Hi Peter
I am not sure I understand your point here. If the extension around the
Ilan Plain is related to gravitational collapse of the northern part of
the Taiwan mountain belt, then it seems that you are also saying that the
Okinawa Trough is related to this gravitational collapse. At least where
it comes ashore in northern Taiwan. Or, are you saying that is it just
coincidence that the Okinawa Trough, which, as you know, extends all the
way south from the Japan Sea comes ashore in exactly the location where
very localised gravitational collapse (extensional faulting is maybe a
better term here) of the Taiwan mountain belt is taking place? I would
suggest that the extension and volcanism around the Ilan area is related
to extension in the Okinawa Trough, which in turn is related to the larger
scale plate tectonic processes taking place to the east and north of
Taiwan rather than to gravitational collapse of this part of the Taiwan
mountain belt.
As you also know, there is another strange coincidence in the Ilan area.
That is, the Lishan Fault, a crustal-scale fault that extends north -
south through almost all of Taiwan, terminates at just this point in the
north. The kinematics of this fault are still a bit unclear, but over most
of its strike length it seems to be some sort of oblique east-directed
thrust.
Dennis
> That's right Dennis, I am referring to the extension in the Ilan Plain
> basin in northern Taiwan and the SW end of the Okinawa. The lag time
> between collision in the south and collapse in the north is around 4-5 Ma
> in this case. You are totally right that the island is basically in
> compression and uplifting and that only the north is extending. I guess we
> have to agree to disagree that this extension in the north is or is not
> related to gravitational collapse. If it is as I would argue then the
> during of this collapse is very short indeed, just 1-2 Ma.
> Peter
>
>
> On 09/12/2011 16:48, "Brown_Dennis" <[log in to unmask]> wrote:
>
> Peter
> I don't know what data you have to say that Taiwan is collapsing.
> Seismicity and GPS data clearly demonstrate that it is under compression
> and, east of the foreland basin, building topography at a rate of about 6
> cm a year (locally more). Where it has been measured, compressional (or
> transpressional) slip on sevaral of the active faults in the foreland is
> about 45mm per year. While seismicity shows that there is extensional
> faulting active in places, the majority of these types of fault mechanisms
> are in the upper 10 km of crust and local. In general, the dominate fault
> mechanisms are thrust to strike-slip.
>
> My guess is that you are talking about extension along the northern tip of
> the island, in the Ilan area where the Okinawa Trough comes ashore. But,
> this is a different process (i.e., not gravitational collapse) and it
> can't be extrapolated across the entire island.
>
> Cheers
> Dennis
>
>
>> I am not sure that Mike and I really disagree here except on the basis
>> of
>> what collapse means. I concur that the Tibetan Plateau has been getting
>> bigger and maybe even taller over the past 20 Ma or so but as he notes
>> it
>> has been experiencing E-W extension since at least 20 Ma. This is being
>> more than made up for by the continued growth but to my mind at least
>> that
>> does not mean it is not also collapsing while also being built. The
>> South
>> Tibetan Detachment is a different animal altogether. While it has a
>> normal
>> sense of motion it can be viewed as just the lid of an erosionally
>> controlled extruding unit and does not really involve collapse of the
>> plateau edifice. If collapse in Tibet is taking a long time compare to
>> other examples it is only because of the relentless collision between
>> India and Asia that keeps the mountains up.
>>
>>
>>
>> On 09/12/2011 10:13, "mike searle" <[log in to unmask]> 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
>> *******************************************
>>
>>
>> -------------------
>>
>> 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.
>>
>
>
> -------------------
>
> 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.
>
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