A walk in cold desert: Retracing the arc

Co-author: Ian Watson

The trans Himalayan cold desert is a delight for geologists and earth scientists. The Himalaya-Karakoram-Tibet orogen system is one of the most fascinating tectonic belts on the Earth to serve as natural laboratory for subduction, obduction, inverted metamorphism, and neotectonics. Every year travellers flock these barren landscapes to enjoy the beauty of the variegated coloured mountains. Most miss the subtle whispers of geology. In corporate speak, we hear the phrase - walk the talk. In Ladakh, one can walk the geology. The Ladakh arc was formed due to Indian and Asian plate collisions through late Cretaceous and Cenozoic Periods. It is mainly characterised by – the subduction of continental crust in the Tso Moriri region, and subduction of neo Tethyan oceanic lithosphere along the Shyok (SSZ) and Indus Tsangpo Suture (ITSZ) Zones.

In one of my previous articles (link ), I discussed on walking through the Konka Wangpo La, Zulong Karpo La to Kang Yatse summit. It was a 72 km long trip through the boundaries between the Zanskar limestone, Tso Moriri crystalline complex crossing the ITSZ, along which river Zanskar flow (Figure 1). Different regions can be demarcated using a Google Map image colour and rock types were correlated with the trekking difficulty in the regions. From Konka Wangpo La onwards, the lithologies are dominantly colourful deep marine flysch deposits, with near vertical dips due to major compressive tectonic activities in the region.

To appreciate the geology of the major tectonic history, multiple traverses in different orientations is a requisite. A traverse through SSZ and ITSZ zone is a good option due the ability to observe the structural complexity and intermixing of different lithologies. The interval is dotted with tectonic melanges of ultra-mafics, gabbro, basalt, sediments and calc alkaline island arc volcanics (Ladakh-Kohistan batholith). The relatively easy Shyam valley trek, aka baby trek of Ladakh, gave the opportunity to traverse through the unique landscape. It’s a 27 km long 3 days trek, through idyllic mountain villages and gompas. The region is famous for shy snow-leopards, and one can make the trek in winters to meet these mountains ghosts face-to-face.

In addition to the Ladakh batholith, which compositionally is calc-alkaline granites, and granodiorites, I have seen northerly dipping fore arc rocks and continental molassic sedimentary rocks derived from transitional environment between the Kohistan Island arc and continental Asian margin (Figure 2).

These sandstones were quite coarse grained, but mineralogically immature with numerous small caves -nature’s way of providing shelters to the high-altitude faunas.? The sandstone percentage in the traverse were very low, in the range of 5-10%. In Figure 3 deposits of Fuller’s earth were observed, a weathering product of deep-water flysch deposits (?) or mineralogically immature sandstone. Interstratified with the sedimentary rocks are acidic (?) and intermediate Khardung volcanics.

The batholith is intruded by numerous orthogonal dark coloured (dolerite?) dykes, possibly amphibole, clino-pyroxene, rich ultra mafic gabbros (Figure 4).

The typical complete ophiolites sequences (Figure 5) are not observed in the traversed section, only melanges of different individual ophiolitic units are observed. Nidar ophiolites, ESE of the traverse and Spongtang ophiolites south of it, are the best locations to visualize the near complete set.

Present structures observed in the traversed section are a result of Indian and Tibetan plate collisions around 50-million years ago.?Major tectonic forces have created a mashup of structures amongst the 55-million-year-old Ladakh batholith, Khardung volcanics and 118-million-year-old ophiolite complexes. One can easily identify the thrust planes along which the rocks were deformed and a characteristic ramp anticline or fault-bend fold (Fig 6).

Geologists learn to adapt their techniques and approaches based on real-world conditions. Field observations allow geologists to see and study geological formations and structures in their natural context. Hands-on experience helps the so-called “Desktop Geologists” to understand rock formations, the structures in the context of the evolution of the area, and sedimentology.

Many times, through hydrocarbon production trends, the reservoirs whisper us back to subsurface challenges. Trying to solve unexpected geological conditions or complexities in a discovered or producing fields hones our problem-solving skills. My friends and I don’t miss the chance to interpret the roadside geological structures. We can be 100% wrong in interpretations, but the exercise will ensure the grey cells are active and rehearsed to give a quick, usually plausible response to a subsurface anomaly.