14 research outputs found
Tectonic evolution of the High Himalaya in upper Lahul (NW Himalaya, India)
The Upper Lahul region in the NW Himalaya is located in the transition
zone between the High Himalayan Crystalline (HHC) to the SW and the
Tethyan Zone sedimentary series to the NE. The tectonic evolution of
these domains during the Himalayan Orogeny is the consequence of a
succession of five deformation events. An early D1 phase corresponds to
synmetamorphic, NE verging folding. This deformation created the Tandi
Syncline, which consists of Permian to Jurassic Tethyan metasediments
cropping out in the core of a large-scale synformal fold within the HHC
paragneiss. This tectonic event is interpreted as related to a NE
directed nappe stacking (Shikar Beh Nappe), probably during the late
Eocene to the early Oligocene. A subsequent D2a phase caused SW verging
folding in the HHC. This deformation is interpreted as contemporaneous
with late Oligocene to early Miocene SW directed thrusting along the
Main Central Thrust. In the Tethyan Zone, a D2b phase is marked by a
decollement thrust, a system of reverse faults, and gentle folds,
associated with SW directed tectonic movements. This deformation is
related to an imbricate structure, characteristic of a shallow
structural level, and developed in the frontal part of a nappe affecting
the Tethyan Zone units of SE Zanskar (Nyimaling-Tsarap Nappe). A later
D3 phase generated the Chandra Dextral Shear Zone (CDSZ), a large-scale,
ductile, dextral strike-slip shear zone, located in the transition zone
between the HHC and the Tethyan Himalaya. The CDSZ most likely
represents a part of a system of early Miocene extensional and/or
dextral, strike-slip shear zones-observed at the HHC-Tethyan Zone
contact along the entire Himalaya. A final D4 phase induced large-scale
doming and NE:verging back folding
Himalayan inverted metamorphism and syn-convergence estension as a consequence of a general shear extrusion.
Two paradoxical geological features of the Himalaya are the syn-convergence extension
and the inverted metamorphic isograds observed in the crystalline core zone of this orogen. This High
Himalayan Crystalline Sequence corresponds to an up to 40 km thick sequence of amphibolite to
granulite facies gneiss, bounded by the Main Central Thrust at the base, and by the extensional faults
of the South Tibetan Detachment System at the top. Geochronological and structural data demonstrate
that coeval movements along both the Main Central Thrust and South Tibetan Detachment
System during Early to Middle Miocene times were related to a tectonically controlled exhumation of
these high-grade metamorphic rocks. The High Himalayan Crystalline Sequence systematically shows
an inverted metamorphic zonation, generally characterized by a gradual superposition of garnet, staurolite,
kyanite, sillimanite + muscovite and sillimanite + K-feldspar isograds, from the base to the top
of the unit. Recent kinematic flow analyses of these metamorphic rocks demonstrate the coexistence
of both simple shear and pure shear during the ductile deformation. The simple shear component of
such a general non-coaxial flow could explain a rotation of isograds, eventually resulting in an inversion.
The pure shear component of the flow implies a thinning of the metamorphic sequence that must
be balanced by a perpendicular stretching of the unit parallel to its boundaries. Inasmuch as seismic
data show that both the Main Central Thrust and South Tibetan Detachment System converge at
depth, a thinning of the wedge-shaped High Himalayan Crystalline Sequence should induce a ductile
extrusion of these high-grade rocks toward the surface. Rapid extension at the top of the sequence
could thus be the consequence of a general shear extrusion of this unit relative to its hanging wall.
Moreover, this extensional movement should decrease with depth to become zero where the boundaries
of the unit meet, accounting for the paradoxical convergence of the South Tibetan Detachment
System toward the Main Central Thrust. Furthermore, a general flow combining simple shear and
pure shear can reconcile inverted isograds with the lack of inverted pressure field gradient across the
High Himalayan Crystalline Sequence, despite an intense non-coaxial deformation. In good agreement
with the seismic, kinematic and P–T–t constraints on the Himalayan tectono-thermal evolution,
general shear extrusion provides a consistent model accounting for both inverted isograds and rapid
extension in a compressional orogenic setting.</jats:p
Tertiary Himalayan structures and metamorphism in the Kulu Valley (Mandi-Khoksar transect of the Western Himalaya) - Shikar-Beh-nappe and crystalline nappe
The Crystalline Nappe of the High Himalayan Crystalline has been
examined along the Kulu Valley and its vicinity (Mandi-Khoksar
transect). This nappe was believed to have undergone deformation related
only to its transport towards the SW essentially during the `'Main
Central Thrust event''. New data has led to the conclusion that during
the Himalayan orogeny, two distinctive phases, related to two opposite
transport directions, characterize the evolution of this part of the
chain, before the creation of the late NE-vergent backfolding.
The first phase corresponds to an early NE-vergent folding and
thrusting, creating the Tandi Syncline and the NE-oriented Shikar Beh
Nappe stack, with a displacement amplitude of about 50 km. Two
schistosities, together with a strong stretching lineation are developed
at a deep tectonic level under amphibolite facies conditions
(kyanite-staurolite-garnet-two mica schists). At a higher tectonic level
and in the southern part of the section (Tandy Syncline and southern
Kulu Valley between Kulu and Mandi) one or two schistosities are
developed in the greenschist facies grade rocks (garnet-biotite and
biotite schists). These structures and the associated Barrovian type
metamorphism are all related to the NE-verging Shikar Beh Nappe. The
creation of the NE-verging Shikar Beh Nappe may be explained by the
reactivation of a SW dipping listric normal fault of the N Indian
flexural passive margin, during the early stages of the Himalayan
orogeny.
In the second phase, the still hot metamorphic rocks of the Shikar Beh
Nappe were folded and thrust towards the SW (mainly along the MBT and
the MCT with a displacement in excess of 100 km) onto the cold,
low-grade metamorphic rocks of the Larji-Kulu-Rampur Window or, near
Mandi, on the non-metamorphic sandstones of the Ganges Molasse
(Siwaliks). Sense of shear criteria and a strong NE-SW
stretching-lineation indicate that the Crystalline Nappe has been
overthrusted towards the SW. Thermometry on synkinematically
crystallised garnet-biotite and garnet-hornblende pairs reveals the
lower amphibolite facies temperature conditions related to the
Crystalline Nappe formation.
From the muscovite and biotite Rb-Sr cooling ages, the Shikar Beh Nappe
emplacement occurred before 32 Ma and the southwestward thrusting of the
Crystalline Nappe began before 21 Ma. Our model involving two opposite
directions of thrusting goes against the conventional idea of only one
main SW-oriented transport direction in the High Himalayan Crystalline
Nappes
Formation of aluminosilicate-bearing quartz veins in the Simano nappe (Central Alps): structural, thermobarometric and oxygen isotope constraints
We combined structural analysis, thermobarometry and oxygen isotope
geochemistry to constrain the evolution of kyanite and/or
andalusite-bearing quartz veins from the amphibolite facies metapelites
of the Simano nappe, in the Central Alps of Switzerland. The Simano
nappe records a complex polyphase tectonic evolution associated with
nappe stacking during Tertiary Alpine collision (D1). The second
regional deformation phase (132) is responsible for the main penetrative
schistosity and mineral lineation, and formed during top-to-the-north
thrusting. During the next stage of deformation (D3) the
aluminosilicate-bearing veins formed by crystallization in tension
gashes, in tectonic shadows of boudins, as well as along shear bands
associated with top-to-the-north shearing. D2 and D3 are coeval with the
Early Miocene metamorphic peak, characterised by kyanite + staurolite +
garnet + biotite assemblages in metapelites. The peak pressure (P) and
temperature (T) conditions recorded are constrained by
multiple-equilibrium thermobarometry at 630 +/- 20 degrees C and 8.5 +/-
1 kbar (similar to 27 km depth), which is in agreement with oxygen
isotope thermometry indicating isotopic equilibration of quartz-kyanite
pairs at 670 +/- 50 degrees C. Quartz-kyanite pairs from the
aluminosilicate-bearing quartz veins yield equilibration temperatures of
645 +/- 20 degrees C, confirming that the veins formed under conditions
near metamorphic peak. Quartz and kyanite from veins and the surrounding
metapelites have comparable isotopic compositions. Local intergranular
diffusion in the border of the veins controls the mass-transfer and the
growth of the product assemblage, inducing local mobilization of SiO2
and Al2O3. Andalusite is absent from the host rocks, but it is common in
quartz veins, where it often pseudomorphs kyanite. For andalusite to be
stable at T-max, the pressure in the veins must have been substantially
lower than lithostatic. An alternative explanation consistent with
structural observations would be inheritance by andalusite of the
kyanite isotopic signature during polymorphic transformation after the
metamorphic peak
Thrusting, extension, and doming during the polyphase tectonometamorphic evolution of the High Himalayan Crystalline Zone in NW India
In the NW Himalaya of India, high-grade metamorphic rocks of the High Himalayan Crystalline Zone (HHCZ) are exposed as a 50 km large dome along the Miyar and Gianbul valleys. This Gianbul dome is cored by migmatitic paragneiss formed at peak conditions around 750 degreesC and 8 kbar, and symmetrically surrounded by sillimanite, kyanite +/- staurolite, garnet, biotite, and chlorite Barrovian mineral zones. Thermobarometric and structural investigations reveal that the Gianbul dome results from a polyphase tectono-metamorphic evolution. The first phase corresponds to the NE-directed thrusting of the Shikar Beh nappe, that is responsible for the Barrovian prograde metamorphic field gradient in the southern limb of the dome. In the northern limb of the dome, the Barrovian prograde metamorphism is the consequence of a second tectonic phase, associated with the SW-directed thrusting of the Nyimaling-Tsarap nappe. Following these crustal thickening events, exhumation and doming of the HHCZ high-grade rocks were controlled by extension along the north-dipping Zanskar Shear Zone, in the frontal part of the Nyimaling-Tsarap nappe, as well as by coeval to late extension along the south-dipping Khanjar Shear Zone, in the southern limb of the Gianbul dome. Rapid syn-convergence extension along both of these detachments induced a nearly isothermal decompression, resulting in a high-temperature/low-pressure metamorphic overprint, as well as enhanced partial melting. Such a rapid exhumation within a compressional orogenic context appears unlikely to be controlled solely by granitic diapirism. Alternatively, large-scale doming in the Himalaya could reflect a sub-vertical ductile extrusion of partially melted rocks