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| Titel |
A cross section through the eastern Pamir and Tien Shan obtained by teleseismic receiver functions |
| VerfasserIn |
Felix Schneider, Xiaohui Yuan, Christian Sippl, Bernd Schurr, James Mechie, Christian Haberland, Mustafo Gadoev, Ilhomjon Oimahmadov, Ulan Abdybachaev, Sobit Negmatullaev |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250051384
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| Zusammenfassung |
| To investigate the mechanisms of mountain building processes in continental collision zones,
the Pamir region is a prominent example, that shows several interesting features. In this
region comparatively high convergence accommodated over a small meridional orogenic
width causes extreme crustal shortening. This leads to crustal thickening, high deformation
rates and maybe continental subduction. In contrast to the adjacent regions of the
Indian-Eurasian collision zone such as the Himalaya and Tibet, in the Pamir-Hindu Kush
mountain range intermediate depth seismicity occurs in the mantle, the origin of which is not
yet well understood.
Besides, these mantle earthquakes form a narrow down-dipping band resembling a
Wadati-Benioff zone with changing dipping directions, pointing northwards beneath the
Hindu Kush and southwards beneath the Pamir. Different subduction scenarios with different
geometries and compositions of either one or two slabs have been proposed to explain this
formation, including remnant and decoupled oceanic lithosphere, one partly overturned slab
of either continental or oceanic composition and two opposing slabs of continental
lithosphere.
Within the framework of the multidisciplinary Tien Shan Pamir Geodynamic program
(TIPAGE) we deployed a temporary network of 32 broadband and 8 short period seismic
stations across the Pamir in two different network configurations from 2008 to 2010. In
the first year 24 stations were set up in the eastern Pamir forming a 350Â km long
north-south profile, with a station spacing of approximately 15Â km. In the second year we
rearranged the deployment in order to get an equally distributed network with a station
spacing of approximately 40Â km. We extended this dataset with data of a temporary
network in the Ferghana Valley in Kyrgyzstan, which ran for one year from 2009 to
2010.
We have performed a receiver function (RF) analysis that images seismic discontinuities
to get insights into the gross crustal and upper mantle structure. A cross section along the
profile through the eastern Pamir, the Tien Shan and the Ferghana Valley is obtained by
migration of the RF-amplitudes using a common conversion point stacking algorithm. This
shows a prominent southward-dipping structure in the mantle south of the Main
Pamir Thrust, which coincides with the Wadati-Benioff-type zone formed by the
hypocenters of the intermediate-depth earthquakes, that we had localized by manual
picking.
By comparing the arrival times of direct Moho conversions and those of reverberated
phases (that is crustal multiples), we derive the average crustal Vp/Vs ratio and its lateral
variation. We find low values of around 1.68 for crustal Vp/Vs ratios beneath the Pamir,
which corresponds to a Poisson’s ratio of 0.225 indicating an overall felsic crust, whereas
towards the Tien Shan the Vp/Vs ratio increases, reaching values around 1.77 (Poisson’s
ratio of 0.265). Vp/Vs then slightly decreases towards the Ferghana Valley to 1.73
corresponding to an intermediate Poisson’s ratio of 0.25. Crustal thickness varies along
the profile from 75Â km beneath the southern Pamir to 64Â km beneath the Tien
Shan and then decreases abruptly in a distinct step to 45Â km towards the Ferghana
Valley. |
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