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Geology 110 Syllabus

SHEAR ZONES

FaultsReverse/Thrust Faults

I. HOW TO DETERMINE SHEAR OR ROTATIONAL SENSE (KINEMATIC INDICATORS)

A. Offset markers - May be useful in determining the sense of movement if slip line is known. Without knowing the slip line this is only a separation which can be misleading.

B. Drag folds - if slip is known

C. Shear-sense indicators in shear zones (read Simpson, C., 1986, J. Geol. Ed., v. 34, p. 246-261)

1. Brittle shear zones

a. Slickensides (steps & slicken lines)

b. Tension gashes

c. Fault breccia (cataclasite)

2. Ductile shear zones

a. Found in mylonitic rocks

b. Must know slip line (elongation lineation)

c. Must look parallel to elongation lineation and perpendicular to mylonitic foliation

d. Sorby (1850's) x-y plane = foliation

* Shear bands - small scale ductile shear zones

* Mica fish - shear band and quartz-feldspar composites

* Asymmetric folds

* Asymmetric porphyroclasts

sigma (s) type (e.g., kgn-1)

delta (d) type