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Mark, I keep an open mind and welcome more work!! Especially since it's hard to figure out rates of processes in deeper crust by looking at upper-crustal geology or geophysical images. But I'll keep the same bet that I made when I was an insecure Asst. Prof in 2004, Geology, Indian Punch Rifts Tibet. It's really the simplest scenario based on Intro to Structural Geology basics. Then recently, there are GPS data indicating that Tibet is rising (which I won't bet on given complexities like lakes drying up resulting in isostatic rebound), and people are shocked because the plateau is supposed to be collapsing.... So many misconceptions. Like extension occurs because something pulls it apart. No. It gets pushed apart by gravity. Vertical compression. Only real tensile structures I'm aware of occur near surface (joints) and in tearing, subducting slabs (due to negative bouyancy). Everywhere else, gravity is pushing rocks toward lower elevations. The result is horizontal extension (strain) but due to vertical compression (stress). Paul ________________________________________ From: Tectonics & structural geology discussion list [[log in to unmask]] On Behalf Of Brandon, Mark [[log in to unmask]] Sent: Saturday, December 10, 2011 4:39 PM To: [log in to unmask] Subject: Re: Orogenic collapse Paul, Thanks for your comments. Sounds like you have your money on the option that the surface of Tibet is either steady or rising. At a seminar a year ago at ETH Zurich, Peter Molnar put his money on the option that Tibet is "collapsing". If I remember correctly, he suggested that the modern rate of elevation change is about 1 mm/a. It is bit unfair to focus exclusively on elevation, because GPE is associated with the topography of both the surface and the root. Another unknown is changes in dynamic topography (as related to mantle convection). In any case, the issue seems far from settled. Good thing.... it means that there is more work to be done. Best, Mark On Dec 10, 2011, at 5:45 PM, Kapp, Paul A - (pkapp) wrote: > Mark, > > There is no direct evidence. Some recent GPS suggests uplift, but I take that with a grain of salt. > > But here is history: ~18 Ma appearance of N-S dikes in S. Tibet, but no major extension. Sigma 1 went vertical but differential stress not large enough for normal faulting. 13-8 Ma: onset of significant extension based on thermochron and basin record. Today, extension is rapid- up to 20 mm/yr E-W in central plateau. Extrapolate that to >10 Ma, would suggest >200 km of finite extension. We don't see that much. My conclusion = extension has accelerated with time as vertical stress has increased as a result of progressive crustal thickening. The evolution in stress makes sense. How else could you maintain, let alone accelerate, extension? Need to keep the GPE up. Keep removing mantle lithosphere beneath the system or keep thickening the crust. I favor the latter given geophysical images of the system and that >40 km thick Indian crust is underthrusting the Himalaya at a rate of ~20 mm/yr. There is the added complexity of plate boundaries to the east. This could have decreased sigma 3 and accelerated extension. But you still have to keep vertical stress in Tibet greater than collisional stress. > > To be fair, if eastward crustal flow is winning, then Tibet elevation should have been higher in past. Existing paleoelelevation studies indicate elevations as high or higher than today. But in this scenario, I would expect a decrease in extension rate with time and an eventual return to strike-slip faulting. > > Paul Kapp > Associate Professor > Department of Geosciences > Gould-Simpson Building #77 > University of Arizona > Tucson, AZ 85721 > (O) 520-626-8763 (F) 520-621-2672 > > -----Original Message----- > From: Tectonics & structural geology discussion list [mailto:[log in to unmask]] On Behalf Of Brandon, Mark > Sent: Saturday, December 10, 2011 11:59 AM > To: [log in to unmask] > Subject: Re: Orogenic collapse > > Paul: Is there direct evidence to support your statement? Do we know that the surface of Tibet is either steady or rising, but definitely not falling? > Mark > > On Dec 10, 2011, at 1:58 AM, Kapp, Paul A - (pkapp) wrote: > >> 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 >>> *******************************************