Friday, April 28, 2017

Himalayan Gravel Flux And Flood Risk

Why should an understanding of sediment transport distance and whether that sediment gets broken down into coarser gravel or finer sand be of any practical use?

Here is a good example from the Himalaya.

Abrasion-set limits on Himalayan gravel flux- Elizabeth H. Dingle, Mikaƫl Attal & Hugh D. Sinclair

Rivers sourced in the Himalayan mountain range carry some of the largest sediment loads on the planet, yet coarse gravel in these rivers vanishes within approximately 10–40 kilometres on entering the Ganga Plain (the part of the North Indian River Plain containing the Ganges River). Understanding the fate of gravel is important for forecasting the response of rivers to large influxes of sediment triggered by earthquakes or storms. Rapid increase in gravel flux and subsequent channel bed aggradation (that is, sediment deposition by a river) following the 1999 Chi-Chi and 2008 Wenchuan earthquakes reduced channel capacity and increased flood inundation. Here we present an analysis of fan geometry, sediment grain size and lithology in the Ganga Basin. We find that the gravel fluxes from rivers draining the central Himalayan mountains, with upstream catchment areas ranging from about 350 to 50,000 square kilometres, are comparable. Our results show that abrasion of gravel during fluvial transport can explain this observation; most of the gravel sourced more than 100 kilometres upstream is converted into sand by the time it reaches the Ganga Plain. These findings indicate that earthquake-induced sediment pulses sourced from the Greater Himalayas, such as that following the 2015 Gorkha earthquake, are unlikely to drive increased gravel aggradation at the mountain front. Instead, we suggest that the sediment influx should result in an elevated sand flux, leading to distinct patterns of aggradation and flood risk in the densely populated, low-relief Ganga Plain.

Behind paywall, but I thought this is a good illustration of how insights into very fundamental earth processes can potentially help save lives.

Wednesday, April 19, 2017

Evolution Of The Konkan-Kanara Coastal Plain

The Konkan coastal plains is a beautiful getaway from west coast city life. Palm fringed beaches, quiet rivers and estuaries, betel nut plantations and forest tracts. Small villages and settlements dot the landscape. To the east, the coastal plains abut against the imposing Western Ghat escarpment.

How did this coastal plain of Maharashtra form? (Kanara refers to the stretch south of Maharashtra in the state of Karnataka).  I came across a paper by Mike Widdowson on the evolution of laterite in Goa. It also has a broader discussion on the conditions that led to the formation of geomorphology of the coastal lowlands extending all along the west coast of India.

Here it is summarized nicely in this figure below:

Source: Evolution of Laterite in Goa: Mike Widdowson  2009

After Deccan Volcanism ended, rifting of the Indian west coast and down faulting of the western side led to the formation of a west facing fault scarp. Erosion of this scarp over the early mid Cenozoic (from about 60 million years ago) has caused it to retreat eastwards. The Western Ghat escarpment is this retreated scarpThe coastal plain formed as an erosional surface that became broader and broader with the progressive eastward retreat of this cliff to the current location. The fault which caused the western side to subside thus lies in the Arabian Sea along the west coast.

In Mid-Late Miocene (~10 million years ago), a phase of humid climate resulted in intense chemical weathering of the basalts and pediment (rock debris) exposed along the coastal plains. This alteration of the basalts formed thick iron rich soils. The reddened and indurated crust of this soil is commonly termed laterite. In the Western coastal lowlands this laterite may be a few meters thick.

Subsequent uplift of the west coast and concomitant down cutting by west flowing rivers formed a dissected landscape composed of laterite capped mesas (table lands) and entrenched meandering streams. These mesas reach altitudes of 150-200 m in the eastern parts of the coastal plain. Nearer the coast they are about 50 -100 m above sea level. 

The western margin of India has seen multiple episodes of extensive laterite formation. The famous table lands of the hill stations of Panchgani and Mahabaleshwar are also made up of laterite. They occur at altitudes of around 1200 m to 1500 m.  However, this upland or high altitude laterite is much older, having formed about 60- 50 million years ago in the early Cenozoic, soon after Deccan volcanism ended. The Konkan and Goa lowland laterites point to another younger phase of laterization. Sheila Mishra and colleagues have identified two more surfaces in the Deccan Traps at 650 m ASL and 850 m ASL that preserve remnants of laterite cover. This suggests a complex polyphase history of denudation and chemical weathering and tectonic stability of the Sahaydri ranges of the Western Ghats.

The sea cliffs that one encounters as you travel along the Konkan and Goa coastline are a result of a late Cenozoic uplift. I remember with fondness a trek I did during my college days from the town of Ratnagiri south to the town of Malvan. There were absolutely majestic sections where we walked on the edge of laterite capped sea cliffs with the Arabian Sea heaving and thundering below us. Little coves and beaches of sparkling white sand lay between the cliffs. Here and there local fisherman had kept their fish catch to dry out in the sun. The pungent smell urged us on!

The satellite imagery below shows a section of the coastal plains from Ratnagiri in the north to Devgarh in the south. White arrows point to the laterite capped table lands dissected by stream networks. Orange arrows point to sea cliffs. Black arrows shows the Western Ghat escarpment.

This is a very interesting paper. Open Access.

Thursday, April 13, 2017

Oceanic Crustal Thickness Since The Breakup Of Pangea

Of interest:

Decrease in oceanic crustal thickness since the breakup of Pangaea - Harm J. A. Van Avendonk, Joshua K. Davis, Jennifer L. Harding and Lawrence A. Lawver

Earth’s mantle has cooled by 6–11 °C every 100 million years since the Archaean, 2.5 billion years ago. In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches. Here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time. We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1.7 km thicker on average than crust produced along the present-day mid-ocean ridge system. If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15–20 °C per 100 million years over this time period. The difference between this and the long-term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss. We also find that the increase of ocean crustal thickness with plate age is stronger in the Indian and Atlantic oceans compared with the Pacific Ocean. This observation supports the idea that upper mantle temperature in the Jurassic was higher in the wake of the fragmented supercontinent Pangaea due to the effect of continental insulation.

Continental insulation refers to the idea that an unbroken continental crust such as that provided by a supercontinent may act as a blanket resulting in a slow build up of heat over tens to hundreds of millions of years in the underlying mantle. Eventual continental breakup will lead to enhanced magmatism and thicker ocean crust along these previously insulated regions.

The Pangaean paleogeography of the Triassic (252 million to 201 million years ago) is depicted in the map below. The distribution of continents is lopsided covering the sites of the future Atlantic and Indian Oceans.

 Source: Paleobiology Navigator