The world’s fourth largest island, covering
388,740 square miles (627,000 km2) in the western Indian
Ocean off the coast of southeast Africa. Madagascar is in a
great many ways one of the most distinctive locations in the
world. It is recognized as biologically distinguished because
of its singular community of fauna and flora and its unique
role in the evolution and sustenance of life as evidenced in the
geologic record. Madagascar is a valuable natural laboratory
for the study of many phenomena ranging from biological
systems, cultural adaptations, and fundamental geologic and
geomorphic structures and material that form the foundation
of the island and the platform on which all other systems
have been situated and developed. Although, much effort has
been focused in recent years on many aspects of Madagascar,
from a geologic and geomorphic material perspective the
island is not sufficiently understood in detail. Unlocking the
secrets to the geology of this region could reveal information
about the formation, breakup, and dispersal of several supercontinents
including Rodinia, Gondwana, and Pangea.
French colonial geologists pioneered by Bernard Moine
mapped and described the principal geologic elements of
Madagascar.
The island consists of a highland plateau fringed by a
lowland coastal strip on the east, with a very steep escarpment
dropping thousands of feet from the plateau to the
coast over a distance of only 50 to 100 miles. The highest
points in Madagascar are Mount Maromokotro in the north,
which rises to 9,450 feet (2,882 m), and the Ankaratra
Mountains in the center of the plateau, which rise to 8,670
feet (2,645 m). The plateau dips gently to the western coast
of the island toward the Mozambique Channel, where wide
beaches are located. There are several islands around the
main island, including Isle St. Marie in the northeast and
Nosy-Be in the north. Most of the high plateau of Madagascar
was once heavily forested, but intense logging over the
last century has left most of the plateau a barren, rapidly
eroding soil and bedrock-covered terrain. Red soil eroded
from the plateau has filled many of the river estuaries along
the coast.
Precambrian rocks underlie the eastern two-thirds of
Madagascar, and the western third of the island is underlain
by sedimentary and minor volcanic rocks that preserve a
near-complete record of sedimentation from the Devonian to
Recent. The Ranotsara fault zone divides the Precambrian
bedrock of Madagascar into two geologically different parts.
The northern part is underlain by Middle and Late Archean
orthogneisses, variably reworked in the Early and Late Neoproterozoic,
whereas the southern part, known as the Bekily
block, consists dominantly of graphite-bearing paragneisses,
bounded by north-south trending shear zones that separate
belts with prominent fold-interference patterns. All rocks
south of the Ranotsara fault zone have been strongly
reworked and metamorphosed to granulite conditions in the
latest Neoproterozoic. Because the Ranotsara and other sinistral
fault zones in Madagascar are subvertical, their intersections
with Madagascar’s continental margin provide ideal
piercing points to match with neighboring continents in the
East African Orogen. Thus, the Ranotsara fault zone is considered
an extension of the Surma fault zone or the Ashwa
fault zone in East Africa, or the Achankovil or Palghat-
Cauvery fault zones in India. The Palghat-Cauvery fault zone
changes strike to a north-south direction near the India-
Madagascar border and continues across north Madagascar.
The Precambrian rocks of northern Madagascar can be divided
into three north-south trending tectonic belts defined, in
part, by the regional metamorphic grade. These belts include
the Bemarivo block, the Antongil block, and the Antananarivo
block.
The Bemarivo block of northernmost Madagascar is
underlain by calc-alkaline intrusive igneous rocks (Andriba
Group) with geochemical compositions suggestive of rapid
derivation from depleted mantle sources. These rocks are
strikingly similar in age, chemistry, and isotopic characteristics
to the granitoids of the Seychelles and Rajasthan (India).
The Andriba granitoids are overlain by the Daraina-Milanoa
Group (~750–714 million years old) in the north, and juxtaposed
against the Sambirano Group in the south. A probable
collision zone separates the Sambirano Group from the
Andriba Group. The Daraina-Milanoa Group consists of two
parts: a lower, largely clastic metasedimentary sequence and
an upper volcanic sequence dominated by andesite with lesser
basalt and rhyolite. Like the Andriba Group, volcanic rocks
of the Daraina-Milanoa Group are calc-alkaline in chemistry
and have Nd isotope signatures indicating a juvenile parentage.
Cu and Au mineralization occurs throughout the belt.
The Sambirano Group consists of pelite schist, and lesser
quartzite and marble, which are variably metamorphosed to
greenschist grade (in the northeast) and amphibolite grade (in
the southwest). In its central part, the Sambirano Group is
invaded by major massifs of migmatite gneiss and charnockite.
The depositional age and provenance of the Sambirano
Group is unknown.
The Antongil block, surrounding the Bay of Antongil
and Isle St. Marie, consists of late Archean biotite granite and
granodiorite, migmatite, and tonalitic and amphibolitic
gneiss, bound on the west by a belt of Middle Archean
metasedimentary gneiss and migmatite. The tonalitic gneisses
of this region are the oldest rocks known on the island of
Madagascar, dated (U-Pb on zircon) at 3,187±2 million years.
The older gneisses and migmatites are intruded by circa
2,522±2 million-year-old epidote-bearing granite and granodiorite.
Late Archean gneisses and migmatites near the coast
in the Ambositra area may be equivalent to those near the
Bay of Antongil, although geochronological studies are sparse
and have not yet identified middle Archean rocks in this area.
Rocks of the Antongil block have greenschist to lower-amphibolite
metamorphic assemblages, in contrast to gneisses in the
Antananarivo block, which tend to be metamorphosed to
granulite facies. This suggests that the Antongil block may
have escaped high-grade Neoproterozoic events that affected
most of the rest of the island. Gneisses in this block are
broadly similar in age and lithology to the peninsular gneisses
of southern India. High-grade psammites of the Sambirano
Group unconformably overlie the northern part of the
Antongil block and become increasingly deformed toward the
north in the Tsarantana thrust zone, a Neoproterozoic or
Cambrian collision zone between the Bemarivo block and
central Madagascar. The western margin of the Antongil
block is demarcated by a 30-mile (50-km) wide belt of pelitic
metasediments with tectonic blocks of gabbro, harzburgite,
and chromitites, with nickel and emerald deposits. This belt,
named the Betsimisiraka suture, may mark the location of the
closure of a strand of the Mozambique Ocean that separated
the Antongil block (and southern India?) from the Antananarivo
block.
The Antananarivo block is the largest Precambrian unit
in Madagascar, consisting mainly of 2,550–2,490-millionyear-
old granitoid gneisses, migmatites, and schist intruded
by 1,000–640-million-year-old calc-alkaline granites, gabbro,
and syenite. Rocks of the Antananarivo block were strongly
reworked by high-grade Neoproterozoic tectonism between
750 million and 500 million years ago and metamorphosed
to granulite facies. Large, sheet-like granitoids of the stratoid
series intruded the region, perhaps during a phase of extensional
tectonism. Rocks of the Antananarivo block were
thrust to the east on the Betsimisiraka suture over the
Antongil block around 630–515 million years ago, then
intruded by post-collisional granites (such as the 537–527-
million-year-old Carion granite, and the Filarivo and Tomy
granites) 570–520 million years ago.
The Sèries Quartzo-Schisto-Calcaire or QSC (also known
as the Itremo Group) consists of a thick sequence of Mesoproterozoic
stratified rocks comprising, from presumed bottom to
top, quartzite, pelite, and marble. Although strongly deformed
in latest Neoproterozoic time (~570–540 million years ago),
the QSC is presumed to rest unconformably on the Archean
gneisses of central Madagascar because both the QSC and its
basement are intruded by Early Neoproterozoic (~800-million-
year-old) granitoids, and no intervening period of tectonism
is recognized. The minimum depositional age of the QSC
is ~800 million years ago—and its maximum age of ~1,850
million years ago—is defined by U-Pb detrital zircon
geochronology. The QSC has been variably metamorphosed
(~570–540 million years ago; greenschist grade in the east;
amphibolite grade in the west) and repeatedly folded and
faulted, but original sedimentary structures and facing-directions
are well preserved. Quartzite displays features indicative
of shallow subaqueous deposition, such as flat lamination,
wave ripples, current ripple cross lamination, and dune crossbedding,
and carbonate rocks have preserved domal and pseudo-
columnar stromatolites. To the west of the Itremo Group,
rocks of the Amboropotsy and Malakialana Groups have
been metamorphosed to higher grade, but include pelites, carbonates,
and gabbro that may be deeper water equivalents of
the Itremo Group. A few areas of gabbro/amphibolite-facies
pillow lava/marble may represent strongly metamorphosed
and dismembered ophiolite complexes.
Several large greenstone belts crop out in the northern
part of the Antananarivo block. These include the Maevatana,
Andriamena, and Beforana-Alaotra greenstone belts,
collectively called the Tsarantana sheet. Rocks in these belts
include metamorphosed gabbro, mafic gneiss, tonalites,
norite, and chromitites, along with pelites and minor magnetite-
iron formation. Some early intrusions in these belts
have been dated by Robert Tucker to be between 2.75 billion
and 2.49 billion years old, with some 3.26-billion-year-old
zircon xenocrysts and Middle Archean Nd isotopic signatures.
The chemistry, age, and nature of chromite mineralization
all suggest an arc setting for the mafic rocks of the
Tsarantana sheet, which is in thrust contact with underlying
gneisses of the Antananarivo block. The thrust zone is not yet
well documented, but limited studies indicate east-directed
thrusting. The 800–770-million-year-old gabbro cut early
fabrics but are deformed into east-vergent asymmetric folds
cut by east-directed thrust faults.
The effects of Neoproterozoic orogenic processes are
widespread throughout the Antananarivo block. Archean
gneisses and Mesoproterozoic stratified rocks are interpreted
as the crystalline basement and platformal sedimentary cover,
respectively, of a continental fragment of undetermined tectonic
affinity (East or West Gondwanan, or neither). This
continental fragment (both basement and cover) was extensively
invaded by subduction-related plutons in the period
from about 1,000 million to ~720 million years ago, that
were emplaced prior to the onset of regional metamorphism
and deformation. Continental collision related to Gondwana’s
amalgamation began after ~720 million years ago and
before ~570 million years ago and continued throughout the
Neoproterozoic with thermal effects that lasted until about
520 million years ago. The oldest structures produced during
this collision are kilometer-scale fold and thrust-nappes with
east or southeast-directed vergence (present-day direction).
They resulted in the inversion and repetition of Archean and
Proterozoic rocks throughout the region. During this early
phase of convergence warm rocks were thrust over cool rocks
thereby producing the present distribution of regional metamorphic
isograds. The vergence of the nappes and the distribution
of metamorphic rocks are consistent with their
formation within a zone of west- or northwest-dipping continental
convergence (present-day direction). Later upright
folding of the nappes (and related folds and thrusts) produced
km-scale interference fold patterns. The geometry and
orientation of these younger upright folds is consistent with
east-west horizontal shortening (present-day direction) within
a sinistral transpressive regime. This final phase of deformation
may be related to motion along the Ranotsara and related
shear zones of south Madagascar, and to the initial phases
of lower crustal exhumation and extensional tectonics within
greater Gondwana.
South Madagascar, known as the Bekily block, consists
of upper amphibolite and higher-grade paragneiss bounded
by north-south–striking shear zones that separate belts with
prominent fold interference patterns. Archean rocks south of
the Ranotsara shear zone have not been positively identified,
but certain orthogneisses have Archean ages (~2,905 million
years) that may represent continental basement to the paragneisses
of the region. All rocks south of the Ranotsara shear
zone have been strongly reworked and metamorphosed in the
latest Neoproterozoic. The finite strain pattern of refolded
folds results from the superimposition of at least two late
Neoproterozoic deformation events characterized by early
subhorizontal foliations and a later network of kilometerscale
vertical shear zones bounding intensely folded domains.
These latest upright shears are clearly related to late Neoproterozoic
horizontal shortening in a transpressive regime under
granulite facies conditions.
The western third of Madagascar is covered by Upper
Carboniferous (300 million years old) to Mid-Jurassic (180
million years old) basinal deposits that are equivalents to
the Karoo and other Gondwanan sequences of Africa and
India. There are three main basins, including from south to
north, the Morondova, Majunga (or Mahanjanga), and
Diego (or Ambilobe) basins. Each has a similar three-fold
stratigraphic division including the Sakoa, Sakamena, and
Isalo Groups, with mainly sandstones, limestones, and
basalts, overlain by unconsolidated sands in the south and
along the western coast. These basins formed during rifting
of Madagascar from Africa and have conjugate margins
along the east coast of southern and central Africa. The
base of the Morondova basin, the oldest of the three, has
spectacular glacial deposits including diamictites, tillites,
and glacial outwash gravels. These are overlain by coals and
arkoses, along with plant fossil– (Glossopteris-) rich mudstones
thought to represent meandering stream deposits.
Marine limestones cap the Sakoa Group. Fossiliferous deltaic
and lake deposits of the Sakamena Group prograde (from
the East) over the Sakoa Group. The uppermost Isalo Group
is 0.6–3.7 miles (1–6 km) thick, consisting of large-scale
cross-bedded sandstones, overlain by red beds and fluvial
deposits reflecting arid conditions. Mid-Jurassic limestones
(Ankara and Kelifely Formations) mark a change to subaqueous
conditions throughout the region.
See also GONDWANA; SUPERCONTINENT CYCLE.
mafic See IGNEOUS ROCKS.
magma See IGNEOUS ROCKS.
magnetic anomalies See PALEOMAGNETISM; PLATE TECTONICS.
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