Sunday, July 22, 2018

Mount Abu Geology

A reader asked me about the geology of Mount Abu and how it relates to the Aravalli fold mountains. Mount Abu is a popular tourist destination in the Sirohi district of Rajasthan. It forms a distinct elevated area ( ~ 5,500 feet ASL) and provides a much needed respite from the heat of the Rajasthan plains.

Here is my short note.

Mount Abu is a batholith. That means it is a big body of granite, made up of magma which cooled deep in the subsurface.

The satellite imagery below shows the Mount Abu hills in relation to the Aravalli fold mountains.

This magmatic episode is part of what is known as the Malani Igneous Suite. The term refers to an assortment of mostly felsic igneous rocks (magmas enriched in silica, aluminum and potassium) ranging from large intrusive bodies like the Mt Abu batholith, lava fields resulting from volcanism (Malani Rhyolites), and mafic (iron, magnesium, calcium rich magmas) and felsic dikes intruding the margins of the province. This was the result of a fairly prolonged phase of magmatism that affected the western margin of the Aravalli orogenic belt. The magmatic activity occurred between 780 -750 million years ago.  So, it is much younger than sedimentation and orogeny of the Delhi Supergroup (which ended about 1 billion years ago) and is not considered as part of the Aravalli/Delhi Supergroups. The thinking is that after Delhi orogeny, this part of the Indian craton underwent extension (was pulled apart), triggering granitic magmatism.

The detailed geological map shows Mount Abu batholith and other stratigraphic and tectonic elements of the Aravalli craton (continental block) and fold mountains. The Western Margin Fault roughly marks the zone of contact between the Aravalli craton to the east and the Marwar craton to the west. The Delhi orogeny is believed to have resulted from a convergence and suturing of these two cratons about 1 billion years ago.

Source: Joseph Meert and Manoj Pandit 2014

Here is a link  to an article on the Malani Igneous Suite with a useful map of the distribution of various igneous bodies in this province:

One additional point is that there were earlier magmatic episodes affecting the western margin of the Aravalli craton marking the culmination of the Delhi orogeny. This includes the 970 million year old granitoids of the Ambaji region, believed to represent crustal melting at the terminal stages of the Delhi orogeny. Younger than these are the post orogeny Erinpura Granite. This too was a protracted magmatic episode lasting the time span from 870 to 800 million years ago. Erinpura granites are collectively shown as post-Delhi granites in the map by Meert and Pandit. Note that both the Erinpura Granites and the Mt Abu batholith occur at the juncture between the Aravalli and Marwar cratons. Such a region would have lines of weaknesses inherited from earlier collision and deformation episodes making it susceptible to be reactivated as rifts and loci of magmatism.

Finally, why did the Mt. Abu granites that formed in the subsurface get elevated to heights of around 5,500 feet? The Aravalli mountains too have ridges which reach around 3000 feet. The Aravalli craton is bounded by two great NE-SW trending fault systems. The Great Boundary Fault to the east and the Western Margin Fault to the west. There are also innumerable NE-SW and some NW-SE trending fracture and fault systems (lineaments) which break up the crust into rigid blocks. Many of these originated during Archean and Proterozoic crustal deformation. Some new lineaments may have formed during the Jurassic breakup of Gondwanaland (A B Roy 2006).  Mount Abu may be an example of a block uplift, resulting from movements of the crust along these lineaments, perhaps in response to stresses originating from the India-Asia collision. The Aravalli fold mountains too have likely been rejuvenated and seen some recent uplift (Bhu 2014) These movements took place in the Neogene to Quaternary times, in the past 20 million years or so.

No comments:

Post a Comment