 |
 |
The influx of Indian crust into the Tibetan system thickens the Tibetan crust as fast or faster than it is thinned by E-W extension or Estward crustal flow. Collapse is a misleading term. Convergence/thickening is winning. Collapse/High GPE is the the consequenc of ongoing collision. It all goes back to thinking about a sandpile. ________________________________________ From: Tectonics & structural geology discussion list [[log in to unmask]] On Behalf Of Brandon, Mark [[log in to unmask]] Sent: Friday, December 09, 2011 11:23 AM To: [log in to unmask] Subject: Re: Orogenic collapse Mike Searle brings up a very important issue. At present, people assume that if there is horizontal extension, then the topography must be collapsing. The orogenic collapse idea is widely invoked.... but there is often little direct evidence for the collapse part of the idea. Hopefully we will have this information soon as GPS vertical velocities become better resolved. As for other areas, Neil Mancktelow has made the argument that E-W extension in the Alps (Simplon normal fault, and others) was active during convergence so that that the Alps maintained a steady balance in topography. I suspect that this result may be more common. The Apennines and Crete are other example where shortening and extension are ongoing, in a fashion as originally proposed by John Platt in 1986. The core argument for orogenic collapse is the positive GPE anomalies in orogenic regions should be associated with outward viscous flow. As proposed, GPE-drive flow should occur at all times, regardless of whether there is active plate convergence or not. Olivier Vanderhaghe's definitions are good ones, but they are hard to apply for non-modern orogenic zones since the structural and metamorphic geology do not provide the information needed for his criteria. Best, Mark Brandon On Dec 9, 2011, at 5:13 AM, mike searle wrote: > I beg to differ Peter. > If you define 'extensional collapse' as extension as a result of crustal thickening, then it is not happening in Tibet at all. Tibet is not collapsing; the plateau is high (average elevation 5 km) has thick crust (70-90 km) and has been under North-south compression since the India - Asia collision 50 m.y. ago and still is today from GPS. The high-angle normal faults aligned N-S in Tibet do not result from 'collapse' because India is still underthrusting beneath the plateau and jacking it up from below, and there is still net convergence. > > The E-W aligned low-angle normal fault (South Tibetan Detachment) was operative during late Oligocene-Miocene crustal thickening, ky- sill ± crd metamorphism-migmatisation of the Greater Himalayan sequence between ca 24-15 Ma, but is not active today. The STD was active during continual N-S compression so does not indicate 'collapse' at all. The STD low-angle normal fault cannot have been rotated from high-angle to low-angle as this would imply unreasonable crustal thickness (>120 km, the distance you can walk along the low-angle fault north of Everest for example) in Tibet, so it was a 'passive roof fault' or 'stretching fault (in the sense of Means) with the hanging-wall (Tethyan Himalaya) locked above as southward extrusion >120 km occurred in the footwall - Channel Flow with a mid-crustal partially molten core bounded by ductile shear zones top (STD - right way-up isograds) and below (Main central Thrust, inverted isograds). > > Neither set of normal faults in Tibet indicate 'collapse' (lowering of surface elevation, or thinning of crust) at all. > > Searle, M.P., Eliott, J.R. Phillips, R.J. & Chung, S-L. 2011. Crustal-lithospheric structure and continetal extrusion of Tibet. Journal of Geological Society, London 168, 633-672, doi: 10.1144/0016-76492010-139 > > tashi delek! > > Mike Searle > > Clift, Peter wrote: >> Dear Yvette >> Well some good examples of collapse following arc-continent collision are the Irish Caledonides in Connemara where collapse lasted ~15 to 20 my, but it is even faster in Taiwan where the whole process of collision is only last ~5 Ma and the extensional collapse phase may be even faster. Extensional collapse of the Betic-Rif Orogen lasted ~20 my to form the Alboran Sea. Extension here continued to drive thinning and formation of a deep basin probably because of the delamination of the mantle lithospheric root. Extensional collapse in Tibet appears to also have been ongoing since ~20 Ma but will be longer lasting than that because the process is still ongoing. >> I hope this is useful >> Best wishes >> Peter >> On 09/12/2011 03:39, "Yvette Kuiper" <[log in to unmask]> wrote: >> Dear experts and creative thinkers, >> In my Thursday evening ponderings I am wondering how long orogenic >> collapse can continue after convergence ends. 10 Ma? 50 Ma? What makes >> orogenic collapse continue for a longer time (e.g., size of orogen, >> certain tectonic settings)? Do we know? Any good examples? Ideas? I'd >> love to hear. >> (not talking about crustal extension that follows orogeny in some >> places, but solely collapse as a result of the crustal thickening) >> I'm hoping for a fruitful discussion! >> Cheers, Yvette >> -- >> Yvette D. Kuiper >> Assistant Professor, Structural Geology >> Department of Geology and Geological Engineering >> Colorado School of Mines >> 1516 Illinois Street >> Golden CO 80401 >> Tel. 303-273-3105 >> Fax 303-273-3859 >> http://geology.mines.edu/econgeol/ykuiper.html >> ------------------- >> Peter Clift >> School of Geosciences >> University of Aberdeen >> United Kingdom >> http://www.abdn.ac.uk/~wpg008/PChomepage.html >> ------------------- >> After 9th January 2012 >> Department of Geology and Geophysics >> Louisiana State University >> Baton Rouge, LA 70803, USA >> *New email: [log in to unmask] * (now active) >> The University of Aberdeen is a charity registered in Scotland, No SC013683. > > -- > ****************************************** > Professor Michael P.Searle > Dept. Earth Sciences > Oxford University, > South Parks Road., > Oxford, OX1 3AN > England > Professor of Earth Sciences, and > Senior Research Fellow, Worcester College, Oxford. > > Tel: +44 1865 272022 > Fax: +44 1865 272072 > > Mike Searle's Home Page: http://www.earth.ox.ac.uk/~mikes > *******************************************