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

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. LAICPMS 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

Rate and processes of river network rearrangement during incipient faulting: The case of the Cahabon river, Guatemala

Deeply incised river networks are generally regarded as robust features that are not easily modified by erosion or tectonics. Although the reorganization of deeply incised drainage systems has been documented, the corresponding importance with regard to the overall landscape evolution of mountain ranges and the factors that permit such reorganizations are poorly understood. To address this problem, we have explored the rapid drainage reorganization that affected the Cahabon River in Guatemala during the Quaternary. Sediment-provenance analysis, field mapping, and electrical resistivity tomography (ERT) imaging are used to reconstruct the geometry of the valley before the river was captured. Dating of the abandoned valley sediments by the Be-10-Al-26 burial method and geomagnetic polarity analysis allow us to determine the age of the capture events and then to quantify several processes, such as the rate of tectonic deformation of the paleovalley, the rate of propagation of post-capture drainage reversal, and the rate at which canyons that formed at the capture sites have propagated along the paleovalley. Transtensional faulting started 1 to 3 million years ago, produced ground tilting and ground faulting along the Cahabon River, and thus generated differential uplift rate of 0.3 +/- 0.1 up to 0.7 +/- 0.4 mm . y(-1) along the river's course. The river responded to faulting by incising the areas of relative uplift and depositing a few tens of meters of sediment above the areas of relative subsidence. Then, the river experienced two captures and one avulsion between 700 ky and 100 ky. The captures breached high-standing ridges that separate the Cahabon River from its captors. Captures occurred at specific points where ridges are made permeable by fault damage zones and/or soluble rocks. Groundwater flow from the Cahabon River down to its captors likely increased the erosive power of the captors thus promoting focused erosion of the ridges. Valley-fill formation and capture occurred in close temporal succession, suggesting a genetic link between the two. We suggest that the aquifers accumulated within the valley-fills, increased the head along the subterraneous system connecting the Cahabon River to its captors, and promoted their development. Upon capture, the breached valley experienced widespread drainage reversal toward the capture sites. We attribute the generalized reversal to combined effects of groundwater sapping in the valley-fill, axial drainage obstruction by lateral fans, and tectonic tilting. Drainage reversal increased the size of the captured areas by a factor of 4 to 6. At the capture sites, 500 m deep canyons have been incised into the bedrock and are propagating upstream at a rate of 3 to 11 mm . y(-1) deepening at a rate of 0.7 to 1 5 mm . y(-1). At this rate, 1 to 2 million years will be necessary for headward erosion to completely erase the topographic expression of the paleovalley. It is concluded that the rapid reorganization of this drainage system was made possible by the way the river adjusted to the new tectonic strain field, which involved transient sedimentation along the river's course. If the river had escaped its early reorganization and had been given the time necessary to reach a new dynamic equilibrium, then the transient conditions that promoted capture would have vanished and its vulnerability to capture would have been strongly reduced

The recording of floods and earthquakes in Lake Chichój, Guatemala during the twentieth century

International audienceLaguna Chicho'j (Lake Chicho' j) is the only deep permanent lake in the central highlands of Guatemala. The lake is located in the boundary zone between the North American and Caribbean plates. The lake has been struck by devastating earthquakes and tropical cyclones in historical times. We investigated the imprint of twentieth century extreme events on the sedimentary record of this tropical lake using a bathymetric survey of the lake, coring the lake floor, and providing a chronology of sediment accumulation. The lake occupies a series of circular depressions likely formed by the rapid dissolution of a buried body of gypsum. 210Pb and 137Cs inventories and varve counting indicate high rates of sedimentation (1-2 cm year-1). The annually layered sediment is interrupted by turbidites of two types: a darker-colored turbidite, enriched in lake-derived biogenic constituents, and interpreted as a seismite, and a lightercolored type, enriched in catchment-derived constituents, interpreted as a flood layer. Comparison of our 137Cs-determined layer ages with a catalog of twentieth century earthquakes shows that an earthquake on the Motagua fault in 1976 generated a conspicuous darker-colored turbidite and slumped deposits in separate parts of the lake. The entire earthquake inventory further reveals that mass movements in the lake are triggered at Modified Mercalli Intensities higher than V. Tropical cyclonic depressions known to have affected the lake area had limited effect on the lake, including Hurricane Mitch in 1998. One storm however produced a significantly thicker flood layer in the 1940s. This storm is reportedly the only event to have generated widespread slope failures in the lake catchment. It is thus inferred that abundant landsliding provided large amounts of concentrated sediment to the lake, through hyperpycnal flow