I enjoyed reading Rob¹s note. We are all coming to similar realizations
about what is taking place during deformation at the larger scale.

The degree of partitioning of multiple successive foliation producing events
that we observe using porphyroblasts is extreme within single large
outcrops. This generates local highly non-coaxial strains against
porphyroblast rims, but not necessarily distributed away from them into the
matrix as each foliation developed.  That is, away from porphyroblast
margins the deformation intensity may be minimal for each event. However,
the actual strain accumulated across many such zones could be significant,
over many deformations and periods of porph growth. It could even be
different  in its non-coaxial large-scale displacive effects to that
suggested by the porphyroblast inclusion trail asymmetry. The reason for
this is that porphs always grow very early during deformation.  They do not
track the deformation as it intensifies in one event. The non-coaxial
component of the deformation always increases as the strain increases.
Potentially it could go more non-coaxial in the matrix with the opposite
shear sense after the porph has formed and as the deformation intensifies.
 
However, the most important point with regards to Robs comments is that
matrix foliations preserve almost none of the very lengthy history that is
trapped in the porphyroblasts. Sections perpendicular to lineation or
parallel to lineation but perpendicular to foliation commonly show inclusion
trails that appear to be continuous with the matrix when they are actually
truncated if one looks at other section orientations. This is dealt with to
some degree in Cihan (2004) just out in JSG on web. This lengthy history has
all been wrapped into the inevitable schistosity parallel to compositional
layering in the rocks that we deal with from orogen cores. Such S0//S1 hides
the bulk of the deformation history in our experience. If S0 is //S1 in
non-porphyroblastic rocks then there is probably a very lengthy deformation
and thus displacive history that cannot be extracted. The displacive history
can be extracted, to some degree, by probing garnet cores that formed in
successive FIA sets from a confined region, and plotting Mn, Ca and Fe
isopleths on pseudosections constructed using THERMOCALC. The resulting path
of PTs is much more extended than available by other means. Where we have
done this it tracked an extended downwards path into the orogen in some
detail. Porph growth stopped soon after uplift began.
 
Regionally, the distribution pattern of all FIA trends on a total rose plot
can be remarkably similar (Bell et al, 2004 ­ JSG) with overall relatively
uniform  distributions of inclusion trail asymmetries between FIA sets.
Where we have seen regional variation in partitioning in terms of FIA
distribution (same paper as above) is around terrains underlain by thick
large feldspar granitic gneisses that apparently behaved more competently at
lower temperatures earlier in the metamorphic history.
 
In summary, 
1. Locally the deformation can be very intense, and very non-coaxial
particularly at small scales. Yet the distribution of this small-scale
non-coaxiality as preserved by porphs is such that the bulk deformation
appears to be relatively coaxial.
2. If the same thing happens at a whole range of scales then local
mesoscopic shear zones (which only track very late history) may average out
relatively coaxial at a bigger scale, or have minimal translational effects
compared to the bulk, but much lower strains occurring in intervening zones.
3. Since the bulk of such a prolonged foliation development history
undergone by multiply deformed rocks now lies in the foliation parallel to
S0, so may too the bulk of displacement history. In that case matrix shear
zones would tell us absolutely nothing about the large-scale displacement
history! 
This might help with some of what Rob told us.
Cheers
Tim

> Interesting discussions. I¹d like to re-iterate what my fellow convenor of
> the "Styles of continental compression" symposium at IGC stated as to its
> background. Stefano and I were interested (and remain interested!) in
> examining the different styles of continental deformation. As Stefano points
> out ­ the really key issue seems to be about localisation. So here¹s my
> tuppence-worth to the debate. It¹s 2-dimensional (some may say one).
> If we buy into a Dewey & Bird (for the old-timers) or Houseman/England view
> of Tibet for example ­ we don¹t need very large strains (at an outcrop
> scale) to thicken the crust ­ and hence accommodate say 75% of the total
> convergence in the India-Asia collision system ­ because the strain is
> distributed across (now) 2000+km across-trend crust. In contrast, the
> Himalayas (say 25% of the convergence) is narrow  and the thrust zones (e.g.
> MCT) narrow too (with locally very high strains  appropriately consistent
> sense-of-shear indicators).  Of course we can argue that distributed strain
> is accommodated through an array of anastamosing simple-shear-dominant shear
> zones ­ but is that just a convenience? The localisation behaviour is
> different ­ the controls on partitioning, the timing and evolution of
> partitioning remain interesting issues.
> Looking through my slide collect I find I¹ve lots photos of narrow
> deformation zones that have the qualitative/semi-quantitative aspects of
> dominant simple shear. They probably have integrative displacements across
> the whole lot of <1km!  By way of illustrationŠon the field trip to the
> Outer Hebrides that preceded the Mike Coward meeting in May, we visited the
> North Uist coast home to the classic Ramsay and Graham shear zones. Rod led
> the visit. Something like 40 man hours were spend scouring the immediate
> vicinity of Caisteal Odair ­ the two classic examples are still there (one¹s
> in a boulder). They¹re very beautiful of course and we all photographed
> them. But there are only 2. Of the dozens of other, narrow deformation
> zones, no others seemed to have a simple foliation pattern ­ so perhaps do
> not approximate closely to simple shear zones. They¹re less elegant (and
> largely unstudied). Furthermore ­ the area sits in a tract of gneisses ­
> many km acrossŠ with very little asymmetry evident. Mike Coward interpreted
> this lot (and many other examples, as have others since) as very broad (and
> therefore crustal-scale, big-displacementŠso orogenically important) simple
> shear-dominated deformation zones. But are they?
> I¹ve spent lots of time in thrust belts ­ only have had only passing
> interests in slate belts. Both types of structure existŠ.and they are
> different beasts. Even bits of thrust belts are different ­ parts of the
> Moine Thrust Belt for example show km-wide zones of layer-parallel
> shortening, others have none at all. For me the issues are not whether
> various end-member strains exist but the application of mixtures to real
> settings. 25 years ago there was a bandwagon looking at strain in thrust
> belts (a few brave souls have continued). Then this became unfashionable.
> People "realised" that thrust displacements are more important (in thrust
> belts!) at accumulating the bulk convergence. 20 years on there are still
> cross-sections drawn on a crustal scale that extrapolate discrete thrusts to
> the Moho (although everyone recognises that these, if they exist, will
> actually be shear zones rather than cataclastic, discrete faults at depth).
> There are advantages in this simplicity, but surely not if they generate
> simple kink-geometry dip-changes at the surface (an issue the John Ramsay
> raised way back). How sensitive are these models to subtle changes in the
> model at depthŠ? What if the thrusts passed back down into distributed
> strain (check out Adrian Pfiffner¹s papers from the mid 80s on this one) -
> or even down onto sub-vertical stretchingŠ?
> When Dave Prior and I worked on the structure of Nanga Parbat in the mid80s
> we were taken (distracted?) by lots of shear criteria and a dramatic
> discrete fault. But it only represents the edge of the massif. There¹s lots
> of moderate sub-vertical stretching throughout which, if you
> speculate/integrate the strains, proves to accommodate more shortening than
> the attractive "shear zone" on the edge. Doubtless old hands will chuckle at
> this "road to Damascus" like conversionŠ.
> To return to the issue ­ what are the key controls and their sensitivities ­
> for influencing the degree of deformation localisation (partitioning) within
> the continents? Is it the number, orientation and linkage of pre-existing
> weakness? Is it the influx of fluids (or the hydrated state of minerals
> within the crust), is it heat,, are the reasons intracrystalline? Or is it
> down to erosion or other "external" factors? I suspect that the phenomena we
> should be discussing are not merely those exhumed deeper crustal materials
> we photograph at outcrop but also information from geodetic surveys, seismic
> reflection data, seismology (anisotropy and earthquake distributions). But
> surely we must be open to the wide range of approaches ­ particularly in
> trying to link across scales, not to mention make adequate mechanical
> descriptions/predictions of structures. A bit of an
> apple-pie/grandmother/eggs statement I know.
> These are some of the issues our session tried to capture. We¹re putting
> together a proceedings ­ with provisional acceptance for a Special Paper of
> GSA. We can take some extra contributions (subject to approval/review etc)
> so if you¹ve been aroused by the discussions­ send a title and abstract to
> Stefano (Stefano Mazzoli <[log in to unmask]>) who is taking the lead
> in editing the volume. The article submission deadline will be the end of
> December 2004.
> 
> Cheers
> Rob Butler





Prof. T.H. Bell
School Earth Sciences
James Cook University
Townsville
Qld 4811
Australia
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