Geological-geomorphological analysis of the northwest region of São Sebastião Island, São Paulo, Brazil: basic and intermediate Mesozoic dikes as morphogenetic determinants

Abstract

In São Sebastião Island (SP), geomorphological diversity is directly associated with distinct geological units, such as granite-gneiss precambrian basement and Mesozoic intrusions represented by two main sets: tholeiitic rocks connected to the South Atlantic rift (Eocretaceous — approx. 130 Ma) and alkaline magmatism (Neocretaceous — approx. 70 Ma) from the Brazilian southern shelf. The aim of this work is to analyze the lithostructural characteristics and properties of the basic and intermediate dykes intruded in the granitoids of the northwestern part of the island, correlating them to the different geomorphological patterns, that is, positive features such as the elongated crests more resistant to weathering and erosion, and less resilient negative features such as the embedded valleys. It was observed, through field measurements, petrographic analysis and geoprocessing, that the mineralogical composition and fracture pattern of the basement and dikes, as well as their thickness, are the main factors responsible for the different responses to the superficial processes. The dykes associated to drainage are represented by diabase of varied texture, presenting mafic minerals (clinopyroxene and olivine) and calcium plagioclase (labradorite). Such mineralogy is more susceptible when compared to that present in the dykes in the elongated ridges, represented by porphyritic andesites with lower proportion of mafic minerals, in addition to the plagioclase being predominantly of sodium (oligoclase) composition. Diabase dikes are less thick and have higher fracture density. The geological units that occur in the study area are classified according to their susceptibility to exogenous processes, from the most susceptible to the most resistant one, as follows: diabase dikes, granite-gneiss basement, and andesitic dikes.