Tuesday, July 31, 2018

Papers: Himalaya Foreland Basin Evolution

Foreland basins are moat like depressions that form in response to the loading of the crust by a rising fold and thrust mountain belt. They get filled up by sediment derived from the erosion of the mountain chain. The sediment composition of foreland basins and how it has changed through time is therefore an archive of the uplift and erosional history of the orogen. The Himalayan foreland basin contain a few kilometer thick pile of sediments. One broad scenario of foreland basin evolution goes like this. The oldest of these foreland sedimentary successions of Palaeocene-Mid Eocene age (~ 55- 45 my) are marine deposits formed in the ever narrowing Tethys Ocean. Uplift of the region due to the ongoing India-Asia collision resulted in the complete withdrawal of the sea. There was no sedimentation for 10-12 million years. After this depositional hiatus, sedimentation resumed in Early Oligocene times (~ 31 my) in a continental fluvial setting and continues till today.

This timing of events representing the transition from marine to continental conditions has been challenged by some workers. They have proposed that the depositional hiatus is of different magnitude at different places depending upon the effect of local tectonics. Estimates of the duration of the unconformity between marine strata and the overlying fluvial deposits range from 25 million years (early withdrawal of the sea) in the Kohat Basin of Pakistan to 3 million years (marine conditions persisting until around 31 million years ago) in the Subathu Basin of Himachal Pradesh, India. Continental depositional environments then formed between 28 million years ago to around 20 million years ago in different basins. River systems have deposited a thick succession of sediments since.

About 1 -0.5 million years ago these sediments were uplifted to form the well known Siwalik ranges. The locus of deposition shifted southwards. The alluvial plains of the Ganga river system is the present day foreland basin.

In this map of the Himalayan orogen, the southernmost belt, abbreviated as Ts, is the deformed foreland basin.

Source: An Yin 2006

..and in the satellite imagery, the foreland basin are the hill ranges to the south of the brown line. This line is a system of thrust faults that place the Lesser Himalaya on top of the foreland basin strata.

Provenance fingerprinting of the oldest foreland sediments indicate a Himalayan source. That tells us that incipient ranges in the collisional zone formed by around 50 million years ago. Since then, successive pulses of thrusting have uplifted terrains of differing composition. Their exhumation and erosion is recorded in the changing sediment mixture of the foreland basin strata. The schematic below reconstructs the Early Miocene to Pliocene uplift history of the Nepal Himalaya. The Dumri Formation and the Siwalik Group are the foreland basin sediment archive of this uplift.

Source: Peter G DeCelles et. al. 1998.

I've come across quite a few papers on these Himalaya foreland basin sediments. They focus on using a variety of techniques like sedimentary facies analysis, petrography, geochemistry and geochronology to unveil foreland basin geometry, foreland drainage patterns, paleo-climate and soil formation, and the timing of emplacement and erosion of different thrust sheets.

1) Evolution of the Paleogene succession of the western Himalayan foreland basin - B.P. Singh 2013.

2) Evolution of the Himalayan foreland basin, NW India : Yani Najman et. al. 2004.

3) Eocene-early Miocene foreland basin development and the history of Himalayan thrusting, western and central Nepal:  Peter G DeCelles et. al.  1998.

4) Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal: Peter G DeCelles et. al. 1998. (Behind Paywall).

5) Detrital geochronology and geochemistry of Cretaceous–Early Miocene strata of Nepal: Implications for timing and diachroneity of initial Himalayan orogenesis: Peter deCelles et.al 2004. (Request full text).

6) The Os and Sr isotopic record of Himalayan paleorivers: Himalayan tectonics and influence on ocean chemistry: John Chesley et.al. 2000. (Request full text).

7) Early Oligocene paleosols of the Dagshai Formation, India: A record of the oldest tropical weathering in the Himalayan foreland: Pankaj Srivastava et. al. 2013.

Update: August 1 2018

Geologist Vimal Singh reminded me of some more studies on early foreland basin evolution as well as the excellent work of Rohtash Kumar on various aspects of the later Siwalik Group sediments. I am adding these papers to the list.

 8) Marine to continental transition in Himalaya foreland - Bera et.al 2008

9) Reconstructing early Himalayan tectonic evolution and paleogeography from Tertiary foreland basin sedimentary rocks, northern India- Yani Najman and Eduardo Garzanti 2000 (Behind Paywall)

10)  Sedimentary  Architecture  of  Late Cenozoic  Himalayan  Foreland  Basin  Fill: An  Overview:  Rohtash Kumar et. al. 2011

A collection of Rohtash Kumar's papers on the Siwaliks can be viewed here.

I was somewhat unfamiliar with the Eocene-Early Miocene age deposits of the Himalayan foreland, and so was glad to have found studies that deal with the record of the earliest stages of Himalayan uplift.

Open access except where indicated.

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