We report a paleomagnetic and rock-magnetic study of Miocene volcanic rocks from the Trans-Mexican Volcanic Belt. A total of 32 sites (238 oriented samples) were collected from three localities: Queretaro, Guadalajara and Los Altos de Jalisco basaltic plateaux, which span from 11 to 7.5 Ma. Several rock-magnetic experiments were carried out in order to identify the magnetic carriers and to obtain information about their paleomagnetic stability. Microscopic observation of polished sections shows that the main magnetic mineral is Ti-poor titanomagnetite associated with exsolved ilmenite. Continuous susceptibility measurements with temperature yield in most cases reasonably reversible curves with Curie points close to that of magnetite. Judging from the ratios of hysteresis parameters, it seems that all samples fall in the pseudo-single domain (PSD) grain size region, probably indicating a mixture of multidomain (MD) and a significant amount of single domain (SD) grains. Based on our paleomagnetic and available radiometric data, it seems that the volcanic units have been emplaced during a relatively short time span of 1 to 2 My at each locality. The mean paleomagnetic directions obtained from each locality differ significantly from that expected for the Middle Miocene. The mean paleomagnetic direction calculated from 28 sites discarding those of intermediate polarity is I = 32.46 • , D = 341.2 • , k = 7.2 and α 95 = 11.6 •. Comparison with the expected direction indicates some 20 • anticlockwise tectonic rotations for the studied area, in accordance with the proposed left-lateral transtensional tectonic regime already proposed for this period.
In spite of the geomorphological diversity of Central America and Costa Rica, there are few detailed geomorphological maps. A 1:25,000 scale map of the Upper General River Basin (UGRB) in Costa Rica is presented, based on the interpretation of aerial photographs and field geomorphological mapping. A total of 43 types of landforms divided genetically into endogenic (tectonic features), and exogenic (fluvial, gravitational, and glacial features) are mapped for an area of 1560 km 2. This cartography is important in terms of disaster prevention and mitigation (floods and hillslope processes), as well as for land use planning.
Several regions of tropical America show imprints of past glacial activity. These relict landforms can support the understanding of past climate conditions, such as during the Last Glacial Maximum (LGM), and the implications that these paleoclimatic conditions could have had on landscape change. Here, we present and analyze glacial morphologies for the Chirripó National Park in Costa Rica based on aerial imagery (1:25,000), detailed Digital Elevation Models, geomorphic mapping, as well as geomorphic assessments in the field to determine and validate landforms. This study adds valuable insights into the reconstruction of the maximum expansion of tropical glaciation during the LGM in Costa Rica and into tropical America glacial landscapes in general.
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