrology. Through the inversion of 3D aeromagnetic anomalies, we estimate depths from 5.58 to 8.49 km for the magma chamber, which is in good agreement with the studies mentioned. The RAPS is consistent with a depth of 4.1 km for the deep sources in El Chichón (Fig. 10a–c). Jutzeler et al. (2011), used the algorithm of Li and Oldenburg (1996) to invert the RMF anomaly derived from a ground-based geomagnetic carried out on the crater and apron of El Chichón. They obtained three vertical ellipsoidal bodies of susceptibility 0.025 and 0.033 SI at shallow depths (tens to hundreds of m). They further inverted in 3D the regional aeromagnetic data from the SGM data. They found a large, somewhat vertical cylindrical body with a diameter roughly similar to that of the Somma crater. Three branches stem from the main body and reach very shallow depths. The authors identify these branches as the bodies found with the ground data. Our study encompassed 3D inversions at a semi-regional model that covers the area comprising the Santa Fe diorite, la Minita, and El Chichón and Catedral volcanoes. The causative body that we found for El Chichón has a susceptibility of 0.040 SI (one of the highest susceptibilities measured in the area) and reaches ~ 4.3 km in the E-W direction, and 2.7 km in the N–S direction. Furthermore, it is located at depths from 0.29 to 1.1 km from the surface, with its bottom between 2.5 and 2.8 km deep. Although more irregular than the body found by Jutzeler et al. (2011), it also shows four large protrusions almost reaching the surface. Thus, although somewhat different, both models generally agree (Figs. 12, 13, 14). For their inversion of the TMI data, Jutzeler et al. (2011), used software from the UBC-GIF, which solves for 3D discretized magnetic susceptibility models by simultaneously minimizing a data misfit function that favors “small” and “smooth” models (Li and Oldenburg 1996). We used software based on a different methodology, thus the differences can be attributed to the different criteria of inversion imbued in the software used.
Our 3D model of the Santa Fe intrusion uses two magnetic susceptibilities (0.02 and 0.0053 SI), which are average values for the dioritic intrusive and the country sedimentary rocks, respectively. These values yield a large intrusive body of more than 27 km in the NNE direction and more than 14 km in the W–E direction with depths ranging from surface to 1.48 km and the bottom at depths from 14 to 20 km (1662 km3). The dimensions of this body (Fig. 11) dwarf those of El Chichón causative body, showing that the magmatic processes at depth are larger than those reaching the surface as volcanoes. It also shows that volcanoes are nor the result of isolated processes but part of a wider phenomenon that can be explained only with some knowledge of the subsurface structure on volcanic areas. As stated by Marsh (2015), magma chambers can be of any imaginable shape and size. And they are present at almost any depth in the crust, but those directly related to volcanic eruptions are located in the first few km from the surface. The structure and state of these bodies can be very complex as they could arise from multiple intrusions along a substantial period. The 1662 km3 estimated volume for the Santa Fe intrusion qualifies it as a rather moderate-size intrusion compared to global examples. For example, the estimated volume of the Mull Tertiary intrusive complex in NW Scotland falls between 2000 and 3600 km3 (Bott and Tantrigoda 1987), and the Mount Givens pluton in the central Sierra Nevada batholith, California has more than 6000 km3 (McNulty et al. 2000). Small-size magma chambers are usually present under polygenetic volcanoes, although they are frequently associated with larger magmatic bodies, as for the case of El Chichón with the MCVA. Thus, all volcanoes are part of volcanic systems related to particular geologic processes such as subduction, rifting, hot spots, or other tectonic environments. Surface features and events can reveal these processes, but the entire processes leave their mark mostly through structures below the surface, which are sometimes unknown but can be exposed by geophysical techniques such as the geomagnetic method. In Mexico, the internal structure of several volcanic volcanoes and volcanic fields has been inferred with the help of aeromagnetic studies such as those of the Pinacate volcanic field in Sonora (García-Abdeslem and Calmus 2015), Ceboruco volcano in Nayarit (Fernández-Cordoba et al. 2017; Sawires and Aboud 2019), and the Acoculco caldera in Puebla (Avellán et al. 2019).
We obtained a geomagnetic model of the subsurface structure of an area about 25,000 km2 centred roughly on El Chichon volcano. We obtained a regional view of volcanism in this area and the relationship of El Chichón to the general volcanic and intrusive structures in the area. The geochemical and geomagnetic analyses reveal that El Chichón volcano is not an isolated occurrence of volcanism but part of a broader volcanic and intrusive process. Our aeromagnetic analyses allowed the identification of different volcanic areas that had not been previously recognized. We also review large intrusive bodies, and the extent of various volcanic bodies, such as the Santa Fe diorite, that crops out to the E-SE of El Chichón and the Catedral volcanic fan to the NW. Based on the chemical similarities and 40Ar/39Ar ages (ranging from 2.2 Ma to Holocene) reported here, these volcanic bodies belong to the MCVA, are enriched in incompatible elements and depleted in the compatible ones, and show pronounced negative anomalies in Nb, Ta, and Ti. Based on Sr and Nd isotopic ratios, this suggests that El Chichón magmas were produced by fractional crystallization from a mafic melt, whereas those from the Catedral and Santa Fe diorite were generated by fractional crystallization and crustal contamination (AFC process). The alignments detected by the processing of the geomagnetic data can be interpreted as a zone of weakness trending NE-SW that crosses El Chichón’s crater and suggests the influence of the tectonic framework. The aeromagnetic information allowed us to identify sources of understudied magmatic activity around El Chichón.
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We thank F. García for his technical support during the laboratory analyses and G. Cisneros for his support during image processing and some map generation. We are also grateful to Dr. Robert I. Tilling and two anonymous reviewers for their constructive comments, which greatly improved the original manuscript.
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Whole-Rock Geochemical Analysis
We collected three samples from structures that to our knowledge had not been analyzed before. One of the samples was collected from an intrusive body (MCH516) that correlates with rocks of the Santa Fe mining district described by Damon and Montesinos (1978), which are defined here as Santa Fe Intrusive. The second sample is a block (MCH505B) from the Catedral volcanoclastic fan, and the third is a mafic enclave (CH0902E) hosted in an andesitic block of El Chichón volcano. These three samples were collected for petrographic and geochemical analysis and 40Ar/39Ar dating.
X-Ray Fluorescence (XRF) Analysis
The XRF analyses of major and some trace elements (Rb, Sr, Ba, Y, Zr, Nb, V, Cr, Co, Ni, Cu, Zn, Th, and Pb) have precisions of < 1% for all elements (Table 1). These analyses were carried out at the Laboratorio Nacional de Geoquímica y Mineralogía, UNAM, México, following the methodology described in Lozano-Santacruz and Bernal (2005). Trace elements were determined by inductively coupled plasma mass spectrometry, at the Laboratorio de Estudios Isotópicos (LEI) UNAM, using a Thermo Series XII spectrometer, following Mori et al. (2007).
Sr, Nd, and Pb Isotopic Analysis
The Sr, Nd, and Pb isotopic analyses of selected samples were carried out at the Jackson School of Geosciences, University of Texas at Austin, USA, using a Thermo Triton TI thermal ionization mass spectrometer (TIMS), following standard procedures on whole-rock chips (Table 1). Groundmass chips that lacked visible alteration or phenocrysts were handpicked under a binocular microscope. Chips were first leached in hot 6 N HCl to remove any caliche or weathered material and then digested in HF:HNO3. Sr, Nd, and Pb were extracted and purified following standard procedures after Lassiter et al. (2003). Sr-Spec, REE-spec/HDEHP, and AG1-X8 resins were used to separate Sr, Nd, and Pb, respectively.
Measurements were carried out in manual and programme modes and included two total procedural blanks and two sample duplicate analyses. Sr was loaded on single Re filaments with Ta2O5; mass fractionation of 87Sr/86Sr ratios was corrected using 88Sr/86Sr = 8.375209. Neodymium was run as metal on double Re filaments. 143Nd/144Nd ratios were corrected for mass fractionation using 146Nd/144Nd = 0.7219. During the course of this study, 32 analyses of Sr standard NBS 987 yielded a value of 0.710263 ± 9 (2σ). An in-house UT Ames Nd standard yielded a 143Nd/144Nd = 0.512069 ± 6 (2σ). Pb-isotope analyses were corrected for mass fractionation using a double-spike technique modified from Hamelin et al. (1985), House and McMahon (2000). Seven double-spiked fractionation corrected NBS 981 analysis carried out in this study yielded mean Pb isotopic ratios of:
The samples were dated with the 40Ar/39Ar method used at the University of Alaska Fairbanks geochronology laboratory by laser step-heating of phenocryst-free groundmass chips (1 to 0.5 mm size fraction) following the procedures outlined in Layer (2000) and Layer et al. (2009). The mineral TCR-2 with an age of 28.6 Ma (Renne et al. 2010) was used to monitor neutron flux and calculate the irradiation parameter, J, for all samples. The measured argon isotopes were corrected for system blank and mass discrimination, and for the irradiated samples, calcium, potassium, and chlorine interference reactions, following the procedures of McDougall and Harrison (1999). Ages were calculated using the constants of Renne et al. (2010) and are reported at the 1-sigma level. Typical full-system, fifteen-minute laser blank values (in moles) were generally 6 × 10–16 mol 40Ar, 3 × 10–18 mol 39Ar, 3 × 10–18 mol 38Ar and 1 × 10–17 mol 36Ar, which are 10 to 50 times smaller than the sample/standard volume fractions. Mass discrimination was monitored by running calibrated air shots. The mass discrimination during this method was 1.0% per mass unit. Two runs of each sample were done, and the most precise run was chosen for presentation and discussion. A sample is considered to have defined a plateau age if it has 3 or more contiguous fractions constituting at least 50% 39Ar release and is significant at the 95% confidence level (as indicated by a Mean Square Weighted Deviates; MSWD < ~ 2.5). For the two samples analyzed, plateau ages yielded the most interpretable results due to little isotopic variation between the steps. The results are quoted to the ± 1 sigma level and calculated using the constants of Steiger and Jaeger (1977). The results are given in Table 2.
AMD I This domain is located in the central portion of the study zone and is characterized by a dipolar anomaly of large extent spanning, ~ 52 km in the N–S direction and ~ 40 km in the E–W direction. The magnetic high is elongated in the NNE–SSW direction with a length of ~ 27 km and ~ 24 km in the E–W direction. The magnetic low runs for ~ 37 km along the E–W direction and is ~ 19 km in the N–S direction. The anomaly is associated with a large granodioritic-dioritic intrusion cropping out around the central portion of the anomaly. In the surrounding areas, the anomaly is associated with sedimentary rocks, mostly intercalations limestone-sandstone and limonite-limestone, except to the west where it is associated with andesite breccias and andesites from the El Chichón volcano complex.
AMD II comprises the area to the north, where it meets the edge of the study area, and diffuses into nearby areas of smaller magnetic field intensity, which we labeled as domains IIa and IIb.
AMD II is characterized by its large size (~ 152 km in the E–W direction and ~ 78 km in the N–S) and seems to be associated with a magnetic high toward the N, beyond the study area. The anomaly has a somewhat slant L-shaped form. The magnetic high has gradients that increase at a rate of 0.00257 nT/m from S to N, 0.0051 nT/m from the west to the center, and 0.00078 nT/m from the center to the east. The topography in this area varies from 619 masl to the S to 25.5 masl to the N. It correlates with shale-limestone to the center and west, with shales-sandstones to the S, sandstone-polymictic conglomerates to the N, and alluvium and andesitic breccias to the E.
AMD IIa is found south of AMD II, it is elongated in an NW–SE trend measuring approximately 54 km, and 30 km in the NE–SW direction. Its magnetic intensity shows values between − 163 nT and − 112 nT, with an average of − 129 nT. Its topography varies from 230 1273 masl, with an average of 339 masl, and 230 masl toward the NW to the center, after that increasing to reach 1273 masl at the center and 552 masl to the SE. Geologically, it correlates with limestones-shales, limestones-dolomites, alluvium, and limonite-sandstone around the center.
AMD IIb lies to the east of AMD II and is characterized by similar magnetic field intensity to AMD IIa, with a gradient of only 0.00068 nT/m. It has an irregular elongated shape toward the S, and narrows toward the N. It is geologically associated with limestone-sandstones to the S and sandstones-shales to the N. The topography increases from 5 masl to the north to 189 to the south, but it reaches an altitude of about of 665 masl somewhere between the center and the south.
AMD III is located to the E and SE of AMD I and extends for 86 km to the NW–SE. A minor axis oriented NW–SE has a length of about 33 km. Magnetically it shows low values ranging from − 266 to − 206 nT, with a mean value of − 224 nT. Geologically, most of the domain correlates sandstones-shales and outcropping shales-limestones. The altitudes in the domain vary from 281 to 1882 masl to the SE, 1288 masl to the NW (where it meets AMDI) and 281 masl in the center.
AMD IV is a domain defined in three separated zones, two of them neighboring AMD III. They are banded with the long axis striking roughly NW–SE. The three sections are described in the following sub-domains.
AMD IVa comprises the south-center of the area, with a length of 41 km and a width of 23 km in the NE–SW direction. Magnetically, it shows a discrete gradient from NE to SW, going from − 200 to − 158 nT, with a variation of 0.0019 nT/m. It is associated with dolomites and alluvium. Topographically, it varies from 119 to 2073 masl, with an average of 1545 masl, the lowest values in the central portion and the highest in the NE.
AMD IVb is located in the NE portion of the zone, in the vicinity of AMD IIb and III. It has a length of the order of 68 km and 23 km in the perpendicular direction. Toward the NW portion, in the limits with the AMD II, it shows some small anomalies with magnetization intensities averaging 7.4 nT (157.3 to 164.7 nT) with a horizontal gradient of 0.0064 nT/m, and 12.8 nT (156.9 to 169.7 nT) with a horizontal gradient of 0.010 nT/m, respectively. In the NW–SE direction the domain presents magnetization intensity values ranging from − 186.3 to − 127.3 nT, with an average of − 172.4 nT. With the exception of the NW portion, where the small anomalies appear, the section shows two groups of intensities: one toward the NW central portion with values from − 169.2 to − 156.7 nT (with altitudes ranging from 1151 to 929 masl), and another from the center to the SE with values of − 183 to − 164.4 nT and a gradient of 0.0005 nT/m. Topographically, it exhibits two morphologies: one in the NW portion, running from 11.5 to 190.7 masl, and another one from the center to the NE going from 1158 to 1595 masl with an average of 1189 masl.
The NE–SW section of this domain presents magnetization intensities ranging from − 205 to − 140 nT, decreasing toward the NW and increasing to the NE, and a horizontal gradient of 0.00276 nT/m. The relief shows two morphologies, one from center to the NW of 272 masl to 1537 masl, with an average of 1032 masl, and another from center toward the NE from 272 to 34.3 m, with an average of 145 masl. The surface rocks are fundamentally shale-limestone, limestone-sandstone, and alluvium.
AMD IVc is defined by the SW portion of the study area and bounded by AMDs II and IIa to the NE and E, respectively, by AMD VII to the SW, and by AMD VIII to the NW. It is elongated, inclined toward the NW and consists of a long and wide section and two branches, one trending roughly N–S, and the second to the west. Its dimensions are 77.5 km in the NW–SE direction, up to the intersection of the two branches. At about 3.3 km from the intersection, the N–S branch has a width of 6.8 km in the NE–SW direction. The west branch has a length of 12.3 km and a width of 3.9 km.
The long section of the domain presents variations in the intensity of magnetization ranging from − 212 to − 163.7 nT and a horizontal gradient of 0.00062 nT/m. The topography varies from 152 masl to the NW to 1239 masl to the SE. The intensity of magnetization in the N–S branch range from − 205 (SSE) to − 157.7 nT (NNW), with an average of − 176.4 nT. The topography varies from 6 to 416 masl from the NNW to the SSE. The western branch spans 10 km and ranges in its magnetization intensity from − 190 to − 181 nT. Topographically, it has heights of 770 to 965 masl, which increase toward the west.
AMD V is located near the SE, in the vicinity of AMDs III, IVa and VI. It is cut off to the S by the limit of the study area and is characterized by a series of magnetic anomalies with a preponderance of magnetic lows. It has an amorphous shape, although somewhat elongated in the north direction with a length of 31.6 km. In the E–W direction, it varies from 36 km in the southern part to 20 km in the northern portion.
The anomalies have different intensities of magnetization; one shows an amplitude of 175.4 nT (varying from − 119.6 to − 295 nT) with a polar distance of 395 m. A second one has an intensity of 221.4 nT (− 130.1 to − 351.5 nT) with a polar distance of 5300 m. The topography changes from 1356 to 2383 masl from the SW to the NE. Geologically, the domain is associated with limestones-shales, and limonites-sandstones; the anomalies, however, are correlated with andesitic tuffs.
AMD VI is located in the southern part of the area and is incomplete as it is cut by the edge of the study area. It is elongated with a length of 21 km in NW–SE direction, and 10.7 km, wide. Magnetically it is a monopole with a magnetic high of 134 nT (− 158.6 nT at − 24.7 nT). The magnetic intensity in the domain ranges from − 180.1 to − 24.7 nT. In the domain it is possible to observe part of another monopole, showing values up to 63 nT. The terrain presents heights going from 1049 to 1908 masl, increasing in intensity from NW to SE. The domain is associated with andesites-andesitic tuffs. The monopoles are interpreted as due to protuberances on an intrusion of basic to intermediate composition.
AMD VII is identified in the SW corner of the study area and is cut both to the south and west. It is elongated with a length of 57.4 km in the NW–SE direction and 40 km in the NE–SW direction. At the center and south part, it contains a succession of small anomalies resembling a stone pavement showing various magnetization amplitudes ranging from − 175.4 to − 6.8 nT. Individual intensities are: 15.2 nT (− 6.8 to 22 nT), 56.1 nT (− 119.3 to − 175.4 nT), 85.5 nT (− 17.7 to − 103.2 nT) with a polar distance of 1.7 km. The highs are aligned in an NW–SE direction. Altitudes in the area range from 353 to 1256 masl.
A profile in an NE–SW direction reveals a series of small anomalies with magnetization intensities of 204.1 nT (− 16.2 to − 220.3 nT) and a polar distance of 3.4 km. The magnetic low is associated with a lineament in the NW–SE direction. Other magnetic anomalies have amplitudes of the order of 59.7 nT (− 63.8 to − 123.5 nT) with a polar distance of 1.45 km and 17.2 nT (− 102.9 to − 120.1 nT). The zone of magnetic lows has an NW–SE trend and is associated with zones of weakness, faults and (or) fractures. Topographically, the altitude varies from 319 to 1397 masl, from NE to SW. The area contains andesitic breccias and tuffs to the SW, and limestones and dolomites to the NE.
AMD VIII is located toward the NW end of the study zone, where it is cut off by its north and west margins. It is elongated with a length of 63 km in the NNW-SSE direction and an average width of 13.4 km. The entire domain is characterized by values associated with magnetic lows shown in three subdomains. One is located in the NNW portion with values from − 255 to − 226 nT, averaging − 232 nT. It correlates topographically with values going from 6 to 89 masl, from NW to SE. The second subdomain with magnetization intensities from − 284 to − 252 nT can be identified in the central portion of the domain. Topographically, it ranges from 40 to 419 masl. A third subdomain reckons magnetization intensity values between − 263 and − 210 nT, with a horizontal gradient increasing to the SW of 0.0041 nT/m. Topographically, it varies from 209 to 828 masl, rising toward the SE. The domain is geologically associated mostly with shale and limestone-dolomite.
Detailed information and attributes of the seven aeromagnetic domains (AMDs) recognized from the study of the 2nd Derivative in Z direction; additional information given in Table 5.
AMD I is located mostly in the central portion of the study area, it is elongated with an average length of 31 km in the N–S direction, and 20 km in length to the E–W direction. It encloses a magnetic high surrounded, except to the west limit, by magnetic lows (− 0.00255 × 10−2 to − 0.000742 × 10−2 nT). On the surface, it is associated with limestones-sandstones, and a small outcrop of intrusive granodiorite-diorite, which suggests the presence of a sizeable granodioritic body below.
AMD II comprises a good part of the study area. Its main location is somewhat west of the center and its shape is amorphous, formed by a series of magnetic highs and lows aligned in such a way that a preferential direction cannot be distinguished. Toward the W, the dominant direction is NW–SE; at the center and the NE the direction is E–W, and toward the S the direction is N–S. This series of magnetic highs and lows have wavelengths spanning from 1850 to 4100 m. The magnetic intensity values fall in the interval from 0.013 × 10−2 to − 0.02 × 10−2 nT. El Chichón volcano is located in this domain. Geologically most of the surface rocks are shales and limestones, but there are also outcrops of andesitic breccia. For a better description this AMD was divided into the following subdomains:
AMD IIa is a section located to the north of the study area and limited by its edge. It is elongated, approximately 65 km along the E–W direction, and approximately 11 km wide. It is characterized by a sequence of highs and lows, mainly with an NE–SW and E–W trends. Their magnetic values oscillate between − 0.002 × 10−2 nT and 0.0037 × 10−2 nT; their wavelengths between 1900 and 2950 m with an average of about 2250 m. Most of this subdomain is covered by alluvium, with some outcrops of andesitic breccias.
AMD IIb is located in the SW corner of the study area and is cut off by the margins to the south and west. It has, therefore, a triangular shape with maximum dimensions of 51 km in the NW–SE direction and 27 km in the NE–SW. Magnetically it shows the same characteristics as AMD II. It is arranged in a series of consecutive highs and lows, with a preferential NW–SE trend, and wavelengths of the order of 2000 m. The magnetic values are in the range − 0.00217 × 10−2 nT and 0.0037 × 10−2 nT. The area is composed of breccias and andesitic tuffs.
AMD IIc is an elongated area situated toward the NW of the study area. With dimensions of 44 km to the NW–SE and 15 km to the NE–SW directions, it is cut off to the west by the study area´s edge. Magnetically, it is similar to other parts of the domain, consisting of stripes of magnetic highs and lows trending in the NW–SE direction. It shows magnetic values oscillating between − 0.00435 × 10−2 and 0.0023 × 10−2 nT, and terrain wavelengths of the order of 1800 m to 2, 325 m. The surface geology displays alluvium, sandstones-shales, shales-limestones and andesites-andesitic tuffs.
AMD III covers an almost square region to the east of the study area. Its dimensions are 73 km and 67 km in the in the E-W and N-S directions, respectively. Its shape is given by a series of banded highs trending NW–SE in a matrix of average values. The magnetic intensities vary between − 0.000430 × 10−2 nT and 0.000246 × 10−2 nT, and can be classified into three groups of different wavelengths, one of length 6.1 km topographically associated with elevations of 854 to 975 masl, another one of 7.9 nT with altitudes that go from 414 to 731 masl, and a third one of 9 km in length with height going from 1155 to 1336 masl. Most of the area of the domain contains sedimentary rocks: shales-limestones, limestones-sandstones, and limonites-sandstones. Around the center, however, there are mineral deposits.
AMD IV is composed of three sections: to the NW, NE and S of the study area, almost surrounding AMD II. These sections are cut off toward their NW, NE, E and S margins. It is characterized by showing a series of magnetic heights. The magnetic values range from − 0.00064 × 10−2 to 0.0013 × 10−2 nT in a setting of average values. There are high frequencies that can be classified into two groups: one with wavelengths from 1450 to 1550 m, another with wavelengths from 2050 to 2250 m. The NW portion of the domain has an area of 69 by 35 km in the E–W and N–S directions, respectively. The southern section is 40 km long in the N–S direction and 43 km along the E–W. The NE portion spans an area 35 km long along the N–S direction and 34 km along the E–W. Geologically, the NW part is associated with sandstones and polymictic conglomerates and shales-limestones. The NE part it is associated with limestones, sandstones, shales, and sandstones-limonites. The SW section is related to shales, limestones, and sandstones-shales.
AMD V is a somewhat elliptical domain is located in the SE, where it is cut off by the southern edge of the study area. Its dimensions are 61 and 2 km in the E–W and N–S directions, respectively. It is characterized by series of well-defined magnetic anomalies, showing evident bipolarity. Individual anomalies can be identified with values of 0.002988 × 10−2 nT and 0.001169 × 10−2 nT and polar distances of the order of 3.5 km. It contains wavelengths from 6450 to 7000 m. Geologically, it is associated with andesitic tuffs and sedimentary rocks, such as limestones, shales, and sandstones.
AMD VI is located toward the SW portion of the study area and is elongated with a long axis in the NW–SE direction, cut off in its extremes by the limits of the study area. It is 74 km in length along the NW–SE direction and, approximately 28 km wide. It shows bands of elongated magnetic highs of 34 km × 8.7 wide with values of up to 0.0006227 × 10−2 nT, embedded in a matrix with values ranging from − 0.00012 × 10−2 nT to − 0.00024 × 10−2 nT. It is related to limestones-dolomites, limestones-shales, and alluvium. See Table 2.
AMD VII is located toward the north central part of the studied área. It is limited to the W by AMD IV, to the E by AMD IIa and to the S by AMD II. It presents a shoe shape being elongated toward the W portion and in the NW–SE direction where it shows the dimensions of the order of the 26 km. See Table 2. Geologically it is correlates superficially with sandstones, polimictic conglomerate and with limestone-shale.
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López-Loera, H., Macías, J.L., Espíndola, J.M. et al. The Santa Fe Intrusion and Other Magmatic Bodies Under the Chichón Volcano Area (Mexico): Inferences from Aeromagnetic and New Petrologic-Geochronologic Data. Surv Geophys 41, 859–895 (2020). https://doi.org/10.1007/s10712-020-09591-8
- Modern Chiapanecan Volcanic Arc
- El Chichón volcano
- Santa Fe diorite
- Aeromagnetic surveys
- Structure beneath volcanoes
- 3D inversion susceptibility models