CHANNELING NEPAL? P-T-T DATA SAY NO

Exposure of the Himalayan metamorphic core is commonly considered prima facie evidence for channel flow, but no definitive test has yet evaluated whether the thermal effects of channel flow are reflected in P-T-t evolution of these rocks. In central Nepal, P-T-t data consistently repudiate channel flow, and instead support classical thrust kinematics. Numerous tests are possible in high- and low-grade rocks, including metamorphic field gradients, P-T paths, peak ages, durations at high-T, cooling ages, and cooling rates. Of 16 metamorphic and geochronologic tests, channel flow accurately predicts only 3 (P at peak T for migmatites; migmatite peak ages; high T-gradients across the MCT); none of these is required by channel flow. In contrast, at least 9 predictions fundamental to channel flow, i.e., resulting directly from extra heat advection by a flowing channel, are refuted by P-T-t data. Within the Greater Himalayan Sequence, predictions vs. contrasting observations include: (1) decreasing T structurally upward vs. increasing T, (2) P-T paths that show isothermal exhumation vs. isobaric cooling, (3) durations at peak T of ~15 Myr vs. ~5 Myr, (4) initial cooling ages of 8-9 Ma vs. ages of 15-20 Ma and (5) rapid cooling rates of 70 °C/Myr vs. relatively slow rates of ~35 °C/Myr. Within the Lesser Himalayan Sequence, channel flow predictions vs. contrasting observations include: (1) P at peak T of 4-6 kbar vs. 8-10 kbar, (2) a high field T-gradient vs. a low T-gradient, (3) isobaric heating P-T paths vs. “clockwise” and probable hairpin P-T paths, and (4) a high (125 °C/kbar) metamorphic field gradient vs. a low (50 °C/kbar) gradient. In contrast to channel flow's broad petrologic and chronologic failure, classical thrust kinematic models, e.g. critical taper, can reproduce 14 of these tests, although 4 are sensitive to assumed underplating rates. In sum, there is little petrologic support for channel flow of the Greater Himalayan Sequence, but good support for critical taper, at least in central Nepal. Additional petrologic analysis elsewhere in the Himalaya is needed to determine the orogen-scale generality of these results.