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| Titel |
On the disappearance of the Greater Himalayan crystalline in the northwestern Himalaya |
| VerfasserIn |
Konstanze Stübner, Djordje Grujic, Talat Ahmad |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250044315
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| Zusammenfassung |
| The apparent continuity of the major tectonic units and tectonic contacts along the Himalayan
orogen is intriguing. Numerous studies substantiate a common first order extrusion history of
the crystalline core of the orogen (the Greater Himalayan crystalline) along the length of the
orogen, or elucidate second-order lateral variations thereof. However, little is known about
the lateral termination of the crystalline core. The major shortening structures below the
Greater Himalayan crystalline (Main Central thrust, Main Boundary thrust, and Main Frontal
thrust) have been mapped as far west as the Nanga Parbat syntaxis in Pakistan, but significant
debate exists as to how far west the Greater Himalayan crystalline crops out at the
surface.
In Himachal Pradesh, northwest India, the hanging wall of the Main Central thrust (MCT)
consists of chlorite- to biotite-grade metamorphic metasedimentary rocks, which have been
interpreted as lower-grade equivalents of the Greater Himalayan crystalline in the framework
of both “Channel-flow” and general shear extrusion models. More recently, proponents of a
“tectonic wedging” scenario of extrusion challenge this view and contend that the Greater
Himalayan crystalline does not surface west of ~ 077-E, except in the Kishtwar
tectonic window. However, none of these models account for the observed steep
lateral metamorphic gradient from up to migmatite grade (-¥ 750 -C) in the Sutlej
valley to biotite grade west of the Beas valley, within 100 km along the strike of the
orogen.
New muscovite 40Ar/39Ar cooling ages from the Beas valley indicate early Miocene
emplacement of the MCT hanging wall in Himachal Pradesh. Emplacement is concurrent
with the onset of extrusion of the crystalline core further east. Our cooling ages are
significantly older than comparable ages measured in the central and eastern Himalaya,
indicating that in Himachal Pradesh, unlike elsewhere, the amount of exhumation since the
early Miocene is negligible.
Based on structural mapping and our preliminary thermochronology data, we tentatively
propose the existence of a lateral ramp in the Main Boundary thrust (MBT) in Himachal
Pradesh. East of the Beas valley shortening between India and Asia has been accommodated
by thrusting along the MCT and related faults. Thrusting was in part counteracted by early
Miocene normal-sense slip along the South Tibetan detachment, which acted as a passive
roof fault to the extruding Greater Himalayan crystalline. West of the Beas valley
down-stepping of the MBT across the proposed ramp provided accommodation space for the
MCT hanging wall. Extrusion of the Greater Himalayan crystalline is therefore
limited to the east of the ramp; post-early Miocene exhumation is negligible west of
the Beas, and no passive roof thrust comparable to the South Tibetan detachment
developed. |
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