ABSTRACT We present a summary of information on seismically active faulting in Chiapas, Mexico, related to North America–Caribbean plate-boundary zone deformation. We collected data from published works, and we also present new data collected from reporting agencies. Several active structures were identified as part of the deformation of the plate-boundary zone in the states of Chiapas and Veracruz, including 18 large (up to 175-km-long) strike-slip faults belonging to three tectonic realms: the Tonalá realm, the Depresión Central, and the strike-slip fault province. Available fault-plane solutions indicate left-lateral, strike-slip displacement along these faults. The reverse-fault province is also found to be part of the plate-boundary zone and seismically active, with thrust-faulting fault-plane solutions. Deformation extends to the northwest, along the Veracruz coastal plains region, which is also seismically active.

ABSTRACT In 1935, Krynine postulated that first-cycle arkose in the humid tropical setting of southern Mexico can be rapidly eroded with minimal chemical weathering and redeposited as second-cycle arkose. Modern quantitative data confirm this hypothesis and highlight exceptions where first-cycle arkosic sediments have been diagenetically altered by intense weathering to yield second-cycle quartz arenites. In this study, extensive sampling of upland source rocks and their derived sediments provided a robust data set with which to quantitatively evaluate the composition and provenance of Holocene sediments. Three upland source terrains were identified: Paleozoic crystalline basement of the Chiapas Massif; Mesozoic to Cenozoic siliciclastic and carbonate rocks of the Chiapas fold belt; and Cenozoic sedimentary rocks in the foothills of the fold belt. Holocene sediments from these source terrains are grouped into seven facies (A–G) based on their provenance and geographic location. Facies A consists of feldspathic sediments from the Mezcalapa-Grijalva River that are sourced from the Chiapas Massif. Facies B consists of lithic-rich sediments from the same area that are derived from the Chiapas fold belt. Facies A and B consist predominantly of first-cycle sand capable of yielding arkosic deposits. Facies C represents a mixture of Facies A and B sands deposited along the course of the Mezcalapa-Grijalva River. Facies D (from Rio Sierra) and Facies E (from Rio Pedregal) represent second-cycle feldspathic sands of the coastal-plain delta and were derived from Cenozoic sedimentary rocks of the foothills. Mild chemical weathering due to rapid mechanical erosion enabled the creation of these arkosic deposits. They are less feldspathic than their parents and have limited occurrence due to mixing with less feldspathic first-cycle sands downstream from their sources. Facies F (from Rio Zanapa) and Facies G (from Lagunas Rosario and Enmedio) represent second-cycle quartzose sands of the low-lying savanna that were also derived from Cenozoic sedimentary rocks in the foothills of the fold belt. Intense, long-term (>10,000 yr) chemical weathering of these sands has precluded the formation of arkoses, instead yielding quartz arenites. They are more weathered than the delta sands (Facies D, E) with a greater loss of feldspar and carbonate detritus. They are enriched in silica and depleted in alumina, CaO, Na 2 O, and K 2 O relative to Facies A arkoses due to loss of feldspars and mafic minerals. Second-cycle sediments eroded from Tertiary sedimentary rocks in the foothills (Facies D–G) contain detrital serpentine and chromite with high abundances of Cr and Ni, suggesting an ultramafic component in their provenance. Cr and Ni are effective tracers for second-cycle components in sands of mixed provenance.

Abstract High permeability and rapid recharge in karst aquifers make them susceptible to contamination. We combined a groundwater vulnerability map with an environmental disturbance index to give an adaptable spatial tool for developing management strategies for a karst environment in the Reserva de la Biosfera Selva el Ocote (el Ocote), Chiapas, Mexico. Seventy-two per cent of the study area is classified as an area of least concern for management, with 60% falling within el Ocote. Consequently, although there are concerns regarding the vulnerability of the karst ecosystem, the lack of development and the natural protection of the ecosystem, the immediate need for remedial action by the area’s managers is currently minimal. About 27% of the study area is classified by the composite model as of moderate concern, with 34% within el Ocote. This reflects a balance between areas of moderate and high vulnerability, but little disturbance. Based on the management zones created by this study, much of the sub-catchment is zoned as of least or moderate concern, where disturbance has not occurred. As such, the opportunity exists to prevent major human impacts on vulnerable areas and the entire ecosystem, but only if local stakeholders are incorporated into this process of limiting development.