Dear Yvette,
I wish that the exercise of deciphering the trigger/cause of extension in
orogens (especially old orogens, where the lack of time markers becomes a
significant hurdle to detailed time/space reconstructions) was so
straightforward to always allow a confident separation between "crustal
extension that follows orogeny in some places, and collapse as a result of
the crustal thickening".
I myself have often troubles with this very point. An extensional shear zone
looks the same irrespective of what the cause for that extension is.
Having said this and coming to the point of the duration: we recently
published a paper on the Mesoproterozoic Sveconorwegian orogen where we
discuss geochronological and structural evidence for an extremely protracted
phase of extension - possibly more than 70 Ma (Viola et al., 2011,
Precambrian Research: The GrenvillianSveconorwegian orogeny in
Fennoscandia: Back-thrusting and extensional shearing along the ³Mylonite
Zone²).
Was this all collapse? We certainly know that the orogen was large and that
overthickening is very likely. Moreover, spatial relationships between
compression and extension are complex and extension began in the east when
in the west shortening was still being accommodated.
Also, when working with the Pan African Mozambique belt, we proposed that
collapse was the likely trigger of extension at about 535 Ma. Some late
leucosomes in the footwall of large detachments were dated at 480 Ma, and
Ar-Ar ages on amphibole confirm these numbers (Viola et al., 2008,
Tectonics: Growth and collapse of a deeply eroded orogen: Insights from
structural, geophysical, and geochronological constraints on the
Pan-African evolution of NE Mozambique).
So, even there we believe that "extension" lasted at least 55 -60 Ma.
I am thus convinced that 50 -60 Ma can be a reasonable time span - the real
question is how long the actual collapse lasts, that is, how much thinning
do you need to vanish the effects of the gravitational instabilities? And,
in addition to that, how long can extension carry on after you zero the
instabilities?
The importance of far-field extensional plate boundary stresses are hard to
decipher in an already extending orogen.
Just some food for thought.
Ciao
Giulio
-------------------------------------------
Giulio Viola, PhD
Senior structural geologist
Geological Survey of Norway - NGU
Leiv Eirikssons vei 39
74911 Trondheim
Norway
Tel: 0047-73-904236
and
Adjunct Professor of Structural Geology
Department of Geology and Mineral Resources Engineering
Norwegian University of Science and Technology - NTNU
Sem Sælands veg 1
7491 Trondheim
Norway
On 12/12/11 5:40 AM, "Yvette Kuiper" <[log in to unmask]> wrote:
> Hi All,
>
> I just want to thank you all for an excellent discussion! I am certainly
> learning a lot and getting more of a feel for what is and is not
> possible, even if there is no straight answer to my original question
> (as expected). I'll be spending time reading your references you sent me
> and learning more. Meanwhile please do keep those interesting thoughts
> coming :-)
>
> Thanks a lot!
> Yvette
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