Digging through my collection of Himalaya geology papers, I came across this geological map of the Himachal Himalaya. It is from a study on the tectonic history of the Himachal mountains by A. Alexander G. Webb and colleagues, published in Geosphere in the year 2011.
The paper itself is quite a detailed work using both field mapping and various geochemical and geochronologic methods. It will be hard reading and likely incomprehensible for non-geologists. I won't discuss the specifics here. I will however simplify the most interesting findings.
1) The Greater Himalaya are the tallest of the ranges, and they are made up of rocks which were buried the deepest during the mountain building process. Based on the geometric relationship of the fault zones that contain this rock unit, and the timing of fault activity, the authors propose a mechanism for the emplacement of these rocks from deeper to shallower levels.
2) By establishing a chronology and comparing their geochemistry, rock units displaced by faults and dismembered by tectonics and erosion are shown to have been contemporaneously deposited.
3) There are rare instances of preservation of the depositional contacts between major rock groups. Using this as a guide to their original location and supplementing it with geochronologic information, the pre mountain-building geographic locations of these rock units in relation to the northern shoreline of the Indian subcontinent is proposed. I found this paleo-geographic reconstruction most useful. It really helped clarify my thoughts about the origin and relationships of the different Himalayan sub-divisions.
4) Himalayan mountain building beginning around 35-45 million years ago led to metamorphism of sedimentary layers that were deposited on the northern margin of India. The mineral monazite (phosphate mineral) forms during such metamorphic reactions. It also contains radioactive elements like thorium which geologists can use to estimate the timing of its formation. Dates of monazite formation from some low grade metamorphic rocks has unearthed an even older phase of metamorphism that affected these rocks. It shows that the northern margin of India was involved in an earlier phase of mountain building around 500-600 million years ago.
Travelers, do download a high resolution version of this map here - Geological Map of the Himachal Himalaya.
Kangra, Chamba, Manali, Spiti, and Shimla. These are all popular places to visit in the Himachal region. Here are a few tips for a broad understanding of the terrain that you will be driving or trekking through. The descriptions below are by reference to the map legend where all the rock groups are tabulated.
1) Sub-Himalayan Sequence. These are sedimentary rocks deposited in the Himalayan foreland basin. As the Himalayan mountain building progressed from around 45 million years ago, the crust in front of the rising mountains bent to form a depression. Debris from this eroding mountain chain was deposited in these foreland basins and then folded and uplifted to form the frontal ranges, including the familiar Siwaliks. Jwalamuki Temple, where a flame is powered by natural gas emanating from deeply buried strata, is located within the Siwaliks. These sub-Himalayan ranges were uplifted between 5 to 0.5 million years ago. Lookout for a lot of sandstone, shale, and pebbly and gravel rich layers.
2) Tethyan Himalayan Sequence. These are low grade metamorphic and sedimentary rocks ranging in age from 800 million to around 70 million years old. The fossil bearing strata that you find in the Spiti valley belong to this group of rocks. The Tethyan Himalaya were the first mountain ranges to form following the India-Asia collision, beginning about 45 million years ago. You will see slates, limestone, sandstone. The older part of this sequence is made up of metamorphic rocks like shiny phyllites and mica schists with garnets.
3) Igneous Rocks. These are of various ages , ranging from 1.8 billion years ago to about 470 million year ago and point to magmatic activity that affected the northern margin of the subcontinent from time to time. The spectacular Dhauladhar range near the town of Dharamsala is made up mostly of granites which intruded the crust around 500-470 million years ago.
4) Greater Himalaya Crystalline Complex. As the name suggests these rocks are high grade crystalline metamorphic varieties like gneiss and schist. They range in age from about 1000 million to 500 million years ago. These were rocks that formed at depths of about 25 kilometers and then were uplifted about 25-16 million years ago. These rocks have a typical banded appearance and contain pink and red garnets and shiny mica rich layers.
5) Tertiary Leucogranite. This granite formed by partial melting of metamorphic rocks during mountain building. It ranges in age from about 40 million to 8 million years ago. You will see them in the higher reaches of the Greater Himalaya. They are easy to spot. Look for white bands cutting across (dikes) dark banded rock. At places the white bands will be parallel (sills) to the rock layers.
6) Outer Lesser Himalaya. These are low grade metamorphic and sedimentary rocks ranging in age from about 1000 million to 500 million years ago. The common rock types will be the familiar slate and limestone.
7) The rest of the Lesser Himalayan units are among the oldest rocks in the Himalaya. The oldest among them, the Munsiari Group, have been dated to about 1.9 billion years ago. They comprise high to low grade metamorphic rocks. Keep a watch for banded metamorphic rocks as well as quartzites and limestones.
The Lesser Himalayan rocks were uplifted between 16 and 5 million years ago.
8) Indus Suture Zone. If you wander into the Indus valley. These are rocks that formed in the zone of collision between India and Asia. Ophiolites are fragments of the ocean crust thrust up when the Indian plate dove underneath Asia. The Indus Molasse are beds of sand and gravel derived from the erosion of nearby mountain ranges and deposited in lakes and streams.
To briefly summarize the geology. The Greater Himalaya, the Lesser Himalaya, and the Tethyan Himalaya are rock groups made up of sediments that were deposited on the northern continental shelf of India and intermittently intruded by granitic magmas. This sequence developed across a vast time span ranging from 1.8 billion years ago to 70 million years ago. Sediments of the older units of the Tethyan Sequence (Haimanta), the rocks of the Greater Himalaya, and the units making up the Outer Lesser Himalaya were deposited roughly at the same time but at different geographic locales. All these rock units were metamorphosed to varying degrees during Himalayan mountain building.
Two cross sections from the paper depicts these units restored to their original locations across the Indian margin and then subsequently disrupted by tectonics.
Compressive forces have deformed this stratigraphy into a complicated structure made up of folded rock sheets stacked by thrust faults. Erosion, by selectively removing portions of these thrust sheets, and by exhuming deeper levels of the crust, has played a big role in producing the present day rock outcropping pattern.
The paper is open access if you want to dive in. A. Alexander G. Webb et. al. 2011 : Cenozoic tectonic history of the Himachal Himalaya (northwestern India) and its constraints on the formation mechanism of the Himalayan orogen.
