geo structures
natural phenomena of deformed or fractured rock units (strata) in 3D
geology goals (geo structures)
use measurements of rocks to find the history of a deformation and to understand the stress fields causing it
geo engineering goals (geo structures)
use measurements to find stability of deformation, and to predict the future stability
fold
- ben or curved rock strata
- formed due to plastic deformation
fold belt
- set of folds distributed on a regional scale
- common with orogenic zones (mtns)
Components of a fold
axial surface
-|-
/ -|- \
/ / \ \ limb
hinge line
fold axis runs parallel the plain of the foldlook at pics for better description
types of folds
-Anticline: upside down U
-Syncline: U
-plunging fold: fold starts out horizontal than the fold axis comes to an angle with the ground called the “angle of plunge”
-basin: dish-like
-dome: …..
-monocline: slight step down in elevation
_________
_____/
horse mountain (He Lan Shan) Chongqing & 3 Gorges
mountains with numerous folds in China
Fault
- planar fracture in a rock
- the rock one side moved with respect to the other.
- can be small to tectonic plate boundaries size
fault zone
series of faults in a zone with complex deformation
Components of a fault
- slip vector: direction of slipping strata
- footwall: the strata that doesn’t move
- hanging wall: strata that does move
types of faults
- normal: where hanging wall slides down decline
- reverse/thrust: hanging wall is pushed up incline
- shear/tear/strike-slip: no change in elevation, just slipping strata passed each other
types of strike slip faults
- sinistral/left-lateral: when viewed from above, strata make ccw motion
- dextral/right-lateral: when viewed from above, makes cw motion
series of faults
- step faults: look like stairs
- block faults: misaligned block shaped faults
- horse/rift valley or graben: trapezoid shaped strata that form valleys above ground
slickensided fault surface
- smoothly polished surface caused by frictional movement between rocks along the two sides of a fault
- striations are the lines in the direction of the sliding on the fault face.
Tension Crack
-opening in the rock due to tensile failure in slope with downward displacement
joints
- planar discontinuities in rock that show or have fracture that aren’t displace with respect to each other
- slight displacement can occur
fissures
- microscopic cracks in rocks that are smaller than joints
- usually don’t have pattern
loading conditions for geo structures
look at page q07-8-9
geo structures in HK
-Fault shown in cliff at ma shi chau
geo structure engineering considerations
- geo structures are internal weaknesses of rocks and sites of penetration/ground water to screw up other foundational sites
- principal stresses are essential for cave/tunnel making
geo structure types
fold fault fracture joint fissure
