Incision migration across Eastern Tibet controlled by monsoonal

climate, not tectonics

1. Department of Earth Sciences, Durham University 2. Department of Geography, University of Glasgow

Katharine Groves 1

Mark Allen1 - Chris Saville1 - Martin Hurst2 - Stuart Jones1

The central Tibetan Plateau hashigh elevation and low relief andis internally drained.

The eastern Tibetan Plateau isdrained by major rivers,including the Yangtze, Mekongand Salween.

In this study we analyse themodern geomorphic andprecipitation data to highlight atransition zone in thelandscape.

We interpret this transitionzone in terms of competingmodels of Tibetan Plateau upliftand growth.

Salween

Mekong

Yangtze

InternallyDrained

Eastern Tibetan Plateau

CentralTibetan Plateau

In the eastern Tibetan Plateauthere are competing tectonicmodels of plateau uplift.

These suggest either:

• early Cenozoic plateaugrowth

or

• a late phase (Miocene) ofcrustal thickening, surfaceuplift and plateau growthdriven by lower crustal flow(“channel flow”) from thecentral Tibetan Plateau.

The results of this study suggestthat the plateau formed by earlyplateau growth followed bymonsoon intensification.

Salween

Mekong

Yangtze

InternallyDrained

Eastern Tibetan Plateau

CentralTibetan Plateau

Precipitation is controlled by theEast Asian Monsoon

Precipitation decreases fromeast to west from over 1000mm/yr at the Longmen Shan toless than 200 mm/yr in thecentral, internally drainedregion.

Longmen Shan

InternallyDrained

Geomorphic analysis show thehigh elevation, low relief areas,within the internally drainedTibetan Plateau:

Surface Roughness (SR):

High SR (yellow) indicatesvariable topography within eachdrainage basin and is usuallyassociated with steeper slopesoccurring in areas with highincision.

Low SR (purple) indicates lessvariable topography, showingflatter landscapes and indicatinglow uplift and incision.

The standard deviation of slope withineach second order drainage basin,calculated from 90m SRTM data.

17 swath profiles were drawnorthogonal to and centred onthe internal-external drainageboundary. Swath profiles were of30 km width. Profiles were takenof elevation and mean annualprecipitation (GPM, 2002-2018)and SR.

Along each profile the maximumchange in trend (shown by thepurple vertical lines) from south-east to north-west to find:• A change to constant high

elevations• Decline in SR• Sharp decline in rainfall

(interpreted as the westernextent of the East Asianmonsoon)

The location of these changeswas mapped

A broad transition zone ispresent in the landscape,where changes in landscapeand precipitation are groupedand in alignment.

It represents, from East toWest, a sharp decline inprecipitation below ~650mm/yr (interpreted as thewestern extent of the EastAsian monsoon), a change froma high relief landscape tosmoother elevations at 4500-5000 and a decrease in SR .

This zone is not a drainagedivide: the main rivers havetheir headwaters further West,in the interior of the plateau.The zone cuts across structuralboundaries.

Strike-slipThrust

Normal

Compiled thermochronology datashows an increase in exhumationfrom ˜25 Ma in the area to thesouth east of the monsoon limit(blue line). There is no evidence ofthis increased exhumation to thenorth west of the monsoon limit.

The channel flow model predicts awest to east wave of uplift andexhumation across the plateau asthe landscape is uplifted during theMiocene (< 23 Ma).

This is not shown by thethermochronology data andtherefore does not support thechannel flow model.

Exhumation Probability Distributions

Data compiled by Li et al., (2019).

Within Monsoon Limit

NW of Monsoon Limit

Internally Drained

Incised

Non-incised

Early plateau growth predicts littleexhumation in the plateau interiorduring the past ˜25 Ma, as is seen tothe NW of the monsoon limit.

We suggest that increasedexhumation since ˜25 Ma to thesouth of the identified geomorphic-climatic transition zone is due tointensification of the monsoon atthis time.

The transition zone thereforerepresents a change from an incisedto non-incised landscape, controlledby the western extent of themonsoon (blue line).

Erosion of the plateau would lead toa east to west regression of thesteepest part of the landscape. Thezone may represent the currentposition of an east to west migratingwave of incision driven by monsoonintensification.

Data compiled by Li et al., (2019).

Exhumation Probability Distributions

Incised

Non-incised

In conclusion:

• We find a geomorphic andclimatic transition in the eastern-central Tibetan Plateau. Wesuggest that the precipitationtransition represents the westernlimit of the East Asian monsoon.

• Thermochronology data isconsistent with early uplift of theeastern Tibetan Plateau withincreased exhumation from ˜25Ma, consistent with lateCenozoic intensification of themonsoon climate.

• This work supports a model ofearly Cenozoic growth of theeastern Tibetan Plateau,superimposed by incision drivenby climate change; it does notsupport the channel flow model.

Data compiled by Li et al., (2019).

Exhumation Probability Distributions

References:

Duvall, A.R., Clark, M.K., Avdeev, B., Farley, K.A. and Chen, Z., 2012. Widespread late Cenozoic increase in erosion rates across the interior of eastern Tibet constrained by detrital low‐temperature thermochronometry. Tectonics, 31.

Li, H.A., Dai, J.G., Xu, S.Y., Liu, B.R., Han, X., Wang, Y.N. and Wang, C.S., 2019. The formation and expansion of the eastern Proto-Tibetan Plateau: Insights from low-temperature thermochronology. Journal of Asian Earth Sciences, 183, 103975.

Shahzad, F. and Gloaguen, R., 2011. TecDEM: A MATLAB based toolbox for tectonic geomorphology, Part 1: Drainage network preprocessing and stream profile analysis. Computers & Geosciences, 37, 250-260.