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| Titel |
Mechanical Behaviour of Phyllosilicate-Rich Faults |
| VerfasserIn |
Cristiano Collettini, André Niemeijer, Cecilia Viti, Chris Marone |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040577
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| Zusammenfassung |
| A number of observations indicate that phyllosilicate-rich faults are widespread within the
brittle crust. Here, we report on laboratory experiments designed to investigate the
frictional behavior of 1) intact foliated solid rocks sheared in their in-situ geometry
and 2) powders obtained from crushing and sieving the samples used in the solid
experiments; 3) powders obtained from non foliated samples. The samples were collected
from the Zuccale low-angle normal fault (Italy) and are characterised by different
amounts of phyllosilicates (from 60 to 10 % of talc, smectite biotite and chlorite)
plus calcite, hornblende and tremolite. Friction of powders, at room humidity and
temperature, is in the range 0.27-0.62 and it is controlled by the abundance of weak
phyllosilicates (talc and smectite). Over a range of normal stresses from 10 to 150 MPa
and sliding velocities from 1 to 300 μm/s, the powdered material with a) 60% of
phyllosilicates exhibits velocity strengthening behaviour associated to calcite grain rotation
and translation in a phyllosilicate-rich groundmass; b) < 40% of phyllosilicates
exhibits an evolution, with increasing sliding velocity, from velocity strengthening to
velocity weakening associated to grain size reduction and localization along B and Y
shear planes. Friction of solid samples (20-35% of phyllosilicates) is in the range
0.35-0.25 and for each normal stress solid rocks have a friction coefficient 0.2-0.3
lower than powders with identical mineralogical composition. In addition, the solid
samples become more and more velocity strengthening with increasing sliding
velocity.
Field observations along exhumed and phyllosilicate-rich faults worldwide suggest that
phyllosilicates-rich strands bound lenses of stronger and non foliated lithologies. Within
this fault structure fault creep and aseismic slip along weak and phyllosilicate-rich
foliated surfaces (velocity strengthening behaviour) might increase stress on lenses
of stronger materials (velocity weakening behaviour) from which a rupture can
nucleate. In this view, some crustal faults can behave as weak structures over long
timescales (millions of years) and be intermittently seismogenic on shorter timescales. |
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