Carlos Ortega-Obregón

The Maya-Chortís Boundary: A Tectonostratigraphic Approach

This work presents an updated revision of the complex stratigraphic and tectonic relationships that characterize the geologic boundary between the Chortís and Maya continental blocks of the Caribbean region. Based on field, petrologic, structural and geochronological work in key areas of central Guatemala, as well as analysis of the relevant literature, we propose a new tectonostratigraphic structure that more fully appraises the fundamental tectonic role played by major faults that cut across the continental isthmus between the Americas, and bound separate tectonostratigraphic terranes (or fault blocks according to author JDK). Accordingly, we subdivide the area into seven of these units, from south to north: Chortís, Yoro, Sula, El Tambor, Jacalteco, Achí, and Maya, bounded respectively by the Agúan-La Ceiba, Jocotán-Chamelecón, Motagua, Baja Verapaz (defined in this work), and Chixoy-Polochic fault zones. Unfortunately, the extreme paucity of modern geologic data bearing on the pre-Cretaceous cover and basement units in the entire region constitutes a major obstacle for building convincing paleogeographic models to explain the complex tectonic evolution of the area from Precambrian to Cenozoic time. Consequently, this work should be taken as an attempt line to understand more clearly the nature and contact relationships between deep crustal blocks in nuclear Central America, and as a contribution to interpret their geologic evolution in plate tectonic terms.

Middle-Late Ordovician magmatism and Late Cretaceous collision in the southern Maya block, Rabinal-Salamá area, central Guatemala: Implications for North America-Caribbean plate tectonics

The Rabinal-Salama area in central Guatemala provides critical data bearing on the relationships between the North American and Caribbean plates because it lies within the Polochic-Motagua fault zone that separates the two plates. The cumulative Cenozoic sinistral displacement across this zone that separates the Maya and Chortis terranes has been variously estimated to be ~125 km or ~1100 km, evidence for which should be recorded in the rocks of the studied area. The Rabinal-Salama area lies between two of the east-west faults within the Motagua fault zone, the Polochic fault, and the Baja Verapaz shear zone. The shear zone separates the Maya block from eclogitic rocks of the Chuacus Complex that pass southward into ophiolitic rocks and melanges that define a suture between the Chuacus Complex and the Chortis block. The following sequence of events is recorded in the Rabinal-Salama area: (1) low-grade, pre-Silurian siliciclastic metasedi-mentary rocks (San Gabriel unit), that are intruded by (2) ca. 462–453 Ma calc-alkaline, peraluminous, S-type Rabinal granite suite, and unconformably overlain by (3) very low grade clastic and calcareous metasedi-mentary rocks (Santa Rosa Group) containing Mississippian conodonts and pebbles of granite, sandstone, and phyllite derived from the older units. The Rabinal granite suite is inferred to be rift related, inheriting its calc-alkaline signature from its source, along with the ca. 1 Ga xenocrystic zircons (upper intercept U-Pb data). Deformation in all these Paleozoic rocks produced a steeply south-southwest–dipping cleavage (chlorite and sericite) and a stretched quartz lineation. These fabrics become more intense adjacent to the Baja Verapaz shear zone, where C-S fabrics and rotated porphyroclasts indicate a reverse sense of motion with a sinistral component. White mica in the shear zone yields 74–65 Ma K-Ar ages, which are inferred to closely postdate the time of crystallization. Thus, although evidence for major sinistral displacement is absent, the kinematics are consistent with uplift and exhumation of the Chuacus Complex during obduction of the Baja Verapaz ophiolite onto the Paleozoic rocks of the Rabinal-Salama area in latest Mesozoic-Paleocene. This is inferred to have been produced during collision of the Cuban arc and Chortis block with the southern Maya block. Restoration of the Early Mesozoic ~70° anticlockwise rotation of the Maya block places the Rabinal-Salama area adjacent to northeastern Mexico, where comparable continental-shallow marine, Paleozoic rocks occur near Ciudad Victoria overlying the ca. 1 Ga Oaxaquia basement.

Geochronology and Geochemistry of the ~917 Ma, Calc-alkaline Etla Granitoid Pluton (Oaxaca, Southern Mexico): Evidence of Post-Grenvillian Subduction along the Northern Margin of Amazonia

The post-tectonic Etla pluton intrudes the ~1 Ga granulitic Oaxacan Complex that cooled through 450°C by ~945 Ma. The Etla pluton consists of massive, coarse, porphyritic granodiorite-monzogranite (plagioclase, K-feldspar, quartz, biotite ± hornblende) with fine-grained felsic rocks along the margin. Geochemistry indicates that it is a peraluminous, I-type, medium-K, calc-alkaline, volcanic-arc granite-trondjemite with relatively low contents of high-field-strength elements and flat REE patterns. U-Pb zircon isotopic analyses fall on a chord with intercepts at 180 ± 50 Ma and 920 ± 25 Ma: the latter is similar to the 207Pb/206Pb age of 917 ± 6 Ma of the least discordant (1%) analysis and is inferred to date the time of intrusion. This pluton is synchronous with similar igneous activity in Avalonia (eastern Appalachians) and in Tocantins Province of central Brazil, which may form parts of a peri-Amazonian magmatic arc. 40Ar/39Ar laser step-heating analyses of biotite and K-feldspar yielded plateau ages of 207 ±5 Ma and 221 ± 3 Ma, respectively, that may be related to Phanerozoic reheating.

Geology and geochronology of Paleozoic rocks in western Acatlán Complex, southern Mexico: Evidence for contiguity across an extruded high-pressure belt and constraints on Paleozoic reconstructions

The Acatlan Complex straddles a high-pressure belt previously interpreted as either: (1) a suture zone within the Iapetus or the Rheic oceans, which would have a contrasting geological record across the suture; or (2) a tectonic slice extruded into the upper plate, which would imply contiguity across the complex. Distinguishing between these hypotheses is critical to paleogeographic reconstructions. Examination of the western Acatlan Complex reveals the following: (1) deposition of clastic rocks between 654 and 464 Ma; (2) intrusion of bimodal Ordovician bodies at ca. 464 Ma; (3) high-grade deformation with cooling through 400 °C by ~360–335 Ma; (4) deposition of clastic rocks and pillow lavas after ~350–400 Ma; (5) deformation accompanied by greenschist facies metamorphism at ca. 335 Ma; (6) deposition of clastic and bimodal volcanic rocks at ca. 327 Ma; (7) ~320–270 Ma subgreenschist deformation; (8) deposition of the Middle-Upper Permian sedimentary rocks; and (9) intrusion of a 61 ± 1 Ma diorite followed by early Cenozoic (Laramide) ENE folding and faulting. Zircon ages (~350–400, 570–505, 827–890 Ma, 0.9–1.3 Ga) suggest both local and Amazonian sources with deposition above a local Mesoproterozoic (Oaxacan) basement on the southern margin of the Rheic Ocean. This geological record is very similar to that of the eastern Acatlan Complex, which supports the extrusion hypothesis, a model that may be applicable to other orogens.

U-Pb zircon geochronology of Palaeozoic units in Western and Central Guatemala: insights into the tectonic evolution of Middle America

Abstract Precambrian and Palaeozoic basements are present in southern Mexico and Central America, where several crustal blocks are recognized by their different geological record, and juxtaposed along lateral faults. Pre-Mesozoic reconstructions must take into account the nature of such crustal blocks, their geological history, age and petrology. Some of those crustal blocks are currently located between southernmost north America (the Maya Block) and Central America (Chortís Block).To better understand the geology of these crustal blocks, and to establish comparisons between their geological history, we performed U–Pb dating of both igneous and metasedimentary key units cropping out in central and western Guatemala. In the Altos Cuchumatanes (Maya Block) granites yield both Permian (269±29 Ma) and Early Devonian (391±7.4 Ma) U–Pb ages. LA-ICPMS detrital zircon ages from rocks of the San Gabriel sequence, interpreted as the oldest metasedimentary unit of the Maya Block, and overlain by the Late Palaeozoic Upper Santa Rosa Group, yield Precambrian detrital zircons bracketed between c. 920 and c. 1000 Ma. The presence of these metasedimentary units, as well as Early Devonian to Silurian granites in the Mayan continental margin, from west (Altos Cuchumatanes), to east (Maya Mountains of Belize) indicates a more or less continuous belt of Lower Palaeozoic igneous activity, also suggesting that the continental margin of the Maya Block can be extended south of the Polochic fault, up to the Baja Verapaz shear zone. A metasedimentary sample belonging to the Chuacús Complex yielded detrital zircons with ages between c. 440 and c. 1325 Ma. The younger ages are similar to the igneous ages reported from the entire southern Maya continental margin, and show proximity of the Complex in the Middle-Late Palaeozoic. The S. Diego Phyllite, which overlies high-grade basement units of the Chortís Block, contains zircons that are Lower Cambrian (c. 538 Ma), Mesoproterozoic (c. 980 to c. 1150 Ma) and even Palaeoproterozoic (c. 1820 Ma). Absence of younger igneous zircons in the San Diego Phyllite indicates that either its sedimentation took place in a close range of time, during the Late Cambrian, or absence of connection between Chortís and Maya Blocks during the Early–Mid-Palaeozoic. The Precambrian zircons could have come from southern Mexico (Oaxaca and Guichicovi Complexes), or from Mesoproterozoic Massifs exposed in Laurentia and Gondwana. Palaeogeographic models for Middle America are limited to post-Jurassic time. The data presented here shed light on Palaeozoic and, possibly, Precambrian relationships. They indicate that Maya and the Chortís did not interact directly until the Mesozoic or Cenozoic, as they approached their current position.

Detrital-zircon record of major Middle Triassic–Early Cretaceous provenance shift, central Mexico: demise of Gondwanan continental fluvial systems and onset of back-arc volcanism and sedimentation

New stratigraphic and petrographic data and zircon U–Pb geochronology from sandstones and volcanic rocks in the states of Queretaro and Guanajuato in central Mexico indicate an important provenance change between Late Triassic and latest Jurassic–Early Cretaceous time. The Upper Triassic El Chilar Complex consists of pervasively deformed, deep-marine olistostromes, and debris-flow deposits of arkosic and subarkosic composition. Detrital-zircon populations range from latest Palaeoproterozoic (1.65 Ga) to Middle Triassic (240 Ma), all predating the depositional age of the strata. The detrital-zircon populations are similar to those previously reported from turbidites of the Potosi fan complex of north-central Mexico and interpreted as derived from Grenville and Pan-African (Maya block) basement and Permo-Triassic arc of continental Mexico directly to the east of the basin. A single sample with a dominant Proterozoic population at ∼1.65–1.30 Ga was likely derived either from the Rio Negro-Juruena province of the Amazonian craton or from a local source in the Huiznopala Gneiss, and indicates that El Chilar strata were likely deposited in the proximal part of a submarine-fan system separate from the Potosi fan. Uppermost Jurassic–Lower Cretaceous strata of the San Juan de la Rosa Formation unconformably overlie the El Chilar Complex and likewise consist of deep-marine olistostromes, slump deposits, debris-flow deposits, and proximal fan-channel fills, but are volcanogenic litharenites with abundant felsic and vitric volcanic lithic fragments. Detrital-zircon populations are dominated by Early Cretaceous grains (150–132 Ma) with no known sources in eastern Mexico. Abundant young grains indicate a maximum depositional age of ∼134 Ma (Valanginian–Hauterivian). The San Juan de la Rosa Formation is overlain by deepwater carbonates with interbedded siliciclastic beds of the Peña Azul Formation, which contains detrital-zircon ages as young as ∼130 Ma, indicating possible equivalence with similar strata of the Las Trancas Formation, with a maximum depositional age of ∼127 Ma and lying to the east in the Zimapan Basin, now part of the Sierra Madre Oriental fold and thrust belt. Decreasing content of volcaniclastic strata eastward indicates a volcanic source to the west. Upper Cretaceous marine strata in the Mineral de Pozos area to the northwest in the state of Guanajuato contain litharenites with a maximum depositional age near 92 Ma, and are thus part of a younger depositional system. Composition and detrital-zircon content of the Upper Triassic and Lower Cretaceous successions in central Mexico indicates an important shift from Gondwanan continental sediment sources in the Triassic to western volcanic sources, probably on the edge of the newly opened Arperos basin, by the end of the Jurassic. This important sediment-dispersal change records the break-up of Pangea and concomitant development of arc-related sedimentary basins on the western edge of Mexico.

The origin of intraplate magmatism in the western Trans-Mexican Volcanic Belt

Alkaline basalts with geochemical features similar to those of intraplate ocean islands have been emplaced along the main trace of the Tepic-Zacoalco rift (TZR), a unique tectonic structure of the western Trans-Mexican Volcanic Belt in which extension is superimposed to a convergent margin. New geochemical and petrologic data on mafic volcanic rocks along the rift indicate the existence of a highly heterogeneous pre-subduction mantle wedge that has been slightly overprinted by slab-derived chemical agents. Most mafic volcanic rocks display geochemical and isotopic compositions that are indistinguishable from those of the Pacific islands Socorro and Isabel, and confirm the existence of an ancient, recycled, high-μ component (HIMU; μ = 238 U/ 204 Pb) in their mantle source. Olivines separated from samples carrying the HIMU signature have NiO and CaO contents similar to olivines from mid-ocean ridge basalt (MORB), indicating that the source of enrichment must be entirely hosted in peridotite. In contrast, more evolved rocks within the TZR have stronger subduction signatures and water contents, and display a distinctive isotopic array that points to slab-derived contributions. Olivines from these rocks are slightly less forsteritic but also extend to higher NiO and lower CaO contents than those from more mafic magmas, suggesting provenance from a secondary pyroxenite source. The overall geochemical evidence thus indicates that the pre-subduction background mantle wedge in western Mexico must be identical, and just as diverse, as that below the Pacific basin. Extension-driven mantle upwelling in a continental setting can only melt a dry peridotitic mantle to its lowest extents, and therefore preferentially sample its most enriched and easily fusible components. Yet the addition of even a small amount of slab-derived silica promotes a secondary petrologic transformation to pyroxene-rich lithologies that upon melting create magmas with compositions that are more akin to a volcanic arc setting.

LA-ICP-MS-based apatite fission track dating of the Todos Santos Formation sandstones from the Sierra de Chiapas (SE Mexico) and its tectonic significance

Abstract This study reports the first LA-ICP-MS-based apatite fission track age data from the Toarcian–Oxfordian Todos Santos Formation sandstones exposed in the Sierra de Chiapas (SCH), SE Mexico. Single-grain fission track ages obtained from four rock samples vary between 232 ± 31 (1σ) and 40 ± 3 (1σ) Ma, indicating partial resetting of detrital apatite populations by post-depositional heating during diagenesis. Decomposed data were interpreted as follows: (a) cooling of the sediment source area at 203 ± 7 (1σ) and 163 ± 3 (1σ) Ma, which are respectively coincident with the late Permian–Triassic thermo-tectonic event and Early–Middle Jurassic arc volcanism that affected the Chiapas Massif Complex; and (b) a post-burial cooling period (with variable cooling rates of <3°C Ma–1) in the range of ~83–51 Ma controlled by a Late Cretaceous–early Eocene tectonic activity, which can be correlated with the Laramide orogeny that occurred along central and northern Mexico during the same period of time (i.e. from 85–80 to 50–40 Ma). The results obtained in the present study, as well as previously published data, indicate that the SCH has experienced a multi-episodic thermo-tectonic evolution with at least four main stages.

New stratigraphic, geochronological, and structural data from the southern Guanajuato Mining District, México: implications for the caldera hypothesis

Abstract The Cenozoic stratigraphy of the southern Guanajuato Mining District (GMD) was established 40 years ago. The existence of a caldera structure that produced the Cenozoic volcanic cover was postulated and the world-class silver ore deposit of the Oligocene age has been closely related to magmatism. In this context, we present a new geological map of the southern GMD, U–Pb and Ar–Ar ages of the volcanic units, and structural data for the Cenozoic faults. Our results document that the volcanic centre was active between ca. 33.5 Ma and ca. 31.3 Ma, coeval with NW–SE normal faulting. We propose that the Bufa, Calderones, and Cedro formations are stratigraphic units directly related to the volcanic centre. Although the younger Chichíndaro Rhyolite scarcely crops out within the study area, it appears to be more extensive outside of the study area, forming part of the rhyolitic volcanism of the Mesa Central of Mexico. In the study area, the Chichíndaro Rhyolite buries major faults, demonstrating that it was emplaced after the peak of faulting. The two main structures are the El Cubo and Veta Madre grabens; also there are several faulted and brecciated zones where silver–gold mineralization was emplaced. The extension direction changed from NE to NW producing normal faulting, reactivating older structures and allowing dike intrusion. The extensional phase continued to be active throughout the Oligocene. The age of the volcanic event and a new K–Ar age of the Veta Madre vein of 29.8 ± 0.8 Ma (K–Ar in adularia) indicate that the hydrothermal event began immediately after the emplacement of the Cedro Formation. The emplacement of the Chichíndaro Rhyolite allowed hydrothermal activity to be active for two million years or more.