 |
| Titel |
Towards quantification of the interplay between strain weakening and strain localisation in granular material |
| VerfasserIn |
Malte C. Ritter, Matthias Rosenau, Karen Leever, Onno Oncken  |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250090908
|
| Publikation (Nr.) |
 EGU/EGU2014-5169.pdf |
|
|
|
|
|
| Zusammenfassung |
| Strain weakening is the major agent of localisation of deformation into shear zones and faults
at various scales in brittle media. Physical analogue models using granular material are
especially apt to investigate both phenomena, because they are able to reproduce them
without the need of any assumptions concerning the physics behind. Several attempts have
been made to quantify either strain weakening (e. g. Lohrmann et al., 2003, using
Ring-Shear tests) or strain localisation (e. g. Schrank et al., 2008, using a variation of
the classical Riedel-experiment). While Ring-Shear tests yield excellent data on
strain weakening through measuring shear stress during localisation, they do not
allow monitoring the process of strain localisation in-situ because of experimental
inaccessibility of the small scale kinematics. In Riedel-type strike-slip experiments, on the
other hand, no direct measurements of shear stresses have been available so far.
Furthermore, they contain a strong boundary condition in form of a pre-defined
linear discontinuity at the base. This forces the formation of Riedel-Shears, i. e. a
complex fault system, that makes it difficult to define strain localisation on single
faults.
We developed a new experimental set-up, in which the formation of a strike-slip shear
zone in granular material is induced using an ndenter with stress and strain monitored at high
accuracy and resolution. In a first set of experiments we used a horizontal sand layer indented
by a vertical wall. The sand layer is laterally unconfined and rests on low-viscosity silicone
oil in order to minimize basal shear strength. Compared to the Riedel experiments, this avoids
the boundary condition of a pre-existing basal discontinuity allowing one single,
hrough-going shear crack to form and propagate. The indenter moves at a constant
rate and is equipped with a force sensor that measures the applied push, which
integrates over shear stresses along the fault and the base of the sand pack. Therefore
simultaneous monitoring and analysis of strain weakening and strain localisation becomes
feasible.
Here we present results from first tests of this setup demonstrating its capability and
limits. Various granular analogue materials with differing mechanical properties are tested
and compared to results in ring shear and axial tests. Preliminary results show a principal
pattern of correlated strain weakening and localisation that is characterized by a
phase shift between stress and strain evolution during shear zone formation: Strain
localisation has its maximum when the weakening rate is highest. Strain then delocalises,
however, and reaches a steady state when the material strength reaches a stable
value.
References:
Lohrmann, J., Kukowski, N., Adam, J., and Oncken, O., 2003, The impact of
analogue material properties on the geometry, kinematics, and dynamics of convergent
sand wedges: Journal of Structural Geology, v. 25, no. 10, p. 1691–1711, doi:
10.1016/S0191-8141(03)00005-1.
Schrank, C.E., Boutelier, D.A., and Cruden, A.R., 2008, The analogue shear zone: From
rheology to associated geometry: Journal of Structural Geology, v. 30, no. 2, p. 177–193, doi:
10.1016/j.jsg.2007.11.002. |
|
|
|
|
|
|