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

• Published in: Geology (Boulder) 2001-07, Vol.29 (7), p.571-574
• Main Authors: Kohn, Matthew J, Catlos, Elizabeth J, Ryerson, Frederick J, Harrison, T. Mark
• Format: Article
• Language: English
• Subjects: absolute age; Asia; Cenozoic; central Nepal; Darondi River valley; dates; faults; garnet group; geologic barometry; geologic thermometry; Geological time; Geology; igneous and metamorphic rocks; Indian Peninsula; Indian Plate; ion probe data; Main Central Thrust; mass spectra; metamorphic rocks; metamorphism; metapelite; metasedimentary rocks; Miocene; monazite; Neogene; Nepal; nesosilicates; orthosilicates; P-T conditions; P-T-t paths; Pb/Th; Petrology; phosphates; plate tectonics; Pressure; reactivation; relaxation; silicates; spectra; Structural geology; Temperature; Tertiary; thrust faults
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/0091-7613(2001)029<0571:PTTPDI>2.0.CO;2

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.