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| Titel |
Can kinematic approaches be used to predict shear fracture orientations for use in discrete fracture network models? |
| VerfasserIn |
Jonathan Imber, Richard Jones, Tom Perry, Ruth Wightman |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250084452
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| Zusammenfassung |
| Structural geologists typically invoke the Mohr-Coulomb failure criterion to explain
the orientations of shear fractures. Nevertheless, there have been recent attempts
to explain shear fracture orientation in terms of finite strain or strain increment.
In these models, spatial variations in shear fracture orientation are explained by
spatial variations in the orientations of lines of no finite elongation (LNFE), and/or
zero extension directions. We test these models by analysing the orientations of
conjugate cataclastic deformation bands that cut unconsolidated sand and gravel
at McKinleyville, California, and dip shallowly towards the north-northeast and
south-southwest. The acute dihedral angle between the two sets of deformation bands is 47Ë
and is bisected by the sub-horizontal, north-northeast directed incremental and
finite shortening directions. Trishear models of fault propagation folding above the
McKinleyville fault predict two sets of LNFE that plunge steeply and shallowly to the south
and north. These predictions are inconsistent with deformation band orientations
and suggest that deformation bands did not form parallel to these LNFE. During
plane strain, zero extension directions with acute dihedral angles of 47Ë develop
when the dilatancy rate is 4.3. Experimental dilatancy rates for Vosges sandstone
(cohesion > 0) and unconsolidated Hostun sand suggest the deformation bands
either developed parallel to zero extension directions or in accordance with the
Mohr-Coulomb criterion, assuming initial porosities of 22% and 39%, respectively. However,
an empirical relationship between dilatancy rate, relative density and mean stress
suggests that dilatancy rates for Vosges sandstone overestimate the dilatancy rate
at McKinleyville. Deformation bands at McKinleyville likely developed either
in a Mohr-Coulomb orientation, or an intermediate orientation bounded by the
Mohr-Coulomb and Roscoe angles. These results suggest that approaches based
on strain or strain increment (i.e. kinematic models) should be used with caution
when predicting shear fracture orientations, and should not be used to populate
discrete fracture network (DFN) models without careful consideration of their validity. |
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