Dear Mail-list owner - Rob Twiss cannot send material to this bulletin but
can read it. Can you help him please. Thx

Colleagues,

	I agree with Tim Bell's comment about scale and the study of
heterogeneities in rock as a means of understanding local deformation
processes. Although his comment seemed to be taking issue with point (5) in
my earlier communication (thanks, Tim, for forwarding that to geotectonics
for me!), what he says is not inconsistent, I think, with what I was trying
to say.  Allow me to expand a bit on that point by showing the analogy in
method of study between the investigation of two very different scales of
geologic process.

	If one is interested in diffusion as a process in foliation
formation, for example, then one talks about diffusion constants and
material migration down a potential gradient, but the details of the local
paths of individual ions, which are very irregular, or local ionic
interactions, are not of immediate interest.  The diffusion constant and
chemical potential are simply descriptions of the ion motions and
interactions that average out the heterogeneities of the smaller-scale
processes, the details of which are not relevant.  These averages are
appropriate for studying processes at the larger scale of foliations, and
one can discuss whether the diffusion constant or the chemical potential is
homogeneous or heterogeneous at the foliation scale.
	Similarly, if one wants to study or model the flow within an
orogenic belt, for example, one talks about the strain distribution at the
orogenic scale and the rheologic properties of the rock, but the specific
local processes by which deformation is accomplished and rheology
determined are not of immediate interest.  The strain and rheological
equations are simply descriptions of the motions and processes that average
out the heterogeneities of the smaller-scale processes, the details of
which are not relevant.  These averages are appropriate for studying the
flow at the larger scale of orogenic belts, and one can then discuss
whether the strain and the rheology is homogeneous or heterogeneous at the
orogenic scale.
	Thus the heterogeneities of material migration that one might want
to study at the foliation scale must be averaged out in studying the flow
of rock at an orogenic scale in the same way that the heterogeneities in
ionic motions and interactions must be averaged out to study material
mirgration at the foliation scale.  So what seems like heterogeneities at
one scale become averaged out, or 'homogenized', in studying a larger scale.

	I understood the pure-shear/simple-shear discussion to relate to
the description of larger scale deformation of rocks, and my comments were
motivated by that understanding.  There was no implication that study of
smaller-scale effects and processes is unimportant, or that large-scale
strain can be used to study small-scale processes.  To the contrary, my
point was that it is important to define the scale at which one is
interested before the concept of heterogeneity and homogeneity has any
meaning.  Tim's comment, it seems to me, merely emphasizes the point I was
trying to make, as illustrated by the two examples above.

	As for the 100-year-old approach to understanding large-scale
deformation in rocks: If one is interested in the details of local
processes, then clearly heterogeneities at the scale of foliations, for
example, are important, and the 'new approaches' that focus attention on
process at that scale have provided major advances to our understanding.
But to some of us, rock deformation at a larger scale is also of interest.
At that scale, the details of local process are not so important - the
local processes are just the mechanisms by which the large-scale
deformation is accomplished.  Although the study of larger scale strain may
not have gotten us very far in understanding local processes, it is
incorrect then to infer that the larger scale is not useful in
understanding other aspects of rock deformation and tectonics, or that
study of strain at the large scale has not been productive.

	In the end, both scales are important to understand.  Not only are
the processes at the foliation scale of interest in their own right, but by
understanding such local processes, we can understand how local-scale
structures relate to the average deformation at the larger-scale and then
use these structures to improve constraints on the larger-scale studies.
Our models and understanding only become richer by including information
from all scales of study, but we must keep in mind the dependence on scale
of the concepts of homogeneity and heterogeneity.

Rob Twiss


Robert J. Twiss                         Internet: [log in to unmask]
Geology Department                      telephone: (530) 752-1860
University of California at Davis       FAX:       (530) 752-0951
One Shields Ave.
Davis, CA 95616-8605, USA

Prof Tim Bell
School of Earth Sciences
James Cook University
Townsville
QLD 4811
Australia
ph: +61 7 47814766
fax: +61 7 47251501
email: [log in to unmask]




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