Saturday, July 8, 2017

Field Photo: A Bend In The Rocks

I saw this textbook example of a fold in the Lassar Yankti valley, about 2 kilometers south of Tidang village in the Kumaon Himalaya.

Consider how rocks bend and deform in response to stress. Blue arrows denote the direction of maximum compressive stress perpendicular to the fold axis. As rocks fold, the convex portion of the fold will experience tensile forces and fractures parallel to the axial plane develop. Notice also conjugate stress fractures (black arrow). Since this is a loose boulder I cannot assign actual directions to the stress field.

The graphic below summarizes the typical fracture patterns found in folded rocks. How many of these can you identify in the fold above?

Source: Applied Hydrogeology of Fractured Rocks

My Himalayan treks over the past few years have taken me on a walk across almost the entire thickness of the Greater Himalayan Sequence. As I mentioned in an earlier post, the GHS is bounded at its base by the Main Central Thrust and at the top by the South Tibetan Detachment. It shows an "inverted" metamorphic sequence. This means that the grade of metamorphism increases as one climbs to higher structural levels. Finally, sillimanite and kyanite grade metamorphic rocks transition into a zone of partial melting and leucogranite intrusions. Above this level the grade of metamorphism decreases to biotite grade and then to a finer grained phyllite grade. One conspicuous structural feature of the GHS is that large folds are very rare. Instead, from the base right up to the zone of partial melting the GHS exhibits a homoclinal northerly dip as seen in the picture below.

Within these northerly dipping slabs, small scale ductile folding in high grade gneiss and migmatites can be seen (picture below), but the slabs themselves are not contorted into mountain face scale folds.

Large isoclinal and recumbent folding is present only in the uppermost structural levels of the GHS in the phyllite grade rocks above the zone of partial melting. The picture below shows tightly folded phyllite grade metamorphic rocks north of the village of Baaling in the Darma Valley.

And this splendid recumbent fold is exposed at village Dantu.

Why is large scale folding rare to absent over much of the thickness of the GHS? Could the movement of the South Tibetan Detachment cause folding in the underlying phyllites?

These are some of the niggling questions I am struggling with. I still have much to learn about Himalayan geology. I need to go there with a structural geologist!

Finally, a view of the outcrops from which was derived the textbook quality folded phyllite.

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