4. Shear Zones Flashcards
What must you give to specify the geometry of a fault (shear zone)?
- give the orientation (strike/slip) of the fault plane
- give the orientation (plunge/plunge direction) of the slip vector
- give the magnitude and sense of displacement of both the strike-slip and dip-slip components of the slip vector
Draw sketches showing the distinction between simple shear, transtensional and transpressional shear zones
What are the defining features of mylonite?
a well-developed foliation (Z) that results from tectonic grain size reduction
stretching lineation (X)
Deflection markers
intrafolial folds - sheath folds
S - C fabrics
shear sense indicators
What are the defining features of breccia?
contain more than 30% clasts that are over 2mm in size
crackle (>75%), mosaic (60-75%) and chaotic (30-60%)
What are the defining features of cataclasites?
non-foliated rocks with less than 30% clasts over 2mm
protocataclasites (0-50% matrix), mesoctaclasites (50-90% matrix) and ultracataclasites (>90% matrix)
What is a pseudotachylyte?
cataclasites that contain dark glass (or its devitrified product)
forms by localized melting during rapid sliding (earthquakes)
mm to cm wide zones that have sharp contacts with the host rock
What is fault gouge?
fault rocks with less that 30% clasts over 2mm that are incohesive at the present outcrop
they are a strongly comminuted version of the protolith and may or may not be foliated depending upon their mica/clay mineral content
The foliation generated in a shear zone is parallel to which plane of the strain ellipsoid? The slip direction is parallel to which axis of the strain ellipsoid? (Answer both of these using the XYZ nomenclature we have used to label the axes of the strain ellipsoid.)
The foliation tracks the principal flattening plane (XY plane) across the shear zone
slip direction is parallel to the X axis
Understand the geometry of the stereograms
If I were to draw a pair of lazy arrows indicating a particular shear sense, could you show the orientation of the strain ellipse on the diagram (or vice versa)?
What is a sheath fold?
some intrafolial folds may amplify into tubular shapes with hinge lines that swing into parallelism with the X direction as strain increases, these are described as sheath folds
If I were to give you pictures of the following commonly encountered features in ductile shear zones, could you indicate the sense of shear on the picture: S-C fabrics, σ-clast, δ-clast, sigmoid, mica fish, grain shape fabric? Alternatively, if I were to draw a pair of lazy arrows indicating a particular shear sense, could you show the orientation of these features on the diagram?
If I were to give you pictures of the following commonly encountered features in semi-brittle shear zones, could you indicate the sense of shear on the picture: an en échelon array of tension gashes, pressure solution cleavage? Alternatively, if I were to draw a pair of lazy arrows indicating a particular shear sense, could you show the orientation of these features on the diagram?
If I were to give you pictures of the following commonly encountered features in a fault gouge, could you indicate the sense of shear on the picture: P-foliation, R1 Riedel shears, Y shears? Alternatively, if I were to draw a pair of lazy arrows indicating a particular shear sense, could you show the orientation of these features on the diagram?
