Protolith affiliation and tectonometamorphic evolution of the Gurla Mandhata core complex, NW Nepal Himalaya

Abstract

Assigning correct protolith to high metamorphic-grade core zone rocks of large hot orogens is a particularly important challenge to overcome when attempting to constrain the early stages of orogenic evolution and paleogeography of lithotectonic units from these orogens. The Gurla Mandhata core complex in NW Nepal exposes the Himalayan metamorphic core (HMC), a sequence of high metamorphic-grade gneiss, migmatite, and granite, in the hinterland of the Himalayan orogen. Sm-Nd isotopic analyses indicate that the HMC comprises Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS) rocks. Conventional interpretation of such provenance data would require the Main Central thrust (MCT) to be also outcropping within the core complex. However, new in situ U-Th/Pb monazite petrochronology coupled with petrographic, structural, and microstructural observations reveal that the core complex is composed solely of rocks in the hanging wall of the MCT. Rocks from the core complex record Eocene and late Oligocene to early Miocene monazite (re-)crystallization periods (monazite age peaks of 40 Ma, 25–19 Ma, and 19–16 Ma) overprinting pre- Himalayan Ordovician Bhimphedian metamorphism and magmatism (ca. 470 Ma). The combination of Sm-Nd isotopic analysis and U-Th/ Pb monazite petrochronology demonstrates that both GHS and LHS protolith rocks were captured in the hanging wall of the MCT and experienced Cenozoic Himalayan metamorphism during south-directed extrusion. Monazite ages do not record metamorphism coeval with late Miocene extensional core complex exhumation, suggesting that peak metamorphism and generation of anatectic melt in the core complex had ceased prior to the onset of orogen-parallel hinterland extension at ca. 15–13 Ma. The geometry of the Gurla Mandhata core complex requires significant hinterland crustal thickening prior to 16 Ma, which is attributed to ductile HMC thickening and footwall accretion of LHS protolith associated with a Main Himalayan thrust ramp below the core complex. We demonstrate that isotopic signatures such as Sm-Nd should be used to characterize rock units and structures across the Himalaya only in conjunction with supporting petrochronological and structural data.

Megathrust Heterogeneity, Crustal Accretion, and a Topographic Embayment in the Western Nepal Himalaya: Insights From the Inversion of Thermochronological Data

“…Many previous thermokinematic models that highlight the role of crustal accretion in other Himalayan regions correlate the accretion in their models with the LHS duplex in the footwall of the MCT. Unlike these models, our models of the western Nepal Himalaya suggest the midlower crustal accretion causes the thickening of the northern Himalayan anticline (Figure 9), where high‐grade metamorphic rocks with protolith from both the GHS and LHS are exposed (Godin et al., 2021; Murphy, 2007). Modified from the original critical taper model that considers only friction material (Dahlen, 1990; Dahlen et al., 1984; Davis et al., 1983), a brittle‐ductile taper model has predicted that a wedge with brittle‐ductile transition in both the wedge and decollement parts can maintain a high‐slope zone connecting a taper‐like outer wedge and a plateau‐like inner wedge at a critical state (Williams et al., 1994).…”

“…These discontinuities approximately mimic the PT2 on the map and usually correspond to the areas on the foreland limb of the antiformal crustal accretion in the thermokinematic models discussed above, indicating midlower crustal strain accumulation. Underplating of the material from the Indian plate to the upper plate is supported by isotopic signatures of the high‐grade crystalline rocks in the Gurla Mandhata area along the Northern Himalayan antiform (Godin et al., 2021; Murphy, 2007). Direct observation of a crustal‐scale duplex structure on seismic‐reflection profiles across the Yarlung‐Zangbo suture was documented (Gao et al., 2016; Guo et al., 2018) or reinterpreted (Laskowski et al., 2018).…”

Unravelling the evolution of regional-scale shear zones with the aid of microstructures and textural analysis: an example from the central Himalaya

“…It is not unambiguously defined if the MCTz sheared both the LHS and GHS L or represents just a tectonic boundary between them (Godin et al, 2021 with references). Moreover, a clear break between the LHS and the GHS L is very rare along the belt, and this feature makes even more difficult to define, for example, the GHS and the location of the shear zone (Searle et al, 2008;Godin et al, 2021).…”