Saturday, April 21, 2018

Everest Summit Limestone

Most people I talk to about geology are aware that the Himalaya formed by the buckling and uplift of crust caught up in the India-Asia collision. But, I do see eyebrows raised when I tell them that the summits of some of the highest peaks are made up of marine sedimentary rocks.

The summit of Mount Everest is a fossil bearing limestone of Ordovician age.

What happened to these sediments as they got caught up in Himalayan mountain building? A recent study published in Lithosphere has teased out the deformation and metamorphic history of this limestone.

Polyphase deformation, dynamic metamorphism, and metasomatism of Mount Everest’s summit limestone, east central Himalaya, Nepal/Tibet - Travis L. Corthouts, David R. Lageson, and Colin A. Shaw

The geologists trained Nepalese Sherpa climbers to recover samples from the Everest summit. The location of the samples and the basic geological divisions of the summit is seen in the annotated photograph posted below

 Source: Travis L. Corthouts, David R. Lageson, and Colin A. Shaw 2018

The Everest region is made up of high grade metamorphic rocks of the Greater Himalayan Sequence. These are intruded by leucogranite dikes and sills. Towards the upper levels, the grade of metamorphism decreases gradationally to upper greenschist facies. The contact between the two metamorphic grades is a shear zone termed the Lhotse Shear Zone. The greenschist faces rocks are termed the Everest Series.  On top of the Everest Series is the 'Yellow Band'. This is a coarse grained marble and calc-schist. The summit limestones (Qomolangma Formation) rests on this Yellow Band. The boundary between them is a fault zone known as the Qomolangma detachment. This fault zone is a strand of the South Tibetan Detachment (STD) that puts the Tethyan Sedimentary Sequence (TSS) on top of the Greater Himalaya Sequence throughout the extent of the Himalaya.

A schematic cross section depicting this stratigraphy is shown below.

Source: Travis L. Corthouts, David R. Lageson, and Colin A. Shaw 2018

Researchers used three types of analysis to figure out the geologic history of the limestone.

a) Microfabric analysis of the samples gave the geologists clues to the deformation and stress regime experienced by the summit limestone. The limestones have been converted into a mylonite. This means that increased temperatures and pressures from faulting resulted in a new textural arrangement in which the original calcite grains of the limestone were recrystallized and deformed. New calcite crystals grew flattened and stretched along one direction, resulting in a foliated (layered) streaky appearance to the rock. This texture forms during ductile deformation in a compressive stress regime. Geologists found that near the vicinity of the Qomolangma fault, a set of dilational fractures indicating extensional forces cut across these ductile deformation textures. This indicates that the summit limestone was subjected to tensile forces and normal faulting at a later stage.

b) Titanium content of quartz and biotite from samples close to the South Summit (EV6) indicated the temperature of metamorphism. This is so because the amount of Ti incorporated in to growing crystals of quartz and biotite increases with increase in temperature of crystallization. Results indicated that the limestones at the base of the Qomolangma Formation experienced temperatures as high as 500 deg C. 

c) The age of metamorphism was estimated by dating muscovite crystals using Ar40/Ar39 technique. Muscovite crystals grew in response to the increased temperature and pressure the limestone was subjected to during Himalayan orogeny. Dates show that there were two phases of mineral growth. The first at 28 million years ago, and a younger phase at about 18 million years ago, indicating separate events of movement and heating along the Qomolangma fault zone.

The leucogranite sills and dikes, which intrude the underlying Greater Himalaya Sequence, also merit a mention. They formed by the partial melting of the crust during Himalaya orogeny.  As this magma intruded and solidified inside the Greater Himalaya Sequence, they expelled fluids with volatile elements which permeated into the overlying limestone. This caused metasomatism and crystallization of secondary minerals in the limestone. Boron, potassium, titanium and H2O were introduced into the limestone and were incorporated into minerals like muscovite, biotite and quartz. This activity is dated to about 28 million years ago based on the age of secondary muscovite in the lower parts of the summit limestone.

The sequence of geologic events is summarized in the graphic below:

Source: Travis L. Corthouts, David R. Lageson, and Colin A. Shaw 2018

And an excerpt of the conclusions from the paper-

The different fabrics and metamorphic temperatures observed between the upper and lower parts of the Qomolangma Formation are the result of distinct events that influenced the summit limestone at different times throughout Himalayan orogenesis. Fabrics seen in summit samples are the result of Eohimalayan deformation and low-grade metamorphism associated with initial thrust faulting, folding, and crustal thickening of the Tethyan Sedimentary Sequence in the Eocene. In contrast, the fabrics and elevated temperatures preserved in South Summit samples are the result of events that occurred in the late Oligocene and early Miocene, including metasomatism associated with Neohimalayan metamorphism and normal faulting on the South Tibetan detachment. This means that several significant tectonic events in Himalayan orogenesis are preserved in the Qomolangma Formation, a succession of deformed Ordovician limestone that now comprises the top of Mount Everest.

Open Access.

Wednesday, April 11, 2018

Dating Rock Art

I love it when science is explained with a well thought out and cleanly drawn illustration.

A commentary in Nature News and Views by David G. Pearce and Adelphine Bonneau presents this diagram on dating rock art.

Two recent studies on dating cave paintings from Spain are discussed.

Who were the artists?

The oldest minimum age for the paintings is 66 ka (thousand  years) leading to speculation that they might have been drawn by Neanderthals. The earliest presence of modern humans in Spain is from  40 ka. This would imply independent evolution of symbolic behavior in Neanderthals.  However, the same painting throws up a range of minimum dates from ~ 60 ka to ~ 3 ka making an exclusive link  to Neanderthals problematic.

Friday, April 6, 2018

Crisis In Indian Palaeontology

This is incredibly sad reading.

Two recent articles highlight the utter state of disarray Indian paleontology finds itself in.

Less offical importance, low budgets, low career prestige, no legal protection for fossil sites, no local fossil repositories to store collections and no national museum with an attached well funded research program.

From the article by Sanjay Kumar in Science:

With few legal protections, sites often fall victim to looting and development. And although funds are scarce for all science in India, the plight of paleontology is particularly acute. Little money is available for excavations and for acquiring and curating specimens, and the country lacks a national institution in which its natural heritage can be studied and preserved.

All of this discourages young people from entering the field. Cash-strapped universities are curtailing or axing paleontology courses, says Ashok Sahni, of Panjab University in Chandigarh, a leading figure in Indian paleontology. Sahni, best known for his finds of dinosaur nesting sites in Jabalpur and insects trapped in amber in Vastan, in Gujarat state, says he has watched waves of colleagues retire—with few young talents stepping in to replace them. "There is no critical mass of researchers left," he says. "Indian paleontology is dying."

..and Sreelatha Menon in The Wire writes about the problems in palaeontology, and more broadly, geology education:

“In well known centres of paleontological teaching and research, such as BHU, Lucknow University, Panjab University, Jadavpur University, etc., the number of palaeontologists has gone down drastically and new, prestigious educational institutions like the IISERs are not showing much interest in hiring palaeontologists,” Prasad said (IISERs: Indian Institutes of Science Education and Research). “So the country has very few palaeontologists working on large invertebrate fossils at present.”

Pratul Saraswati, a micropalaeontologist in the department of earth sciences, IIT Bombay, thinks it’s not about people not being interested in palaeontology. “If you ask me to name some micropalaeontologists other than myself, I won’t be able to give you more than five names. If you ask Prof Sahni to name some vertebrate palaeontologists, he won’t be able to name more than three or four.”

“The problem is with geology departments as a whole across the country. Except in the IITs and central universities, just one or two faculties teach all the subjects coming under geology – and that includes palaeontology,” Saraswati said. “There is no faculty for geology across India. So it is not just palaeontology but all the subjects coming under geology that are taking a beating.”

At the IITs, every subject is taught by a specialist – which is good because, according to Saraswati, “It is difficult for a non-specialist to teach palaeontology.” But in the other universities, “One or two teaching all the subjects in geology is fine till graduation. For post-graduation and research,” that will not be enough.

A couple of weeks ago I visited the Dept. of Geology at Sinhagad College of Science in Pune. One of the faculty there is working on the sequence stratigraphy of the Cretaceous deposits of the Cauvery Basin in Tamil Nadu, South India. She mentioned that many of the famous outcrops and fossil sites are being destroyed as farmlands and small villages and towns expand. This story is repeated elsewhere across India.

That really struck me hard. During my undergraduate years I had visited that area on a field trip. I saw and collected ammonoids, echnoids, molluscs, corals and plant fossils in the field and had come back with a finer appreciation of the stratigraphic and sedimentologic context in which fossils are entombed and preserved. In retrospect, we should not have collected so many fossils. But in those days we weren't taught, and neither did we introspect, about ethical issues regarding fossil collection and outcrop integrity.

India's natural history must be given more importance. It will be a real tragedy if these localities are lost to future generations.