Tectonic evolution of the Paleozoic Alto Tapajos intracratonic basin - A case study of a fossil rift in the Amazon Craton

摘要

The Alto Tapajos Basin is an intracratonic basin located in the Amazon Craton, consisting predominantly of Paleozoic siliciclastic sedimentary rocks cutted by Mesozoic dyke swarms. With an area of about 97,000 km(2), the size of Portugal, some parts density cover by the Amazon forest, it has few detailed studies regarding geological structures and potential for hydrocarbons, with scarce surface mapping, absence of seismic surveys and only one well drilled to the SE. It is a NW-SE oriented branch of the ENE-WNW Amazonas Basin. Geological maps, topographic, gravimetric and magnetometric data are here interpreted and integrated to analyse its tectonic evolution. The sedimentary strata are deposited on reactivation zones related to NW-SE normal faults and NNE-SSW oblique faults, that control the spatial arrangement of the basement highs. The former is parallel, and some places coincident, with a major tectonic Paleoproterozoic crustal boundary that separates the Tapajos-Parima and Rondonia-Juruena provinces within the Amazon Craton. The NNE-SSW oblique faults are interpreted as representing a transfer zone that accommodates the Paleozoic oblique rifting mode. However, the rifting age is still not well stabilished. Cambrian mafic dikes could link the initiation of this basin to the breakup of a supercontinent in the early Cambrian (Pannotia), in the dawn of the Gondwana. To account for the Devonian to Permian basin subsidence, crustal stretching due to intraplate passive rifting might have played the role, in response to farfield stresses regarding a convergent tectonic setting on the western margin of Gondwana (Terra Australis and Alleghanian orogenies). This is corroborated by the shift on the basin depocenter axis, from NW-SE (Devonian-Carboniferous) to NNE-SSW (Permian), that might be related to the change on orientation of stresses due to these two orogenies. In addition, the Paleozoic sedimentary units occur basically within the same area of the Paleoproterozoic cratonic sedimentary units from a ca. 1.7 Ga rift. This suggests that the Alto Tapajos Basin Paleozoic rifting reactivated an ancient fossil rift, also described in other cratons throughout the world. In the Jurassic, Alto Tapajos Basin was submitted to a post-sedimentation tectono-thermal event, related to the disintegration of Pangea and expansion of the Central Atlantic Magmatic Province (CAMP). During this episode, 180-220 Ma mafic dike swarms intruded its central-southwest compartment, following the NNE-SSW structures. The actual positive topographic feature, Cachimbo Range and plateau, and the absence of thick Cenozoic deposits, unlike the Amazonas Basin, indicate that the recent evolution of the Alto Tapajos region is related to uplift within an intraplate setting.