And the answer to the earlier puzzler I posed about the thick dark layer seen capping a basalt summit is... Ferricrete - popular name Laterite. These iron enriched hard crusts cap high basalt summits along the Western Ghats and along with similar surfaces of chemical weathering which occur at different altitudes tell us about the long and episodic uplift history of the Western Ghats.
The west coast of India is a high elevation passive margin that formed when India broke away from first Madagascar around 88 my ago and then Seychelles around 65-70 my ago. The Western ghats is a north south trending mountain range parallel to the west coast of India. It is characterized by a spectacular west facing escarpment and very youthful relief which suggest recent and ongoing uplift. The ghats are composed of Deccan Basalts in the north and Precambrian granulites in the south.
Right, so here is what a typical ferricrete cap on basalt looks like. It forms mesas, like the one in the image below of Panchgani tableland, a popular tourist destination.
Source: E-Journal Finn-India
So, how does ferricrete / laterite form? This material is formed by the impregnation of weathered rock by iron oxides and hydroxide. During prolonged chemical weathering, more soluble elements within the basalt like, Calcium, Sodium, Magnesium, eventually Silicon get transported away in solution and the more insoluble iron hydroxides are left behind. These iron composites get hardened on exposure to air and form crusts.
There are two explanations of how they form Mesas like the Panchgani tableland. The schematic below shows the two mechanisms.
Source: Ollier and Sheth 2008
1) A period of chemical weathering along a widespread area of basalts formed a continuous blanket of ferricrete. Subsequent erosion of most of it left discontinuous patches. 2) The ferricrete formed in shallow flat bottomed river valleys that developed on the basalt surface. The alluvium and colluvium of these basalt valleys got converted to ferricrete. The surrounding highlands got eroded away being softer than the ferricrete. The mesas represent an "inversion of relief". Former valley bottom now standing in higher relief.
I am a little biased towards the inverted relief as far as the ferricrete mesas in the north part of the Western Ghats go. The southern laterites developed on the Precambrian granulites are most probably remnants of blankets. But in this case I am persuaded more by the outcrop geometry of the ferricrete mesas.
Take a look.
Source: Ollier and Sheth 2008
Now, isn't that cool! They occur as a long stringy line of mesas (dark patches) outlining a dendritic pattern, just like a river system. This ancient river system if the interpretation is correct flowed in a northerly direction. Today the drainage east of the continental divide is in a southeasterly direction. So as the Western Ghats rose there must have been differential uplift which reversed the drainage direction as well.
What was once a river system is now preserved as a chain of Mesas standing at 1500 m ASL. Tell me you don't think earth processes are awesome after thinking through this.
Okay, so ferricretes are chemical weathering products but what has that to do with the uplift of the Western Ghats. The problem with the Western Ghats both the basaltic section and the PreCambrian section is that there are no obvious structural indicators like fault scarps or structually offset marker horizons that can indicate uplift history.
But surfaces of intense chemical weathering are important. They form during times of tectonic stability. The Panchgani ferricrete tells us that after the eruption of the Deccan Basalts in the latest Cretaceous there was a long period of intense chemical weathering in a tectonically stable regime. No uplift at all. If blocks of crust are being actively uplifted they will erode into facets and form sharp peaks and V shaped valleys. Weathering on a stable block of crust will form flat surfaces or peneplains or planation surfaces which is what the Panchgani tableland and other such surfaces further south are.
If there is episodic uplift alternating with periods of stability then there will be development of such flat weathering surfaces at various altitudes as mountain ranges rise and get weathered and beveled and then rise again. The younger surfaces will occur at lower altitudes.
The diagram below helps conceptualize this.
This is precisely the pattern you see along the length of the Western Ghats. The older planation surfaces occur at the highest altitude and the younger ones at lower altitude. Along the western coastal plains there are ferricrete surfaces of mid Tertiary age which occur at higher relief than the surrounding plains and testify to the ongoing uplift of the region.
The image below shows a north south transect along the summits of the Western Ghats from Panchgani (1500 m) in the north to Anamalai (2695 m) in the south.
And the figure below is the cross section.
Look at the multistory benches that have developed episodically as the Western Ghats rose to their present heights. A series of planation surfaces (S0 - S4) indicating periods of stability and chemical weathering followed by uplift.
Most of this uplift occurred in the Cenozoic. Planation surfaces below the Deccan pile (S0 - Mesozoic age) tells us that there was little pre-volcanic uplift. Planation surfaces directly above the volcanics (Panchgani ferricrete - latest Cretaceous- Paleocene) tells us that after eruption there was a period of stability. Uplift started later in the Cenozoic. An intial period of uplift in early Cenozoic which lifted the Ghats as well as the coastal plains was followed by a period of stability. Younger ferricrete on the coastal plains developed during this period of stability in the mid-Tertiary sit unconformably on coastal plain rocks indicating deformation and tilting of the coastal plain during the earlier phase of uplift.
We can time that uplift by looking at the sediments in the west coast basins in the Arabian sea. They show two major pulses of sedimentation in the Cenozoic, the first in the Paleocene and the second post Miocene. These basins contain about 109,000 cubic km of sediment. Numerical modelling of the west coast and ghat evolution suggests that most of the great elevation of the Western Ghats is best explained as an isostatic response of the lithosphere to denudational unloading of the crust onshore along the now coastal plains and sediment loading offshore. As erosion removed great volumes of sediment and dumped them in adjoining basins, the crust rebounded in response to the unroofing.
This theoretical work is supported by Apatite fission track thermochronometry which indicates that nearly 3-4 km of crust has been removed from the coastal plains and about 1-2 km from further inland. There probably was a elevation difference between the hinterland and coast, an artifact of the rift flank structure formed when India rifted from the Seychelles around 65-70 million years ago. But then differential erosion between the coast and the hinterland must have accentuated this existing topographic difference in effect generating the steep west facing escarpment and causing the escarpment to retreat eastwards forming the present coastal plains. Isostatic uplift contributed by further lifting the entire region resulting in the present day great heights and the youthful rejuvenated topography.
Isostacy doesn't explain all the elevation. Post Miocene compressional stresses from the Himalayan collision also affected this region resulting in minor deformation and uplift.
That's a lot to take in from a starting point of a flat weathered surface made up of ferricrete. I have been trekking in the Western Ghats long before I took up geology seriously. Now I look at these great heights with a more mature understanding but with an even greater sense of excitement.