Deformaciones neotectónicas en el relieve fluvial de la Llanura Sur de Pinar del Río, Cuba

Abstract

Research findings in neotectonics (the most recent movements of the earth's crust) are fundamental to the analysis of modern landforms. In the 1970s, a significant number of structural and geological investigations were carried out in Cuba, all complemented by geophysical data and perforations, Allowing the first division in morphostructural blocks of different hierarchical orders: megabloque, macroblock, mesobloque and smaller blocks. The southern plain of Pinar del Río is located in the mesobloque Batabanó, bordered by the mesobloques Pinar and Habana-Matanzas, and controlled by the Southern Cuban fault system. In the southern plain of Pinar del Río, recent vertical velocities of the order of +2.0 to +4.0 mm / year are reported, according to the geodetic investigations of successive leveling made by others authors in 1989, which influence the conformation of the current relief. The existence of different local morphostructures, anomalies in the drainage network and a coast with submersion characteristics, which condition its marshy nature, are elements that point to the existence of these movements. In general, the main methods used to recognize neotectonic deformations have been instrumental (seismological and geodetic), historical-archeological and geological-geomorphological investigations. In Cuba, sedimentary-structural criteria have been applied to differentiate the tectonic styles of the pliocene-quaternary deposits with respect to the folded substrate. Simultaneously, these hypotheses were corroborated by geophysical criteria and deep drilling data. The development of digital elevation models allows the quantitative characterization of the relief and to recognize the tectonic effect in its evolution, especially in very active tectonic zones. As an essential support for the neotectonic investigations of the Cuban archipelago, quantitative geomorphological investigations and teledección techniques were also included. In these works, morphothectonic methods were applied in a very limited way, since there were no numerical models of the terrain that facilitated their use. In the case of Cuba, the criteria for placing the neotectonics stage in geologic time vary greatly. While some authors identify its inception at the end of the Paleogene, others place it between the late Eocene and the Miocene. From the geomorphological perspective, the neotectonics stage is divided into two sub-stages: early neotectonics (Oligocene-Miocene) and late neotectonics (late Miocene to Pleistocene). In relatively flat regions, such as the southern plains of the Pinar del Río Province, in western Cuba, evidence of neotectonic deformation is not always easily identifiable, especially when attributed to recent geological history. A compounded approach is therefore necessary. During the last prospecting works for sand and gravel deposits, certain regularities were established in the distribution of alluvial sediments, the former riverbeds and the diversion of major rivers. All these elements led to the development of new lines of research, of which this work partakes, and which aim at defining active tectonic processes, especially during the Pliocene-Quaternary. These processes are evident in present landforms and the existing drainage system, as well as in their influence on the morphological evolution of the Pinar del Rio southern plains. From a methodological point of view, the efficiency of combining geological, geomorphological and pedological methods in the study of the neotectonics of relatively flat areas was demonstrated. These methods included the morphometric evaluation of landforms, the analysis of drainage anomalies, the study of the spatial distribution of stratigraphic complexes and alluvial deposits, and the spectrometry of the potassium channel there in. Within the perimeter of the southern plains, a total of 15 river basins were identified and delimited. Those basins directly connected to the southern coast were singled out for morphometric evaluation, and in some cases, divided for closer study. In order to identify recent neotectonic activity, morphometric indices were used. Longitudinal blocks of the horst-and-graben type were highlighted in confirmation of the results obtained by geophysical and drilling tests, and three overlapped blocks from west to east were also identified with different amplitudes of vertical displacement of the order of 6 to 10 m. Located at the center and towards the east, blocks II and III are subjected to a sustained tectonic tilting, which has resulted in a progressive migration of the river network in a southwesterly direction, as evidenced by the asymmetry of watersheds. Finally, among the rivers in the axial part of the plain a clockwise deflection pattern was established, probably associated with shearing neotectonic movements along the dislocation at the southern end of the Los Palacios depression.