On the
geologic timescale,
the Precambrian describes the time - span from the formation of earth about 4,5 billion years (Ga) ago to the evolution
of abundant macroscopic hard - shelled fossils, which marked the beginning of the Cambrian, about
542 million years (Ma) ago. It is divided into the eons Hadean (4,5 - 3,8 Ga),
Archaean (3,8 - 2,5 Ga) and Proterozoic (2,5 Ga - 542 Ma).
During the Hadean (4,5 - 3,8 Ga), the chemical and physical basic
structure of the Earth was formed.
The heavier elements, such as iron and nickel, sank into the Earth´s
core (6.371 - 2.900 km)
and the lighter elements, such as Oxygene, Silicon and Aluminium rose into the Earth´s viscous
mantle (2.900 km - 40 km).
At the end of the Hadean, the Earth´s outer silicate solid crust
(40 - 0 km) was formed.
The implication is that at this time the entire Earth was covered by a primeval ocean.
During the Archean (3,8 - 2,5 Ga),
molten material from the Earth's mantle was released to the surface and the first (micro)
continents are supposed to have emerged out of that ocean.
Ur
is believed to be the first known continent that probably formed
3 billion years ago followed by Arctica (2,5 Ga), Atlantica (2Ga) Baltica (2Ga) and Nena (1,8 Ga).
Today, fragments of Archean continents, so - called
cratons,
can be found all over the world and are common places for
diamonds to form.
During the Proterozoic (2,5 Ga - 542 Ma),
Ur, Arctica, Atlantica, Nena and fragments of them interacted
and reassembled in different configurations to so - called
supercontinents
such as Columbia (1,8 - 1,5 Ga), Rodinia (1,1 Ga - 800 Ma), Gondwana (750 - 500 Ma) or Pangaea (Pangea) (450 - 250 Ma).
This interaction can be described by the so - called
Wilson Cycle
which is divided in a divergent stage involving
rifting, drifting and ocean building and a convergent stage involving subduction (island arc and cordilleran type),
collision (arc - continent and continent - continent) and mountain building
(orogeny).
The development of Gondwana took place during the
Neoproterozoic (1 Ga - 542 Ma)
through fusion of East and West Gondwana.
While Eastern Gondwana was formed around the cratonic components of Ur, Western Gondwana was
predominantly composed of cratons of Atlantica.
The so - called
Brasiliano / Pan - African
cycle (850 - 500 Ma) describes the assembly of West Gondwana
from the break - up of Rodinia to the closure of all oceanic basins,
over a series of subduction and collisional events
up to its final fusion/agglutination.
At the end of this cycle, a large continuous landmass was formed,
comprising the protoypes of South America (
the so-called South American Platform)
and Africa prior to the
opening of the South Atlantic.
During the divergent stage of this cycle, Rodinia break - up into different pieces which drifted away
from each other. The open spaces inbetween these fragments were filled with oceans.
One of those fragments, the
São Francisco -
Congo craton (descendant of Atlantica) was surrounded by
the Pharusian - Borborema Ocean to the north, the Goianides Ocean to the southwest
and the Adamastor Ocean to the southeast.
During the convergent stage of this cycle, subduction
gave rise to magmatic arc suites and was followed by arc - continent and continent - continent collisions
which resulted in the closure of these oceans and the docking / amalgation of a number of
tectonic blocks and orogenic belts to the border of the craton. The
Atlantic Shield
was formed.
For descriptive purposes, the orogenic belts around the
São Francisco - Congo craton
(on the Brazilian side) were divided into 3 geotectonic
provinces:
Borborema (Northeast), Tocantins (Centraleast)
and Mantiqueira Province
(East - Southeast).
The Mantiqueira Province is composed of the orogens:
Araçuaí in the north
Ribeira
(northern,
central and
sothern segment)
in the center and the Dom Feliciano orogen in the south.
The Neoproterozoic geological evolution of the
Atlantic
Rainforest /
(Mata Atlântica) region
in southern Bahia / Espírito Santo
(Discovery Coast),
Rio de Janeiro and São Paulo / Paraná
(Southeast Reserves)
is recorded in the crystalline core and mobile fold belts of the Araçuaí and Ribeira orogen.
In the course of the Brasiliano / Pan - African orogeny,
granitic magmatism
in the coastal crystalline core of these two orogens took place.
Molten rock (magma) intruded into the Earth´s crust,
where it slowly cooled, solified and crystallized, forming ingeous rock. As
this happened below the surface, these rocks are called intrusive or plutonic rocks
(e.g. granite). As the orogeny continued, high - pressure and high temperature
metamorphism altered the granite into metamorphic rock (e.g. gneiss).
Over time, the topographic surface expression (volcanoes) of the Brasiliano / Pan - African orogens
eroded away and only the deep roots of the mountains remained as a series of belts of
igneous and metamorphic rock that outcrop in southeastern and southern
Brazil.
Some of the most famous granite and gneiss outcrops are the
Marumbi massif in Paraná,
the Cantareira batholith
in
São Paulo,
the Sugarloaf Mountain (Pão de Açucar) and Corcovado in
Rio de Janeiro
/
Tijuca
National Park or
the rock formations related to the
Serra dos Orgãos
batholith.
In the early Paleozoic (450 Ma), Gondwana started to merge with Laurasia
(North America, Greenland and Europe)
in the north and the assembly of the supercontinent
Pangaea begun. The formation was completed in the late Paleozoic (250 Ma).
Pangaea started rifting in the early Jurassic
(200 Ma) and split into Laurasia (North America, Europe and Asia) in the north
and Gondwana (South America, Africa, India, and Australia) in the south.
Gondwana started to break up in the mid Jurassic (170 Ma) with the separation
of its eastern part. The western part containing the
prototypes of South America and Africa started to break - up in the early Cretaceous
(130 Ma).
The South America - Africa break up
was probably caused by the uplift and break of the lithosphere (continental rifting)
due to crustal expansion due to heating caused by
the hotspot activity of mantle plumes situated below the Earth´s mantle.
The hotspot activity was associated with intensive
mafic volcanism that flooded the large basins in western Gondwana, especifically the
Paraná Basin (today in Brazil) and
the Etendeka Basin (today in Africa), with massive lava flows.
The Paraná Flood Basalts (or
Serra Geral Formation)
extend through southern Brazil, Paraguay and Uraguay and
are reputed to cover over a million sq km. They nowadays represent
the main
ecotouristic attraction in serveral
national parks
in southern Brazil
(e.g. São Joaquim,
Serra Geral,
Aparados da Serra)
or Iguazu.
With the beginning of the upper Cretaceous (90 Ma) the prototypes of South America and Africa separated entirely
and South America drifted westwards until hitting the Nazca plate in the Pacific. The subduction of
the Nazca Plate under the
South American Plate
gave rise to the Andes (40 Ma).
During the drift phase (85 - 55 Ma), several
alkaline magmatic intrusions
occurred in the crystalline core of the central Ribeira orogen between Rio de Janeiro and
São Paulo.
There is evidence that the plutonic continental complexes Poço de Caldas,
Itatiaia /
Passa Quatro and the offshore complexes of São Sebastião
(Ilhabela)
are associated with
felsic magmatism due to crust melting caused by the
Trindade Mantle Plume.
Several studies demonstrate that South America on its way westwards must have passed over this plume.
Nowadays, the Trindade "plume - head" is located beneath the volcanic islands of
Trindade / Martim Vaz at the eastern end of the Vitória - Trindade seamount chain,
715 km east of Vitória (Espírito Santo).
It is believed that during the Cretaceous (55 - 11 Ma)
, these magmatic events were followed by continental rifting, which in turn gave rise to the spectacular
Southeastern Continental Rift.
The Serra do Mar at the
eastern margin of this rift and the
Serra da Mantiqueira
at the western margin were uplifted up to 2,000 m at this time while the
the oceanic margin (along the Santos fault line) was downdropped.
Strong and successive climatic variations during the Neogene (20 Ma),
with alternations between very cold and very warm periods, resulting in strong oscillations
of the ocean are responsible for the final shape
of the coastal region.
120.000 years ago (period of advance) the waters of the Atlantic Ocean flooded large areas
of the continent, reaching places where today the village of Eldorado is located.
15.000 years ago (period of retreat), the sea level was 100 m lower than actually and today's
coastal islands were continental hills and could be reached by feet.
The following picture
shows the actual geological configuration of the coastal mountain ranges in Southeastern
Brazil.
 |
