Geology

Taper or Channel?

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Science  25 Apr 2008:
Vol. 320, Issue 5875, pp. 426
DOI: 10.1126/science.320.5875.426a

Near the base of the Himalayas, a major fault system (the Main Central Thrust and related faults) is exposed that accommodated the collision and subduction of India into and beneath Asia around 50 million years ago. The metamorphic minerals there record the pressure and temperature conditions before and during the displacement along the thrust; dating of minerals and recognition of zoning or overgrowths provide a time sequence that in turn can be used to infer how the collision and thrusting occurred. Two major models have been proposed, one involving formation and maintenance of a taper extending back from the thrust fault, and the other extrusion through a lubricating channel formed from hot weak crust along the fault. Kohn has synthesized the metamorphic data in rocks near the Main Central Thrust to test these models. Just below the fault, rocks were heated to about 550°C at depths of about 25 km, whereas above it the rocks were heated to about 725°C at depths of ∼35 km. The data support the first model over the second, suggesting that the active fault progressively deepened as erosion stripped material from the top of the Himalayan taper. — BH

Geol. Soc. Am. Bull. 120, 259 (2008).

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