Tectono-Magmatism in the NW Indian shield: A fragmenting continent
Government Postgraduate College, Sirohi
(Rajasthan) 307001 India
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Continental fragmentation under an extensional
tectonic regime during Cretaceous-Tertiary (K-T) time resulted
in magmatism and basin tectonism in the northwestern Indian shield.
Sedimentary rift basins developed in the Barmer, Jaisalmer and
Bikaner regions at this time in western Rajasthan. Rift evolution
resulted in alkaline magmatism along the rift margin at Mundwara,
Sarnu and Tavidar. Mundwara and Sarnu magmatism is dated at 68.53 ± 0.16
Ma and interpreted by some to be pre-outburst plume activity associated
with Réunion plume activity (Basu et al., 1993; Raval & Veeraswamy,
2000, 2003; Raval, 2003; Roy & Jakhar,
2002; Roy 2003, 2004). However, the presence of ultra-basic,
calc-alkaline circular plutons, carbonatite and lamprophyre dykes
at Mundwara and Sarnu signify the development of deep crustal fractures.
This initiated decompressional melting in the northwestern Indian
shield under an extensional tectonic regime during K-T time. Geodynamic
changes in the northwestern Indian shield from the Jurassic to
the K-T boundary were a response to fragmenting continents rather
than plume-lithosphere interactions.
K-T magmatism occurs at Mundwara, Tavidar and
Sarnu along the northern margin of the Barmer-Sanchor-Cambay rift
basin (Figure 1). The Mundwara igneous complex consists of plutonic,
hypabyssal and volcanic members ranging in composition from ultrabasic
to alkaline, and forming three ring complexes. These are the Musala,
Mer and Toa complexes and they occupy an area of ~12 km2.
The presence of lamprophyre and carbonatite dykes indicates a deep-seated
crustal fracture system in the region. Basu et al. (1993)
dated olivine gabbro of the Mundwara complex at 68.5 Ma. This age
has been interpreted as indicating an early phase of Réunion hotspot
activity (Basu et al., 1993; Raval & Veeraswamy,
& Jakhar, 2002; Roy, 2003). Rathore et
al. (1996), using the 40Ar/39Ar
method, analyzed igneous rocks of the Mundwara region and reported
an age of 70 Ma for the mafic rocks and 64 Ma for the syenite.
The ages reported for the Mundwara rocks are inconsistent and
cannot be interpreted as Réunion plume activity.
Figure 1 Map of northwest-central India showing
the K-T magmatism and rift basins (after Campbell & Griffiths,
The Tavidar volcanics (64-66 Ma; Rathore,
1995) form an almost north-south-running outcrop along the Precambrian
Malani fracture system (Sharma,
2005; Ed: See also Malani page). They are divided into two groups:
rhyolite, quartz-trachyte and trachyte,
basalts, including hawaiite and mugerite
(Upadhyaya et al., 1988).
The igneous rocks outcropping in the Sarnu-Dandali
region evolved in two separate phases:
- the mildly alkaline igneous rocks developed at ~120
Ma, below Lower Cretaceous sandstone in the Sarnu region (Chandrasekaran,
- a variety of acidic, intermediate and alkaline magmatism
occurred during K-T time.
Basu et al. (1993) obtained
a mean age for the Sarnu-Dandali alkali pyroxenite of 68.57 ± 0.08
Ma and interpreted the volcanism as Réunion plume activity.
K-T sedimentary basins
The evolution of Jaisalmer, Barmer, and the Bikaner-Nagaur
basins (Figure 2) in western Rajasthan began in the Mesozoic and
continued into the Tertiary. The Jaisalmer basin covers over 30,000
km2 in the northwestern Indian shield, extending as
far as the Mari region of Pakistan, and forms part of the Indus
Basin (Figures 1 & 2). The basin is controlled by wrench-fault
tectonics (Misra et al., 1993) and divided into
the Mari-Jaisalmer high shown by the Kanoi and Ramgarh faults,
the Shahgarh sub-basin, the Miajalar sub-basin and the Kishangarh
sub-basin (Figure 2). The basin fill indicates sedimentation from
the Jurassic to the Tertiary.
Figure 2 Tectonic map of western Rajasthan
(after Misra et al. 1993).
Sedimentation in the Bikaner-Nagaur basin began
with the coal bearing Palana Formation, which was deposited during
the Paleocene in subtropical swampy conditions on the continental
part (Ghosh, 1983). Marine sedimentation indicates encroachment
of the sea during the Upper Paleocene to Lower Eocene.
The Cambay rift basin extends northward through
the Sanchor and Barmer basins (Figure 2). The presence of Cretaceous-Paleocene
volcanogenic sediments indicates that the Barmer basin developed
as a composite, second-order graben.
The plume model
K-T magmatism and sedimentary basin development
in northwestern Indian have been considered to represent the pre-outburst
phase of the Réunion plume, with continental lithosphere interaction
(Raval & Veeraswamy, 2000), with the main plume outburst
phase represented by Deccan volcanism at 65.5 ± 0.5 Ma (Baksi,
1994; Basu et al., 1993). The K-T magmatism at Mundwara
and Sarnu (Figure 1) is interpreted as representing the earliest
expression of the Réunion mantle plume in northwest India (Basu
et al., 1993).
Roy & Jakhar (2002) and Roy (2003,
2004) approved of the Réunion plume model (Raval &
Veeraswamy, 2000, 2003) and opined that the Cambay-Barmer, Jaisalmer
and Bikaner-Nagaur basins, Deccan volcanism and alkaline magmatism
in Sarnu, Tavidar and Mundwara, represent manifestations of plume
activity. High gravity anomalies, high heat flow and a seismic
low-velocity zone beneath the Cambay-Barmer region were interpreted
as the geophysical characteristics of Réunion plume impingement
on the base of the Indian lithosphere.
An alternative model
The mantle plume model has been invoked to explain
various phenomena of northwest India including development of the
Cambay-Barmer rift, Deccan volcanism, sedimentary basins in the
Jaisalmer, Barmer and Bikaner regions, and alkali magmatism on
the rift shoulders at Barmer, Tavidar and Mundwara. However, important
factors such as continental fragmentation, the origin of the Arabian
Sea, plate tectonics, and pre-K-T basin tectono-magmatism in western
Rajasthan have not been adequately addressed.
Basin evolution began in western Rajasthan prior
to Deccan volcanism and K-T magmatism. The Jurassic basin of Kutch
Jaisalmer in Rajasthan (Figure 1) resulted from separation of the
Indian continent from eastern Gondwana. The next phase of basin
renewal and alkali magmatism in western Rajasthan coincides with
separation of the Seychelles from India, large-scale Deccan volcanism
(Sheth, 2005a, 2005b) and formation of the Arabian Sea
at K-T time. Extensional tectonics resulted in deep fractures
as manifested by the development of rift basins, alkali magmatism
and lamprophyre-carbonatite dykes.
All the three K-T sedimentary basins of western
Rajasthan have a different type of tectonism, unrelated to one
another. The Jaisalmer basin is tectonically related to Indus basin
evolution. It is separated from the Bikaner-Nagaur basin (Figure
2) by the Pokaran-Nachana high to the northwest (Figure 2) and
from the Barmer basin by the Barmer-Devikot-Nachana high in the
south (Figure 2). A pronounced NW-SE-trending regional step-faulted
Jaisalmer-Mari high zone traverses the center of the basin. The
Indus basin developed as an extensional basin (Zaigham
& Mallick, 2000) as a result of divergence of the Indo-Pakistan
subcontinent from Gondwanaland. The origin of Jaisalmer basin
is related to Indus basin evolution at the beginning of the Triassic
(Pareek, 1984) and not to Réunion plume activity.
The Barmer basin and associated magmatism at Mundwara,
Tavidar and Sarnu along basin margins during the K-T period is
interpreted as evidence for Réunion plume activity in western Rajasthan
(Basu et al., 1993; Raval
& Veeraswamy, 2000; Roy, 2003, 2004). Sharma (2004a,
2004b, 2005), however, has explained these Tertiary alkaline
complexes as the result of reactivation of the Precambrian Malani
fracture system during the development of the Cambay-Barmer rift
under an extensional tectonic regime.
The Bikaner-Nagaur basin in northwest Rajasthan
is attributed to a plume by Roy & Jakhar (2002) and Roy (2003).
The basin developed along east-west striking fault blocks in the
Bikaner-Nagaur region, which extends westward into Pakistan. Mafic
and alkaline magmatism and other plume manifestations (Campbell,
2005) are also absent in the region, disagreeing with the model
of plume-lithosphere interaction there. Naqvi (2005) opined
that the Bikaner-Nagaur basin formed in relation to collision between
the Indian plate and Tibet at around 55-50 Ma.
Separation of the Indian landmass from eastern
Gondwana during the Jurassic resulted in formation of rift basins
in the Kutch (Gujarat) and Jaisalmer (Rajasthan) regions, and no
plume has been hypothesized for this. This fragmentation caused
alkaline magmatism at ~120 Ma in the Sarnu region. This date is
not within the 65-68 Ma timeframe and thus excluded from Réunion
plume activity. Sedimentary basin evolution in plume-affected
regions is generally explained as the result of crustal sagging
after volcanism. However, Barmer and Jaisalmer basin tectonism
started during the early Mesozoic, and thus cannot be interpreted
as a consequence of K-T plume-lithosphere interactions.
The geodynamic changes in the northwestern Indian
shield from the Jurassic to the K-T boundary were a result of fragmenting
continents rather than plume interactions. Gondwana breakup at
K-T time resulted in an extensional tectonic regime and separation
of the Seychelles from India, formation of the Arabian Sea, Deccan
volcanism, Cambay-Sanchor-Barmer rift basin formation and associated
magmatism. The presence of ultrabasic circular plutons, carbonatite,
and lamprophyre dykes at Mer-Mundwara and Sarnu-Dandali signify
the development of deep crustal fractures. This initiated decompressional
melting in the northwestern Indian shield under an extensional
tectonic regime during K-T time.
I express a deep sense of gratitude to Prof. Gillian
R. Foulger for assisting and encouraging me
in various ways in preparing this webpage.
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last updated 24th March, 2007