Eclogites in Bhutan with Clare Warren

Bhutan: Elusive eclogites and their importance in Himalayan mountain-building
 
Clare is a senior research fellow at the Open University, UK and holds a NERC Advanced Research Fellowship.  Her research interests focus on the metamorphic and temporal evolution of plate collision zones: from subduction zone eclogites to melting during mountain-building episodes.  She her research profile here.

My research interests in the Himalaya revolve around trying to determine how how, and how quickly Indian crust got buried under Tibet during the on-going India-Asia collision, metamorphosed, deformed, melted and then transported back to the surface.  Equally, how much of the India-Asia collision was accommodated by deformation in the Himalayas, and has that relative proportion changed over time?

Tom Argles inspecting the Laya Thrust, Bhutan

 
Bhutan is one of two independent Himalayan Kingdoms, bordered on the east, south and west by India and to the North by Tibet (China).   It is geologically interesting because the rocks exposed there appear to record a change in metamorphic evolution compared to similar rocks in Nepal, further west. 
 
My first foray to Bhutan, in 2006, came about because of a chance conversation on a wet and windy February walk with Djordje Grujic, a professor at Dalhousie University in Canada.  He invited me to join him on an expedition along the “Paro-Jhomolari-Laya-Gasa trek’ in Bhutan, crossing three near-5000m passes over 12 days. 
Clare at the top of a 5000m pass, Bhutan

At the base of a glacial moraine, NW Bhutan

We were on the hunt for rare and elusive high-pressure rocks called eclogites, which had previously been reported from NW Bhutan.  These rocks were unusual given what was previously known about the metamorphic evolution of the eastern Himalayas.  In particular we wanted to find out how old these rocks were and how they had exhumed to the surface after forming.  Did they represent metamorphism in the lower Himalayan crust?  Or remnants of the subduction zone that existed before India and Asia collided?

 
We found that these rocks must have formed under eclogite-facies conditions (P >1.4 GPa; T >550°C), at around 15 million years ago.  They then suddenly got very hot, (~850°C) and started recrystallizing under granulite-facies conditions at about 13 million years ago, before being exhumed to the surface by about 8 million years ago.  

Retrogressed granulite boudin, Bhutan

It is still unclear how and why these rocks got transported back to the surface so late during the Himalayan evolution and so quickly.  They are not positively buoyant compared to everything else around, and they are incredibly young compared to most of the other metamorphic rocks that surround them (~24-18 million years old).  There is also no obvious structure to explain their transport.  One option is that a particularly cold and strong piece of Indian crust burying its way into the collision zone acted as a ramp, forcefully expelling lower-crustal rocks over the top.  Strong erosion may also have helped.

 

So what is fieldwork in Bhutan like?  Bhutan tries to limit the number and ‘type’ of tourists by imposing a hefty daily tourist tax, which sadly, even as researchers, we have to pay.  But once the tax is paid, the only things extra are tips for the crew, beer, postcards and, for the geologists, rock shipment.  It’s a high altitude country, with few roads, so most travel is on foot, with horses or yaks carrying the gear.  The best times of year to visit are October/November, which are cold but dry, or April/May which are warmer but may be damper.  In the winter there is too much snow and in the summer it rains.  All the time.  And the rain brings out the leeches…  yuk! 

Typical camp scene, Bhutan

Camping in the snow, Laya

Since that initial expedition, I have been back to Bhutan three more times: once back to Laya, once to the high mountains in the centre of Bhutan searching for the outcrop trace of the fault that may have been responsible for bringing the eclogites to the surface (the Laya-Kakthang Thrust), and once to the Jhomolari Massif in western Bhutan.  It’s a country of many surprises, and  I hope that it won’t be too long before I manage to get back out there….
 
 

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Videos on YouTube from the Himalayan-Tibet research group at the Open University:  https://www.youtube.com/user/HiTGroupOU


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