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Journal of Volcanology and Geothermal Research 170 (2008) 135 – 152 www.elsevier.com/locate/jvolgeores
Research paper
Geophysical surveys of the Joya Honda maar (México) and surroundings; volcanic implications Héctor López Loera a , José Jorge Aranda-Gómez a,b,⁎, Jorge A. Arzate b , Roberto Stanley Molina-Garza b a
Instituto Potosino de Investigación Científica y Tecnológica, A.C., División de Geología Aplicada, P.O. Box 3-74, San Luis Potosí, S.L.P., 78216, México b Universidad Nacional Autónoma de México, Centro de Geociencias, Campus UNAM-Juriquilla, Querétaro, Qro. 76230 México Received 6 March 2006; accepted 31 August 2007 Available online 19 September 2007
Abstract Joya Honda (JH) is a Quaternary maar excavated in Mesozoic limestone. It is located in central Mexico and belongs to the Ventura volcanic field (VVF), which is composed by cinder cones and maars made of intraplate-type mafic alkalic rocks. Volcanoes in the region form ∼ N20W lineaments, roughly parallel to a regional set of normal faults, but there is no obvious relation between these faults and vent distribution in the exposed geology around the maar. The volcanic rock volume is small in the VVF, and most volcanoes and their products are scattered in a region where outcrops are dominated by limestone. The near-vent tephra associated to the JH maar lies north of the crater. This relation suggests that the crater was formed by directed hydromagmatic explosions and may indicate an inclined volcanic conduit near the surface. The tephra stratigraphy suggests that the initial explosions were relatively dry and the amount of water increased during the maar forming eruption. Therefore, the existing model of the maar– diatreme formation may not be applicable to Joya Honda as it requires the formation of a cone of depression in the aquifer and deepening of the focii of the explosions as the crater and underlying diatreme grew. Thus, it is unlikely that there is a diatreme below Joya Honda. Aeromagnetic data shows a boundary between two regional magnetic domains near the elongated volcanic cluster of the VVF. The boundary is straight, with a distinct kink, from NE- to NW-trend, near JH. The limit between the domains is interpreted as fault contacts between mid-Tertiary volcanic rocks and marine Mesozoic sedimentary rocks. Hence, magma ascent in the area may have been facilitated by fractures near the surface. Magnetic and gravimetric ground surveys show that the anomalies associated with the maar are not centered in the crater, which could be consistent with an inclined volcanic conduit. A magnetic profile measured on exposed limestone across the volcanic lineament failed to show an anomaly such as that caused by a connecting dike between the volcanoes. Therefore, either the dike does not exist or it is so deep or so thin that it is beyond the limit of detection of the method and/or equipment used. Thus, the volcanic conduit immediately below Joya Honda can be reasonably modeled in the shape of a plug. A 2-D model of the crater anomaly is consistent with a roughly tabular deposit formed by fall-back pyroclasts and slump deposits near the surface. Based on this result we propose an alternative model for the formation of maar-type volcanoes excavated in hard rock, where there is no evidence of a gradual decrease of the water/magma ratio as the eruption advanced. © 2008 Published by Elsevier B.V. Keywords: feeder dikes; diatreme; Mesa Central; tuff-cone; hydrovolcanism; intraplate magmatism
1. Introduction
⁎ Corresponding author. Universidad Nacional Autónoma de México, Centro de Geociencias, Campus UNAM-Juriquilla, Querétaro, Qro. 76230 México. Fax: +52 442 238 1101. E-mail addresses:
[email protected] (H. López Loera),
[email protected] (J.J. Aranda-Gómez). 0377-0273/$ - see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.jvolgeores.2007.08.021
Mid- to late-Tertiary intraplate-type volcanism occurs throughout central and northern Mexico. Most localities are in the southern part of the Basin and Range Province, and in few places it is possible to relate, both in time and space, the xenolith-bearing mafic alkalic volcanism with normal faulting (e.g. the Camargo volcanic field: Aranda-Gómez et al., 2003).
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However, most intraplate-type volcanic fields in this region do not present an obvious relation with extensional structures, other than they are located in a region that was extended during the late Cenozoic (Aranda-Gómez et al., 2005). The Ventura volcanic field (Fig. 1) is composed by several maars, cinder cones and isolated lava flows of Quaternary age (K–Ar = 1.6 ± 0.21Ma: Aranda-Gómez and Luhr, 1996) on the southeastern portion of the Basin and Range province. The xenolith-bearing olivine nephelinites, basanites and alkali olivine basalts (Luhr et al., 1989) rest atop a pre-volcanic basement composed by a Mesozoic calcareous sequence or Pleistocene alluvial deposits. Xenoliths found in the volcanic products include spinel-bearing upper mantle peridotites and felsic to mafic granulites interpreted as samples of the deep crust. Regional mapping (Labarthe-Hernández et al., 1982) has shown that the Ventura volcanic field is located north of a NEtrending, ∼ 140 km long Villa de Reyes graben (Fig. 1), but it is uncertain if the graben ends ∼ 30 km south of field, or if it changes to a N–S trend (or intersects another independent structure) and continues for an additional 150 km to the north (Tristán-González, 1986; Ramos-Leal et al., 2007). The Quaternary volcanoes in Ventura locally form ∼ N20W-trending alignments (Fig. 2) that were used by Suter (1991) to infer the orientation of the least horizontal component of the far field stress in the region. The volume of the volcanic products in Ventura and the number of volcanoes are small and limestone and/or alluvium are exposed between the vents and associated lava flows and pyroclastic deposits. The best studied volcano in Ventura is the Joya Honda maar, which is a large elliptical
(1200 × 800 m) crater excavated more than 200 m below the premaar surface by hydrovolcanic explosions in a folded limestone sequence (Fig. 3). Worldwide, monogenetic volcanoes frequently form distinct alignments. It is commonly assumed that the vents observed on the surface lie above feeding dikes, and these in turn, usually occupy tensional fractures or faults that facilitated magma ascent to the surface (e.g., Nakamura, 1977). A model for the formation of maars and diatremes developed in solid rock, based in part on observations made in deep mines, propose that the maar–diatreme system occurs at the sites where a feeder dike intersected a hydraulically active zone, such as a fault or fracture zone. These older faults and fractures produced secondary permeability in the host rock of the diatreme. The close association between maars and cinder cones along an alignment of vents is explained in terms of availability of water near the surface. Cinder cones and associated lava flows form where the dike intersects “dry” regions in the rock mass, and maar/diatremes where groundwater is present in regions with secondary permeability caused by fractures (Lorenz and Kurszlaukis, 2003) and/or karstic features. It has been argued that the maar–diatreme system is caused by the formation of a cone of depression in an aquifer with limited hydraulic conductivity (e.g. Lorenz, 1986). In this model, both the maar crater and the diatreme grew as the locii of the explosions deepened with time. A stratigraphic study in the pyroclastic sequence around Joya Honda (Aranda-Gómez and Luhr, 1996) suggests that the maar forming eruption began with hydrovolcanic explosions in an environment with a limited
Fig. 1. Geologic sketch map of the southern portion of the Mesa Central (modified after Aranda-Gómez and McDowell (1998)) showing the location of the Ventura volcanic field (VVF). The most conspicuous localities in the Ventura volcanic field are three maars: Joya Honda, Joyuela and Laguna de los Palau. These volcanoes are located ≈ 30 km north of an area where the Villa de Reyes graben intersects the N35W-trending Enramadas–Santa María fault system and close to the San Luis Potosí graben proposed by Tristán-González (1986). Inset A shows the approximate location of the map in central Mexico. Inset B is a rose diagram of the orientation of the Cenozoic faults in the San Luis Potosí and Guanajuato 1:250,000 quadrangles (e.g., Stewart et al., 1998). Abbreviations: G — Guanajuato; SLP — San Luis Potosí; S — Salinas de Hidalgo; SF — San Felipe; DH — Dolores Hidalgo; L — León; SM — San Miguel de Allende.
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Fig. 2. Regional geology around Ventura volcanic field and location of the Villa Hidalgo 1:50,000 quadrangle (Modified from Aranda-Gómez and Luhr (1996)). The vents around Ventura define a conspicuous alignment. Similar alignments are also present near Cerro Verde (CV, n = 3) and Pozo del Carmen (PC, n = 4). The heavy dashed line in the central part of the Villa Hidalgo Quadrangle represents the limit between the regional aeromagnetic domains I and II (see Fig. 4). Note that domain I corresponds to exposures of mid-Tertiary felsic volcanic rocks and alluvium-filled areas of the San Luis Potosí graben. It is inferred that the magnetic basement below the sediment fill is formed by volcanic rocks. Domain II corresponds to areas where outcrops are dominated by Mesozoic marine sediments, except where the Ventura volcanoes and their products are located. Inset A. General trends of mapped Cenozoic faults in the southern portion of the Mesa Central in the region between San Luis Potosí and Guanajuato (Stewart et al., 1998). Inset B. Sketch map that shows the Ventura cluster of monogenetic volcanoes and the location of the buried vent discussed in the text.
supply of groundwater. These blasts caused the initial formation of relatively “dry” pyroclastic surges. As the eruption progressed, a sudden influx of water contained in fractures and/or
karstic structures, changed the water/magma ratio in the system producing “wet surges” that formed a tephra sequence similar to those observed in tuff cones (Wohletz and Sheridan, 1983).
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Fig. 3. Geologic map of Joya Honda (modified afterAranda-Gómez and Luhr (1996)).
Therefore, if the interpretation of the stratigraphy (ArandaGómez and Luhr, 1996) and the existing model for formation for some maars (Lorenz, 1986) are correct, there is no compelling reason to believe that the explosions were significantly deeper with time nor that a diatreme underlies the Joya Honda crater, as the driving mechanism for the formation of the diatreme is missing.
and fragments of mafic alkalic rocks) and the country rock (limestone), as well as the low volume of the volcanic products in the area, favored the formation of distinct magnetic anomalies that might be analyzed to infer the form of hidden igneous bodies without the interference produced by a thick volcanic pile around the vents. 2. Geology
1.1. Purpose and scope 2.1. Regional setting The goals of this paper are: 1) evaluate if the location of the Ventura volcanoes was influenced by mid- to late-Cenozoic (extensional?) structures that facilitated the magma ascent, 2) establish if it is possible to detect magnetic anomalies in the areas between the volcanic vents. 3) infer the shape of the magnetic body beneath the maar's crater. We address these questions based on the interpretation of the exposed geology, aeromagnetic, magnetic and gravimetric data collected in ground surveys. Magnetic anomalies may indicate the existence at depth of a dike connecting these isolated volcanoes. Furthermore, models of a magnetic body immediately underneath the maar crater may establish if its geometry could be similar to the shape of a diatreme (an inverted cone), dike (a tabular body) or a plug (a cylinder). Additional control on the probable shape of the magnetic body underlying the crater was inferred from models derived from gravimetric data collected at the bottom of the crater. The geometry inferred from the models is also used to test Aranda-Gómez and Luhr (1996) hypothesis that the volcanic conduit is inclined, and the tephra was preferentially accumulated north of the crater as a consequence of directed blasts. The sharp contrast between the magnetic susceptibilities of the mafic alkalic rocks, the pyroclastic deposit around the maar (a heterolithologic tuff-breccia composed mainly by limestone
The Quaternary Ventura volcanic field is located in Central México (Fig. 1A) near the southern end of the Mexican portion of the Basin and Range province (Henry and Aranda-Gómez, 1992). The region where the monogenetic volcanoes occur is a high plateau known as the Mesa Central, and the volcanic field is near the eastern end of the plateau. Outcrops in the immediate vicinity of the volcanic field are dominated by a folded sequence of marine sedimentary rocks composed by limestone with bands and nodules of black chert and shaly partings. West and south of the Ventura volcanic field, the sedimentary rocks are covered by a volcanic sequence formed by mid-Tertiary (K– Ar ∼ 32–27 Ma: Labarthe-Hernández et al., 1982) felsic ignimbrites and/or rhyolitic lava flows (Figs. 1 and 2). The Tertiary volcanic sequence is up to 1000m thick between Guanajuato and San Luis Potosí (Fig. 1), but outcrops near Ventura are usually less than 100m thick. The Mesozoic prevolcanic basement at the Ventura volcanic field represents the shelf and foreslope facies of a large calcareous platform located toward the east. The Mesozoic sedimentary sequence was deformed during the early Tertiary Laramide Orogeny. The Mesa Central was extended during the middle and late Tertiary and the region is characterized by a complex array of
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normal faults, which in places form a roughly rhombohedral pattern in map view (e.g. Aranda-Gómez et al., 1989; NietoSamaniego and Alaniz-Alvarez, 1997). Regional geologic
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mapping has shown two dominating sets of conjugated normal faults with N45E (± 15 °) and N35W (± 15°) trends (LabartheHernández et al., 1982). These faults are conspicuous in the
Fig. 4. Aeromagnetic maps of the Villa Hidalgo 1:50,000 quadrangle. a. Magnetic field after subtraction of the corresponding IGRF values for 2000. b. The data was reduced to the pole. The map also shows the location of the known cinder cones, maars and intra-plate type lava flows, as well as sections measured in the field; these are labeled 01–07. The heavy white line represents the approximate limit between the two regional magnetic domains discussed in the text. South of Joya Honda, within domain I, a water well cut 87m of mid-Tertiary felsic volcanic rocks which were buried under 90m of unconsolidated granular materials.
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areas covered by the mid-Tertiary volcanic rocks (Fig. 1), but they are less obvious in regions where outcrops are dominated by the Mesozoic marine rocks, which form NW-trending folds. In addition to these fault sets, in some areas of the Mesa Central also occur N5W (± 5 °) normal faults (Fig. 1B). The NE-trending Villa de Reyes graben (Fig. 1) is a large structure that extends for nearly 140 km in the southern part of the Mesa Central. The graben apparently ends 30 km south of the Ventura volcanic field in an area where it intersects a NWtrending, down to the SW, array of normal faults that form a domino-style set of fault blocks, which is well exposed at the Sierra de San Miguelito (Xu et al., 2004, 2005) and along the Enramadas and Santa María river basin (Fig. 1). TristánGonzález (1986) proposed the existence of the ∼N–S trending San Luis Potosí graben (Fig. 1), which extends for more than 150 km, from the northeastern end of the Villa de Reyes Graben to the western slope of Sierra de Catorce. South of sierra de San Miguelito, all the way to the southern end of the Mesa Central, outcrops are dominated by mid-Tertiary felsic volcanic rocks (Fig. 1), which facilitate the recognition and mapping of the normal faults. North of the city of San Luis Potosí, folded calcareous rocks, in places partially covered by thin volcanic sequences, dominate the panorama and the location of the extensional structures is less certain. The normal faults are often inferred from the geomorphology, assuming that the old inactive faults are buried under the alluvium close to the mountain fronts.
2.2. Local geology The Ventura volcanic field, as originally defined by LabartheHernández (1978), is formed by three maar-type volcanoes and several cinder cones with associated lava flows. The volcanic field is located in the inferred footwall of the eastern margin of the San Luis Potosí graben (Fig. 1). The most conspicuous volcano in the field is Joya Honda maar, a large elliptical crater (1200 × 800, and 300 m deep) completely excavated in the calcareous basement of the region. Six and 25 km, respectively, south and southeast of Joya Honda are La Joyuela tuff cone and Laguna de los Palau maar (Fig. 2). Other volcanic manifestations in the area are deeply eroded cinder cones sometimes accompanied by short lava flows and few isolated lava flows with no obvious relation with the cinder cones (Fig. 2b). Ventura's volcanoes lie at the intersection of a N20Wtrending overturned anticlinorium crest (Aranda-Gómez et al., 2000) with a NE or ENE-trending fault zone with a right-lateral component. The existence of this poorly documented mid- to late-Cenozoic structure was inferred by Aranda-Gómez and Labarthe-Hernández (1975) from the apparent displacement of an upper Cretaceous sandstone and shale sequence exposed on the eastern slopes of the sierras El Coro and Alvarez (Fig. 2). In the surroundings of Joya Honda we have identified an elongated cluster of eleven volcanic vents (Fig. 2b). Nine of them form a N20W, 15 km long linear trend, roughly parallel to one of the sets of normal faults mapped in the southern part of
Fig. 5. Configuration of the residual magnetic field based on the data collected in the ground surveys. The map is more reliable near the crater where there is a higher density of points. Note the location of the magnetic anomaly associated to the crater. The white lines show the location of magnetic profiles 01–07. White circles are the sites where electrical vertical and audio-magnetotelluric soundings were performed. The white triangle inside the crater shows the location of two vertical electrical soundings done; one was with current electrodes along profile 02 and the other along profile 07. Dotted black lines within the crater are the gravity profiles.
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the Mesa Central (Fig. 2a and b) and to the axes of the Laramide folds. Near Joya Honda also occur two relatively large, isolated basanitic lava flows with no obvious relation to the exposed volcanoes; they are clearly outside of the linear trend defined by the cones, but they are located in the general area where the ENE fault is inferred (Fig. 2). Of particular interest is a small outcrop of volcanic agglutinate located ∼ 800 m ENE of the crater (Fig. 3). This agglutinate is exposed at the top of a small rounded hill blanketed by distal surge deposits from Joya Honda. Two quarries, located on the eastern and northern portions of the base of the hill (Fig. 3), are excavated in a distinctly bedded, unlithified scoria-fall deposit, which is nearly monolithologic (alkali olivine basalt and a small amount of accidental limestone, Luhr et al., 1989). The lithology of this pyroclastic sequence contrast with the heterolithologic tephra from Joya Honda, where clasts derived from the pre-volcanic basement are abundant and may form up to 50% (vol.) of the deposit. Based on these field relations, it is believed that this hill is an older cinder cone, partly buried by Joya Honda products (Figs. 2b and 3).
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The bulk of the pyroclastic sequence associated with the formation of Joya Honda lies north and northwest of the crater (Fig. 3). The deposit is up to 100m thick and rapidly thins out on the western and eastern borders of the maar, and it is absent or very thin south of the crater. This distribution of the surge beds and fall deposits, which in the near vent area are formed by heterolithologic tuff-breccias, is interpreted as evidence of a series of directed blasts to the north and northwest, through a set of south- or southeast-dipping fractures that acted as the muzzle of a gun (Aranda-Gómez and Luhr, 1996). The stratigraphic characteristics of the upper part (80%) of this tephra deposit are similar to those commonly found in “wet” surges associated with tuff cones as they are described by Wohletz and Sheridan (1983). Near the base of the deposit, the tephra (20%) is like that commonly associated with “dry” surges in tuff rings and maars. Therefore, it is believed that the water/magma ratio increased during the eruption, contrary to Lorenz' (1986) general model for the formation of the maar–diatreme system. Thus, it is reasonable to question the existence of a diatreme underneath the maar.
Fig. 6. Magnetic profile 01 (see location in Fig. 3). a) Residual magnetic field (continuous line), low pass filter (discontinuous line); b) Horizontal gradient (calculated); c) Vertical gradient; d) Topographic profile and qualitative geologic interpretation. The symbols in the geologic cross-sections are consistently used in all the profiles (Figs. 6–12).
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3. Magnetic surveys 3.1. Analysis of aeromagnetic data We analyzed aeromagnetic data acquired by the Servicio Geológico Mexicano (SGM), both as printed 1:250,000 maps (Mérida-Montiel et al., 1997, 1998) and digital information corresponding to the 1:50,000 Villa Hidalgo quadrangle (see Fig. 2 for location). According to the data supplied by SGM, the aeromagnetic data was obtained in 1995 with a Geometrics G-803 magnetometer along a set of north-south flight lines, 1000m apart, with a mean terrain clearance of 450m. The digital information we processed consists of a XYZ (latitude, longitude, magnetic intensity) ASCII file with 1,442,633 points for an area of ∼ 1000 km2. We processed the digital data with the software OasisMontaj 5.1.6, by Geosoft generating several maps and 3D diagrams with different filters offered by the software, such as upward and downward continuations, analytical signal, apparent density and susceptibility calculations and derivatives in X, Y, and Z directions, low and- highpass band. As examples of these maps, we include Fig. 4a and b, which show the configuration of the corrected magnetic field after subtraction of the corresponding IGRF 2000 values
(Fig. 4a) and the data reduced to the magnetic pole (Fig. 4b) The reduction to the pole filter is applied in the Fourier domain, and the result is such that the observed bipolar magnetic field is migrated to a simpler monopolar field, similar to the gravity field, making magnetic anomalies easier to interpret. Details about the applied filter can be found in Baranov (1957), Baranov and Naudy (1964) and Spector and Grant (1971). The elongated N20W-trending cluster of Quaternary volcanoes (circles, Fig. 4) is located near the east end of a regional aeromagnetic domain (I in Fig. 4b characterized by wavelengths in the order of ≤500 m and series of highs and lows with superimposed high-frequency anomalies of varied intensities (440 nT–280 nT). This aeromagnetic domain I contrast sharply with an adjacent area to the east, where the magnetic pattern has longer wavelengths (>2 km) and has magnetic intensities between 320 and 380 nT (II in Fig. 4b. A visual comparison between the geologic map of the Villa Hidalgo quadrangle (Fig. 2) and the aeromagnetic map (Fig. 4) shows that aeromagnetic domain I corresponds with outcrops of mid-Tertiary felsic volcanic rocks and extensive areas covered by alluvium. Aeromagnetic domain II matches with outcrops of the calcareous pre-volcanic basement at sierras de Alvarez and El Coro and/or with alluvium. It is assumed that the magnetic basement
Fig. 7. Magnetic profile 02. Note the subtle magnetic anomaly centered in the crater and the presence of a buried cinder cone in magnetic sector 5.
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under the alluvium in aeromagnetic domain I is mid-Tertiary volcanic rocks and in aeromagnetic domain II sedimentary marine rocks lie under the alluvium. Additional support for this interpretation comes from deep wells that had documented (Aguirre-Hernández, 1992; Martínez-Ruíz, 1997) the existence of igneous rocks below the alluvium in several places in domain I (e.g. Fig. 4b). Notable exceptions within aeromagnetic domain II are the area where the intraplate-type monogenetic volcanoes are located, and a thick remnant (≈150 m) of mid-Tertiary volcanic rocks at the northeastern corner of the quadrangle, near the town of Villa Hidalgo (Figs. 2 and 4). 3.2. Ground magnetic survey The ground survey was carried out along 32 km, distributed in 7 magnetic profiles (see location in Fig. 4b) with reading stations spaced 25m (profiles 02, 03, 06, and 07) and 50m (profiles 01, 04, and 05). The geometric pattern defined by the profiles is perpendicular to the magnetic and tectonic trends, as inferred from the aeromagnetic (Fig. 4) and geologic maps (Fig. 2), respectively. Two of the profiles (02 and 07) cross the elliptical Joya Honda crater, roughly parallel to its major and minor axes. The effects of the daily variation of the magnetic field were registered with a precession proton magnetometer
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Geometrics G-856-A, and for the measurements along the profiles we used an overhauser GEM GSM-19 v 5.0. The residual magnetic field for all the profiles was obtained by subtracting a linear trend to the corrected data. We also applied a low pass filter to the data in order to eliminate the higher frequency content, which is often related to the presence of shallow magnetic sources such as magnetized boulders. A five point low pass filter allowed us to better observe the lower frequencies associated to deeper sources. Finally, we calculated the vertical and horizontal gradients for every profile in order to detect zones with pronounced lateral changes. A discreet normal dipolar magnetic anomaly is associated to Joya Honda (Fig. 5). This anomaly is part of the N20W trend of magnetic highs produced by the Quaternary volcanoes (Fig. 4). The most pronounced anomalies along the trend occur in the area between Joya Honda and Joyuela, where the volcanic vents are tightly clustered (Fig. 2b). The volcano cluster occurs near the site where the straight boundaries between aeromagnetic domains I and II make a sharp change from ∼N25E to ∼N30W (Fig. 4). 3.3. Magnetic ground survey results The results obtained in the ground survey are summarized in Figs. 6–12. Graph a) in all these figures shows the residual
Fig. 8. Magnetic profile 03. Compare magnetic signature of the Cerro Quemado agglutinate with sector 5 in profile 02 (Fig. 7).
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magnetic field (thin continuous line) and the data after applying a low pass filter (heavy discontinuous line). Graphs b) and c) are the horizontal and vertical gradients of the residual magnetic field respectively. The sectors shown in graph c) of all figures were established based mainly on the signature of the vertical gradient, but also on the non-smoothed residual field and the horizontal gradient of the corresponding profile (Table 1). The qualitative geologic interpretation (graphs d) was based on the contrasting magnetic signatures correlated to the recorded surface geology along the measured magnetic profiles. For example, limestone is exposed in the surface throughout the entire length of profile 05 (Fig. 10) and in sectors 1 and 3 of profile 04 (Fig. 9). Likewise, the near-vent Joya Honda tuffbreccia (limestone + basanite) outcrops in parts of sectors 5 of profile 02 (Fig. 7) and 2 of profile 07 (Fig. 12). Basanitic agglutinate and lava occur at sectors 6 and 7 of profile 03 (Fig. 8). Based on these magnetic signatures and mapped surface geology, we propose qualitative geologic interpretations of the magnetic data for each profile (Figs. 6–12). The geologic interpretation along some profiles (02 and 07; Fig. 5) were partly constrained with data from four electrical vertical sound-
ings and two audio-magnetotelluric soundings collected in the area (Fig. 5). Remarkable features in the interpreted profiles, in terms of the goals pursued in this investigation are: 1) there is no magnetic anomaly across the N20W volcanic lineament in the area where only limestone is exposed (magnetic profile 05; Fig. 10). Therefore, either the feeder dike that according to volcanic cone alignment models should run between the vents does not exists, or it is covered by a thick section of limestone that effectively masks the expected magnetic anomaly produced by it. A synthetic model of a 10m thick dike with a large magnetic susceptibility of 1emu, buried under 100m of limestone, produces an anomaly with maximum amplitude less than 2nT. This value is within the resolution threshold of the instrument used during the survey. 2) The magnetic anomaly at the bottom of the crater is very subtle (≈100 nT), compared with the anomalies produced by exposed basalt, tuff-breccia or midTertiary volcanic rocks (several hundred nT). Furthermore, this anomaly is almost centered in the crater as seen in magnetic profile 02 (≈ east-west, Fig. 7), and it is distinctly off-centered in profile 07 (≈ north-south, Fig. 12), where it peaks close to the southern border of the crater. 3) The small agglutinate outcrop
Fig. 9. Magnetic profile 04.
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Fig. 10. Magnetic profile 05.
located east of the crater (Fig. 3) produces a magnetic anomaly signature (Fig. 7) similar to that of an exposed, partially eroded volcano (e.g., Fig. 8). This, and the nature of the small outcrop (a near-vent agglutinate deposit), conical shape of the hill, and the scoria fall deposit at the quarries (Figs. 3 and 7) suggest that there is an eroded cinder cone, older than the maar, buried under intermediate surge deposits from Joya Honda. This vent is a likely source for the scoria-fall deposit documented by ArandaGómez and Luhr (1996) at the base of the pyroclastic section exposed on the NE border of the crater. The fact that this scoriafall deposit is covered by a paleosol is also consistent with a cinder cone where the outer slope was profoundly modified by erosion prior to the formation of the Joya Honda maar. 4) Sector 2 in profile 04 (Fig. 9) corresponds to a previously unknown lava flow. The olivine nephelinite is buried under soil and alluvium and its presence is not obvious in the air photos. The existence of this lava was confirmed in outcrops located near the Salto Prieto creek, located ∼ 2 km east of the center of the maar. The most likely source of this lava is Cerro Quemado, which is the eroded cinder cone exposed in sector 6 of profile 03 (Fig. 8).
The continuous presence of a thin cover of mafic volcanic rocks, as inferred from sector 1 in the magnetic profile 06 (Fig. 11) is now interpreted as evidence that one of the isolated lava flows southeast of Joya Honda (Fig. 3) was issued from the Cerro Quemado volcano. Based on the general arguments presented by López-Loera and Urrutia-Fucugauchi (1999) and López-Loera (2002), the sharp, straight boundaries between regional magnetic domains I and II in the aeromagnetic Villa Hidalgo quadrangle (Figs. 2 and 4b), and the marked contrast between sectors 1 and 2 of profile 01 (Fig. 6) and 2 and 3 of profile 02 (Fig. 7) we infer a faulted contact between the mid-Tertiary volcanic rocks (domain I) and the calcareous basement (domain II). The residual magnetic anomalies within the crater along profiles 02 (Fig. 7) and 07 (Fig. 12) were modeled using a 2-D algorithm with the WinGLink™ software. The results are shown in Fig. 13a and b. Magnetic susceptibilities and natural remanence (Table 2) were obtained through laboratory measurements in oriented cores collected from limestone, heterolithologic tuff-breccia, basalt (sensu lato) and rhyolite(sensu
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Fig. 11. Magnetic profile 06.
lato) in representative sites (Figs. 2 and 3). The combination of an irregular body, with a roughly tabular shape, composed by post-eruptive sediments (lake beds, talus deposit and alluvium) with a heterogeneous layer of fall-back pyroclasts mixed with slump material underneath – with a composition similar to the Joya Honda tuff-breccia – produces matches with the residual magnetic anomalies along both profiles. Better fits between the measured and calculated curves were obtained introducing vertical and lateral variations in the magnetic susceptibility inside the body (Fig. 13 a and b). It is highly uncertain if these correspond to the shallow lithozones underneath the maar crater using Lorenz (2003) terminology; see Fig. 14a) Adding to these models bodies equivalent to unbedded diatreme and/or feeder vent lithozones and/or a feeder dike (Fig. 14a), with magnetic susceptibilities up to twice as big as that for body of fall-back and slump material, does not produce a noticeable change in the calculated curves. 3.4. Gravity survey A gravity survey was carried out within the maar's crater using a Scintrex CG-3 gravimeter. Measurements were done along two
nearly orthogonal profiles that follow the magnetic profiles 02 and 07 (Fig. 5). A total of 47 stations were measured and corrected to obtain the Bouguer gravity along each profile. A terrain correction was also performed using topographic data extracted from a digital elevation model with a spatial resolution of 30m generated by INEGI (http://www.inegi.gob.mx/lib/ usuarios/default.asp?s=geo&sistema=mde) in order to correct for the effect of the crater walls, which affect mainly the measurements close to them. The corrected anomaly profiles were then modeled using a standard 2-D algorithm (Talwani and Ewing, 1960) and densities corresponding to the type of rocks inferred from geologic inspection around the crater and density variations similar to those used in available models for the underground structure underneath maar-type volcanoes (e.g. Lorenz, 2003; Schulz et al., 2005). The results are shown in Fig. 13c and d, where it can be seen that the overall geometry of the bodies composed by post-eruptive sediments and fall-back and slump material closely resemble those obtained from the magnetic modeling at both profiles. However, as in the magnetic models, a better fit is obtained introducing vertical and lateral changes of the density values within the body. The presence of unbedded diatreme and/or feeder vent lithozones and/or feeder
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147
Fig. 12. Magnetic profile 07.
dike below the bodies of post-eruptive sediments (Fig. 14a) has no positive effect on the curve fitting. Despite this, the asymmetry of the crater vent is consistent with both sets of geophysical data, and supports the inferred shape of the hidden bodies. 3.5. Discussion of results The overall geometry of the bodies that appear to produce the magnetic and gravimetric anomalies below the crater deserves special attention. They reflect the form and nature of the conduit used by the magma that reached the surface, its possible attitude (vertical or inclined), and depth under the present-day crater floor. Considering the inferred mechanism of maar formation, trough a series of hydrovolcanic explosions, the persistence of the aquifer for an unknown time after the volcanic activity ceased, and the mass-waste modification processes of the steep walls of the crater during and after the eruption, it is reasonable to assume that the composition of the rocks underneath the crater floor could be: 1) a volcanic tuff-breccia, related to the formation of a diatreme (i.e. with the shape of an inverted cone
and lithozones characteristic of these subvolcanic structures: Lorenz, 1986, 2003. See Fig. 14a); or 2) if a diatreme was not formed underneath Joya Honda, a basanitic dike or plug surrounded by limestone at depth and covered or intruding a nearly tabular body of tuff-breccia of unknown thickness closer to the surface (Fig. 14b). Either way (1 and 2), this rock body must be dominantly composed by limestone and basanite fragments and its magnetic properties should be similar to the tuff-breccia exposed north of the crater. A third possibility is a combination of 1) and 2). In this interpretation a diatreme was formed during the initial stages of the eruption. After water was temporarily depleted in the system, magma continued its ascent to the surface intruding the diatreme (Lorenz and Kurszlaukis, 2003), or invading a tabular body of tuff-breccia forming a cinder or spatter cone at the bottom of the maar. This last case is unlikely because when this happens there is evidence of the late magmatic stage either at the bottom of crater or in the pyroclastic deposits around the crater as it has been documented, for instance, of La Breña maar (Aranda-Gómez et al., 1992). In all three cases, the sub-crater configuration must be
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Table 1 Criteria used to establish the magnetic sectors along profiles 01–07 Profile Average Length Filter Altitude trend (m)
(masl)
01
N55E
5425
L. P.
02
N60E
5720
L. P.
03
N60E
6830
Magnetic Stations sector
L. P.
N57E
3565
L. P.
05 06
N69E N25W
1000 4677
L. P. L. P.
07
N10W
4467
L. P.
RMF
Horizontal gradient
Vertical gradient
Wavelength Exposed geologic unit
(masl)
(nT)
(nT/m)
(nT/m)
(m)
1844–2006 1 2 3 1991–1716 1 2 3 4 5
0–38 38–76 77–101 0–19 19–32 32–64 64–105 105–154
1844–1867 1867–1963 1963–1947 1833–1824 1824–1834 1834–1766 1766–1869 1869–1934
359 to − 226 80 to 220 −2 to 103 169 to − 21.6 294 to − 615 115 to − 96 92 to − 155 621 to − 899
10 to −8 3 to − 9 6 to − 6 1 to − 2 12 to −7 3 to − 3 14 to 1 23 to −32
80 to − 64 147 to − 193 171 to − 187 111 to − 62 227 to − 195 33 to 64 7 to − 2 55 to − 50
625 to 650 100 to 300 500 450
1815–1953 1 2 3 4 5 6
0–23 23–76 76–104 104–134 134–196 196–215
1820–1818 1818–1829 1829–1828 1828–1840 1840–1922 1922–1930
491 to − 590 250 to − 279 250 to − 347 107 to − 144 106 to − 206 966 to − 625
13 to −17.6 9 to − 5 4 to − 5 3 to − 3 9 to − 3 19 to −19
15 15 14 12 14 15
215–242 1930–1926 659 to − 752
19 to −31
28 to − 32
150 to 200
1865–1915 126 to − 104 1915–1920 901 to − 919
5 to − 5 45 to −68
14 to − 11 15 to − 14
N1000 50 to 100
7 04
Altitude
to − 15 to − 13 to − 5 to − 5 to − 10 to − 15
300 500 500 225 N500 N1000 225
1865 –1989 1 2
0–28 28–97
3 1899–1942 1 1884–1977 1
97–133 1910–1958 187 to − 26 0–19 1899–1942 9.4 to − 10.6 0–41 1844–1942 837 to − 917
5 to − 47 0.1 to −0.2 25 to −28
15 to − 12 N1000 5 to − 8 N1000 190 to − 244 100 to 200
2 3 1742–2019 1 2 3 4
41–56 56–81 0–20 20–54 54–84 84–99
462 to − 185 56 to − 64 52 to − 29 176 to − 35 102 to − 408 −2.4 to − 91
9 to − 10 6 to − 6 1 to − 2 1 to − 2 4 to − 7 0.5 to −0.3
39 24 13 20 54 78
1942–1964 1964–1976 1911–1939 1939–1825 1825–2014 2014–1991
to − 81 to − 59 to − 58 to − 58 to − 62 to − 3
150 to 250 N1000 N1000 N500 300 N500
Alluvium Fall deposit Limestone Alluvium Alluvium Limestone Talus & alluvium Tuff-breccia & scoria Alluvium Alluvium Alluvium Alluvium Limestone Tuff-breccia & scoria Alluvium & basalt float Alluvium Alluvium & basalt float Limestone Limestone Basalt & tuff-breccia Tuff-breccia Limestone Limestone Alluvium & talus Tuff-breccia Limestone covered by ash-fall deposit
RMF = Residual Magnetic Field.
covered by post-volcanic deposits related to the possible formation of a lake, colluvium formed by post-volcanic masswasting of the inner walls of the crater, and a thin layer of soil. Thick talus deposits, formed mainly by limestone fragments, are obvious near the vertical walls of the Joya Honda maar and may exceed 50m near the walls, thinning out rapidly toward the center of the crater (Figs. 3, 7, and 12). The existence of a diatreme underneath the tabular body of fall-back and slump materials in the models shown in Fig. 13 can not be ruled out as the magnetic and gravimetric models are insensitive to its presence. The rock bodies in Fig. 13 are capable of producing the anomalies by themselves (i.e. adding the root of a diatreme underneath them does not improve the fit between the measured and calculated anomalies). This phenomenon could be originated by low magnetic and gravimetric contrast of the supposed diatreme with the overlying rock bodies, as they must have similar composition. In relation with this point, it is important to note that the stratigraphic evidence points out toward an increase in the water/magma ratio in the system as the eruption advanced. Therefore, the driving mechanism for a progressive deepening of the explosion chamber – and formation of a cone-shaped diatreme (Fig. 14b) – proposed by Lorenz
(1986) can not be applied to Joya Honda. We envision a less pronounced and more gradual deepening of the explosion chambers as the bottom of the crater was partially emptied by the violent expulsion of water vapor and a mixture of juvenile pyroclasts and slump material (Fig. 14d). Fall-back and slump material, together with a large and continuous supply of water provided by a conduit-dominated aquifer must have formed a dense slurry at the bottom of the crater between the explosions and acted both as the fluid for the fuel coolant reactions and as the material that caused the confining pressure for the explosions. This process may too be responsible of the relatively homogenous, non-stratified, nature of Joya Honda tuff-breccia; in contrast to the alternation of surge and fall deposits commonly observed in other maar sequences. Our gravimetric and magnetometric models can not reproduce in detail the internal structure of the post-eruptive sediments and fall-back and slump deposit that fill the crater (Fig. 13), but give an overall idea of the shape of this body and show that the anomalies inside the crater can be produced without a cone-shaped diatreme with a deep root. An unexpected result of the models in Fig. 13 is the asymmetry of the buried bodies within the crater. The shape and
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149
Fig. 13. Models of the structure underneath the crater as inferred from magnetic (A and B) and gravimetric (C and D) data collected along two nearly orthogonal profiles. The combination of an irregular body, with a roughly tabular shape (as compared with the shape of a diatreme), composed by post-eruptive (alluvium, talus and lake) sediments with irregular layers of fall-back pyroclastic and slump material produces a close match of both the residual magnetic (A and B) and gravity anomalies (C and D) along both profiles. The addition of a diatreme and/or an inclined feeder dike below the tabular body does not influence the gravity or the magnetic curve fitting.
location is not consistent with Aranda-Gómez and Luhr (1996) hypothesis of an inclined conduit to explain the distribution of the tephra north and northwest of the crater. A probable explanation for the modeled asymmetry in bodies is sketched in Fig. 15, where it is suggested that the attitude of the folded limestone caused the observed asymmetric structure underneath the crater. We assume the initial blast occurred near the surface with a limited amount of water contained in fractured limestone. A small, shallow crater was produced; ideally the initial shape of the crater was hemispherical. However, the asymmetric anticlinal structure in the limestone sequence influenced the mass-waste modification of the first, and subsequent, transient craters formed during the eruption. Likewise, the mass-waste process should have continued at a slower rate after eruption ceased, controlling present day's form of the crater. Note in Fig. 15 that the structural attitude of the limestone strata is different at each side of the fold's axial plane. Slumping at the northeastern limb of the fold was favored by hundreds of steep bedding planes dipping towards the bottom of the crater, while Table 2 Magnetic susceptibility and remanence measurements Sample
JH1 JH3 JH6 JH7 JH2 a JH4 a JH5 a a
Lithology
Joya Honda tuff Dry surge (base) Oligocene latite Massive basalt Limestone Tuff breccia Air fall tuff
Hand samples.
Susceptibility
NRM
(cgs units)
(emu)
0.006229 0.001921 0.006918 0.004957 − 0.000006 0.001803 0.001658
2.924 0.599 3.135 4.452
Q-ratio
2.13 3.207 2.206 1.113
on the southwestern side mass-waste process was slowed by the attitude of the strata. The shape of the rock body composed by fall-back, slump deposits, and post-maar sediments (lacustrine and talus deposits) underneath the bottom of today's crater must be controlled by the shape of the transient asymmetric craters formed at the time of eruption. 4. Conclusions 1. The Ventura volcanic field is located near the limit between two distinct regional magnetic domains. West of Ventura the outcrops are dominated by mid-Tertiary volcanic rocks and alluvium-filled basins. The magnetic pattern in this region is characterized by wavelengths in the order of ≤500 m and series of highs and lows with superimposed high-frequency anomalies of varied intensities (440nT–280nT). It is assumed that the magnetic basement underneath the alluvium filled valleys is also composed of mid-Tertiary volcanic rocks. The almost straight boundaries between the aeromagnetic domains (Figs. 2 and 4) are consistent with post-Oligocene (normal?) fault contacts between the Mesozoic rocks and mid-Tertiary volcanic rocks. It is worth noting that the alignment of volcanic vents in the Quaternary Ventura volcanic field is similar to other vent alignments in the region, but it is not strictly parallel to either one of the interpreted faults. However, the highest Quaternary vent density in the region (Fig. 2) and the only volcanoes (Joya Honda and Joyuela) with large mantle xenoliths (up to 10cm in diameter) in their products occur close to the intersection between these interpreted structures. We speculate that rapid magma ascent was facilitated in this area by a more fractured upper crust compared to surrounding areas.
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Fig. 14. Idealized diatremeless model for Joya Honda. (a) Lithozones in buried diatreme. (b) Model for the formation of the maar–diatreme system (Lorenz, 2003). (c) The magnetic and gravimetric anomalies above the crater can be modeled with a tabular body composed by fall-back and slump material, covered by post-volcanic sediments. (d) We assume that the water/magma ratio increased as the eruption developed; therefore, there is no need to invoke the pronounced deepening of the explosion chambers below the crater as in (B). The explosion chambers were always shallow under bottom of the crater. Confining pressure for the hydrovolcanic explosions developed by fall-back and slump material mixed with an increasing amount of water provided by a fracture controlled aquifer. The depth of the explosion chamber increased as a consequence of fragmentation and removal of material caused by the blasts.
2. A magnetic profile measured across the vent alignment, in an area where there is not a volcanic cover lying on top of the limestone, failed to show a magnetic anomaly produced by a buried dike (Fig. 10). Therefore, it is reasonable to model the Joya Honda conduit as a vertical or nearly vertical chimney close to the surface. The hypothesis that there is a connecting dike between aligned monogenetic volcanoes may not always be true, or the dike might lie deep underneath the surface, beyond the distance where it can be detected with the method used. 3. The magnetic and gravity models of the crater do not directly support the existence of a large diatreme underneath the maar. The modeled data is insensitive to the presence of the expected relatively low density and moderate magnetization diatreme. The most successful attempt to reproduce the measured magnetic and gravity anomalies suggest a roughly asymmetric basin filled with fall-back pyroclasts and slump
deposits near the surface (Fig. 13). The assumed values for the physical parameters in the models must be close to the real ones, as suggested by direct measurements of hand samples; thereby the combined thickness of the crater-fill deposit may exceed 500m below the floor of the crater. 4. The magnetic and gravity models associated to the Joya Honda maar suggests an SW–NE asymmetry in the lithozones buried underneath the crater. This asymmetry is not consistent with the interpretation that Joya Honda was formed by a series of freatomagmatic blasts directed towards the north and northwest, which is based on the distribution of the near-vent pyroclastic sequence (Fig. 3). Instead the documented asymmetry is interpreted as the product of a passive control of the Laramide folds on the mass-waste modification or the crater (Fig. 15). 5. The ground magnetic survey showed a previously unknown lava flow (Figs. 9 and 11) probably issued by the Cerro
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Acknowledgments This investigation was carried out during the sabbatical of Aranda-Gómez at the Instituto Potosino de Investigación Científica y Tecnológica (IPICYT). Financial support was provided by CONACYT (project 47071 to Aranda-Gómez) and COPOCYT (project 4722 to López-Loera). The IPICYT students David Torres and Raúl Paz, UNAM, CGEO Irving Arvizu, Paula González, and Ienisei Peña Díaz, and exchange students from UNAM, CCH Azcapotzalco: Aura Zepeda; UANL: Iván Galván; Strasbourg University (France): Emilie Renoux, Marion Recordon and Benjamin Dubois assisted in the field during the magnetic ground survey. Gabriel Origel and Rodolfo Díaz Castrejón elaborated a DEM of the crater area that was used to correct the gravimetric data. References
Fig. 15. Conceptual model proposed to explain the asymmetry of the inferred rock body underneath the crater of the Joya Honda maar. The shape of the body is attributed to the structural attitude of the folded Cretaceous limestone. (a) Conditions prior to phreatomagmatic activity. Pre-maar relief is eliminated, and number of bedding planes is considerably reduced for simplicity. (b) Initial phreatomagmatic blast occurred near the surface, fracturing rocks around the focus of the explosions. (c) Mass-waste modification of the first transient crater. (d) As the eruption continued, magma-water interactions occurred between the shallow muddy lakes at the bottom of the transient craters. Deepening of the explosion focii was very slow and independent of the formation of a cone of depression in the aquifer. (e) After the end of eruption a lake formed and masswaste continued always influenced by the pre-maar structure. Later, local hydrologic conditions changed and the lake finally dried out. The final result is an asymmetric crater with a steep, nearly vertical wall in the eastern side.
Quemado vent. Likewise, the agglutinate body east of the crater appears to be a buried cinder cone, almost completely covered by Joya Honda tephra (Fig. 7).
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