 |
 |
I just wanted the chance to reply to some of Mike’s points First the MCT (and STD) are not active faults; they were during the Miocene but are they not active now That’s true but there is evidence that thrust faulting is focused by erosion patterns in the recent past and could have done so during the Miocene too (likewise Channel flow, the exhumation of a layer of partially melted mid-crust in the Himalaya during the Miocene is probably not active now). The active faults are along the southern boundary of the Himalaya (MBT, MFT). Indeed, but that does not mean that the MCT was not climatically located when it was active, Indeed it is hard to see how it could not have been since the Channel Flow only works when erosion removes the shallower rocks allowing the channel to flow. Second Rainfall most certainly did not control initiation of the MCT (or the STD). Tectonics controlled this, not Rain. That is not the way that the Channel Flow model is described. The channel onlu flows because of the gravitational potential caused by the thicker than normal crust but no exhumation would occur without the erosion along the mountain front. Earthquakes generally initiate faults and most earthquakes initiate from above the brittle-ductile transition and at depths down to ~30-40 km and propagate up through the brittle crust. The MCT and STD initiated well down into the ductile mid-lower crust. You are surely not saying that rain/climate penetrates down there (I hope!). I might be. To be precise I am arguing that stresses caused by surface processes (erosion) result in rock uplift which has be compensated for throughout the crust. At shallow levels this results in faulting by these must extend to ductile shear zones at depth. It is only the focused erosion that allows the Greater Himalayan slab to flow so without that erosion there would be no large thrust like the MCT (or STD). This is why we only see these structures on the wet south side of Tibet and not on the north side. The climatic differences between the two Himalayan syntaxes are enormous, both are controlled entirely by tectonic processes with maximum compressive stress in all directions accounting for the rapid vertical exhumation of rocks and the young metamorphic ages in the syntaxes. The big rivers (Bhramaputra in the east; Indus in the west) cut at right angles across both syntaxes, across the areas of high uplift and exhumation. The rivers have nothing whatever to do with the rock uplift (or surface uplift) of either syntaxis. Tectonics control these structures entirely. I agree that tectonic forces are responsible for driving the rock uplift but without the erosional influence I don’t see how you would explain the deep exhumation unless you think there are large extensional faults in the syntaxes too. It rains like hell in the Amazon (and in Oxford sometimes) but I see no major MCT type fault induced here. No of course not because we need rock uplift as well as erosion to generate significant exhumation and thus the development of major faults. However, rock uplift in the absence of erosion does not cause exhumation which is a signature feature of the Greater Himalaya Rainfall has nothing to do with mountain building. Tectonics make mountains and rain etc erodes them away. I don’t agree with that at all. Compressional tectonics causes crustal thickening and both rock and surface uplift but without erosion, modulated by climate there is no exhumation until the orogenic belt experiences large scale gravitational collapse.Thus Tibet is the product of plate tectonic forces but the Greater Himalayas are only possible in the form that we know them because of surface processes Likewise in the Karakoram exhumation of kyanite and sillimanite grade rocks formed at depths of 12 kbar or more are not controlled by glaciation. Glaciers eroded the top 5-10 km of present day topography but they cannot be responsible for exhumation of deep crustal rocks from 12 kbar. Then the exhumation must be caused by other erosional processes unless you think that normal faulting is dominating. I would curious to know why you think that glaciers only account for 5-10 km of erosion. If the rocks were metamorphosed at 12 km then you have to remove that overburden to get them exposed. That seems to require either extensional faulting or erosion. You choose. Greenland, Northern North America and Siberia are covered in huge ice sheets and glaciers, theres no active mountain building going on there. No, because there is no significant rock uplift to drive deep exhumation Whatever induced the Asian monsoon (Tibetan plateau uplift?) and when noone really knows, but for sure the Himalaya are the northern barrier to the monsoonal rainfall today. Indeed they are Rock exhumation rates are pretty similar along the length of the Greater Himalaya (and timing too) This I do not agree with. Ar-Ar cooling ages are generally younger in the east than in the western Greater Himalaya. To my mind this is because the Channel Flow is faster in the east than the west, driven by the heavier rain fall I would suggest. but in the east it is wet and rainy and in the west (Ladakh, Zanskar, Nanga Parbat) dry. We are really only talking about Zanskar here, and that has some of the oldest cooling ages in the Greater Himalaya because it is drier and there exhumes more slowly that the eastern ranges Thus in my opinion Tectonics initiates and controls rock uplift I agree 100% and ductile shear zones, faults and mountain building along the Himalaya, not Rain, Glaciers or Climate. This I do not agree with. Tectonics can do all those things but surface processes can play a role too in focusing where rock uplift occurs and thus where large faults that allow deep exhumation develop. Without surface processes, modulated by climate there can be little exhumation in compression systems. best wishes. I am enjoying the debate Peter ====================== Peter D. Clift Charles T. McCord Chair in Petroleum Geology, Department of Geology and Geophysics, E235 Howe-Russell-Kniffen Geoscience Complex Louisiana State University, Baton Rouge, LA 70803, USA Tel: +1 225-578-2153 Fax: +1 225-578-2302 Email: [log in to unmask]<mailto:[log in to unmask]> http://www.geol.lsu.edu/pclift/pclift/Home.html Attend AGU Chapman meeting "Evolution of the Asian monsoon and its impact on landscape, environment and society”, June 15-19th 2015, Hong Kong http://www.geol.lsu.edu/pclift/Monsoon_AGU_Chapman_Meeting/Welcome.html