Tuesday, November 16, 2010

Geological Cross Sections And Profiles Of My Lesser Himalayan Hiking Area

 [Update October 2012: Please refer to my new post on the Lesser Himalayas -Stratigraphy and Structure of the Lesser Himalayas in Gharwal and Kumaon]

Update November 29 2012:

Interactive Geologic Map and Cross Section Of The Kumaon Lesser Himalayas In The Shama Gogina Region


 This has been pending for the last couple of weeks.

I went hiking in the Mukteshwar area of the Kumaun Himalayas and promised I would be posting some geological maps and cross sections and so here goes.

If you are a field geologist looking for some classic fold belt structures to map you are not going to be over excited if you are restricted to the  Mukteshwar area. The outcrop geology given the contorted and complex standards of portions of the Himalayas is quite sedate.

The entire region is made up of a NE steeply dipping block of phyllites, quartzites and mica garnet schists. Great exposures are few due to the lush vegetation. The one on the left are mica-garnet schists and quartzites forming an escarpment which is used for rappelling and rock climbing.

Along road cuts like the one below you do get a good view of the quartzites and schists. This one contains elongate deformed quartz veins.

These north east dipping strata are one limb of a regional synform. The cross section below depicts the internal geology of the synform made up of a sequence of medium to high grade metamorphic rocks intruded by granites. All the thrust faults shown in the cross sections are thought to sole in to the Main Central Thrust. The interpretation is that the Main Central Thrust has splayed or split into these subsidiary faults. I'll explain the role of the Main Central Thrust a little later in the post. My location was close to the label Study Area.

 Source: T. Kulkarni 2008

The larger geological context within which this quartzite-schist terrain stands is quite interesting.

As I mentioned Mukteshwar falls in what is known as the Lesser or Lower Himalayas. Below I've put up a topographic profile across the Kumaun Himalayas. SH - Sub Himalays, LH - Lower/Lesser Himalayas, HH- High Himalayas , ITS - Indus Tsangpo Suture beyond which is the Tibetan plateau.

 Source: An Yin 2005

Early mapping in some areas indicated that these physiographic divisions also corresponded with 4 different geological domains which were in tectonic contact along great thrust faults.

So, the Sub Himalayas or Siwaliks are made up of Cenozoic foreland basin sediments separated from the Gangetic alluvium by the Himalayan frontal thrust and from the Lower Himalayas by the Main Boundary Thrust.

The Lower Himalayas in turn are made up of unmetamorphosed to low grade metamorphic mid-late Proterozoic Indian basement (Lesser Himalayan sequence) separated from the High Himalayas by the Main Central Thrust.

And the High Himalayas are made up of high grade metamorphic mid late Proterozoic Indian basement (Greater Himalayan Crystalline sequence) along with unmetamorphosed Paleozoic sediments (Tethyan Himalayan sequence) and this terrain meets the Asian plate along the Indus Tsangpo suture, the zone of plate collision.

The simplified story is that as the Indian continental crust collided with the Asian crust, the Indian plate buckled, deformed and broke up perhaps along pre-existing zones of weaknesses in the Indian crust. These breakages evolved into the major thrust faults bringing into structural contact different geological terrains. The gradient of compressive forces and vertical uplift increases from SW to NE imposing a conformity between the physiographic and geological divisions.

Physiography and geology do coincide in regions like central Nepal but the correspondence between the physiography and geology does not hold up everywhere. For example the Almora /Mukteshwar area which falls in the physiographic Lower/Lesser Himalayas has exposures of very high grade rocks corresponding to the Greater Himalayan crystalline sequence normally found in the physiographic Higher Himalayas.

Below is a map of the Himalayan orogen. I was located just east of cross section C in the Almora-Dadheldura nappe. 

 Source: An Yin 2005

Nappes are terrains which have moved some distance along thrust faults. In this case the Main Central Thrust has moved the Greater Himalayan crystalline terrain over the Lesser Himalayan sequence along a nearly flat or low angle thrust. During continued orogenic forces the continental crust kept buckling, resulting in the Greater Himalayan crystalline terrain along with the Main Central Thrust in this region becoming folded into a broad synform and antiform. Erosion has removed the antiform leaving exposures of the Greater Himalayan crystalline sequence stranded on top of the Lesser Himalayan sequence- cut off from its root in the Higher Himalayas.
This is how high grade metamorphic rocks of the Great Himalayan crystalline sequence and sediments of the Paleozoic Tethyan sequence which make up the High Himalayas are in many regions exposed in the Lower Himalayas.

They are erosional remnants of thrust sheets.

Cross section C below is a depiction of this scenario. The pink is the Greater Himalayan crystalline sequence, the pale brown is the Lesser Himalayan sequence and the blue is the Tethyan Himalayan sequence. MCT is the Main Central Thrust. The red arrow indicates my approximate location within this synform.

 Source: An Yin 2005

Such blocks of the nappe isolated from its root terrain are called klippen. The Almora /Mukteshwar area high grade crystalline rocks are part of a klippen surrounded by unmetamorphosed to low grade Lesser Himalayan sequence.

This is the consensus view.... put forward after decades of mapping and dating rocks in this region. The movement of the thrust sheets and the erosional formation of klippen according to this view took place in the mid Miocene, synchronous with a major uplift and deformation phase of the Himalayas. Another view is that the rocks of the Almora-Mukteshwar area are not allocthonous or transported from elsewhere but are authocthonous i.e. they are a rooted part of the Lesser Himalayan sequence, the different grades of metamorphism reflecting local metamorphic gradients.

You don't really get a feel for this larger picture if you are hiking within a small area as I was. Still it was pretty awesome thought, that I was standing on a klippen whose root lay in the High Himalayas some 75 km away in front of me.

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