TECTONIC SETTING AND VOLCANO–TECTONIC EVOLUTION OF THE TILZAPOTLA CALDERA

TECTONIC SETTING AND VOLCANO–TECTONIC EVOLUTION OF THE TILZAPOTLA CALDERA

The Balsas–related beds of the Amacuzac–Zacapalco area are distributed northeast from the intersection of two structural highs: the NNE–trending Laramide thrust and fold belt of the Iguala–Cacahuamilpa sector and the NW–trend–ing elliptical dome associated with the Tilzapotla caldera (Figures 2, 5).

The Laramide sector of the Iguala–Cacahuamilpa region is formed by a tectonic stack of overturned folds and thrust sheets with a general east vergence (Figure 2) (de Cserna y Fries, 1981; Rivera–Carranza et al., 1998). This forms a range dominated by extensive exposures of the Morelos Formation that thrust over the Mezcala Formation and the Morelos Formation itself. Deeper erosion of the Mezcala Formation east of the Iguala–Cacahuamilpa front produced an elongated topographic low, approximately 20 km wide, partly filled with the Balsas–related sediments in Paleogene time. The NNE–trending topographic low was broken up by the Eocene–early Oligocene Tilzapotla volcanic edifice and associated NW–trending elliptical dome structure. The dome is 50 km long, and extends from Quetzalapa to the Taxco area where it merges with the Iguala–Cacahuamilpa highs (Figures 2, 5). The structural dome interfered with the older Laramide NNE–trending structures. This is indicated by the attitude of Late Cretaceous fold axes, which plunge to the north and to the south at north and south flanks of the structure respectively (Morán–Zenteno et al., 2004). Based on stratigraphic and structural observations around the Tilzapotla caldera, Morán–Zenteno et al. (2004) considered this dome as a structure developed in the early stages of the caldera evolution, and a result of the tumescence associated with the overpressure in the magma chamber. The elongated shape of the dome is partly controlled by the tectonic lineaments at its northeast and southwest flanks (Figure 2). The deformation and volcanic events related to the caldera evolution, as well as their chronological relationships, have been previously documented by Morán–Zenteno et al. (2004). The Tilzapotla ignimbrite (34–35 Ma) is the major volcanic product of caldera collapse. The studied area, where the Balsas–related rocks are exposed, is located at the northwestern margin of the dome structure near the northwestern limit of the caldera ring. In this area, the dome margin displays a local inflection in the form of a curvature, convex to the northeast, defined by a change in the dip direction of the Morelos Formation and basal Tepetlapa beds from north to east (Figures 4a, 5).
One remarkable feature in the studied area is the relatively high northeast and east dip of the Balsas–related beds, without recognizable shortening at outcrop scale (Figures 4a, 4b, 6). Dip angles become lower eastwards, being up to 50° in the basal beds exposed in the southwestern sector contiguous to the Tilzapotla dome structure, but less than 10° near the top, in the northeastern sector. The basal unconformity with the middle Cretaceous Morelos Formation is tilted as well, and rocks of the Morelos Formation form a steep topographic slope (Figure 4b). The decrease in angle of the Balsas Group toward the east and upsection was previously interpreted by Fries (1960) as the result of syn–sedimentary tilting produced by faulting or large amplitude folding. However, our data suggest that the wide folding and change in dips might be related to the evolution of the Tilzapotla caldera, as it is discussed below.



The angular unconformity separating the basal Tepetlapa beds and the underlying folded beds of Mezcala and Morelos formations (Figure 4b) indicate a time interval between the cessation of the Laramide deformation and the accumulation of the terrestrial deposits. The alluvial fan that originated the Tepetlapa Formation probably originated from an outlet of the topographic front formed by the thrust anticline. This is also suggested by the general eastward decrease in the clast size of the Tepetlapa Formation. The location and origin of the eastern limit of the sedimentary basin is unknown because it is covered by Pliocene epiclastic and alluvial deposits that unconformably overlie the Balsas Group and the Mezcala Formation. The sedimentary facies recognized in the Tepetlapa Formation are indicative of an alluvial fan environment dominated by debris flow and sheet flow deposits. The lower and middle members show distal fan and/or flood plain facies (Figure 7a). The upper member shows a coarsening–upward tendency, which might be related to the rejuvenation of the landscape. This probably was due to a first uplift stage related to the emplacement of silicic magmas in the Taxco volcanic center located at the northwestern side of the Tilzapotla dome structure. This is also suggested by an increment in the percentage of volcanic clasts in onglomerate beds of the same upper member. The slight angular unconformity between the Tepetlapa formation and the overlaying units records a break in the sedimentation and is probably related to the eastward tilting produced by this uplift episode. Regional geocronology of silicic magmatism suggest that the doming process began near the Taxco volcanic center at around 38 Ma and was progressing toward the SE until 34 Ma, when the collapse of the Tilzapotla caldera occurred. There is no evidence of normal or lateral faulting at the piedmont that could have produced a slope break between the accumulation area and highlands to the west. Therefore we suggest that the deposition of the Tepetlapa Formation occurred at the flank of an intermontane basin. The age range of the Tepetlapa Formation is less well constrained than the ages of the overlying units. The age obtained for an andesitic flow comes from the upper member (44.5 ± 0.7 Ma), and indicates a minimum Eocene (Lutetian) age for the sedimentation. Therefore the Tepetlapa Formation was not related to the evolution of the Tilzapotla caldera, but to a previous stage in the evolution of the sedimentary basin. Remarkably coarser textures of the Huajintlán Formation indicate a significant change in the depositional environments. The relatively high abundance of sandstone and siltstone lithic fragments in the Tepetlapa beds, resembling the lithology of the Upper Cretaceous Mezcala Formation, is not found in the Huaj intlán Formation. In contrast, the Huaj intlán Formation is dominated by limestone clasts derived from the Morelos Formation. This change in composition of the lithic components is indicative of a change in the dominant lithology exposed in the source area. In addition to the change in clast composition, there is a major change in the facies and depositional environments in the Huajintlán Formation, compared to those observed in the Tepetlapa Formation. The coarse matrix–supported conglomerate texture and the sedimentary features of rocks of the Huajintlán Formation are typical of proximal alluvial fan deposits (Figure 7d). The change in clast composition and depositional environments recorded in the Huajintlan Formation suggests an uplift of the source area, which occurred in a more proximal position to the studied area. The best candidate for the rejuvenation of the landscape and the change in the sedimentary system observed in the area is the late Eocene doming produced by magma emplacement in the early stages of the evolution of the Tilzapotla volcanic center (Figures 5 and 7d). This is supported by the age of the ignimbrites underlying (Piedras Altas) and overlying (Tilzapotla) the Huajintlan Formation. The best age estimate for the caldera collapse is the 34.29 Ma Ar–Ar date obtained for the Tilzapotla ignimbrite (Morán–Zenteno et al., 2004) at the eastern margin of the caldera. These results, compared to the K–Ar date of 35.2 Ma of the Piedras Altas Ignimbrite, left a time interval of at least 0.5 Ma for the volcanic center evolution since the first pre–collapse ignimbrite emplacement, which is a reasonable time for this kind and size of volcanic center (Smith and Bailey, 1968). The northeastward and eastward relative high dip of the westernmost Tepetlapa beds at the dome border ( 50°) and their strike following the inflection in the contours of this structure indicate a tilting episode related to the tumescence in the Tilzapotla caldera (Figure 4a). The relatively slight angular relationship with the syn– and post– doming Huajintlan Formation farther to the northeast, indicates that the tilting effect produced by the tumescence decreased in a short distance from the dome boundary. The K–Ar ages obtained from intercalated volcanic rocks (35.2, 38.5 and 44 Ma) in the studied succession (Figure 3, Table 1) indicate that the main episodes of deposition occurred during the Eocene. The age estimates for the Balsas Group in the study area are in part similar to those obtained in other areas within the Guerrero–Morelos region. In the nearby Taxco area, Alaniz–Álvarez et al. (2002) documented Eocene volcanic rocks intercalated with continental deposits, similar to those of the study area. There are no indications that the basal beds of the studied succession could be contemporary with the Paleocene beds exposed along the Balsas river in central Guerrero State (Cerca, 2004). Even assuming low rates of sedimentation for the beds underlying the oldest dated andesite (44 Ma) in the studied area, it could hardly be interpreted that the basal beds reach a Paleocene age, unless significant interruptions of sedimentation of the Tepetlapa Formation occurred, which is not evident in the studied sections. This indicates that the evolution of the Paleogene continental basins began diachronously in the region. There are no signs of accumulation in a coeval, fault–bounded, subsidence basin as it has been documented in the neighboring Taxco region. Inclination of Balsas beds in the area are clearly related to doming processes and no listric faults seem to have been involved in the tilting process. Besides the evidence of tilting related to the doming process in the Tilzapotla caldera area, the structural attitude does not support listric or lateral faulting associated with the main slop break, separating the accumulation area from the highlands to the west. There is no evidence of the prolongation in this area of the tectonic lineament that defines the northeastern boundary of the Tilzapotla caldera (Figures 2, 4a).

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TILZAPOTLA-BUENAVISTA CALDERA AND Zapalco–Amacuzac area (first page)
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