 |
| Titel |
Role of folded anisotropic fabric in the failure mode of gneiss: new insights from mechanical, microseismic and microstructural laboratory data |
| VerfasserIn |
Federico Agliardi, Sergio Vinciguerra, Marcus R. Dobbs, Stefano Zanchetta |
| Konferenz |
EGU General Assembly 2015
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106046
|
| Publikation (Nr.) |
 EGU/EGU2015-5687.pdf |
|
|
|
|
|
| Zusammenfassung |
| Fabric anisotropy is a key control of the mechanical behaviour of rocks in a variety of
geological settings and on different timescales. However, the effects of inherited, tectonically
folded anisotropic fabrics on the brittle strength and failure mode of foliated metamorphic
rocks is yet to be fully understood. Data from laboratory uniaxial compression tests on folded
gneiss (Agliardi et al., 2014, Tectonophysics) recently showed that the brittle failure mode of
this rock type depends on the arrangement of two distinct anisotropies (i.e. foliation and fold
axial plane anisotropy), and that rock strength correlates with failure mode. Here we
investigate the effects of confining pressure on this behaviour by performing triaxial
compression experiments with acoustic emission (AE) monitoring, and analyse resulting
fracture mechanisms and their microfabric controls using high resolution microanalysis
techniques.
We tested the Monte Canale Gneiss (Austroalpine Bernina nappe, Central Italian Alps),
characterized by low phyllosilicate content, compositional layering folded at the cm-scale,
and absence of a well-developed axial plane foliation. We used a servo-controlled hydraulic
loading system to test 19 air-dry cylindrical specimens (diameter: 54 mm) that were
characterized both in terms of fold geometry and orientation of foliation and fold axial planes
to the axial load direction. We instrumented the specimens with direct contact axial and
circumferential strain gauges. We performed tests at confining pressures of 40 MPa and
constant axial strain rates of 5*10-6 s-1, measuring acoustic emissions and P- and S-wave
velocities by three wideband (350–1000 kHz) piezoelectric transceivers with 40 dB
preamps, mounted in the compression platens. We carried out post-failure microscale
observation of fracture mechanisms, microcrack patterns and related fabric controls on
resin-impregnated samples, using X-ray MicroCT (resolution: 9 μm), optical microscopy and
SEM.
Samples failed in three distinct brittle modes with different combinations of neat shear
planes parallel to foliation, fractures parallel to fold axial planes, or less localized
mm-scale brittle shear zones. The different failure modes, consistent with those
previously described in uniaxial compression experiments, are associated with distinct
stress-strain and acoustic emission signatures (i.e. overall activity, rate distribution,
frequency and amplitude patterns). Failure modes involving the quartz-dominated axial
plane anisotropy correspond to higher peak strength and axial strain, less brittle
macroscopic behaviour with well-developed fracture process zones, and higher and more
progressive acoustic emission activity than failure controlled by mica-dominated foliation
anisotropy. Experimental and microstructural observations support a decisive control of
folded microfabric on the overall behaviour of the same rock type, through the
activation of Q-dominated vs. M-dominated crack nucleation / propagation mechanisms. |
|
|
|
|
|
|