Stratigraphy and Structure of the Tarbela Lake Region, northern Pakistan
Brian K. Pidduck, Kevin R. Pogue, and Walli Ullah Khattak
A geologic synthesis of the Tarbela Lake region of northern Pakistan is presented based on stratigraphic and structural information derived from recently completed geologic maps. Lower Proterozoic Indian plate crystalline basement (the Kotla Complex and Gandaf Formation) forms the base of the section. Orthogneiss of the Kotla Complex intrudes marble and graphitic schist of the Gandaf Formation. Quartzite and schist of the Upper Proterozoic(?) Tanawal Formation unconformably overlies the basement rocks. The Tanawal Formation is intruded by the Mansehra Granite of Late Cambrian age and is overlain by Lower Paleozoic metasedimentary rocks of the Sherwan synclinorium.
The following sequence of deformational events is suggested based on the structural geology of the region. Following Late(?) Proterozoic deformation and metamorphism of the Kotla Complex and Gandaf Formation, feldspathic tourmaline-bearing sand that comprises the protolith for Tanawal Formation quartzite was deposited via erosion of a granitic highland. Late Cambrian deformation and metamorphism(?) accompanied the emplacement of large sill-like bodies of Mansehra Granite into the Tanawal Formation. Late Paleozoic rifting was responsible for normal faulting and the intrusion of ubiquitous diabase dikes and sills. Himalayan deformation consists of early folding along north-south axes followed by south-vergent folding and thrust faulting. The most recent Himalayan event is the development of the Indus syntaxis, an elongate north-striking anticlinal structure centered in the valley of the Indus River. The Darband fault splits the core of the Indus syntaxis and lies beneath the western abutment of Tarbela Dam, the largest earth-fill dam in the world. Variations in stratigraphic throw along the Darband fault imply a large component of strike-slip. Topographic lineaments and faceted spurs along the trace of the Darband fault suggest that it is a youthful and potentially active structure. The Darband fault and Indus syntaxis are considered to be transpressional structures that developed due to partial coupling of the western margin of the Indian plate with Afghanistan.