The Confluence area

Looking up the Indus Valley towards the mountain massif of Haramosh.   Part of the Confluence granite sheet swarm is evident.

The upper part of the Kohistan island arc contains a suite of granite sheets. These are termed the Parri and Confluence granite sheets. When undeformed these sheets have a distinct geochemistry and yield Rb-Sr ages of 50-30 Ma (for the earlier, biotite-bearing Confluence granites) and c. 26 Ma (for the muscovite-bearing Parri sheets). These sheets form a distinctive set of markers which may be used to chart the strain as one approaches the edge of the Kohistan arc and its contact with the Nanga Parbat massif.

From a distance the two sets of granites are indistinguishable - forming a prominent swarm. Although the sheet network is aligned, this is not due to ductile shearing. In close up the granite sheets form an undeformed net vein array, until within about 500m of the base of the arc.

Detail of the cross-cutting net vein structure of the Confluence granite sheets, seen in a 50m cliff section, Indus gorge at Hanuchal village. Note that the sheets cut a pre-existing fabric within the rocks of the Kohistan arc.