Bacia de Coimbra (Portugal): Anatomia de uma bacia pós-Variscana a partir de dados de campo e modelagem física

Abstract

The aim of this thesis was to better understand the Triassic geodynamic evolution of the Conraria Fm. and Castelo Viegas Fm. To that end, detailed lithological and structural was collected not only where the Triassic outcrops but also in the adjacent basement. The Conraria Fm. was deposited in a braided alluvial system with sheets of gravel and debris flow. Four lithofaciological associations were interpreted, although without absule dating, the interconextion between them hinders a chronostratigraphic sub-division. The Conraria Fm. synsedimentary faults are grouped in three sets, from the most frequent to the rarest: NW-SE faults; NNE-SSW faults, and; WNW-ESE. In order to increase the reliability of the calculation of the active stress field the two main sets were computed seperatly. It was observed that the NW-SE set were formed under a transtensive fields with σ3 orientated to NE-SW. The NNE-SSW set was also formed under a transtensive regime, altouh in this case σ3 is angled to WNW-ESE. The Castelo Viegas Fm. was deposited by meandering rivers on alluvial plain. Compared with the Conraria Fm. the Castelo Viegas synsedimentary faults are more unusual, yet the occurrence of a major set oriented NNW-SSE was observed. The paleotensor data reveals a transtensive stress field, being the main extensional axis (σ3) ENE-WSW oriented. In a second phase, physical modeling experiments were conducted to comprehend the geodinamic models developed in the previous phase. In these experiment it was considered that two main factors control the geometry of synsedimentary faults, the direction of the extension and the geometry of pre-existing structures. Four series of experiments were conducted where these two factors were varied, aiming to recreate structures similar to those recorded in phase one. When was experimented an extension to N230º and two weakness zones were considered, one similar to Porto-Tomar Fault (N170º) and other similar to the Nazaré Fault (N40º), three sets of faults were formed with a geometry identical to that observed in nature. In these experiments (extension to N230º) a primary graben was formed parallel to the N170º weakness zone and this graben is bounded by long and straight NNW-SSE faults. On the inside of this graben an en echelon set is formed, aproximatly perpendicular to the extension direction. The en echelon faults sections the primary graben margins, forming half-grabens separated by relay ramps. The third set of faults (NNE-SSW) is formed over the N40º weakness zone limiting narrow assymetrical grabens, separated by accommodation zones. The data collected here presente indicates that the Conraria Fm. was deposited in a post-orogenic basin, here named Coimbra Basin. This basin is composed by a complex assembly of grabens, limited by sets of NW-SE or NNW-SSE faults, that coalesce into longer NNE-SSW fauls. The tectonic that originated this basin relates to the Variscan Orogen collapse, differing in magnitude, area of actuation, stress field and tectonic regime from the one that would lead later to the formation of the Lusitanic Basin. The Castelo Viegas Fm. registers a transitional period between the Coimbra Basin and Lusitanic Basin tectonics