The structural evolution of the Qomolangma Formation, Mount Everest, Nepal
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.
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2022, Journal of Structural GeologyCitation 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.
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