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LH

HHC

TSS

Structure of the Himalayas

Modified from Lavé & Avouac, 2001 ITS Indus Tsangpo Suture TS Tethyan Sedimentary Series STD South Tibetan Detachment HHC Higher Himalayan Crystalline MCT Main Central Thrust LH Lesser Himalayas MBT Main Boundary Thrust Siwaliks MFT Main Frontal Thrust

Galy, 1999

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LH

HHC

TSS

Structure of the Himalayas

Modified from Lavé & Avouac, 2001 ITS Indus Tsangpo Suture TS Tethyan Sedimentary Series STD South Tibetan Detachment HHC Higher Himalayan Crystalline MCT Main Central Thrust LH Lesser Himalayas MBT Main Boundary Thrust Siwaliks MFT Main Frontal Thrust

S

LH

HHC

TSS

Structure of the Himalayas

Modified from Lavé & Avouac, 2001 ITS Indus Tsangpo Suture TS Tethyan Sedimentary Series STD South Tibetan Detachment HHC Higher Himalayan Crystalline MCT Main Central Thrust LH Lesser Himalayas MBT Main Boundary Thrust Siwaliks MFT Main Frontal Thrust

Looking South towards the high summits

S

LH

HHC

TSS

Structure of the Himalayas

Modified from Lavé & Avouac, 2001 ITS Indus Tsangpo Suture TS Tethyan Sedimentary Series STD South Tibetan Detachment HHC Higher Himalayan Crystalline MCT Main Central Thrust LH Lesser Himalayas MBT Main Boundary Thrust Siwaliks MFT Main Frontal Thrust

A HUGE normal fault in the middle of the largest mountain range on Earth!!!

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N

S

LH

HHC

TSS

Structure of the Himalayas

Modified from Lavé & Avouac, 2001 ITS Indus Tsangpo Suture TS Tethyan Sedimentary Series STD South Tibetan Detachment HHC Higher Himalayan Crystalline MCT Main Central Thrust LH Lesser Himalayas MBT Main Boundary Thrust Siwaliks MFT Main Frontal Thrust

S

LH

HHC

TSS

Structure of the Himalayas

Modified from Lavé & Avouac, 2001 ITS Indus Tsangpo Suture TS Tethyan Sedimentary Series STD South Tibetan Detachment HHC Higher Himalayan Crystalline MCT Main Central Thrust LH Lesser Himalayas MBT Main Boundary Thrust Siwaliks MFT Main Frontal Thrust

S

LH

HHC

TSS

Structure of the Himalayas

Modified from Lavé & Avouac, 2001 ITS Indus Tsangpo Suture TS Tethyan Sedimentary Series STD South Tibetan Detachment HHC Higher Himalayan Crystalline MCT Main Central Thrust LH Lesser Himalayas MBT Main Boundary Thrust Siwaliks MFT Main Frontal Thrust

This is the very front of the Himalayas! MFT

Lavé et al., 2005

III. Erosion controls the structure of mountains? 2) The curious case of the Himalayas Puzzling: a huge plateau behind the range and a gigantic normal fault in the middle of the range?! Localization of erosion could explain both features… The growth of the Himalayas did affect profoundly atmospheric circulation  monsoon + aridification of the zone North of the main divide

Bookhagen and Burbank, 2006

(Tapponnier et al., 2001)

Arid Tibetan plateau: erosion 14 mm/yr), M medium (4-14 mm/yr) or L low (< 4 mm/yr). Effective internal angle of friction for the upper crust: 5 or 15 degrees. Upper crustal rheology: viscosity with respect to Wet Quartzite Flow Law (WQz).

Can focused erosion lead to focused exhumation?

NUMERICAL MODELLING

Beaumont et al., 2001: the CHANNEL FLOW theory S

LH

HHC

TSS

Modified from Lavé & Avouac, 2001

Good agreement between model, thermochronologic and PTt data! But why is the angle of friction for the upper crust so low, and where is the channel flow now? (the MCT is now inactive…)

IV. To which extent does erosion affect deformation in mountains? “Revisiting river anticlines”. Montgomery and Stolar, 2006

Montgomery and Stolar, 2006

Unloading  local rebound / uplift. Can be isostatic (passive) or fed by channel flow (active: focused erosion focus exhumation)

IV. To which extent does erosion affect deformation in mountains? “Revisiting river anticlines”. Montgomery and Stolar, 2006

The growth and development of Himalayan river anticlines are not explained well by classical explanations for relationships between river courses and geological structure. Re-examination of the potential role of differential bedrock erosion suggests that rivers appear able to influence the development of geological structures where there are sustained gradients in erosion rate and either a crustal rigidity low enough to permit localized isostatic rebound, or where facilitated by active feedback between tectonic and erosional processes such as that leading to channeling of crustal flow. Consequently, rivers may be the authors not only of their own valleys, but in some circumstances of the structural geology of the surrounding mountains as well.