Interesting discussions. I’d 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 here’s my
tuppence-worth to the debate. It’s 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 don’t 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 I’ve 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 illustration…on 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 (one’s
in a boulder). They’re 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. They’re less elegant (and
largely unstudied). Furthermore – the area sits in a tract of gneisses –
many km across… 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-displacement…so orogenically important) simple
shear-dominated deformation zones. But are they?
I’ve spent lots of time in thrust belts – only have had only passing
interests in slate belts. Both types of structure exist….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 depth…? What if the thrusts passed back down into distributed
strain (check out Adrian Pfiffner’s papers from the mid 80s on this one) -
or even down onto sub-vertical stretching…?
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. There’s 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 conversion….
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. We’re 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 you’ve 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