Metamorphic history of the central Himalaya, Annapurna region, Nepal, and implications for tectonic models

Abstract

Pressure-temperature-time (P-T-t) conditions of metamorphism have been determined in the Annapurna region of central Nepal that place new constraints on the structural and tectonic evolution of the Himalayan orogenic wedge. Peak P-T conditions increase structurally upward: ~525 °C and 8 kbar in the Lesser Himalayan sequence, 650 °C and 12 kbar at the base of the Greater Hima layan sequence across the Main Central thrust, 750 °C and 12 kbar in the middle of the Greater Himalayan sequence, and 775 °C and 13 kbar near the top of the Greater Hima layan sequence. Metamorphic monazite ages in the Greater Hima layan sequence also increase structurally upward: 16-21 Ma for sub solidus growth at the base of the Greater Himalayan sequence to ~25 Ma for peak-T metamorphism and anatexis near the top of the Greater Hima layan sequence. These ages are several million years older than at equivalent structural levels at Langtang, ~200 km to the east. The P-T-t data recommend reinterpretation of the Bhanuwa fault within the Greater Hima layan sequence as a thrust, and the presence of a different thrust structurally above the Bhanuwa thrust, here named the Sinuwa thrust. The new data are consistent with progressive stacking of tectonic slices, with calculated overthrust rates that are consistent with some (but not all) models that presume ~2 cm/yr convergence across the Himalaya since 25 Ma. Despite differences in absolute ages, similarities among the chemical systematics of monazite, peak P-T conditions, and overthrust rates calculated for Annapurna when compared to Langtang imply that the broad geodynamics in one part of an orogen can be realistically extrapolated within a few hundred kilometers, although the timing and duration of movement on discrete thrust surfaces may differ.

Pressure–temperature–time–deformation path of kyanite-bearing migmatitic paragneiss in the Kali Gandaki valley (Central Nepal): Investigation of Late Eocene–Early Oligocene melting processes

“…Carosi et al, 2007Carosi et al, , 2010Corrie and Kohn, 2011;Imayama et al, 2012;Larson et al, 2013;Montomoli et al, 2013;see Montomoli et al, 2014 for a review) have shown that the GHS has a much more complex crustal architecture compared to simple models where a single coherent tectonic unit is bounded by only two tectonic discontinuities with opposite sense of shear. These recent findings are also compatible with diachronic melting within the GHS (Corrie and Kohn, 2011;Imayama et al, 2012;Kohn et al, 2005;Rubatto et al, 2013). As stated above, our samples are localized c. 1 km north of the MCT location according to Colchen et al (1986) and Vannay and Hodges (1996), recently mapped as CT by Parsons et al (2014), whereas the MCT has been shifted more than 10 km to the south according to Searle (2010).…”

Major role of shear heating in intracontinental inverted metamorphism: Inference from a thermo-kinematic parametric study

“…Both the high metamorphic peak temperatures (500-700˚C at 8-11 kbar; e.g., Guillot, 1999;Pitra et al, 2010;Corrie and Kohn, 2011) and the thermal inversion intensities (generally comprised between 10 and 50˚C.km -1 ; e.g., Kohn, 2008;Pitra et al, 2010) can be reproduced.…”

“…(e.g., Bilham et al, 1997;Lavé and Avouac, 2000). High metamorphic temperature peaks within the MCT reach 600 to 650˚C and pressure peaks are of 8 to 11 kbar (e.g., Macfarlane, 1995;Guillot, 1999;Corrie and Kohn, 2011), corresponding to depths of about 30-40 km (assuming that these pressures correspond to lithostatic pressures). In agreement with the kinematical constraints, Fig.…”

“…The pressure and temperature conditions within such thrust zones attest to a medium pressure, medium-high temperature metamorphism. As an example, inverted metamorphic sequences in both the Himalayas and the Variscan belt have recorded peak temperatures between 500 and 700˚C and peak pressures between 8 and 11 kbar (e.g., Burg et al, 1984;Macfarlane, 1995;Guillot, 1999;Kohn, 2008;Pitra et al, 2010;Corrie and Kohn, 2011). Natural intracontinental metamorphic inversions are thus characterised by inverted thermal gradients between 10 and 50˚C.km -1 (e.g., Kohn, 2008;Pitra et al, 2010).…”

Tectonics of the Himalayas

“…; Imayama et al, 2012 , Corrie andKohn, 2011;, Wang et al, 2013;, Regis et al, 2014 High Himalayan Discontinuity; Figs. 4a, 5a -MCT MCT Kohn et al, 2001;Imayama et al, 2010MCT -Harris et al, 2004Imayama et al, 2010Imayama et al, , 2012 Khaghn valley Nanga Parbat syntaxis Tso Morari Fig Pressure-temperature (P-T) diagram with equilibrium curves for fluid-saturated melting and mica dehydration reactions.…”