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Alpine-Himalayan Orogens |
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Afghanistan lies across the critical syntaxis region where the major ranges of Central Asia converge and effectively terminate, to be replaced laterally by the contrasting structural development of Iran. It is a region which has been less studied than the contiguous countries of Iran, Soviet Asia and Pakistan. While portions of the area show high seismic activity, the centre is relatively aseismic. It is traversed by several large transcurrent faults which are still moving. The description which follows includes the molasse basin and foreland of the Indus Valley in Pakistan.
Segment: this has a length, measured along the strike, of 1300 km. The north-west margin of the orogen (north margin of zone 1), against the Turanian shield, is indefinite due to structural or stratigraphical uncertainties or to lack of exposure. The margin of the belt on the southeast (south-east margin of zone 7) against the Indo-Pakistan shield, is broadly gradational (> 3 km). The belt consists of a complex of folded ranges radiating from the Hindu Kush knot and so is of varying width: within the segment it averages c. 1000 km in width.
Zones: the orogenic belt is described in terms of zones (listed above) from north-west to south-east. The Hari Rud zone in the north-west is an area of metamorphics (presumed Pre-Cambrian gneisses and Lower Palaeozoic slates etc.) overlain by unmetamorphosed U. Palaeozoic. It is faulted against (2) the thick Farah Mesozoic flysch with Tertiary volcanics and late batholithic intrusions. This trough is flanked on its outer side
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