Some studies attribute the origins of the Borborema Plateau or Borborema Hills to weather effects. Over millions of years bad weather is supposed to have molded the craggy relief of this region, formed by highlands that give a mountainous air to parts of the inland areas in Pernambuco, Paraíba, Alagoas and Rio Grande do Norte. Other works attribute the origins of the plateau to geological processes that occurred in the Cretacean period, between 65 and 136 million years ago. The separation of South America from Africa, which until then had formed a single block in the former super-continent known as Gondwana, gave rise to the Atlantic Ocean and, according to the most widely accepted theory, caused the Earth’s crust to stretch in areas in Brazil’s Northeast region. The uppermost layer of the Earth became thinner in the region and one of the consequences of this stretching was the appearance of elevations in certain points, like the Borborema Plateau.
New work from geophysicists Walter Eugênio de Medeiros, from the Federal University of Rio Grande do Norte (UFRN), and Roberto Gusmão de Oliveira, from the Geological Survey of Brazil (CPRM), put forward the hypothesis that another, more recent geological mechanism, which was different from the stretching caused by the separation of the continents, may also have played an important role in forming the northeastern plain. According to a paper by the two researchers to be published in August in the Journal of South American Earth Sciences, the raising of the Borborema Plateau may have been the consequence of magmatic activity and a deep thermal anomaly that initiated around 30 million years ago in that stretch of Brazil’s Northeast region.
As it rose up from the depths to the surface of the globe, hot, molten material, basically basaltic magma, became trapped between the crust and the mantle, which are the outermost and intermediary layers of the Earth, respectively. The difference in density between magma and the neighboring rocks caused a vertical force, the thrust. “This force deformed the crust and pushed the region up, thus giving rise to the Borborema Plateau,” says Oliveira. “We’re not saying that this was the only process that led to the formation of the plain, but we are saying that this mechanism may also be the root cause for the emergence of the Borborema Plateau ,” says Medeiros.
The focus of the study are the so-called isostatic conditions of the Borborema Plateau. In other words, alterations in the gravitational equilibrium between two internal structures in the Earth: the lithosphere, a rigid part that includes the crust and the upper part of the mantle, and the asthenosphere, the mantle’s fluid segment. Variations in this equilibrium produce modifications in the terrestrial relief and may give rise to mountains or depressions. The alteration may be caused by forces located on the surface or inside the planet, or in both. “The system functions like an elastic structure that flexes when loads are placed on it, but recovers its initial condition when they are removed,” is the comparison Oliveira makes.
In the case of the Borborema Plateau the data from the two researchers suggest that these hills were formed by the rising of the crust because of the forces located immediately below the uppermost layer of the Earth. According to the interpretation of the geophysicist, measurements indicate that the forces at the base of the crust are greater than the weight of the topography formed. “Judging by the intense seismic activity in the region the process is apparently still active,” says Medeiros. In other words, the Borborema Plateau has still not reached isostatic equilibrium and as denudation of the rocks occurs the crust slowly returns to its initial condition in an elastic way.
Data collected on trips to the hilly region of the Northeast support the thesis of the geophysicists, whose study forms part of the work sponsored by the Brazilian Geophysical Science and Technology of Oil Institute (INCT). The two researchers measured the intensity of the gravitational field (gravimetry) at several points on the Borborema Plateau and also used information produced by other research centers. From this type of data it is possible to infer the density of the rocks and the thickness of the geological layers in a region. The researchers recorded some types of disturbance, such as the so-called positive geoid anomaly, which can be interpreted as an indication that the topmost geological layer is thicker there. “The crust is some four kilometers thicker under the plateau than outside this region,” says Oliveira. At certain points in the hills it is as thick as 35 km.
From Campina Grande to Caruaru
With an average altitude of 500 meters and high peaks that reach 1,200 meters, the Borborema Plateau is one of the most interesting and challenging natural formations for Brazilian geophysicists. Its domains include well-known cities like Campina Grande in Paraiba and Caruaru in Pernambuco. Its format is similar to an ellipse stretching in a north-south direction, reaching a maximum length of 470km and a width that varies between 70m and 330km. Next to the lowlands in the interior of the state, a plain with altitudes between 0 and 200 meters, located to the north of Brazil’s Northeast region, and the Araripe Plateau, a more modest plain in the inland areas of the states of Pernambuco, Ceará and Piauí, the Borborema Plateau forms part of an enormous block of rocks that covers almost the whole of the Northeast of Brazil: the Borborema Province.
“In the past, tectonism and volcanism were two significant marks of this hilly region, and it is still today the scene of small earthquakes and faults that strengthen indications that the Borborema Province has a lithosphere that is different from the rest of the Brazilian shield,” comments Naomi Ussami, a geophysicist from the University of São Paulo, who studies Borborema. Under the province, the asthenosphere, where the temperature of the mantle exceeds 13000 C, is shallower. Its depth is estimated at 80 km, while in the rest of Brazil, especially in the cratons, old and cold pieces of the continent, it is 200 km deep. As a consequence, the lithosphere of Borborema must be thinner and hotter. “A large region inside the lithosphere that has a higher temperature reduces the density of the rocks, which tend to float and move to shallower depths. This may be an alternative mechanism for the uplift and erosion of parts of the Borborema Province,” ponders Naomi.
Not all academics agree with the hypothesis formulated by Medeiros and Oliveira to explain the formation of the Borborema Plateau. Work carried out with other techniques, such as deep seismic refraction, have led to the formulation of theories that are different as to how these important northeastern hills have risen up, more along the lines that the origins of these hills are associated with the process of separation of South America from the African continent. Seismic refraction consists in generating explosions close to the surface in order to measure the propagation of shockwaves deep inside the Earth. When they move from one environment to another, the waves are partially reflected and refracted and change speed. As the geological layers of the earth have different densities, the speed varies at different points deep within the planet. Therefore, it is possible to estimate where the border between the crust and the mantle is.
For José Eduardo Soares, a geophysicist at the University of Brasília, the analyses using the deep seismic refraction technique show that the birth of the Borborema Hills occurred in a different way from the one proposed by Medeiros and Oliveira. “Our idea is that the formation process of the plain was very simple,” says Soares. “There was delamination of the lithosphere, a loss of material, which led to the plain lifting up.” The process occurred in the Cenozoic Period as a consequence of the separation of the continents that started around 100 million years ago. As can be seen, the techniques used until today for studying this important geographical accident have still not brought about agreement as to the origins of the hills marking the interior of a part of the Northeast of Brazil.
OLIVEIRA, R.G. and MEDEIROS, W.E. Evidence of buried loads in the base of the crust of Borborema Plateau (NE Brazil) from Bouguer admittance estimates. Journal of South American Earth Sciences. v. 37, p. 60-76. Aug. 2012.