No abstract

El método aplicado en el presente artículo para el análisis de deformación está basado en la evolución de pliegues según inclinación de estratos y en el tratamiento de planos con estrías de falla. El estudio se realizó en el flanco que comparten el Anticlinorio de Los Yariguíes (al oriente) y el Sinclinal de Nuevo Mundo (al occidente), estructuras representativas al NW de la Cordillera Oriental de Colombia. Los datos se sometieron a un análisis mecánico de generación de fracturas según los principios de Anderson y Mohr y se aplicó la técnica de rotación de estratos a los planos con estrías de falla que no cumplieran con presunciones mecánicas. Los resultados del análisis estructural permiten soportar la deformación progresiva que se presenta en la zona con la observación de tres etapas asociadas con distintas condiciones tectónicas. La etapa uno corresponde a distensión-transtensión, con una dirección de máxima extensión de 126° (NW-SE) asociada al inicio de la acreción de la Cordillera Occidental al final del Mesozoico. La etapa dos se asocia a deformación por compresión-transpresión en el Paleoceno, la cual presenta un tensor de máximo acortamiento en azimut de 30° (NNE-SSW). Finalmente, la etapa tres corresponde al levantamiento de la Cordillera Oriental en el Oligoceno tardío-Mioceno, asociado a un tensor de acortamiento en dirección 118° (NWW-SEE), posiblemente relacionado al inicio de la acreción del bloque Chocó-Panamá. Por otra parte, se establece un estilo estructural con fallas de mayor ángulo que cortan las unidades del Jurásico y de piel delgada que afecta la secuencia cretácica. La zona también está afectada por fallas de rumbo transversales que desplazan levemente las estructuras longitudinales en sentido dextral y dividen el flanco occidental del Anticlinorio de Los Yariguíes, manifestando una deformación diferencial.

The present study reviews radiometric and thermobarometric techniques used to construct cooling curves or paths to characterize intrusive bodies and to calculate cooling and exhumation rates. To construct these curves or paths, the temperature, time and depth variables must be estimated in intrusive bodies by applying various analytical techniques, including thermobarometry and U-Pb zircon, Ar-Ar hornblende and muscovite, fission track and (U-Th)/He zircon and apatite dating, in combination with a geological framework of reference for each intrusive body. The authors recommend to determine the crystallization age by zircon U-Pb dating, to quantify the emplacement depth using thermobarometry methods according to the composition of the intrusive body, to calculate the initial cooling ages with hornblende and muscovite Ar-Ar methods, as well as to calculate the cooling/exhumation ages in the upper crust using low-temperature thermochronology methods. These cooling curves or paths in intrusive bodies are highly relevant when studying compressive or extensional areas because they partly represent the thermal history of the era, thereby providing data on the magmatic and tectonic evolution of the region. Thus, these studies are highly important for designing geodynamic models and for their possible application in developing the tectonic model of the country.

The description of the fabric elements represented by the linear and planar structures present at different scales is a key component of fieldwork. A scheme is proposed for the systematic registration of planes and lineations, coded as S (planar surfaces), F (folds), and L (lineations), among others, that allows for the orderly storage of the measurements taken. This scheme includes information related to the kinematics, the kinematic indicators, and the certainty or reliability ascribed to the assigned movement. In the fieldwork, the graphic representation of the structural measures in modified projection nets includes concentric circles for each dip. Direct drawing of the outcrop data is undertaken, dispensing with the use of tracing or transparent paper. The stereograms resulting from the graphic representation in the modified Wulff stereographic projection net, and the modified Schmidt equal-area net, can be complemented by rose diagrams for visualization of the spatial ordering. During field campaigns in the outcrops, it is essential to visualize the spatial orientation of the data in the diagrams to determine the main structural trends, the vergence, the kinematic nature of faults and shear zones, paleo-stress tensors, and to differentiate structural domains, among others. This information supports the reconstruction of geological and tectonic history and the establishment of relationships between the different geological processes.

The understanding of each geological-structural aspect in the field is fundamental to be able to reconstruct the geological history of a region and to give a geological meaning to the data acquired in the outcrop. The description of a brittle extensional environment, which is dominated by normal fault systems, is based on: (I)  image interpretation, which aims to find evidence suggestive of an extensional geological environment, such  as the presence of scarp lines and fault scarps, horst, graben and/or half-graben, among others, that allow the identification of the footwall and hanging wall blocks; ii) definition of the sites of interest for testing; and  iii) analysis of the outcrops, following a systematic procedure that consists of the observation and identification of the deformation markers, their three-dimensional schematic representation, and their  subsequent interpretation, including the stereographic representation in the outcrop. This procedure implies the unification of the parameters of structural data acquisition in the field, mentioning the minimum fields  necessary for the registration of the data in tables. Additionally, the integration of geological and structural observations of the outcrop allows to understand the nature of the geological units, the deformation related to the extensional environment and the regional tectonic context of the study area.