Hi all,
first of all, we would need to define exactly the meaning of "orogenic
collapse" vs. "orogenic extension", and this is not an easy task as the
processes are intimately related and overlapping. Even if we do reach a
conclusion as to what these processes should be and how they differ from
each other, distinguishing them from the field and other data is a
completely different ball game, especially in old orogens that are no longer
active. For example, in southern Finland, field and seismic reflection
evidence indicate that the 1850 Ma old Svecofennian orogen extended by
syn-convergence mid-crustal flow lasting up to 40 Ma, but there is no sign
of crustal thinning due to collapse IF you consider the anomalous thickness
of the lithosphere as evidence of that. On the other hand, we don't know how
thick the lithosphere originally was and how the thickness was
(re)distributed during the different orogenic phases, and the evidence of
extension we have from field and seismic reflection data could also be
interpreted to imply collapse, at least partly (again depending on how you
define these terms).
Best regards
Taija
**********************************
Dr Taija Torvela
Lecturer
Structural geology and 3D modelling
University of Helsinki
Department of Geosciences and Geography
Division of Geology
PO Box 64
FI-00014 University of Helsinki
Finland
+358 9191 51716
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-----Original Message-----
From: Tectonics & structural geology discussion list
[mailto:[log in to unmask]] On Behalf Of mike searle
Sent: 9. joulukuuta 2011 12:13
To: [log in to unmask]
Subject: Re: Orogenic collapse
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
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