We conducted a multi-disciplinary study of crustal reworking during orogenesis in the Himalayas of Pakistan. The project combines the techniques of geochronology, petrology, stable-isotope geochemistry, radiogenic tracer-isotope geochemistry, seismology, magnetotelluric sounding, structural geology, and geomorphology to characterize the evolution and current state of the crustal lithosphere beneath the Nanga Parbat-Haramosh Massif in the western Himalayas. Nanga Parbat has undergone young high-grade metamorphism and recent rapid denudation. Identifying the tectonic processes responsible for this recent activity gives us the opportunity to evaluate models of metamorphism and mountain building commonly applied to older orogens.
Extremely rapid exhumation, the presence of hot springs, young intrusive rocks, and young metamorphism suggest an anomalous thermal structure lies beneath Nanga Parbat. Strong thermal anomalies result in seismic velocity and attenuation anomalies in the crust and upper mantel. Joint tomographic inversion of travel time and spectral data allow us to map velocity and attenuation perturbations to characterize the state of the crust beneath Nanga Parbat.
We deployed a 60 station array consisting of 10 broadband and 50 short period
stations deployed in a 30 x 50 km area surrounding the massif order to characterize
the crust beneath the massif and the subsurface geometry and kinematics of faults
bounding the massif. Average station spacing is 5 km, and station elevations
range from 1.5 to 4.3 km. The array recorded local and regional seismicity for
a 4 month period (May-September, 1996). We
used the Hindu Kush events which originate at 200-300 km depth as a beam source
to illuminate the structure beneath Nanga Parbat and we used additional regional
events from a range of azimuths combined with local events to produce a tomographic
image of velocity and attenuation structure beneath the massif.
The topography in the area is
extreme - the world's greatest relief ~7000 m in 21 horizontal km occurs
at Nanga Parbat. Station elevations ranged from 1100 - 4100 m. The short
period stations at NP were deployed flanking the west and east sides of
the massif along the KKH and Astor river sections. Access to the interior
of the massif was obtained along three primary glacial valleys, Bunar Das
were we deployed stations in the Diamir Gah up to base camp, and back toward
Philobat; the Tato Valley - almost up to the German base camp, and the Rupal
Valley back toward the Mazeno base camp. Sites in these three valleys sit
on Nanga Parbat gneisses. Reference sites - not on Indian crust, were deployed
back toward Babasuar pass, north at Telichi and Gilgit, and east toward
Rattu.
25 of the 50 sp sites could
be accessed fairly easily along roads and a short walk. The remaining 25,
could only be accessed by trekking. At the beginning of our field season
a loop up and back one of these valleys took between 5 and 7 days, by the
end it took only 3-4 days. We found ample bedrock sites in the area and
generally lots of interest in what we were doing and lots of help setting
up sites and servicing the instruments.
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| A map of regional seismicity in the northwestern terminus of the Himalayan arc for the last 7 years compliled from the database at the IRIS DMC shows the primary source locations in the region, the Pamirs and Hindu Kush to the northeast, the Karakorum and NE terminus of the Baluchistan arc to the north and northeast, the Himalayan arc to the sw, theHazara arc and Kashmir to the s and se. Blue box outlines the position of our array at NP. |  |
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A typical local event with an s-p delay of 3 sec. This particular record shows three small separate events within the 3 min recording window and locates beneath Tato |
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