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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. [Copyright &y& Elsevier]

Abstract: Leech [Leech, M.L., 2008, Does the Karakoram fault interrupt mid-crustal channel flow in the western Himalaya? Earth Planet. Sci. Lett., 276, 314–322.] proposed that (1) Himalayan granites are significantly more abundant east of the Karakoram fault termination (around Mount Kailas, in SW Tibet) than west of it in the Zanskar–Kumaon region, that (2) the fault may have created a barrier to southward flow of mid-crustal channel flow, and that (3) the fault acted as a vertical conduit for these melts. These inferences are based upon new U–Pb SHRIMP data from the Leo Pargil dome, NW India, and the analysis of published U–Pb ages from additional Himalayan domes. Here we point out the flaws in all these hypotheses and suggest a much closer comparison of granites along the Karakoram shear zone to the widespread Miocene crustal melt granites of the Baltoro Karakoram range in North Pakistan. Field relationships combined with U–(Th)–Pb dating of granites and metamorphic rocks clearly shows that the leucogranites exhumed along the Karakoram fault are related to regional metamorphic and melting events along the Baltoro Karakoram range of the Asian plate and not to Indian plate Himalayan leucogranites at all. We discuss individually the points raised. [Copyright &y& Elsevier]