Hi All,
The same nappes I was talking about yesterday (see references below)
have overturned rocks over a width of 100 km. This is an interpetation
based on fold geometries and repetitions of interfolded Archean basement
and Paleoproterozoic cover. That makes it actually likely that the
>70-80 km strike-parallel length is much longer than 70-80 km.
I highly doubt that those nappes (or any) initiated as upright folds
with 100 km limb lengths. As you indicate it is pretty well impossible
to have a 100 km vertical fold limb. It is much more likely that they
form by simple shear (see Williams and Jiang 2005, JSG 27, 1486-1504).
One can think of the entire nappe pile as existing within one big shear
zone. Fold nappes may initiate as drag folds (or pre-existing folds).
Their limbs rotate into the extension field of the deformation so they
get stretched. That way it is not so unlikely that large fold nappes
with long limbs form.
Cheers, Yvette
Henderson, J.R., 1981. Structural analysis of sheath folds with
horizontal X-axes, northeast Canada. Journal of Structural Geology 3,
203–210.
Henderson, J.R., 1983. Structure and metamorphism of the Aphebian
Penrhyn Group and its Archean basement complex in the Lyon Inlet area,
Melville Peninsula, District of Franklin. Geological Survey of Canada,
Bulletin 324, 50 pp.
Tim Bell wrote:
> Hi All
> Martin’s request and the many responses got me involved, not because
> of the length of the nappe, which to me is not important conceptually.
>
> What is significant for me is the maximum width of upside down rocks
> that exists on our planet in a single coherent sheet parallel to the
> assumed transport direction.
>
> Many of us suspect large nappes form through rotation of initially
> upright regional folds on sub-horizontal shear zones.
>
> If this is the case then the maximum width of upside down rocks would
> be quite limited. I know of a stretch in SE New Hampshire that is
> upside down for perhaps 5 kms in a coherent block parallel to the
> transport direction.
>
> What is the maximum distance that any of you have come across?
>
> Cheers
> Tim
>
>
>
> Prof. T.H. Bell
> School of Earth and Environmental Sciences
> James Cook University
> Townsville
> Qld 4811
> Australia
> Work Phone +61-7-47814766
> Work Fax +61-7-47814020
> Home Phone+61-7-47723017
> Email [log in to unmask]
> _http://www.es.jcu.edu.au/dept/Earth/research/samri/index.html
> _
--
Yvette D. Kuiper
Assistant Professor
Department of Geology and Geophysics
Boston College
Devlin Hall 213
140 Commonwealth Avenue
Chestnut Hill, MA
02467 USA
Tel. 617-552-3647
Fax. 617-552-2462
http://www2.bc.edu/~kuipery/
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