Does the Karakoram fault interrupt mid-crustal channel flow in the western Himalaya?

Abstract

Variations in the volume and age of Miocene granites and in mid-crustal conductance from the northwest Himalaya to southeastern Tibet imply lateral differences in late orogenic processes. The change from west to east occurs near the Gurla Mandhata dome, where the Karakoram fault terminates and merges with the Indus–Yarlung suture zone. The ‘channel flow’ model, developed in southeastern Tibet, predicts that anatectic partial melts beneath the Tibetan plateau are gravitationally-driven south to a topographic erosional front and are exposed as leucogranites in the Greater Himalaya Sequence; upwellings of these channel granites occur as gneiss domes in the Tethyan Himalaya Sequence. Published magnetotelluric profiles show high conductivity 30–40 km deep beneath Tibet from c. 400 km north of the Main Frontal thrust south across the suture zone, beneath the Himalayan gneiss domes, and to the topographic front; this conductive middle crust corresponds to 2–4% partial melt in the northwest Himalaya and 5–12% melt in southeastern Tibet, sufficient in the latter case to weaken rock for flow. East of the Karakoram termination leucogranites are abundant and are as young as 7 Ma; west of the termination, channel granites are less abundant and no younger than 18 Ma. Middle Miocene (16–14 Ma) leucogranites are found in the Karakoram fault zone located north of the suture zone and south of the proposed anatectic melt source. The initiation of motion on the crustal-penetrating Karakoram fault at 25–21 Ma may have created a barrier to the southward flow of mid-crustal melts and acted as a vertical conduit for these same melts.