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| Titel |
Crustal thickening and the onset of extrusion of the crystalline core of the Himalaya revealed in Himachal Pradesh, NW India |
| VerfasserIn |
Konstanze Stübner, Djordje Grujic, Randall Parrish, Talat Ahmad |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250082156
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| Zusammenfassung |
| Understanding extrusion of the Greater Himalayan crystalline (GHC) is a key to the ongoing
debate about mechanisms of middle to lower crustal deformation in the India–Asia and other
collision zones worldwide. In the central and eastern Himalaya exposure of deep crustal
levels of the GHC has removed the rocks preserving record on the prograde evolution of the
crystalline during crustal thickening and early stages of extrusion. Shallower levels of the
orogen’s metamorphic core are exposed in Himachal Pradesh, NW Himalaya, arguably
providing the opportunity to study the early tectonometamorphic evolution of the GHC and
the onset of its extrusion.
In Himachal Pradesh, the GHC forms a recumbent fold, the Phojal nappe. Like in the rest
of the Himalayas the exhumation of the GHC is explained by SW-vergent extrusion between
the basal Main Central thrust and the South Tibetan detachment as the roof; structural
position and geometry of the South Tibetan detachment are, however, debated in the NW
Himalaya. Our new monazite U/Th–Pb data from six metapelite samples of different
structural levels of the GHC document the Eocene/Oligocene prograde metamorphic
evolution with distinct periods of monazite growth at ~42, ~36, and ~27 Ma. The youngest
monazite U/Th–Pb ages (20 ± 2 Ma) are indistinguishable from our biotite and white mica
40Ar/39Ar cooling ages (22–20 Ma) and pinpoint the onset of rapid cooling due to SW-ward
extrusion of the GHC. Onset of extrusion is thus 8–10 m.yr. earlier compared to the eastern
Himalaya.
The samples were collected above and below the structural level that has been
proposed as the South Tibetan detachment by some authors, while others dispute its
very existence in the area. The respective age patterns are the same within error
and thus inconsistent with the existence of a major shear zone at that level. We
propose that the South Tibetan detachment pinches out in central Himachal Pradesh
and reapers as the Zanskar shear zone further west, while in the study area it is
manifested by distributed top-to-the-N shear at the upper structural levels of the Phojal
nappe. |
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