wAbstract
@In the Himalaya of Chamba, NW India, a major orographic barrier
in front of the Greater Himalayan Range extracts a high proportion
of the monsoonal rainfall along its southern slopes and effectively
shields the orogen interior from moisture-bearing winds. Along
a `100-km-long orogen perpendicular transect, 28 new apatite fission
track (AFT) and 30 new zircon (U-Th)/He (ZHe) cooling ages reveal
marked variations in age distributions and long-term exhumation
rates between the humid frontal range and the semi-arid orogen
interior. On the southern topographic front, very young, elevation-invariant
AFT ages of 4 Ma have been obtained that are concentrated in
a `30-km-wide zone; 1-D-thermal modeling suggests a Plio-Pleistocene
mean erosion rate of 0.8-1.9 mm yr-1. In contrast,
AFT and ZHe ages within the orogen interior are older (4-9 and
7-18 Ma, respectively), are positively correlated with sample
elevation, and yield mean erosion rates (0.3-0.9 mm yr-1).
Protracted low exhumation rates within the orogen interior over
the last `15 Myr prevailed contemporaneously with overall humid
conditions and an effective erosional regime within the southern
Himalaya. This suggests that the frontal Dhauladar Range was sufficiently
high during this time to form an orographic barrier, focusing
climatically enhanced erosional processes and tectonic deformation
there. Thrusting along the two frontal range-bounding thrust,
the Main Central Thrust and the Main Boundary Thrusts, was initiated
at least `15 Ma ago and has remained localized since then. The
lack of evidence for localized uplift farther north indicates
either a rather flat decollementiลฬeฬชษLj
with no ramp or the absence of active duplex systems beneath the
interior of Chamba. Exhumational variability within Chamba is
best explained as the result of continuous thrusting along a major
basal decollementiลฬeฬชษLj, with a flat
beneath the slowly exhuming internal compartments and a steep
frontal ramp at the rapidly exhuming frontal range. The pattern
in Chamba contrasts with what is observed elsewhere along the
Himalaya, where exhumation is focused in a zone `150 km north
of the orogenic front. In the NW Himalaya, preserved high Himalayan
Crystalline nappes and Lesser Himalayan windows alternate on a
relatively small scale of 100 km; these alternations are closely
correlated with the pattern of exhumation. Although the spatial
distribution of high-exhumation zones varies considerably between
individual Himalayan sectors, all of these zones are closely correlated
with locally higher rock-uplift rates, sharp topographic discontinuities,
and focused orographic precipitation, suggesting strong feedbacks
between tectonically driven rock uplift, orographically enhanced
precipitation, and erosional processes.
Keywords: apatite fission-track; zircon uranium-thorium-helium;
thermochronology; exhumation; Himalaya; Haimantasx
1. Introduction
2. Geologic and climatic setting of the Indian NW Himalaya
3. Methods
4. Results
5. Discussion and conclusions
Acknowledgments
References