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: http://www.mantleplumes.org/Malani.html

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 et.al. 2014) These movements took place in the Neogene to Quaternary times, in the past 20 million years or so.

Saturday, July 14, 2018

Papers: Global Tectonics, Cryogenian Period, Himalaya Miocene Lakes

I've come across quite a few interesting papers on diverse topics in the past couple of weeks. Most of them are 'big question' themes, dealing with processes taking place on global scales. Here are the links.

Global Tectonics:

1) Subduction Initiation of the Wilson Cycle - In plate tectonics, the Wilson Cycle refers to cyclical ( frequency of 100's of millions of  years) breakup of continents and the opening and closing of ocean basins. But how is subduction initiated and new convergent plate boundaries formed? Some good examples from the Western Pacific margin and eastern Indonesia.

2) How Subduction Broke Up Pangaea - Was it top down forces.. i.e. the pull exerted by subducting slabs or was it the horizontal traction exerted by a convecting mantle (bottom up) that broke up the supercontinent?

3) Why is Africa Rifting? - Insights into the formation of the famous East Africa rift system.

4) Gondwana Large Igneous Provinces: distribution, diversity and significance - Synopsis of several papers that explore the link between prolonged magmatic episodes, tectonics, climate shifts and sedimentation patterns in Gondwana continents.

Neoproterozoic:

1) Snowball Earth climate dynamics and Cryogenian geology-geobiology - In the Cryogenian Period, between around 715 to 635 million years ago, the earth was blanketed in two prolonged glaciations. Before these glaciations, the earth was a microbial planet. The end of these glaciations is associated with the evolution of multicellular complex life. What were the conditions during the Cryogenian Period that influenced the evolution of life?

Himalaya:

1) Oligocene‐Miocene Great Lakes in the India‐Asia Collision Zone - Mount Kailash is an important pilgrimage site for Hindus. The sediments that make up this mountain were deposited in narrow basins in the India-Asia collision zone. They preserve a record of surface environments and geodynamic mechanisms operating within the suture zone during the convergence of India with Asia.

All Open Access.

Friday, July 13, 2018

Sutlej Paleochannels- More Details

A while back I had written a post in response to a paper (Ajit Singh et. al. 2017) on the paleo-Sutlej river. The study used geochemical analysis to identify ancient channels of the river. Today, the Sutlej flows out of the Himalaya and joins the Indus River. But this study showed that until about 8000 years ago, the Sutlej (or at least a strand of the river) flowed along a different course. Its paleo-channels coincide with  the course of the river Ghaggar in Haryana and Rajasthan in northwest India. This topic is of relevance in studying how rivers may have impacted agricultural practices and settlement patterns of the Harappan Civilization. The geography of the river Ghaggar also matches that of the river Saraswati, described in the Rig Ved. There is therefore considerable interest in working out the detailed history of these rivers.

I had pointed out that an earlier paper by Liviu Giosan and colleagues has used topographic criteria to come to a similar conclusion as Ajit Singh and colleagues. Giosan's study stressed that today the Sutlej and Yamuna flow along deeply incised valleys that were cut in the early Holocene (~10,000 to 8700 years ago). The absence of such valleys in the region between the present day Sutlej and Yamuna indicates that the Ghaggar channel was not being fed by glacially sourced rivers since 10,000 -8700 years ago.

The relief rendition below is from Ajit Singh's paper. It shows clearly the incised valleys of the Sutlej and Yamuna. I had overlain the blue line and suggested that if the Sutlej had flowed into the Ghaggar in early Holocene, there should have been an incised valley along the blue line.


As it happens, Sanjeev Gupta, who was the lead scientist of the study by Ajit Singh and colleagues, is a reader of my blog. He emailed me and has provided more insights regarding these paleo-channels.

I am posting his comments below with his permission.

My comment from the earlier post- The modified relief rendition below also shows the course of the abandoned Sutlej incised valley. Note that this valley is much narrower than the Sutlej and Yamuna incised valleys. Also, trace these narrower incised valleys upstream and you can see that they originate in the Siwaliks. There are no deep extensive incised valleys along the route I have marked in blue. The Sutlej would have carved a prominent incised valley roughly along the blue route had it been flowing into the Ghaggar during most of the early and mid Holocene. Its absence suggests to me that the valley annotated as the abandoned Sutlej incised valley was really carved out in the earlier part of the Holocene by the smaller Ghaggar river originating in the Siwaliks.

Sanjeev Gupta's reply - Just to respond... we only see the incised channel in the SRTM where the valley is not completely infilled. Indeed where we see the valley in SRTM it is not the base of the incised valley. but a partially infilled valley. So along the blue line you have drawn there is likely to have been a valley but it is entirely infilled.

My comment (with regards to the 'abandoned Sutlej incised valley' in the above relief rendition) - (it)....was really carved out in the earlier part of the Holocene by the smaller Ghaggar river originating in the Siwaliks.

Sanjeev Gupta's reply- This is not possible because all the geochemistry signature is of the Sutlej - the base of the valley actually occurs in the stratigraphy.

We have some newer data that better constrain the timing of incision but I stress the topographic surface is not the base of the incised valley - that lies in the subsurface.

...end.......

So, Sanjeev Gupta's view is that present day topography is not necessarily a more reliable guide to the course of these ancient rivers. Geochemical fingerprinting is the way to go.

Update July 22 2018:

Liviu Giosan posted a comment which led to an extended exchange with him regarding this topic. I have copied his comments below with his permission.

Me- Hi Liviu- was going to message you about my post when I saw your comment. Regarding Sanjeev Gupta's view, wondering if this is a matter of scale. The subsurface paleo valleys he says exist were much smaller valleys carved by a waning Sutlej and hence got infilled? Not quite the wider incision profile carved later by the river  along the present course.

Liviu Giosan - A matter of scale, time since abandonment, and location relative to a sediment source. Maybe I do not see Sanjeev's point. His work confirmed ours for holocene and in adition showed a pre-Holocene channel. A recent paper by Dave et al. showed the same thing for Yamuna.

Me- thanks. Can you send me the link to Dave et. al.?

Liviu Giosan - I can send you the paper. Please elaborate: what is Sanjeev's point?

Me- He hasn't elaborated. Those two short para's  i put up was all he emailed. I guess he is saying that there are incised valleys in the interfluve subsurface but are now infilled and so not picked  up by SRTM.

Liviu Giosan - That may be true but I doubt they will be all the way in filled unless they are old (preHolocene). After all his work shows a network of holocene streams in the region using SRTM. Further geochemistry we did  downstream so far is pretty (Clift et al. not cited by Sanjeev) shows no holocene contribution from Sutlej. Even more Sutlej incised since early Holocene making it almost impossible to feed shallow channels on the interfluve. This needs to be interpreted with all available evidence. Geochem is no silver bullet in isolation.

..end...

Sanjeev Gupta did mention to me that the point about topography is important but they did not have the space to address it in their paper. So, while there are differences in viewpoints about emphasis on techniques,  the overall data does point to a switch in the course of the Sutlej around 8-10 thousand years ago.

The paper on the paleo-Yamuna that Liviu Giosan mentioned is by Aditi Krishna Dave and colleagues. They examine detailed lithologs of the Chautang channel near the town of Hisar, Harayana, and with OSL (Optically Stimulated Luminesence which indicates the time of burial of sediment) age constraints propose that the Yamuna, which was flowing westwards and joining the Ghaggar-Hakra downstream of its confluence with the Sutlej, changed course and started flowing eastwards by 24 thousand years ago. I'll put up a short post on that shortly.

Friday, June 22, 2018

How Did Homo Sapiens Evolve?

Here is a very good summary of our evolving understanding of this question.

How did Homo Sapiens Evolve? - Julia Galway-Witham and Chris Stringer

Excerpt:

Over the past 30 years, understanding of Homo sapiens evolution has advanced greatly. Most research has supported the theory that modern humans had originated in Africa by about 200,000 years ago, but the latest findings reveal more complexity than anticipated. They confirm interbreeding between H. sapiens and other hominin species, provide evidence for H. sapiens in Morocco as early as 300,000 years ago, and reveal a seemingly incremental evolution of H. sapiens cranial shape. Although the cumulative evidence still suggests that all modern humans are descended from African H. sapiens populations that replaced local populations of archaic humans, models of modern human origins must now include substantial interactions with those populations before they went extinct. These recent findings illustrate why researchers must remain open to challenging the prevailing theories of modern human origins.

..and on the status of Recent African Origin (RAO) versus Multiregionalism, the two competing theories of modern human origins

The apparently more deeply rooted origin of H. sapiens adds to the problem of how scientists can delineate H. sapiens from other species in an evolutionarily meaningful way and suggests that the species did not evolve as recently as previously envisaged. However, evidence of evolutionary novelty in the fossil record of putative H. sapiens still appears relatively recently in Africa, compared to a multiregional view of human evolution (in which H. sapiens began to evolve at least 1.8 million years ago). Within Africa, it is currently unclear whether the origin of H. sapiens was localized, as in some early RAO formulations, or involved a process similar to multiregional evolution, but operating purely within the continent (5, 13).

With only a few dissenters, the strictest versions of both RAO (which denies interbreeding with other lineages or species) and multiregionalism (which argues for an interbreeding network of one species over the past ~1.8 million years) are now generally regarded as falsified. Instead, two intermediate theories best accommodate the complex interactions between hominin taxa ~40,000 to 100,000 years ago (8, 14): RAO with hybridization (RAOWH) and the assimilation model (AM) (see the figure).



Source: How did Homo Sapiens Evolve? - Julia Galway-Witham and Chris Stringer 2018

Open Access.

Sunday, June 17, 2018

Mars Geology

Came across a couple of cracking papers on Mars geology

1) Ancient Martian aeolian processes and palaeomorphology reconstructed from the Stimson formation on the lower slope of Aeolis Mons, Gale crater, Mars - Steven G. Banham et. al. 2017

During its travels, the Mars Curiosity Rover has been taking some exquisite pictures of landscapes and rock outcrops. This study uses field photos of a sandstone body and analyses its ancient depositional setting.  The sandstone is a dune field, created and shaped by aeolian processes. The overall aim is to better understand the sedimentary environments on Mars, how they changed over time, and whether they could have been habitable environments.

Just take a look at the spectacular cross bedding and bed sets of the dune.


Source: Steven G. Banham et. al. 2017

And here is a depiction of the regional setting of the dune field


Source: Steven G. Banham et. al. 2017

2) A Field Guide To Finding Life On Mars- S. McMohan et. al. 2018

Excerpt: This paper reviews the rocks and minerals on Mars that could potentially host fossils or other signs of ancient life preserved since Mars was warmer and wetter billions of years ago. We apply recent results from the study of Earth’s fossil record and fossilization processes, and from the geological exploration of Mars by rovers and orbiters, in order to select the most favoured targets for astrobiological missions to Mars. We conclude that mudstones rich in silica and iron-bearing clays currently offer the best hope of finding fossils on Mars and should be prioritized, but that several other options warrant further research. We also recommend further experimental work on how fossilization processes operate under conditions analogous to early Mars.

I got to relearn a good bit about how depositional settings, mineralogy and geochemistry influence organic matter preservation.

Both Open Access.