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| Titel |
A two-step underthrusting and delamination model that explains deep structures beneath Pamir and Hindu Kush |
| VerfasserIn |
Christian Sippl, Bernd Schurr, Felix Schneider, Xiaohui Yuan, Mustafo Gadoev, Sagynbek Orunbaev, Sobit Negmatullaev, Christian Haberland |
| 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 |
250080805
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| Zusammenfassung |
| The Pamir-Hindu Kush region, situated at the western Himalayan syntaxis, is one of the
tectonically most complex and least well understood regions on earth. Frequently occurring
intermediate-depth earthquakes, which define two seismic zones at mantle depth, attest to
ongoing subduction or delamination processes during continental collision. The presence of
deep seismicity and the complexity of mantle structures set the Pamir-Hindu Kush apart from
Tibet, i.e. imply a different tectonic style between the front and the syntaxes of the Indian
indenter.
Automatically identified and located local earthquakes from the TIPAGE data set
(2008-2010) outline two distinct, seismically active zones at mantle depths. Beneath the
Hindu Kush, earthquake locations to first order define a subvertically northward dipping
planar structure with high internal complexity. At depths greater than 150 km, this plane
appears to be broken into several fragments. Focal mechanisms of intermediate-depth
earthquakes uniformly show downdip extension, i.e. T axes oriented around vertical, whereas
retrieved P axes are horizontal, perpendicular to the strike of the Hindu Kush seismic
zone.
The Pamir seismic zone, in contrast, resembles a single, strongly curved slab dipping
southward at its eastern termination towards the Tarim Basin and progressively changes its
dip direction to purely eastward at its southwestern end. Focal mechanisms of Pamir deep
seismicity are less uniform than for the Hindu Kush, but show a prevalence of along-arc
extensive mechanisms in the shallower part of the slab, where earthquake hypocenters outline
an along-strike continuous structure. At deeper levels, where the slab might be torn (which
could be indicated by an absence of seismic activity), T axes are oriented more
steeply.
A local earthquake tomography study, utilizing a selection of 56,229 P and 25,221 S
phases to perform an inversion for vp and vp-vs throughout the Pamir and its surroundings,
clearly shows the arcuate Pamir slab as a high-wavespeed anomaly (vp = 8.2-8.3 km/s). The
hypocentral locations of earthquakes outline the upper edge of this lithospheric slab, in fact
appear to be confined to an about 10 km wide low-velocity channel atop it, which has been
identified with receiver function analysis. Directly above the updip end of seismicity (at
depths of 70-80 km), very low values of vp (around 7.1 km/s) and high vp-vs ratios (>1.80)
are retrieved, probably indicative of the entrainment of upper or middle crustal
material in the subduction process, which leads to the burial of buoyant material to
mantle depths. South of the Pamir slab, velocities around or slightly below normal
mantle values are observed, no indication of cold underthrusted Indian lithosphere is
found.
We interpret the obtained seismological evidence with a two-step model of continental
overthrusting of the Pamir over the basin material to the north and west of it, followed by the
delamination of the underthrusted material into the mantle, possibly due to the acquisition
of negative buoyancy by phase transformation reactions (e.g. eclogitization). The
entrained upper or middle crustal material imaged with tomography is not involved in
the delamination process due to its low density. However, a thin layer of possibly
eclogitized lower crustal material sits atop the sinking lithospheric slab and hosts the
intermediate-depth seismicity. A pure delamination scenario could not explain the
observed presence of slow material at mantle depths, whereas the retrieved stress
axis orientations in the Pamir slab are hard to reconcile with a classical subduction
process. |
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