TSK 11 Göttingen 2006 Liu & Cao
 Change of deformation mech-
 anisms during low tempera-
 ture flow of rocks — observa-
 tion from micron to nanome-
 ter scales Vortrag
 Junlai Liu1 Shuyun Cao1
 Recent studies on nano-materials in
 materials science revealed that nano-
 materials may have fantastic features
 due mainly to size-effect of the mate-
 rials. For example, nano ceramics may
 have very high ductility at room temper-
 atures and pressures, even though nor-
 mal ceramics is easily deformed by brit-
 tle fracturing.
 What and how much do we know about
 the nature of nano or nano to micron
 scale geological materials? What factors
 contribute to their occurrence? How do
 they flow at geological conditions and
 how do they affect the rheology of rocks?
 Upper crustal deformation is charac-
 terized by low temperature flow of
 rocks under unsteady state, which re-
 sults in progressive grainsize reduction
 and leads to the occurrence of mi-
 cron to nano meter scale materials in
 fault zones. The examples of naturally-
 deformed upper crustal rocks presented
 in the paper help to unravel the impor-
 tance of nano to micron scale rock ma-
 terials during the low temperature flow
 of rocks.
 Cataclastically faulted marble, lime-
 stone and dolomite from the Autseib
 fault zone, Namibia, and micro-breccias
 from detachment fault zones in several
 metamorphic core complexes (mcc’s),
 e.g. the Whipple mountains mcc in
 Western USA, Jinzhou mcc, and Huh-
 1 State Key Laboratory of Geological Pro-
 cesses and Mineral Resources, China University
 of Geosciences, Beijing 100083, China
 hot mcc in North China are stud-
 ied with optical microscope, CL micro-
 scope, SEM, and TEM. It is shown from
 the present study that micron to nano
 scale materials do occur during natural
 faulting at low temperatures in the up-
 per crust. Macroscopic brittle features
 are shown primarily by the field occur-
 rence of zones of microbreccias or cata-
 clasites. Angular clasts with straight or
 irregular boundaries are randomly dis-
 tributed in extremely fine-grained ma-
 trix and they do not show any evi-
 dence of preferred dimensional or lat-
 tice orientation. There is either clear or
 vague transition from clasts to matrix
 along the clast boundaries, which is also
 clearly shown by SEM studies. The ma-
 trix materials are extremely fine grains
 from micron to nano meter scales. They
 have optically irregular or vermiculate
 forms and are either fine clasts of grain
 aggregates or single grains. At the high-
 est strain zones, rare clasts are observed.
 Single grains, however, predominate in
 the matrix and often have polygonal
 forms. TEM studies reveal great dif-
 ferences in dislocation patterns between
 coarse-grained clasts and fine-grained
 matrix. Dislocation substructures are
 widespread in grains of different sizes
 and origins. Tangled dislocations are
 the most common dislocation substruc-
 tures in coarse-grained clasts, although
 free dislocations, dipoles, dislocation
 loops, dislocation walls and irregularly
 connected dislocations are also observed
 either jointly or separately in deformed
 grains in the clasts. A general ten-
 dency is that dislocations are more and
 more regularly organized towards clast
 boundaries. Tangled and irregularly
 connected dislocations occur mostly in
 the central grains of relatively big clasts,
 while walls of well-organized disloca-
 1
Liu & Cao TSK 11 Göttingen 2006
 tions occur mainly near the boundaries,
 constituting subgrain boundaries. It is
 shown from TEM observations that tan-
 gled dislocations occur mainly in clasts
 with sizes greater than several tens of
 microns and well-organized dislocations
 and subgrains predominate clasts with
 sizes smaller than that limit. Fine-
 grains are 0.02 µm to 3µm in sizes
 and characterized by polygonal shapes.
 They have regular and straight bound-
 aries, and are generally dislocation free
 or contain only very few free disloca-
 tions. Grain sizes of the fine grains vary
 in the range from hundreds of nano me-
 ters to tens of microns. The fine grains
 generally have no preferred dimensional
 orientation. Due to extremely fine grain
 sizes their lattice orientation is unde-
 tectable.
 On the other hand, there are often
 micropores along grain boundaries be-
 tween the fine grains. This, together
 with the cathodoluminescence difference
 between big clasts and fine matrix may
 imply the importance of fluid phases
 during flow of the fault rocks.
 The above evidence lead to the follow-
 ing conclusions:
 1. Micron to nano meter scale geo-
 logical materials are common in
 highly deformed crystalline rocks.
 Their occurrence is attributed to
 unsteady state progressive shearing
 and grain size reduction along fault
 zones in the upper crustal level.
 2. Variation of grain sizes at micron
 to nano meter scales in rocks has
 strong effects on the flow of the
 rocks at natural strain rate and de-
 formation conditions. The grain-
 size range from n × 10−1 µm to
 n × 101 µm is an important range
 of grain sizes for the transition of
 deformation mechanisms deformed
 at low temperatures in the upper
 crust. Grains with sizes greater
 than n×101 µm can be deformed by
 either crystalline plasticity or cat-
 aclasis. Grains with sizes smaller
 than that limit are deformed only
 by crystalline plasticity. The min-
 imum size of clasts deformed in a
 brittle manner is n× 101 µm.
 3. Variation in deformation mecha-
 nisms at micron to nano meter
 scales is interpreted as the results
 of grainsize reduction to a very high
 level at unsteady state and the ef-
 fects of fluid involvement in the
 deformation of the extremely fine-
 grained rock materials. There is a
 sharp decrease in surface area with
 grain sizes. Such an increase and
 fluid flow may both enhance diffu-
 sion of atoms or lattice defects to-
 wards grain boundaries and from
 grain to grain. Both effects con-
 tribute to the variation of defor-
 mation mechanisms of naturally de-
 formed rocks at upper crustal lev-
 els.
 2