08 Tectonic deformation

Force: An interaction that tends to change the motion of an object, measured in Newtons (N).
Types of Tectonic Forces:
- Compression: A force that pushes or squeezes rocks together.
- Tension: A force that pulls rocks apart.
- Shearing: A force that causes parts of rock to slide past each other.
- Confining Pressure: Pressure applied equally in all directions.
Stress:
- Definition: The force per unit area experienced by a rock, measured in Newtons per square meter (N/m²).

Definition: The amount of deformation resulting from the application of stress.
Types of Strain:
- Shortening: Decrease in length.
- Stretching: Increase in length.
- Shear: Change in shape without changing volume.

Brittle Deformation: Result of low temperature and pressure conditions, where rocks fracture instead of bending.
Ductile Deformation: Occurs under high temperature and pressure, allowing rocks to bend or flow.
Factors Affecting Deformation:
- Temperature: Warmer conditions lead to ductile behavior, while colder conditions favor brittleness.
- Pressure: Higher pressure promotes ductility.
- Deformation Rate: Fast deformation leads to brittleness, while slow deformation promotes ductility.
- Composition: Different minerals (e.g., halite vs. granite) behave differently under stress.

Displacement: Change in position or location of rocks.
Rotation: Change in orientation of rocks.
Distortion: Change in shape of rocks due to stress.
Examples:
- Greenschist (>2.7 Ga): Rock transported down from original site due to slipping on a fault.
- Quartzite (-2.2 Ga): Initially horizontal layers are now tilted and distorted.

Strike: Compass direction of a rock layer as it intersects a horizontal surface.
Dip Direction: Perpendicular to strike direction in horizontal view.
Dip Angle: The angle of tilting of the layer, measured at right angles to the dip direction.

Fault: A fracture in rocks with movement along the fracture, resulting from brittle deformation.
Types of Faults:
- Normal Fault: Occurs due to tensional stress, resulting in the hanging wall moving downward relative to the footwall.
- Reverse Fault: Occurs from compressional stress, leading the hanging wall to move upward relative to the footwall.
- Strike-slip Fault: Horizontal movement along the fault plane, with no vertical displacement.

Left-lateral Strike Slip Fault: The block on the left-hand side moves backward relative to the observer.
Right-lateral Strike Slip Fault: The block on the right-hand side moves backward relative to the observer.

Types of Deformation Based on Tectonic Settings:
- Divergent Boundary: Causes normal faults due to tensional stress.
- Convergent Boundary: Causes reverse and thrust faults due to compressional stress.
- Transform Boundary: Causes strike-slip faults due to shear stress.

Definition: Folds are bent or curved structures formed from originally flat or planar rocks due to ductile deformation.
Key Terms:
- Axial Plane: The plane that divides a fold into two symmetrical halves.
- Fold Axis (Hinge): The intersection line of the axial plane with the fold's surface.
- Limb: The two sides of the fold separated by the axial plane.

Anticline: A fold where the limbs dip away from the axial plane.
Syncline: A fold where the limbs dip toward the axial plane.
Plunging Fold: A fold where the axis is not horizontal.
Monocline: A fold with one limb horizontal, often associated with blind faults.

Orogeny: Refers to the processes that form mountain ranges due to tectonic deformation.
Orogeny by Collision: Involves compressional stress causing the collision of tectonic plates, leading to crustal shortening and the formation of fold-thrust belts and high-grade regional metamorphism.
Orogeny by Subduction: Involves one tectonic plate being pushed beneath another, resulting in crustal shortening, volcanism, and high pressure-low temperature metamorphism.
Orogeny by Continental Rifting: Tensional stress pulls apart continental lithosphere, leading to the formation of normal faults and contact metamorphism.
Orogeny by Exotic Terrane Accretion: Young, light oceanic lithosphere that does not subduct becomes part of the continental crust, forming accreted terranes and contributing to fold-thrust belts and reverse faults.

Crustal Shortening: Thickening of the crust occurs as rocks are added due to tectonic forces, which can lead to isostatic rebound (rising of orogens due to buoyancy).
Delamination: The removal of lithospheric material beneath the crust can also lead to isostatic rebound.
Removal of Load: Erosion removes upper layers, causing the crust to rebound; this process is known as exhumation, where deeply buried rocks are brought closer to the surface over time.

Understand how rocks deform in response to stress and the characteristics of Joints, Faults, and Folds.
Familiarize yourself with how mountain belts and tectonic deformation are related to plate tectonics, recognizing their implications in geological history.