Granites of the Indus River Syntaxis, Northwest Pakistan

Peter Sak

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 study.

Tectonic Setting
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 Laurasia.

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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