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| Titel |
The first direct dating of Main Central Thrust phyllonite demonstrates exhumation of the Greater Himalayan Crystalline had already taken place |
| VerfasserIn |
Sareh Rajabi, Marnie Forster, Talat Ahmad, Gordon Lister |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250147151
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| Publikation (Nr.) |
 EGU/EGU2017-11265.pdf |
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| Zusammenfassung |
| Here we report the results of step-heating experiments that allow the first direct dating of the
timing of movement on the Himalayan Main Central Thrust (MCT). Timing of MCT
operation has, until now, been inferred based on specific tectonic models, or with data not
directly attributable to MCT movement, e.g., the debatable assertion that leucogranite
formation is invariably related to crustal shortening, and therefore that the MCT must
already have been in operation. However the tectonic evolution may have been more
complex, e.g., at times involving horizontal extension. In any case, many different
thrust systems operated during India-Asia convergence, and the MCT is only one of
them. It is time to move away from models and to bring geology back into the
equation.
Here we apply 40Ar/39Ar geochronology to directly date highly strained, phyllonitized,
muscovite in the MCT above the Kullu-Rampur tectonic window (NW Indian Himalaya),
showing that the timing of the shear movement lasted from 15–9 Ma. We show that these
ages have been preserved because the white mica was sufficiently retentive of argon to be
able to inhibit its diffusional loss at the temperatures and pressures in question.
Arrhenius data from ultra-high-vacuum diffusion experiments show that deformation
occurred below the closure temperature of this muscovite, for moderate cooling rates.
Furthermore, we demonstrate that microscopic shear bands associated with MCT operation
overprinted an earlier decussate mica growth. This decussate growth had taken place
prior to ∼ 18 Ma. The decussate microstructure, together with foam textures in the
host deformed quartzite, demonstrate that low deviatoric stress conditions applied
during a prior period of static annealing under middle- to upper-greenschist facies
conditions.
In this region, therefore, the Greater Himalayan Crystalline had therefore already been
significantly exhumed prior to the onset of MCT operation. The foam textures in quartzite
and the decussate intergrowths of mica match in age and character the effects of the
Oligo-Miocene metamorphic event that had widespread effects across this region, coeval with
the operation of extensional ductile shear zones and faults of the South Tibetan
Detachment (STD) system. This means that regional exhumation of the crystalline
series most-likely occurred as the result of extreme extension during STD time. The
MCT at this location is a relatively late structure that overprinted STD fabrics and
microstructures at least five million years after the main exhumation of the crystalline
series.
It is widely agreed that the MCT had a pivotal role in the evolution and exhumation of the
Greater Himalaya crystalline sequences. This aspect is central to models involving
fold-nappes, channel flow, and wedge extrusion. All of these models imply that the crystalline
core of the Himalaya was exhumed as the result of it being thrust southward by the MCT.
However, there is no evidence that this is the case, and these data allow rebuttal of such
models, at least in terms of the structures currently defined as representing the MCT in NW
India. |
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