Mountains Lecture 4- Poking around the rubbish bin: what we learn from large-scale patterns of erosion Flashcards
Can detritus from a moutain range tell us about its evolution?
Yes
Do mountain building processes generate huge volumes of sediment?
Yes
How is sediment transported from the mountains to the ocean?
Rivers
Where is sediment thickest?
Around continental shelves and near mountain ranges
Where can thick accumulations of sediment be found? What can they tell us?
In line with past and present glacier/ice sheet limits.
Can tell us about glacial processes and dynamics.
What do ocean sediment cores provide a valuable record of?
How the environment has changes over time.
The chemical changes of the ocean (recorded by the sediment) help to reflect the evolution of the earth system.
What can analysis of ocean sediment cores tell us?
Sediment age
Sediment accumulation rate
Sediment size (tell us about transport distance)
Sediment provenance and geochemical composition (can tell us about source area)
What drives mountain building?
Climate and tectonics
Can signatures of climate and tectonics be found in sediment records?
Yes
What can we start to investigate by integrating sediment core records over time?
Effects of large-sclae tectonic/climate events.
E.g. onset of India-Asia collision shows a gradual increase in accumulation rates over time.
What does Clift et al. findings around major river systems draining in SE Asia tell us?
Sediment cores from each river outlet show that the onset of rapid accumulation doesn’t seem to coincide with India-Asia collision at 50 Ma.
‘Glacial’ rates of last 2-3 Myr are not always higher than mid-Cenozoic rates.
Aside from large-scale tectonics and climatic shifts, what are other important controls on sediment flux?
Rivers can be diverted or captures during mountain building, this leaves a record in deep-sea sediments.
What did Clift et al. (2005) find surrounding sediment in the Himalayas?
Using differences in neodymium isotope rations to show that sediment from the Himalayas (as opposed to Pakistan/Arabian shield) became more important in the Indus fan after about 5 Myr ago.
Uplift in the region caused the redirection of the water courses.
On a continental scale, where has uplift caused redirection of rivers?
Uplift of the Rockies after around 60 Ma caused the redirection of rivers south.
This can be shown by sediment.
Can small scale shifts in tectonic and/or drainage have continental-scale impacts? Example.
Yes
E.g. ‘Three Rivers’ area in SE Tibet. (Red- used to carrge large component of Yangtze Craton, Yangtze and Tsangpo-Brahmaputra). Motion along Tiger Leaping Gorge caused shift from Red River to Yangtze around 40 Myr ago. Geochemical composition of sediments collected at river outlets show how drainage of this region has evolved over time.
Can large river systems be diverted, blocked and captured?
Yes
Provide two examples of the types of data we are extract from a sediment core that can tell us something about mountain building.
Sediment age, accumulation rates, size, provenance and geochemical composition
How might accumulation rates (recorded in offshore sediment record) change due to renewed uplift of a mountain range?
Increase! But this is not always the case – rivers can redirect sediment, making the record a little messy
What can neodymium isotopes of sediment cores from large basins in the Himalayan region tell us about past drainage?
Sediment provenance and drainage reorganisation
Explain what caused a continental-scale shift in drainage for E. Tibet from ~40 Ma.
Motion along small fault in Tiger Leaping Gorge caused the Yangtze River to capture much of the area that used to drain into the Red River
Why do we have to exercise caution when interpreting sediment records?
Records are not always direct measures of something. Often can be used to infer things. It is also important to corroborate theories with additional evidence!
