The structural evolution of the Qomolangma Formation, Mount Everest, Nepal

https://doi.org/10.1016/j.jsg.2020.104123Get rights and content

Highlights

  • Rocks at the summit of Mount Everest record a multi-stage deformation history.

  • The deformation record across the summit pyramid on Mount Everest varies spatially and temporally.

  • The Qomolangma detachment fault at the South Summit was active ca. 16.3 ± 5 Ma.

Abstract

Re-examination of specimens collected across the Qomolangma Formation exposed in the summit pyramid on Mount Everest confirms a complex, multi-deformational history. Two pre-Oligocene deformational events are recorded within the calcite-dominated rocks from the main summit (8848 m). The first event is characterized by an RXZ of ~1.3–1.4 in detrital quartz grains, which increases to ~1.6 after the second event. The finite strain recorded in calcite in the same rocks is higher with an RXZ of ~2.7–2.8. The deformation in the main summit appears to have been accommodated dominantly through grain boundary sliding in calcite in a slightly constrictive environment with a vorticity of >0.81. Deformation in the South Summit (8750 m) contrasts with that of the top and is instead interpreted to reflect Miocene movement along the Qomolangma detachment with no earlier deformation preserved. U–Pb geochronology on synkinematic rutile indicates that movement on the structure occurred at 16.3 ± 5.0 Ma. Rocks within the Qomolangma Formation in the summit pyramid of Mount Everest appear to record the basic history of the Himalaya with early deformation likely related to crustal thickening and later, more spatially restricted deformation reflecting the Miocene extrusion of the metamorphic core.

Introduction

The summit rocks of Mount Everest have long been the subject of geological curiosity. It is not only the location of these rocks, atop the highest point on Earth, which makes them interesting, but also their structural position within the kinematic framework of the Himalayan orogen itself. The summit pyramid of Mount Everest is host to two major tectonic features: the structurally lower Lhotse detachment and structurally higher Qomolangma detachment, which together comprise the southernmost extent of the north-dipping, top-to-the-north South Tibetan detachment system in the High Himalaya (see Kellett et al., 2018 and references therein). While the exact mechanism may still be debated, the detachment system, along with the partially coeval Main Central thrust, helped facilitate the southward lateral extrusion of the high metamorphic grade midcrustal core of the Himalaya, the Greater Himalayan sequence (Godin et al., 2006).

The Lhotse detachment (Fig. 1) is a >1 km thick, north-dipping, ductile high strain zone characterized by pervasive, top-to-the-north deformation fabrics (Carosi et al., 1998). It separates rocks of the Greater Himalayan sequence below from those of the Everest Series schist or North Col Formation above (Waters et al., 2018), with no significant break in temperature (Jessup et al., 2008). The Qomolangma detachment (Fig. 1) is a north dipping, top-down-to-the- north, brittle-ductile fault (Carosi et al., 1998) that cuts down structural section to the north. The detachment juxtaposes the Qomolangma Formation (also referred to as the Upper Everest Limestone (Wager, 1933), or the Everest Limestone (Gansser, 1964)) against the structurally lower Yellow Band and North Col Formation/Everest Series schists (Searle et al., 2003) and is characterized by temperatures that decrease from ~600 °C to ~ 350 °C structurally upward across it (Cottle et al., 2011; Law et al., 2011).

The rocks that comprise the summit of Mount Everest were originally thought to be Jurassic or Triassic based on the reported occurrence of ammonites at ~8100 m on the north side (Heron, 1922). In work that followed closely, based on new fossil information and correlations across the region, that age was revised to Permo-Triassic (Odell, 1925) and then Permo-Carboniferous (Wager, 1933; Gansser, 1964). More recently, detailed work in the Mount Everest region led to the summit rocks being classified as Lower to Middle Ordovician in age (Wang and Zhen, 1975; Yin and Guo, 1978; Yin, 1987), an interpretation again supported by subsequent regional correlations (Myrow et al., 2009).

Early observations of specimens collected from the summit pyramid of Mount Everest above the Qomolangma detachment report that the calcareous rocks of the Qomolangma Formation are fine-grained, thin-bedded calc-schists or platy limestones, which contain crinoidal fragments (Gansser, 1964). Subsequent workers (Sakai et al., 2005) further described the specimens as bedded, micritic limestone with local horizons of skeletal debris containing crinoid and brachiopod fragments and peloidal grainstone with abundant crinoid, ostracod and trilobite remains.

Section snippets

Recent research

The most recent studies to examine material from the summit of Everest report that the summit rocks are sheared limestones characterized by abundant detrital quartz grains within a ductilely deformed calcite matrix (Jessup et al., 2006; Corthouts et al., 2016). Given the general lack of primary sedimentary features, the summit rocks may be better described as calc-mylonite (see descriptions in Corthouts et al., 2016).

Corthouts et al. (2016) reported the detailed petrography of a series of

This study

While past work has provided important details about geological evolution of the Qomolangma Formation rocks in the summit pyramid of Mount Everest, questions remain about the detailed strain characteristics, timing, and potential overprinting of multiple deformation events. For example, there are potential issues around the possible use of non-rigid detrital quartz grains in the assessment of vorticity in summit specimens (Jessup et al., 2006).

In addition, Corthouts et al. (2016) attempt to

Rf/Φ analysis

Rf/Φ analysis (Ramsay, 1967) was carried out independently on quartz clasts and the calcite matrix in specimens EV1 and EV2B. Past work has demonstrated that this method may be applied to heterogeneously deformed specimens to the estimate average strain (Holm, 1983; Lisle, 1985). Quartz grain aspect ratios and long-axis orientations relative to the foliation were measured in thin sections both parallel (XZ plane) and perpendicular (YZ plane) to the macroscopic kinematic reference frame—i.e.

Deformation

The summit limestones on Everest contain sheared/transposed and folded strata and stylolites, which is consistent with either multiple deformation events or a single progressive event. The Ri(max) for the quartz grains examined in the summit rocks is ~3.5–3.8. Tunwal et al. (2018) measured the shape of 4000 quartz grains across four different depositional environments including fluvial, glacial, beach and aeolian settings. Their data show that, with the exception of a single outlier, the quartz

Conclusions

The protracted deformation history recorded over ~200 m across the Qomolangma is interpreted to span the development of the Himalaya from deformation related to initial Eocene-Oligocene crustal thickening to the mid-Miocene initiation of normal-sense faulting as part of the South Tibetan detachment system. Early thrust-related deformation is recorded by two events. The first is poorly preserved, recorded as folded stylolites and the minor deformation of detrital quartz grains. The second and

CRediT authorship contribution statement

Kyle P. Larson: Conceptualization, Methodology, Formal analysis, Writing - original draft. Riccardo Graziani: Formal analysis, Writing - review & editing. John M. Cottle: Writing - review & editing. Francisco Apen: Formal analysis, Writing - review & editing. Travis Corthouts: Writing - review & editing. David Lageson: Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by a Natural Sciences and Engineering Research Council Discovery Grant (RGPIN-2016-06736) and a Canada Foundation for Innovation Leaders Opportunity Fund award (31301) to K. Larson. N. Logan is thanked for his assistance with quartz clast measurements. This work benefited greatly from discussions with R. Law and L. Nania. Reviews by R. Soucy La Roche and an anonymous reviewer, and editorial handling by S. Laubach further improved this contribution.

References (71)

  • D.W. Schmid et al.

    Mantled porphyroclast gauges

    J. Struct. Geol.

    (2005)
  • T. Shimamoto et al.

    A simple algebraic method for strain estimation from deformed ellipsoidal objects. 1. Basic theory

    Tectonophysics

    (1976)
  • D.W. Stahr et al.

    Effect of finite strain on clast-based vorticity gauges

    J. Struct. Geol.

    (2011)
  • M. Stipp et al.

    The eastern Tonale fault zone: a “natural laboratory” for crystal plastic deformation of quartz over a temperature range from 250 to 700 °C

    J. Struct. Geol.

    (2002)
  • B. Tikoff et al.

    Simultaneous pure and simple shear: the unifying deformation matrix

    Tectonophysics

    (1993)
  • V.G. Toy et al.

    Quartz fabrics in the Alpine Fault mylonites: influence of pre-existing preferred orientations on fabric development during progressive uplift

    J. Struct. Geol.

    (2008)
  • P. Vermeesch

    IsoplotR: a free and open toolbox for geochronology

    Geoscience Frontiers

    (2018)
  • F.W. Vollmer

    Automatic contouring of geologic fabric and finite strain data on the unit hyperboloid

    Comput. Geosci.

    (2018)
  • S.R. Wallis

    Vorticity analysis and recognition of ductile extension in the Sanbagawa belt, SW Japan

    J. Struct. Geol.

    (1995)
  • T.K. Ambrose et al.

    Lateral extrusion, underplating, and out-of-sequence thrusting within the Himalayan metamorphic core, Kanchenjunga, Nepal

    Lithosphere

    (2015)
  • K.T. Ashley et al.

    Ti resetting in quartz during dynamic recrystallization: mechanisms and significance

    Am. Mineral.

    (2014)
  • F.P. Bretherton

    The motion of rigid particles in a shear flow at low Reynolds number

    J. Fluid Mech.

    (1962)
  • R. Carosi et al.

    Late Oligocene high-temperature shear zones in the core of the higher himalayan crystallines (lower dolpo, western Nepal)

    Tectonics

    (2010)
  • T.L. Corthouts

    Deformation and Metasomatism of the Qomolangma Formation: A Geochemical and Microstructural Analysis of the Summit Limestone, Mount Everest, Nepal

    (2015)
  • T.L. Corthouts et al.

    Polyphase deformation, dynamic metamorphism, and metasomatism of Mount Everest's summit limestone, east central Himalaya, Nepal/Tibet

    Lithosphere

    (2016)
  • J.M. Cottle et al.

    Metamorphic history of the South Tibetan Detachment System, Mt. Everest region, revealed by RSCM thermometry and phase equilibria modelling

    J. Metamorph. Geol.

    (2011)
  • D. Flinn

    On folding during three-dimensional progressive deformation

    Q. J. Geol. Soc. Lond.

    (1962)
  • H. Fossen et al.

    The deformation matrix for simultaneous simple shearing, pure shearing and volume change, and its application to transpression-transtension tectonics

    J. Struct. Geol.

    (1993)
  • A. Gansser

    Geology of the Himalayas

    (1964)
  • A. Gébelin et al.

    Infiltration of meteoric water in the south Tibetan detachment (Mount Everest, Himalaya): when and why?

    Tectonics

    (2017)
  • G.E. Gehrels et al.

    Geologic and U-Th-Pb geochronologic evidence for early Paleozoic tectonism in the Kathmandu thrust sheet, central Nepal Himalaya

    Geol. Soc. Am. Bull.

    (2006)
  • S. Giorgis et al.

    Kinematic and vorticity analyses of the western Idaho shear zone, USA

    Lithosphere

    (2017)
  • A.J.W. Gleadow et al.

    Fission track dating of phosphate minerals and the thermochronology of apatite

    Rev. Mineral. Geochem.

    (2002)
  • L. Godin et al.

    Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction

  • R.K. Goldhammer

    Compaction and decompaction algorithms for sedimentary carbonates

    J. Sediment. Res.

    (1997)
  • Cited by (6)

    • Multi-stage evolution of the South Tibetan Detachment System in central Himalaya: Insights from carbonate-bearing rocks

      2022, Journal of Structural Geology
      Citation Excerpt :

      Well-exposed STDS outcrops in the Everest area (Eastern Nepal) show both deep ductile shear zone and structurally upper brittle fault (Carosi et al., 1998; Searle et al., 2003). Geochronology suggests that the two shear zones were active at different times, i.e., the upper brittle fault (Qomolangma detachment) developed at c. 15–16 Ma (U–Pb geochronology on syn-kinematic rutile, Larson et al., 2020 and references therein) after the ductile shearing (Lhotse detachment) was terminated (c. 24-17 Ma, Jessup et al., 2008; Cottle et al., 2011, 2015). In several areas (including the Lower Dolpo) the STDS brittle fault does not develop and the reasons for this lack are still under discussion.

    View full text