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Pressure-temperature-time path discontinuity in the Main Central thrust zone, central Nepal

¹ Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA
² Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA
³ Institute for Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, California 94550, USA
⁴ Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA

Metapelites collected in central Nepal reveal a discontinuity in metamorphic pressure- temperature-time (P-T-t) paths near the base of the Main Central thrust zone, despite an absence of obvious structural breaks. Garnets in the structurally lowest rocks grew with increasing T and P (loading), whereas garnets 1–3 km upsection grew with increasing T, but decreasing P (exhumation). Monazite grains in structurally lower rocks yield ion- microprobe Th-Pb ages of 8–9 Ma. Structurally higher monazite grains range from 10 to 22 Ma. The P-T-t paths confirm previous interpretations that footwall metamorphism in part resulted from thrust reactivation ca. 8 Ma, but also reflect thermal relaxation following older (20 Ma or older) thrust movement. The Main Central thrust zone formed during pulses of movement that resulted in progressive transfer of material from the lower to upper plate.

Key Words: Himalaya • P-T-t paths • geothermometry • geobarometry • geochronology

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