Spectral evidence of paleozoic structure in the western Precordillera, San Juan (31º15?S), Argentina

JUAN PABLO ARIZA; GRACIELA VUJOVICH; FLORENCIA BOEDO; MYRIAM P. MARTINEZ; SOFIA PEREZ; VICTOR RAMOS

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Lugar: Trieste; Año: 2012 vol. 54 p. 39 - 41

0006-6729

The western Precordillera (Baldis et al., 1982) is located in the western central region of Argentina (Fig. 1). Geologically, it is characterized by the development of an Andean thin- skinned fold and thrust belt (Cristallini and Ramos 2000), where the internal structure of each thrust sheet shows evidence of several overlapping deformational phases (von Gosen, 1992; 1995). The stratigraphy of the western Precordillera in the area (Fig. 1) is represented by slope and deep marine siliciclastic facies (Don Polo, Alcaparrosa, Calingasta and El Codo formations) (Quartino et al., 1971). These units are covered unconformably by both continental and marine neopaleozoic deposits (El Ratón and El Salto formations, respectively). The stratigraphic succession culminates with volcanosedimentary Triassic units (Barredo and Ramos, 2010) and Cenozoic continental units. Based on different techniques of digital image processing (Sultan et al., 1986; 1987; Gad and Kusky, 2006) we have recognized the main structural features. The use of supervised classifications and mathematical operations among bands (5/7, 5/1, 5/4 * 3/4 and 7/5, 5/4, 3/1) show a robust structural framework of the study area. Satellite data from Landsat 7 ETM + were acquired for this work from the website of the Global Land Cover Facility (http://glcf.umiacs.umd.edu) of to the University of Maryland and NASA (National Aeronautics and Spatial Administration). The date of acquisition of the selected image (path: 232, row: 082) was December 3, 1999. The software used for the processing of these images was ENVI 4.5, running on a PC platform. With this program, different techniques for the combination and classification of bands, band ratios and products were applied. The digital image processing favored the spectral contrast among different lithological units. Based on this factor it was possible to highlight geometric features of structural lineaments and characterize geometrically both folds and faults at the surface. In the study area at least two structural systems of different nature, judging by their features, were identified (Fig. 2). These systems may be arranged as follows: I - Fault systems: System Ia - represented by faults that are preferentially northwest?trending and vergence toward SW (Fig. 3); System Ib ? represented by thrusts that truncate the previous structures and are mainly north-trending (Fig. 2). These structures are widely extended along this direction and its vergence is toward E (Fig. 3). II - Fold System: This system is characterized by the development of asymmetric folds which the orientations of the axial traces are approximately NW and dipping toward NE (Fig. 2). There is an increase of bed thickness from a minimum value in the limbs, to a maximum in the area of the hinges of the folds, indicating nonparallel folding. This system fold is developed on lower Paleozoic rocks from the study area. This paper shows the potential application of the satellite image processing techniques to solve geologicstructural problems. Based on the previous geological knowledge of the area and the corresponding field controls it is possible to identify two main structural systems: I) fault systems, II) fold systems. On the basis of genetic, temporal, and spatial features, we have could differentiate each group of structures. The results obtained from the application of remote sensing techniques for areas that have undergone a complex structural evolution, such as the Argentine western Precordillera, are an excellent approximation tool, prior to performing a detailed structural study.