Channel flow, tectonic overpressure, and exhumation of high-pressure rocks in the Greater Himalayas

Fernando O. Marques, Nibir Mandal, Subhajit Ghosh, Giorgio Ranalli, Santanu Bose
2018 Solid Earth Discussions  
<p><strong>Abstract.</strong> The Himalayas are the archetype of continental collision, where a number of long-standing fundamental problems persist in the Greater Himalayan Sequence (GHS): (1) contemporaneous reverse and normal faulting; (2) inversion of metamorphic grade; (3) origin of high- (HP) and ultra-high (UHP) pressure rocks; (4) mode of ductile extrusion and exhumation of HP and UHP rocks close to the GHS hanging wall; (5) flow kinematics in the subduction channel; and (6) tectonic
more » ... and (6) tectonic overpressure, here defined as <i>TOP</i><span class="thinspace"></span>=<span class="thinspace"></span><i>P/P<sub>L</sub></i> where <i>P</i> is total (dynamic) pressure and <i>P<sub>L</sub></i> is lithostatic pressure. In this study we couple Himalayan geodynamics to numerical simulations to show how one single model, upward-tapering channel (UTC) flow, can be used to find a unified explanation for the evidence. The UTC simulates a flat-ramp geometry of the main underthrust faults, as proposed for many sections across the Himalayan continental subduction. Based on the current knowledge of the Himalayan subduction channel geometry and geological/geophysical data, the simulations predict that a UTC can be responsible for high <i>TOP</i> (><span class="thinspace"></span>2). <i>TOP</i> increases exponentially with decrease in UTC's mouth width, and with increase in underthrusting velocity and channel viscosity. The highest overpressure occurs at depths <<span class="thinspace"></span>&amp;minus;60<span class="thinspace"></span>km, which, combined with the flow configuration in the UTC, forces HP and UHP rocks to exhume along the channel's hanging wall, as in the Himalayas. By matching the computed velocities and pressures with geological data, we constrain the GHS's viscosity to be ≤<span class="thinspace"></span>10<sup>21</sup><span class="thinspace"></span>Pa<span class="thinspace"></span>s, and the effective convergence (transpression) to a value ≤<span class="thinspace"></span>10<span class="thinspace"></span>%. Variations in channel dip over time (> or <<span class="thinspace"></span>15º) may promote or inhibit exhumation, respectively. Viscous deformable walls do not affect overpressure significantly for a viscosity contrast (viscosity walls/viscosity channel) in the order of 1000 or 100. <i>TOP</i> in a UTC, however, is only possible if the condition at the bottom boundary is no outlet pressure; otherwise it behaves as a leaking boundary that cannot retain dynamic pressure. However, the cold, thick and strong lithospheres forming the Indian and Eurasian plates are a good argument against a leaking bottom boundary in a flat-ramp geometry, and therefore it is possible for overpressure to reach high values in the GHS.</p>
doi:10.5194/se-2018-37 fatcat:la3pmt2cxvaqflfbys4ybyfpz4