Hi All
I enjoyed the responses - though I strongly suspect the 100km width example
is not one nappe.
So, in alignment with John's thinking, I thought I would let you know where
our voluminous porphyroblast data base is taking me and perhaps others.
When one separates generations of porphyroblasts using the foliation
inflection/intersection axes preserved within them (FIAs), and then
quantitatively documents the inclusion trail asymmetries for each axis, in
our experience both asymmetries occur in approximately equal amounts around
each FIA set in a succession (dating the FIA sets using monazite trapped in
them in 6 regions so far suggests that these successions develop over 60 to
100 Million years - which may surprise many as the age data gets
progressively published). In other words, the deformation history that they
record is essentially coaxial. This is no real surprise. Porphyroblasts
microstructurally only nucleate and/or grow in portions of rock undergoing
that portion of the deformation history. They always stop growing as soon as
non-coaxial deformation partitions close by (e.g., if dissolution and a
cleavage seam begin to form adjacent to them) and will only start again if
that cleavage seam cannot accommodate shear strain. This normally requires
deformation at a high angle to the old one.
The only place where we have ever found rocks containing just one asymmetry
(only 3 so far out over 20 regions where such detailed work has been done)
and usually for only one of the FIAs in a succession of 4 or 5, is in well
documented nappes where the rocks are upside down. So we can tell when the
nappes formed. In all cases an earlier sub-vertical foliation is preserved
in the porphyroblast. This is no surprise if vertical axial plane folds
formed first and the nappe developed by overprinting that structure with a
zone of horizontal shearing and vertical bulk shortening. However,
conceptually for me it indicates that coaxial vertical shortening occurred
at initiation of the nappe, at which stage the porphs grew, with just their
rims recording the beginning of the bulk non-coaxial swing of the
sub-vertical foliation towards the horizontal due to the deformation that
developed the nappe, and of course the associated sub-horizontal foliation
development shuts down further porphyroblast growth during that event.
The biggest vertical axial plane regional folds (from which the largest
width of upside down fold limbs could eventually form if cutting a
horizontal shear zone through is a key process in nappe development) appear
to form first or very early in our experience (apparent young big ones like
gneiss domes, so far where we have done this work, are old structures that
have reactivated limbs and thus only appear to be relatively young). No
surprise yet again. When a thick stratigraphic sequence first deforms,
partitioning of deformation successively vertically and horizontally through
the crust will occur due to the bulk forces operating. Therefore, the
biggest folds should form early while the beds are still relatively thick
and planar and mainly during the first phase of bulk horizontal shortening
to affect these rocks.
Nappes with porphyroblasts containing inclusion trails allow us to test and
demonstrate if any are squeezed out from orogen cores because the
porphyroblasts will always preserve that part of the history!!! There should
be the normal coaxial history mentioned above preserved around a succession
of FIAs that develops as the rocks go down and then are squeezed up, and at
some stage, when the nappe forms, the rare one asymmetry mentioned above
(seen only so far in actual nappes) will develop about a younger FIA in the
succession.
If anyone has nappes where this has been convincingly argued, containing
porphyroblasts with inclusion trails, and wants to collect 60 or more large
oriented samples along and across them, this could be checked out! We would
be happy to help.
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
|