Granites of the Indus River Syntaxis, Northwest Pakistan
Department of Geology, Whitman College, Walla Walla, WA 99362
The Himalayan foothills of Northern Pakistan can be subdivided into
three tectonic provinces. The southernmost extends from the Salt Range
thrust (SRT) north to the Main Boundary thrust (MBT) and exposes unmetamorphosed
Phanerozoic sedimentary rocks. The Khairabad thrust, a west striking,
north-vergent thrust, defines the southern boundary of the Himalayan
foothills. The foothills extend northward to the Main Mantle thrust
(MMT), the next major east-west striking fault associated with the
Himalayan Orogeny. Within this structural block east-west striking
Precambrian through Cretaceous metasediments are exposed; the metamorphic
grade increases to the north within this block. The footwall of this
north-vergent thrust exposes rocks of the Jurassic to Late Cretaceous
Kohistan terrane. The MMT, a diffuse zone of deformation, marks the
suture of the Indian plate with the Kohistan island arc.
Structural trends in the Himalayan foreland are primarily east-west.
However, there are three north-south syntaxes: the Nanga Parbat, Hazara-Kashmir,
and Indus River. The Nanga Parbat and Hazara-Kashmir Syntaxes are
well documented. The Indus syntaxis has received less study.
The Indus syntaxis is located northeast of the Peshawar Basin in
northwestern Pakistan. This syntaxis is an elongate north-striking
anticlinorium centered in the Indus River valley. The southern terminus
of the Indus syntaxis is located in the vicinity of the Tarbela Dam
where the active Darband fault separates northeast-striking rocks
of the Gandghar Range from northwest-striking rocks. The Indus syntaxis
extends northward to the Besham area.
Tarbela Lake obscures a large portion of the syntaxis and its associated
faults. Calkins and others mapped the Darband fault as an oblique
north striking sinstral fault running through the Tarbela Dam and
northward through the Indus River Gorge. Northwest of Darband, evidence
for active, north-striking faults includes faceted spurs and the linear
nature of the Indus River valley.
The oldest rocks exposed in the Indus syntaxis are the Besham Group
basement. The Besham Group is intruded by a calcite bearing monzonite.
This is overlain in places by graphitic schists of the Gandaf Formation.
The Gandaf Formation is unconformably overlain by the Tanawal Formation.
Calkins and others subdivided the Tanawal into four members: a conglomerate,
a lower quartzite, a quartz schist, and an upper quartzite. In the
study area, the basal conglomerate is absent. Megacrystic granitoids
and intrusions into the basement are both present. The megacrystic
granitoid east of the Darband fault is the Manshera Granite. This
megacrystic granitiod has concordant contacts with the Tanawal Formation.
In locations, the contact with the Tanawal is marked by a narrow bands
of mylonitic rocks. The sense of shear indicated by these mylonitic
rocks in the vicinity of Tarbela Lake is the primary emphasis of this
The Hazaran orogeny occurred in the late Precambrian to early Cambrian
and is marked marked by an angular unconformity between the late Precambrian
and early Cambrian rocks. The Precambrian rocks beneath this angular
unconformity show evidence of low-grade metamorphism and associated
folding, and display an axial planar cleavage; both of these aspects
are absent in the overlying strata. Baig and others postulate that
the Mansehra granite with its 516±16Ma Rb/Sr isochron, 500Ma
U/Pb isochron, and 493±1 39Ar/40Ar isochron could have been emplaced
during the latest stages of the Hazaran orogeny. To the west of the
Indus River, the undated megacrystic Ambela Granitic Complex is exposed.
A recently proposed mechanism to account for: discontinuous exposure
of Cambrian through Devonian metasedimentary strata, northwardly derived
clasts in the Jafar Kandao Formation, and emplacement of Permian diabase
dikes is late Paleozoic continental rifting associated with the opening
of the Neo-Tethys. The clasts in the Jafar Kandao Formation were supplied
from "thermally induced uplifts of parts of the formerly passive margin
of the Gondwana". The age of this rifting event has been constrained
both stratigraphically and radiometrically. Baig obtained 39Ar/ 40Ar
dates plateau dates of 284±4 Ma and 262±1 Ma for diabase
intrusions into the Mansehra Granite.
The Himalayan orogeny has received tremendous study and is relatively
well understood. During the Eocene, the Indian subcontinent was drifting
northward at a rate of approximately 100-180mm/yr relative to the
mantle. At this pace, accelerated subduction of oceanic crust beneath
Laurasia occurred. After all of the oceanic crust had been consumed
India slowed to 50mm/yr and the subduction should have jumped outboard
of India. This never occurred, effectively stalling the subduction
because there is no density contrast between the Indian Plate and
Neotectonic studies have revealed recent activity throughout the
Indus syntaxis. Yeats and Hussain documented recent movement along
the Darband Fault in the vicinity of Tarbela Dam. Where Holocene Indus
River gravels have been offset and river terraces tilted. Abandoned
water polished and fluted strath terraces 200m above the present-day
Indus River north of Bimbal have been dated as between 19Ka and 12Ka
and suggest a Quaternary uplift rate of approximately 6mm/yr.
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