Crustal Deformation and Structural Geology Flashcards
Mountains
- Vivid evidence of tectonic activity
- Manifestations of geologic processes
○ Uplift
○ Deformation
○ Metamorphism - Frequently occur in elongate, linear belts
- Orogenesis: mountain building
Orogenesis Involves:
○ Uplift ○ Deformation ○ Jointing ○ Faulting ○ Folding ○ Foliation ○ Metamorphism ○ Igneous activity ○ Erosion ○ Sedimentation - Constructive process build mountains up - Destructive processes tear them back down again
Orogenic Belts
- Mountains have a finite life span
○ Young mountains are high, steep, and still growing
○ Middle-aged mountains are lowered by erosion
○ Old-age mountains are deeply eroded remnants
Deformation
- Changes the character of rocks
○ Undeformed (unstrained): horizontal beds, spherical sand grains, no folds or faults
○ Deformed (strained): tilted beds, metamorphic alteration, folding and faulting - Results in one or all of the following:
○ Displacement - change in location
○ Rotation - change in orientation
○ Distortion - change in shape
Strain
- Change in shape as a result of deformation
- Several types:
○ Stretching - pulling apart
○ Shortening - squeezing together
○ Shear - sliding past
Brittle Vs. Ductile Deformation
- Brittle deformation: rocks break by fracturing, occurs in the shallow crust
- Ductile deformation: rocks deform by flowing and folding, occurs at higher T and P deeper in the crust
- Transition between the two types occurs ~10-15km depth
- Earthquakes do not appear in the deep crust b/c breakage does not occur in the deep crust
Type of deformation depends on:
- Temperature
- Pressure
- Deformation rate
- Composition
Causes of Deformation
- Strain is caused by force acting on rock (stress)
- Stress is applied across a unit area
○ Large force per area results in much deformation
○ Small force per area results in little deformation
Stress
- Compression takes place when an object is squeezed
○ Shortens and thickens the material - Tensions occur when the ends of an object are pulled apart
○ Horizontal tension drives crustal rifting, stretches and thins the material - Shear develops when surfaces slide past one another
○ Shear stress neither thickens nor thins the crust
Geologic Structures
- Geometric features are created during rock deformation
- The 3D orientation of a plane is described by strike and dip
○ Strike - horizontal intersection with a tilted surface
○ Dip - the angle of the surface down from the horizontal
Measuring Structures
- Dip is perpendicular to strike and measured downward
- Linear structures can be similarly measured
○ Bearing - compass direction
○ Plunge - angle from the horizontal
Joints and Veins
- Joints: planar rock fractures without any offset, develop from tensile stress in brittle rock (systematic joints occur in parallel sets), often control weathering of rock
○ Groundwater often flows through joints - Dissolved minerals precipitate = veins
- Chemical weathering and weathering from streams in joints
Faults
- Planar fractures showing displacement
○ Abundant in the crust and occur at all scales
○ Sudden movements along faults cause earthquakes
○ Can be active or inactive
Fault Orientation
- On a dipping fault, the blocks are classified as the:
○ Hanging-wall block (above the fault) = picture walking on it, you can’t = hanging
○ Footwall block (below the fault) = picture walking on it successfully = footwall
Dip slip
blocks move parallel to the dip of the fault (vertical movement = normal faults or reverse faults)
Strike slip
blocks move parallel to fault plane strike (lateral movement)
Oblique slip
components of both dip slip and strike slip (both lateral and vertical, almost all faults are oblique, but identify by major vectors)
Dip-Slip Faults
- Sliding is parallel to the dip of the fault
- Blocks move up or down the slope of the fault
Reverse fault
hanging wall moves up fault slope, accommodate crustal shortening = compression
(COMPRESSIVE STRESS, shortening system, only for reverse/thrust)
Thrust fault
special type of reverse fault, lower angle <35º than reverse = gentle dip, often result of continental collisions
Normal fault
hanging wall moves down fault slope, accommodate crustal extension = pulling apart (TENSILE STRESS, lengthening the system)
