The Origin and Evolution of the Marine Environment (MSCI 112) - Tectonics & Deformation Vocabulary Flashcards
Tectonic Forces and Stress
- Definition of force: an interaction that tends to change the motion of an object (units: N).
- Types of force relevant to rocks:
- Compression
- Tension
- Shearing
- Confining (pressure, uniform compression)
- Stress is the force per unit area experienced by a rock:
- \sigma = \frac{F}{A}
- Units: N/m$^{2}$ (Pascals, Pa)
- Relationship: stress is the internal distribution of force; force is the external action that can cause stress when applied over an area.
Strain
- Definition: the amount of deformation resulting from applied stress.
- Types of deformation (strain):
- Shortening (compression)
- Stretching (tension)
- Shear (sliding deformation)
- Strain is related to the type of deformation; it describes change in size/shape/position due to stress.
- Deformation behavior depends on several factors:
- Temperature: Warmer tends toward ductile behavior; colder tends toward brittle behavior.
- Pressure: Higher pressure (confined conditions) favors ductile deformation; lower pressure favors brittle deformation.
- Deformation rate: Fast (impulsive) tends toward brittle; slow tends toward ductile.
- Rock composition: Different minerals exhibit different ductility (e.g., halite vs. granite).
- Summary: Brittle deformation forms fractures and faults; ductile deformation forms folds and shear structures.
- Displacement: change in location of a rock block.
- Rotation: change in orientation of a rock block.
- Distortion: change in shape of a rock body.
- Example notes:
- A rock transported by a fault slip from a deposition location indicates displacement.
- Beds originally horizontal may become tilted due to deformation.
- >2.7 Ga greenschist: slip on a fault transported rock down from where it was deposited.
- -2.2 Ga quartzite: beds were horizontal when deposited; now tilted.
- These illustrate how rocks record deformation histories over deep time.
Strike and Dip
- Strike: compass direction of a rock layer as it intersects a horizontal surface.
- Dip direction: perpendicular to the strike direction on the horizontal surface.
- Dip angle: amount of tilting of the layer; measured at right angles to the dip direction.
Practice Questions 1–3
- 1. The rocks shown in the image below have undergone which type of deformation?
- 2. The dip direction of the deformed rocks points toward _ of the photo. A. left B. right C. top left D. bottom right
- 3. The deformed rocks strike in direction in the photo. A. left to right B. top to bottom C. perpendicular inside to outside D. diagonal bottom left to top right
Joints and Veins
- Brittle deformation leads to:
- Joints: fractures in the surface with no observable movement on either side.
- Veins: mineral-filled cracks.
Faults
- Faults are fractures in rocks with movement along the fracture.
- Types of faults (based on relative movement):
- Normal fault (tensional stress)
- Reverse fault (compressional stress)
- Strike-slip fault (shear stress)
Fault Anatomy
- Fault surface: planar surface along which slip occurred.
- Fault throw: vertical displacement caused by fault movement.
- Footwall: fault block beneath the fault surface.
- Hanging wall: fault block above the fault surface.
- Examples include Normal and Reverse faults with characteristic throws and wall movement.
Strike-Slip Faults
- Left-lateral strike-slip fault: the fault block on the left-hand side of an observer moving on the fault moves backward relative to the observer.
- Right-lateral strike-slip fault: the fault block on the right-hand side moves backward relative to the observer.
- Types of stress and associated faults:
- Divergent boundary → Normal fault → Tensile stress
- Convergent boundary → Reverse fault / Thrust fault → Compressional stress
- Transform boundary → Strike-slip fault → Shear stress
- This links deformation style to plate boundary context.
Practice Questions 4–5
- 4 & 5 fill-in-the-blank:
- A fault with footwall moved upward relative to the hanging wall is called a Normal.
- A fault with footwall moved downward relative to the hanging wall is called a Reverse__.
Question 6 (Multiple Choice)
- A normal fault is caused by stress while a reverse fault is caused by ___ stress.
- A. compressional, compressional
- B. compressional, tensional
- C. tensional, tensional
- D. tensional, compressional
Folds
- Folds are bent or curved structures formed from originally flat/planar rocks.
- Folds result from ductile deformation.
- Key concepts: axial plane, fold axis (hinge), limbs.
Axial Plane, Fold Axis, Limbs
- Axial plane: plane that separates a fold into two roughly symmetrical halves.
- Fold axis (hinge): line where the two limbs meet; intersection of axial plane with the fold surface.
- Limbs: the two halves on either side of the axial plane.
Types of Fold
- Anticline: limbs dip away from the axial plane; oldest rocks are at the top (in-fold structure).
- Syncline: limbs dip toward the axial plane; youngest rocks are at the center.
Plunging Fold and Monocline
- Plunging fold: fold axis is not horizontal; axis plunges into the subsurface.
- Monocline: one limb is horizontal; often associated with blind faults.
Folds – Domes & Basins
- Folding in multiple mutually perpendicular directions.
- Domes: anticlines in multiple directions (upward).
- Basins: synclines in multiple directions (downward).
Practice Question 7
- Identify syncline and anticline in the following image.
Practice Question 8
- Walking on top of an eroded syncline/anticline, you will find the oldest rocks toward the _ of the syncline/anticline.
- A. center/center B. edge/center C. center/edge D. edge/edge
- Answer: C. center/edge
- Tectonic deformations are common in mountainous regions.
- Orogeny: processes that form mountain ranges through tectonic deformation.
Orogeny by Collision
- Collision (cubic compressional regime) leads to:
- Crustal shortening
- Tectonic deformation: fold-thrust belts
- High-grade regional metamorphism
Orogeny by Subduction
- Subduction involves compressional stress causing one plate to subduct beneath another.
- Effects:
- Crustal shortening
- Volcanism
- Fold-thrust belts
- High-pressure, low-temperature metamorphism
Orogeny by Continental Rifting
- Continental lithosphere is rifted apart under tensional stress.
- Deformation: normal faults.
- Metamorphism: contact metamorphism (localized near faults).
Orogeny by Exotic Terrane Accretion
- Exotic terrane: young and buoyant oceanic lithosphere that does not subduct during convergence.
- Suture with the continent forms accreted terrane.
- Orogeny features: fold-thrust belts and reverse faults.
Change of Orogen Topography
- Crustal shortening and thickening adds crustal rocks, affecting topography.
- Isostasy: rising of the crust (uplift) due to buoyancy changes when mass is added or removed.
Change of Orogen Topography (Delamination)
- Delamination: removal of lithospheric material from below the crust leading to isostatic rebound and topographic changes.
Change of Orogen Topography (Erosion and Exhumation)
- Erosion removes surface material, contributing to rebound (positive feedback).
- Exhumation: erosion plus isostatic rebound brings deeply buried rocks toward the surface.
- Example ranges: Himalayas to Appalachians illustrate long-term exhumation and uplift processes.
Take Home Message
- Understand how rocks deform in response to stress.
- Know the characteristics of basic geologic structures and how to describe them: joints, faults, folds.
- Understand mountain belts and tectonic deformation in relation to plate tectonics.
