Tectonometamorphic discontinuities within the Greater Himalayan Sequence in Western Nepal (Central Himalaya): Insights on the exhumation of crystalline rocks

Abstract

The core of the Greater Himalayan Sequence in the Mugu-Karnali area (Western Nepal) is affected by a thick shear zone with development of nearly 4 km of mylonites (Mangri shear zone). It is a contractional shear zone showing a top-to-the-SW and WSW sense of shear. The shear zone developed during the decompression, in the sillimanite stability field, of rocks that previously underwent relatively high-pressure metamorphism deformed under the kyanite stability field. P-T conditions indicate that the footwall experienced higher pressure (1.0-0.9 GPa) than the hanging wall (0.7 GPa) and similar temperatures (675°-700 °C). U-Pb in-situ dating of monazites indicate a continuous activity of the shear zone between 25 and 18 Ma. Samples from the lower part of the Greater Himalayan Sequence underwent similar ductile shearing at ~ 17-13 Ma. These ages and the associated P-T-t paths revealed that peak metamorphic conditions were reached ~ 5-7 Ma later in the footwall of the shear zone with respect to the hanging-wall pointing to a diachroneity in the metamorphism triggered by the shear zone itself. Mangri Shear Zone, with the other recently documented tectonic and metamorphic discontinuities within the Greater Himalayan Sequence, point to the occurrence of a regional tectonic feature, the High Himalayan Discontinuity, running for more than 500 km along the strike of the Central Himalayas. It was responsible of the exhumation of the upper part of the Greater Himalayan Sequence starting from 28 Ma, well before the activation of the Main Central Thrust and the South Tibetan Detachment. Our data point out that exhumation of the Greater Himalayan Sequence was partitioned in space and time and different slices were exhumed in different times, starting from the older in the upper part to the younger in the lower one

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

“…13). This study testifies for the first time the occurrence of a structural and tectonic discontinuity within the GHS in the Kali Gandaki valley and confirms its regional extent (HHD; Montomoli et al, 2013Montomoli et al, , 2014 and the occurrence of diachronous exhumation of the two portions of the GHS divided by the HHD. Exhumation in the GHS was not triggered only by the MCT but started, before it was formed, in the upper part of the GHS.…”

“…The presented Mnz III ages are older than the quoted ages for the MCT activity in the Kali Gandaki (c. 21-16 Ma in Gibson et al, 2014;c. 22 Ma in Godin et al, 2006) and share similarities with intra GHS in sequence contractional shear zones, like the High Himalayan Discontinuity (HHD) of Montomoli et al (2013Montomoli et al ( , 2014. The kinematics, the P-T-t path and the age of the studied sample testify, for the first time, the occurrence of the HHD in the Kali Gandaki valley (Fig.…”

“…c. 22-15 Ma in Larson et al, 2013;17-13 Ma in Montomoli et al, 2013). This means that kyanite in the MCT zone grew at different times.…”

“…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).…”

Metamorphism and geochronology of the exhumed Himalayan midcrust, Likhu Khola region, east-central Nepal: Recognition of a tectonometamorphic discontinuity

“…The evolution of the midcrustal core has been one of the most contentious issues in recent geo-logic research into the Himalayan orogen (e.g., Law et al, 2006, and references therein;Kohn, 2008;Cottle et al, 2009a;Larson et al, 2010;Carosi et al, 2010;Streule et al, 2010;Corrie and Kohn, 2011;Yakymchuk and Godin, 2012;Corrie et al, 2012;Montomoli et al, 2013;Larson and Cottle, 2014). There has been much debate over which of the "end-member" models proposed for the evolution of the Himalaya, commonly considered to be wedge-taper processes or channel flow, best describes or explains available geologic data (see Beaumont and Jamieson, 2010;Larson et al, 2013).…”