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Late Oligocene high-temperature shear zones in the core of the Higher Himalayan Crystallines (Lower Dolpo, western Nepal)
Full text linkA high‐temperature shear zone, Toijem shear zone, with a top‐to‐the‐SW sense of shear affects the core of the Higher Himalayan Crystallines (HHC) in western Nepal. The shear zone developed during the decompression, in the sillimanite stability field, of rocks
that previously underwent relatively high‐pressure metamorphism deformed under the kyanite stability field. PT conditions indicate that the footwall experienced higher pressure (∼9 kbar) than the hanging wall (∼7 kbar) and similar temperatures (675°–700°C). Monazite growth constrains the initial activity of the shear zone at 25.8 ± 0.3 Ma, before the onset of the Main Central Thrust zone, whereas the late intrusion
of a crosscutting granitic dike at 17 ± 0.2 Ma limits its final activity. Monazites in kyanite‐bearing gneisses from the footwall record prograde metamorphism in the HHC from ∼43 to 33Ma. The new data confirm that exhumation of the HHC started earlier in western Nepal than in other portions of the belt and before the activity of both the South Tibetan Detachment System (STDS)
and Main Central Thrust (MCT) zones. As a consequence,
western Nepal represents a key area where the channel‐flow‐driven mechanism of exhumation, supposed to be active from Bhutan to central‐eastern Nepal, does terminate. In this area, exhumation of crystalline units occurred by foreland propagation of ductile and, subsequently, brittle deformation
Analisi della distribuzione geografica del rischio: un approccio bayesiano mediante Gibbs Sampling.
No full textHigh grade shear zones in the core of the Himalayan belt (western Nepal): consequences on the exumation of the Higher Himalayan Crystallines
No full text20th Himalayan-Karakoram-Tibet Worksho
How is accommodated extrusion in the core of the Higher Himalayas Crystallines in the Bhutan Himalaya?
No full text
Deformation features of the Higher Himalayan Crystallines in Western Bhutan during exhumation
No full text19th Himalaya-Karakoram-Tibet Workshop, Niseko, Japan -July 10-12, 2004
LOCALIZZAZIONE DELLA DEFORMAZIONE NEL CRISTALLINO DELL'ALTO HIMALAYA: VINCOLI GEOCRONOLOGICI SULLA ATTIVITA' DI ZONE DI TAGLIO PRECEDENTI AL MAIN CENTRAL THRUST IN NEPAL OCCIDENTALE.
No full textThe excellent exposure of the Higher Himalayan Crystallines
(HHC) in Western Nepal allowed to constrain the tectonic, metamorphic
and geochronological evolution of previously unrecognized
high-temperature shear zones inside the HHC.
A top-to-the SW high-temperature shear zone has been detected
(Toijem Shear Zone: TSZ) in the middle portion of the HHC, bracketed
between the Main Central Thurst to the botton and the South
Tibetan Detachment System to the top. In the TSZ mylonites abundant
prismatic sillimanite crystals are aligned along the main foliation
and wrap garnet and kyanite porphyroblasts. Abundant muscovite
overgrows both kyanite and sillimanite crystals. Thermo
barometric data indicate prograde metamorphism, in the kyanite
field followed by decompression in the sillimanite field.
U-Pb monazite ages on micaschists and gneisses, from both the
footwall and the hanging wall of the TSZ, span between ca. 43 and
17 Ma, with major peaks at ca. 43, 38 and 33 Ma. The oldest age corresponds
to monazite included in garnet in kyanite bearing gneiss,
and is possibly recording the earliest stages of deformation and
metamorphism of the HHC. Further monazite formation is attributed
to continuing metamorphism and/or fluid activity.
U-Pb ages from zoned monazites in sillimanite bearing
mylonites of the TSZ indicate continuous or multiple metamorphism
with a major episode at around 26 Ma.
The ca. 43, 38 and 33 Ma monazite formation can be attributed
to the underthrusting event and only later, from 26 Ma, exhumation
has been active. The 26 Ma age, has been found only in the monazites
on the TSZ mylonites and does not appear in previous
geochronological studies of the Himalayan belt. The ca. 26 Ma could
be attributed to the beginning of the TSZ activity marking the earliest
stage of decompression of the HHC.
Geometric, kinematic, geochronological and petrological data
indicate that, after the underthrusting phase, the HHC in Dolpo area
recorded the activity of a high-temperature shear zone (TSZ) developed
during the earlier stages of exhumation of the HHC enhancing
the decompression of the upper portion of the HHC before the main
extrusion of the HHC by contemporaneous activity of the MCT and
STDS constrained at 23-17 Ma
A structural transect from Lower Dolpo (Western Nepal).
No full textWe present the results of a structural transect in Lower Dolpo, cross-cutting the upper part of the Lesser Himalaya (LH), the Higher Himalayan Crystallines (HHC) and the lower part of the Tibetan Sedimentary Sequence (TSS). The MCT zone affects the upper part of the LH as well as the lower part of the HHC and shows a later brittle reactivation. Mean vorticity in the MCT points to non-coaxial deformation. These data, together with available kinematic data along the belt, on the South Tibetan Detachment System (STDS) and in the core of the HHC, point to increasing simple shear toward the tectonic boundaries. A top-to-the-SW high-temperature shear zone (Toijem Shear Zone) is recognized in the middle part of the HHC at the boundary between Units 1 and 2. It developed during the earlier stages of exhumation of the HHC, enhancing the decompression of the hanging wall and the emplacement of leucogranite dykes and sills. Its development could be explained by a change in the velocity profile during the extrusion of the HHC, triggered by first order changes in rock types of the tectonic unit. The STDS is marked by a wide zone of high strain and by a metamorphic jump from amphibolite facies in the carbonate rocks of the upper part of the HHC to greenschist facies marbles in the lower part of the TSS. The development of a pervasive foliation towards the bottom of the TSS indicates increasing strain, related to top down-to-the-NE tectonic transport. A Low P metamorphic event, marked by the growth of post-D1 biotite porphyroblasts at the base of the TSS, is related to the conductive heating from the underlying HHC
Kinematics and vorticity of flow associated with post-collisional oblique transpression in the Variscan Inner Zone of northern Sardinia (Italy)
No full textQuartz c-axis fabrics, vorticity of flow and deformation temperatures were analyzed in samples collected in the central sector of the Variscan inner zone in Sardinia (Italy). In order to constrain the kinematics of exhumation of medium- and high-grade metamorphic rocks during D2 post-collisional deformation, we investigated two shear belts with opposing shear senses. These shear belts are developed parallel to the trend of the inner zone which marks the boundary between two metamorphic complexes. Our data suggest that the sinistral shear belt initiated during the early stage of D2 deformation in a simple-shear-dominated regime, whereas the dextral shear belt initiated later in a pure-shear-dominated regime. We document a general non-coaxial flow regime with contemporaneous pure and simple shear components. Geometrical relationships reveal the operation of oblique extrusion during a compressive regime where the coaxial component acted sub-horizontal and orthogonal to the trend of the axial zone. Changes in the kinematics, vorticity of flow and strain geometry that occurred during D2 deformation may be related to post-collisional orogen-parallel displacement caught up in the Variscan orogeny. © 2009 Elsevier Ltd. All rights reserved
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