Structural Geology

GEOLOGICAL STRUCTURES Forces acting on the lithosphere generate stresses that cause rocks to DEFORM for example:• folding • faulting. http://www.uky.edu/AS/Geology/howell/goodies/elearning/module10swf.swf A variety of forces act upon plates generate tectonic stresses that are transmitted through the lithosphere.

Activity find the definitions of:STRESS

STRAIN

TECTONIC FORCES:Generate extensional or compressional stress fields leading to fold and fault formation.

Fold formation is by buckling or by ductile flow. Folds can be studied by analysis of the FOLD GEOMETRY:• limbs • hinges • axis • axial plane, • plunge , • upright, • over turned, • recumbent.

Faults result from brittle rock failure when applied stresses exceed fracture strength. Faults can be analysed by FAULT GEOMETRY:• Dip • throw • displacement, • fault types • normal, reverse, thrust, strike, slip.

Applying stress to a rock or soil causes strain resulting in change in volume and length at right angles to principal stress axis (ie where stress applied is greatest) Principal Stress Axis (Pmax) Minimum l Stress Axis(Pmin) Pmin

Pmax

Pmax

Pmin Activity For the following situations place the correct resultant deformation into the box by copying and pasting.

Pmax Pmin Pmax

Pmax

Pmin Pmax

Geological Structures Use the following resources to fill in the blanks http://earthsci.org/education/teacher/basicgeol/deform/deform.html#Stressand Strain

http://earth.leeds.ac.uk/learnstructure/ Activity Label the features of a fault.

Hanging Wall Foot Wall Throw Fault Plane

Measure the throw if 1cm = 1 m

Faults are fractures that have displacement of rock along them.

Features of faults: •

Create zones of broken ground-weaker and less stable, with implications for foundation bearing, slope stability and tunnel roof integrity.

•

Sudden movement alone faults produce earthquakes.

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Allow movement of liquids and gases (radon). Implication for tunnelling, reservoirs.

Types of Faults Activity Name the faults, mark on the direction of movement. Measure the throw (1cm = 1 metre)

Joints Are rock fractures with no movement along them. They are formed by tectonic stressing and are developed in nearly all rocks.

Groups of sub parallel joints or joint systems are known as JOINTS SETS.

All joints are structural weaknesses, whose density, extent and orientation are major influences on rock mass strength.

Activity How many joint sets?

Measurements:I f (m) Fracture Spacing Index Fracture spacing: - given as index, significance of spacing controls stability of slope or tunnel for example closely spaced joints speed up tunnelling but give more potential for collapse. Extremely High >2m to extremely low 0.006m RQD

Dominant Feature in sedimentary rocks is the bedding plane, mark on the DIP and STRIKE Activity label the Dip and strike

Horizontal FOLDS

FOLDS Fold animation http://www.geology.sdsu.edu/visualstructure/Geo306_webmaterial/foldelemen ts.html

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Activity Label the fold.

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Activity Identify the types of folds

Limb Axis Axial Plane Dip Angle