The detailed geological mapping and structural study of a complete
transect across the northwestern Himalaya allow to describe the tectonic
evolution of the north Indian continental margin during the Tethys ocean
opening and the Himalayan Orogeny.
The Late Paleozoic Tethys rifting is associated with several
tectonomagmatic events. In Upper Lahul and SE Zanskar, this extensional
phase is recorded by Lower Carboniferous synsedimentary transtensional
faults, a Lower Permian stratigraphic unconformity, a Lower Permian
granitic intrusion and middle Permian basaltic extrusions (Panjal
Traps). In eastern Ladakh, a Permian listric normal fault is also
related to this phase. The scarcity of synsedimentary faults and the
gradual increase of the Permian syn-rift sediment thickness towards the
NE suggest a flexural type margin.
The collision of India and Asia is characterized by a succession of
contrasting orogenic phases. South of the Suture Zone, the initiation of
the SW vergent Nyimaling-Tsarap Nappe corresponds to an early phase of
continental underthrusting. To the S, in Lahul, an opposite
underthrusting within the Indian plate is recorded by the NE vergent
Tandi Syncline. This structure is associated with the newly defined
Shikar Beh Nappe, now partly eroded, which is responsible for the high
grade (amphibolite facies) regional metamorphism of South Lahul.
The main thrusting of the Nyimaling-Tsarap Nappe followed the formation
of the Shikar Beh Nappe. The Nyimaling-Tsarap Nappe developed by ductile
shear of the upper part of the subducted Indian continental margin and
is responsible for the progressive regional metamorphism of SE Zanskar,
reaching amphibolite facies below the frontal part of the nappe, near
Sarchu. In Upper Lahul, the frontal parts of the Nyimaling-Tsarap and
Shikar Beh nappes are separated by a zone of low grade metamorphic rocks
(pumpellyite-actinolite facies to lower greenschist facies). At high
structural level, the Nyimaling-Tsarap Nappe is characterized by
imbricate structures, which grade into a large ductile shear zone with
depth. The related crustal shortening is about 87 km.
The root zone and the frontal part of this nappe have been subsequently
affected by two zones of dextral transpression and underthrusting: the
Nyimaling Shear Zone and the Sarchu Shear Zone. These shear zones are
interpreted as consequences of the counterclockwise rotation of the
continental underthrusting direction of India relative to Asia, which
occurred some 45 and 36 Ma ago, according to plate tectonic models.
Later, a phase of NE vergent `'backfolding'' developed on these two
zones of dextral transpression, creating isoclinal folds in SE Zanskar
and more open folds in the Nyimaling Dome and in the Indus Molasse
sediments.
During a late stage of the Himalayan Orogeny, the frontal part of the
Nyimaling-Tsarap Nappe underwent an extension of about 15 km. This phase
is represented by two types of structures, responsible for the tectonic
unroofing of the amphibolite facies rocks of the Sarchu area: the Sarchu
high angle Normal Fault, cutting a first set of low angle normal faults,
which have been created by reactivation of older thrust planes related
to the Nyimaling-Tsarap Nappe.