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By Steve Self, volcanologist, and Paul Renne, geochronologist-petrologist, members of the UCB-BGC NSF-supported Deccan project team

We (Paul and Steve), along with UC Berkeley grad student Andy Tholt and Indian colleagues Kanchan Pande (Indian Institute of Technology, Bombay), Gauri Dole, (University of Pune), Makarand Bodas (Geological Survey of India), and Vivek Kale (private consultant), have been hunting for samples to give us more and more precise age dates for the lava-producing eruptions. Paul has shown that the best targets are the enigmatic giant plagioclase basalts (GPBs), which occur in almost all formations of the Deccan province in west-central India.  The large plagioclase crystals (up to 7 cm long and 1 cm thick!) are slightly more potassium-rich than the smaller regular crystals in the Deccan lavas. This improves the chances of getting more precise ages by the ⁴⁰Ar/³⁹Ar method that Paul has perfected!

In our field work this January and February 2020 in the Western Ghats or Sayhadri mountains, just before COVID-19 was declared a global pandemic, we ended up chasing three GPBs in the oldest recognized formations of the Deccan, nicknamed M1, M2, and M3 (M stands for megacryst, i.e. big crystals!).  We sampled as far under M1 as possible.  We also sampled M2, which is at the top of the Igatpuri formation, and M3 at the top of the Thakurvadi (and/or Bhimashankar) formation. 

Contrary to previous assumptions, our observations suggest that lava flows are not “all or nothing” when it comes to GPBs. We found that the GPB is often a zone within a less-GPB-rich lava.  Most of the GPBs we saw are clearly not whole lava flows; they are parts of lava flow fields in which some lobes (a lava lobe is a finger of lava that oozes out from a flow and may then merge with other fingers from the same flow) have GPB-characteristics and others do not.  This was seen in M1, M2, and M3. 

In 2019, we observed that the GPBs seem to be injections into extensive, thick, flat lobes of lava. Our latest observations still support this, although we did see some thinner lobes rich in GPs. The details of this (e.g., % of big crystals in smaller lobes vs. those in thick-lobe-hosted GPBs) deserve a detailed study.  One thing we have wondered about is, how the heck do these things, which are so chock-full of crystals, flow? We looked at some basalts where the crystal content is about 65% of the volume of the rock! Now, we think the answer might be that they didn’t start out quite so packed with crystals.  Maybe the crystals were injected into a lobe of lava at < 50 % concentration, that liquid then stagnated inside the lobe, and the crystals floated to the top of the injection, making something like we see in the Thalghat GPB (M1) on Thal Ghat itself (see photo below). Anyway, it’s certain that GPBs are quite complex beasts and deserve a lot more attention and mapping out across the Deccan province!

We learned a lot about GPBs on our last road trip, but it wasn’t entirely crystal hunting.  Along the way, we appreciated the natural and cultural beauty of west-central India, the geology of which holds so many clues to Earth’s deep history.

By Dr Loÿc Vanderkluysen, Drexel University

A deadly earthquake

The small town of Koynanagar, nestled among the steep hills of the Western Ghats range of India, attracts visitors from miles around with its lush botanical gardens and waterfall vistas. Shortly before sunrise on the 11^th of December 1967, farmers readied their oxcarts, and at the market, fires were lit to prepare the day’s first cup of chai. Then the ground started to shake. A crack 15 miles long split the ground open as shelves toppled and buildings collapsed. Within a few minutes, the small hill town had been leveled: 80% of buildings had collapsed, killing more than 170 people and injuring thousands.

Geologists have puzzled over the events of that 1967 Monday morning for the past five decades. There are no known major faults in the area and, prior to the 1960s, seismicity was practically non-existent. Below the surface, the ground is made up of basaltic lava flows from the Deccan Traps sequence, directly overlying a thick basement of gneisses and granites; in other words, a stable continental interior far from the destructive tectonic stresses that characterize earthquake-prone regions around the world. What could have caused a magnitude 6.6 quake?

More puzzling still: in the last fifty years, many more earthquakes have struck the area, including 22 of magnitude greater than 5.0, and hundreds of magnitude greater than 4.0. The sharp rise in seismicity following the completion of the nearby Koyna dam and filling of the Koyna reservoir in 1962 offered an easy culprit, and quickly the dam was blamed for the region’s woes. Though contentious in the 1960s, a large body of literature now exists documenting so-called “induced” seismicity, that is, generally minor earthquakes triggered by human activity, including mining, groundwater pumping, and fracking. Reservoir-induced seismicity, known since the 1930s, can be triggered by the accumulation of water in dammed lakes, with the added load of the water column causing changes to local stresses. If caused by the Koyna dam, the magnitude 6.6 Koynanagar earthquake would be the largest reservoir-induced quake ever recorded.

In 1993, the completion of another dam and impoundment of the Warna reservoir just south of Koyna triggered another spike in seismicity, and renewed interest in the mechanisms of the 1967 quake. The Indian government and partner institutions initiated an ambitious project to study the source of seismicity in the Koyna region by installing instruments deep within in the crust, at the bottom of dedicated boreholes.

The case for scientific drilling

Rock drilling has been part of human history for more than 4000 years, primarily for the purpose of reaching sources of drinkable water. Modern drilling tools were developed in the mid-1700s as the first oil-exaction wells were established. Because drilling can access rocks at depths or in environments that humans cannot reach, it has become an invaluable tool for the scientific exploration of the Earth’s crust, and even other bodies in the solar system: in 1971, the crew of the Apollo 15 mission collected a core of the lunar crust almost 8 feet in length. The deepest hole ever drilled is the Kola Superdeep Borehole in northwestern Russia. A point of Cold War pride, it reached a depth of over 12 km (40,230 ft).

Instruments placed deep in boreholes have provided invaluable information about the flow of groundwater and the Earth’s heat flow. More recently, geoscientists have begun exploring the possibility of drilling through some of the Earth’s most active faults. Although this likely evokes Hollywood-level nightmare scenarios, in reality there is much to learn from instrumentation placed near active faults. Measurements of built up stress and deformation, for instance, can help us understand and mitigate earthquake hazards. Started in 2004, the San Andreas Fault Observatory at Depth (SAFOD) pioneered borehole fault observatories, and more have now been installed at some of Earth’s most destructive fault systems, such as the North Anatolian fault in Turkey (source of the devastating 1999 Izmit earthquake). Building on the success of these deep borehole observatories, the Koyna drilling program was initiated with the objective of unraveling the source and mechanisms of the Koyna-Warna earthquakes.

The Koyna drill cores and the Deccan Traps

Perhaps coincidentally, the Koyna program happened to drill through hundreds of meters of lava flows from the Deccan Traps volcanic province. Although the stated aim of the project was to install deep borehole instrumentation to understand and monitor local seismicity, project scientists were well aware of the potential goldmine a drilling project through the massive volcanic lava pile could provide for understanding events at a keystone moment in Earth’s history: the end-Cretaceous mass extinction that saw the demise of non-avian dinosaurs and a wealth of animal and plant species across the world’s continents and oceans.

At first, scientific drilling through massive volcanic provinces like the Deccan Traps can seem like a counterproductive enterprise: drill holes are very small and very few in number compared to the vastness of these provinces, so much so that the venture has occasionally been compared to “pin-pricking an elephant.” But drill cores do provide advantages over surface outcrops: they represent a continuous record of the volcano’s activity without interruptions by roads or vegetation, and can reach depths below sea level that would not be otherwise accessible. Of the nine boreholes drilled for the Koyna project, all exceeded 900 m (2,950 ft) and four reached 1,500 m (4,920 ft), the most continuous sections ever observed across the province, from the contact with the underlying granitic rocks to the summit of the lava pile.

With participation from the International Continental Drilling Program (an organization dedicated to continental drilling to answer scientific questions that would not easily be tackled without drilling), the Indian government has now made the Koyna cores available for study by our team as well as other international scientists. There is little doubt that the end-Cretaceous boundary is located somewhere in the lava flow sequence of the Koyna cores, and we’re thrilled to have access to such an amazing geological record of volcanism at that time. Having access to the Koyna cores opens tantalizing opportunities to test hypothesized links between the Chicxulub impact and a rapid acceleration of volcanism in the Deccan!

References

Gupta, H.K., Arora, K., Rao, N.P., Roy, S., Tiwari, V.M., et al. 2017. Investigations of continued reservoir triggered seismicity at Koyna, India. In: Mukherjee, S., Misra, A.A., Calvès, G. & Nemčok, M. (eds) Tectonics of the Deccan Large Igneous Province. Geological Society, London, Special Publications 445, pp. 151-188.

 

by Joseph Byrnes, Postdoctoral researcher at the University of Minnesota

Leif Karlstrom and I are happy to have our paper on the triggering of mid-ocean ridges published in Science Advances. We found a concentration of gravity anomalies (i.e., regions with a very slightly higher force of gravity caused by extra mass at or below Earth’s surface) on K-Pg aged seafloor in the Indian and Pacific Oceans. The simplest explanation is the eruption of ~200 m of excess basalt approximately 66 million years ago. Extending this value globally, the volume of the excess erupted material is at least several hundred thousands cubic kilometers, and could be as large as several million cubic kilometers.

Based on the age of seafloor where we observe the anomaly and the lack of a similar episode in the last 100 Myr, we think the best explanation is the triggering of magmatism by the Chicxulub impact. Seismic waves from the impact appear capable of liberating the melt stored at mantle depths beneath mid-ocean ridges. This is similar to the proposed triggering of the largest eruptions in the Deccan Traps, and represents yet another line of evidence supporting the idea that the Chicxulub meteor impact dramatically affected not just life on Earth, but Earth’s geological activity as well.

Take a look at the abstract for details.