Brittle shear sense indicators

1. Brittle shear sense indicators Department of Earth Science, IIT Bombay PurushottamGupta

2. Fossen,2010 Brittle shear zone may have two meanings, either frictional deformation at the microscale (no plasticity), or complete discontinuity of structures (no ductility).

3. In the field The simplest and best indicator - displacement of markers such as dykes, veins, xenoliths and bedding along a shear zone. Frontal block moves up and to the left, but the deflected layer indicates an apparent sinistral displacement on a horizontal Surface.(Passchier & Trouw,2005)

4. Presence of a marker layer in the form of the dark bed with a white band at its base. [Photograph by G. H. Davis.,2012] Photograph of normal faulting

5. smoothness test By seeking out the direction that feels most rough to the hand, structural geologists try to know the sense of movement along the fault. Finding the smooth direction is key. The hand, like the fault block,movedin the direction that feels smoothest. [Photograph by G. H. Davis.,2012]

6. Fish flash Most mica fish in a rock will lean over in the same direction, resulting in a maximum reflectivity when looking down the lineation, in the same direction as the sense of shear. Simpson,1986

7. Crystal fiber lineations on a thrust fault. Note black pen for scale in the center of the photograph. (From Marshak and Tabor, 1989)

8. SECONDARY FRACTURES AS SSI Shear sense on brittle faults.Block diagram shows relationship between secondary fractures & SoS on brittle fault.Top plane is shear plane.Extension fracture are unstriated & filled with 2˚ minerals.Striated fracture surfaces are shear fractures. (Twiss & Moore,1992) Twiss & Moores,2012

9. Fault crosssection showing SSI on irregular fault surface. • Solution structure-slickolites • Slicken fibres Twiss & Moores,2012

10. Requires CAUTION Twiss & Moores,2012

11. REIDEL SHEAR Schematic diagram showing the characteristic geometry and shear sense of the most common types of Riedel shears (R-, R'-, P-, Y and T-shears) in a brittle fault zone. Fractures. foliation (S). Mff– microscopic feather (Passchier & Trouw,2005) • Sets of subsidiary shear fractures with distinct orientation and movement sense, known as Riedel shears. • Riedel shears resemble ductile shear bands but form by brittle fracturing. • Extensional fractures without displacement form at 20–50° to Y-shears or boundaries of the gouge zone. These are known as T-fractures • If opened into wedge-shaped tensional veins, as microfault induced microcracksor microscopic feather fractures

12. In the lab Deformation Bands Optical (Top) and Cathodoluminescence (Bottom) image of a dilatational brittle fault zone in a quartzite host rock of the Muth Formation (India). Sense of shear is dextral (see rotated quartz chips that flake off from the larger grains). Created voids are cemented with quartz that is dark in the cathodoluminescence image. (Passchier & Trouw,2005)

13. Fractured and offset grains: Antithetic shear fractures cutting tourmaline in protomylonitic pegmatite. (B) Synthetic and antithetic offset of grains.

14. references George H. Davis, Stephen J. Reynolds, Chuck Kluth.,2012 Structural geology of rocks and regions, 3rd ed., C. Simpson, 1986. Determination of movement sense in mylonites. Journal of Geoscience Education, vol. 34. Passchier, C. W., and Trouw, R. A. J., 2005, Microtectonics: Springer, Berlin Haakon Fossen-Structural Geology , 2010 Cambridge University Press Twiss, R. J. and Moores, E. M., 2007, Structural Geology, 2nd edition. New York: H.W. Freeman and Company