Yvette
You're asking some interesting questions - although it's not easy to
give very quantitative answers.
Just to clarify, I guess you're referring to cases where there is
orogen-parallel stretch of a wide zone of deformation relative to the
undeformed orogen foreland and hinterland – i.e. there is
orogen-parallel extrusion?
Obviously, in shear zones there's usually lots of evidence of extension
(stretch), because (almost) any shear will produce a shape change, and
may involve local rotation and stretch. For example, zones of simple
shear as well as transpression/ transtension zones will all usually show
evidence of localised ‘extension’ – the strain ellipsoid is rotating and
extending in response to the non-coaxial component of deformation.
Therefore, as I’m sure you know, structures seen in the field, such as
rotated lineations and rotated and/or extended fold hinges are
themselves not indicative of one particular set of boundary conditions,
least of all orogen-parallel extrusion; for this reason, in practice it
can be really difficult to be sure whether or not orogen-parallel
extrusion has really taken place.
And as you imply in one of your questions, there are theoretical
problems with large amounts of orogen-parallel extrusion, in terms of
strain compatibility (John Ramsay’s “cream-cake” tectonics). On the
other hand, there are modern-day areas such as Anatolia where there is
plate-scale lateral motion of crustal blocks in response to complex
oblique collision, so it does appear possible on the plate-scale.
Regards
Richard Jones
Geospatial Research Ltd.
Department of Earth Sciences
University of Durham
UK
Yvette Kuiper wrote:
> Hi All,
>
> In the past few years I keep finding myself in shear zones
> (transpression zones) and orogens that seem to show good evidence for
> shear zone-parallel or orogen-parallel horizontal stretch. The
> evidence is based on rotated lineations and fold hinge lines that are
> best explained by such stretch. It makes me wonder how much shear
> zones or orogens can stretch parallel to their strikes in nature. At
> some point, there would be space problems and strain compatibility
> problems. My questions to you:
>
> What is the maximum shear zone-parallel horizontal stretch anyone
> recorded?
> What was the length of that shear zone?
> Was the stretch localized or homogeneous along the entire length of
> shear zone?
>
> What is the maximum orogen-parallel horizontal stretch anyone recorded?
> What was the length of the orogen?
>
> If you have any thoughts on any of these questions, I would be very
> interested to hear.
>
> Thanks, Yvette
>
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