Middle to late Eocene exhumation of the Greater Himalayan Sequence in the Central Himalayas: Progressive accretion from the Indian plate

The Greater Himalayan Sequence (GHS) has commonly been treated as a large coherently deforming high‐grade tectonic package, exhumed primarily by simultaneous thrust‐ and normal‐sense shearing on its bounding structures and erosion along its frontal exposure. A new paradigm, developed over the past decade, suggests that the GHS is not a single high‐grade lithotectonic unit, but consists of in‐sequence thrust sheets. In this study, we examine this concept in central Nepal by integrating temperature–time (T–t) paths, based on coupled Zr‐in‐titanite thermometry and U–Pb geochronology for upper GHS calcsilicates, with traditional thermobarometry, textural relationships and field mapping. Peak Zr‐in‐titanite temperatures are 760–850°C at 10–13 kbar, and U–Pb ages of titanite range from c. 30 to c. 15 Ma. Sector zoning of Zr and distribution of U–Pb ages within titanite suggest that diffusion rates of Zr and Pb are slower than experimentally determined rates, and these systems remain unaffected into the lower granulite facies. Two types of T–t paths occur across the Chame Shear Zone (CSZ). Between c. 25 and 17–16 Ma, hangingwall rocks cool at rates of 1–10°C/Ma, while footwall rocks heat at rates of 1–10°C/Ma. Over the same interval, temperatures increase structurally upwards through the hangingwall, but by 17–16 Ma temperatures converge. In contrast, temperatures decrease upwards in footwall rocks at all times. While the footwall is interpreted as an intact, structurally upright section, the thermometric inversion within the hangingwall suggests thrusting of hotter rocks over colder from c. 25 to c. 17–16 Ma. Retrograde hydration that is restricted to the hangingwall, and a lithological repetition of orthogneiss are consistent with thrust‐sense shear on the CSZ. The CSZ is structurally higher than previously identified intra‐GHS thrusts in central Nepal, and thrusting duration was 3–6 Ma longer than proposed for other intra‐GHS thrusts in this region. Cooling rates for both the hangingwall and footwall of the CSZ are comparable to or faster than rates for other intra‐GHS thrust sheets in Nepal. The overlap in high‐T titanite U–Pb ages and previously published muscovite 40Ar/39Ar cooling ages imply cooling rates for the hangingwall of ≥200°C/Ma after thrusting. Causes of rapid cooling include passive exhumation driven by a combination of duplexing in the Lesser Himalayan Sequence, and juxtaposition of cooler rocks on top of the GHS by the STDS. Normal‐sense displacement does not appear to affect T–t paths for rocks immediately below the STDS prior to 17–16 Ma.

Section: Discussionmentioning

confidence: 50%

Section: Discussionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

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Protracted thrusting followed by late rapid cooling of the Greater Himalayan Sequence, Annapurna Himalaya, Central Nepal: Insights from titanite petrochronology

Walters

Kohn

2017

“…According to Carosi, Montomoli, Rubatto, and Visonà (), Carosi et al. (), and Imayama et al. (), U–Th–Pb in situ LA‐ICP‐MS and SHRIMP dating on monazite point to activation of major ductile shear zones related to stacking of nappes within the HHC in the time interval 41–28 Ma.…”

Section: Discussionmentioning

confidence: 99%

Two Tertiary metamorphic events recognized in high‐pressure metapelites of the Nevado‐Filábride Complex (Betic Cordillera, S Spain)

Massonne

2018

Self Cite

Two phengite–garnet‐bearing metapelites from the Nevado‐Filábride Complex, Betic Cordillera, one from the Ragua unit (sample 23085) and the other (sample 23098) from the Calar Alto unit, were studied in detail to elucidate their metamorphic evolution. We calculated various P–T pseudosections for different O2 and H2O‐CO2 contents and Fe3+/Fe2+ ratios with PERPLE_X . On the basis of the compositions of the garnet core and the highest Si content in potassic white mica, different peak pressures of 12.7 ± 0.7 kbar at 525 ± 10°C (23085) and 17.4 ± 1.2 kbar at 526 ± 17°C (23098) resulted (errors refer to an estimated 1σ range). The suggested clockwise P–T loop was followed in both cases by another loop caused by a reheating process. On the basis of garnet rim compositions, peak temperatures of 596 ± 10°C at 10.7 ± 1.4 kbar (23085) and 629 ± 11°C at 8.5 ± 0.7 kbar (23098) resulted, supported by Zr‐in‐rutile thermometry (23085: 591 ± 11°C; 23098: 607 ± 10°C). In situ dating of monazite (23085) with the electron microprobe yielded ages between c. 48 and 14 Ma. On the basis of the monazite composition and histogram analysis, two age populations with mean ages of 40.2 ± 1.7 (1σ) and 24.1 ± 0.8 Ma could be defined. Low Y2O3 contents (<0.6 wt%) in monazite point to its formation after initial growth of garnet. The older generation was therefore assigned to the formation of the garnet core and the early P–T loop. The younger monazite population was related to the late P–T loop. A geodynamic scenario is hypothesized that assigns the Eocene metamorphism to subduction of a continental margin to depths of 50–65 km, followed by exhumation in an exhumation channel accompanied by mixing of slices of different rock types including mantle rocks. This process ended with stacking of such slices causing reheating of the studied rocks in the Early Miocene. The noted time gap of c. 15 Ma between subduction and nappe stacking is similar to the continent–continent collisional orogeny of the Himalaya.

“…These strike‐parallel discontinuities, identified at various structural levels within the HMC (e.g. Ambrose et al., ; Carosi, Montomoli, Rubatto, & Visonà, ; Carosi et al., ; Corrie & Kohn, ; Imayama et al., ; Iaccarino et al., ; Larson & Cottle, ; Larson et al., ; Martin, Ganguly, & DeCelles, ; Montomoli et al., ; Rubatto, Chakraborty, & Dasgupta, ; Wang, Rubatto, & Zhang, ; Wang, Zhang, et al., ; Wang et al., ; Warren et al., ; Yakymchuk & Godin, and others), rarely have an associated field expression (Larson & Cottle, ; Warren et al., ) and are typically recognized on the basis of abrupt breaks in P–T–t paths of adjacent rock packages (Ambrose et al., ; Larson et al., ; Rubatto et al., ). Whilst most of these discontinuities have been investigated based on a single type of analysis or the combination of few analytical methods (see Larson et al., and references therein), which range from field‐based structural identification to laboratory‐based techniques such as thermobarometry (e.g.…”

Section: Introductionmentioning

confidence: 99%

The P–T–t evolution of the exhumed Himalayan metamorphic core in the Likhu Khola region, East Central Nepal

Shrestha

Larson

Guilmette

et al. 2017

The recent identification of multiple strike‐parallel discontinuities within the exhumed Himalayan metamorphic core has helped revise the understanding of convergence accommodation processes within the former mid‐crust exposed in the Himalaya. Whilst the significance of these discontinuities to the overall development of the mountain belt is still being investigated, their identification and characterization has become important for potential correlations across regions, and for constraining the kinematic framework of the mid‐crust. The result of new phase equilibria modelling, trace element analysis and high‐precision Lu–Hf garnet dating of the metapelites from the Likhu Khola region in east central Nepal, combined with the previously published monazite petrochronology data confirms the presence of one of such cryptic thrust‐sense tectonometamorphic discontinuities within the lower portion of the exhumed metamorphic core and provides new constraints on the P–T estimates for that region. The location of the discontinuity is marked by an abrupt change in the nature of P–T–t paths of the rocks across it. The rocks in the footwall are characterized by a prograde burial P–T path with peak metamorphic conditions of ~660°C and ~9.5 kbar likely in the mid‐to‐late Miocene, which are overlain by the hanging wall rocks, that preserve retrograde P–T paths with P–T conditions of >700°C and ~7 kbar in the early Miocene. The occurrence of this thrust‐sense structure that separates rock units with unique metamorphic histories is compatible with orogenic models that identify a spatial and temporal transition from early midcrustal deformation and metamorphism in the deeper hinterland to later deformation and metamorphism towards the shallower foreland of the orogen. Moreover, these observations are comparable with those made across other discontinuities at similar structural levels along the Himalaya, confirming their importance as important orogen‐scale structures.