crustal

Crustal Deformation Study Guide

Stratigraphic Principles

  • Original Horizontality:

    • Sedimentary rocks are deposited in flat, horizontal layers.

    • Due to gravity, particles tend to settle down flatly, thus if layers are seen tilted or folded, it indicates deformation occurred post-deposition.

  • Lateral Continuity:

    • Layers of sediments extend laterally in all directions until they thin out or encounter a physical barrier.

    • This principle assists geologists in correlating rock layers across valleys or eroded areas.

  • Superposition:

    • In a sequence of undisturbed sedimentary layers, the oldest layer is at the bottom with progressively younger layers above it.

    • This principle is essential as the foundation for the relative dating of rock formations.

Stress vs. Strain

  • Stress:

    • Defined as the force applied to a rock per unit area.

  • Strain:

    • Refers to the change in shape, volume, or orientation of the rock as a response to stress.

  • Types of Stress:

    • Compressional Stress:

    • Pushes rocks together, leading to a shortening of the crust.

    • Tensional Stress:

    • Pulls rocks apart, resulting in an extension or lengthening of the crust.

    • Shear Stress:

    • Causes rocks to slide past one another.

  • Lithostatic Stress:

    • Describes the equal pressure applied from all directions due to overlying rock.

    • Increases with depth, known as confining pressure.

  • Volume Changes:

    • Compression:

    • Causes a decrease in volume.

    • Tension:

    • Results in an increase in volume.

    • Shear:

    • Changes the shape, but the volume remains constant.

  • Plate Boundaries:

    • Convergent Boundaries:

    • Associated with compressional stress.

    • Divergent Boundaries:

    • Involve tensional stress.

    • Transform Boundaries:

    • Characterized by shear stress.

Types of Strain

  • Elastic Strain:

    • Temporary deformation where the rock will return to its original shape once the applied stress is removed.

  • Plastic Strain:

    • Permanent deformation where the rock bends or flows, and does not return to its original shape.

  • Faulting (Brittle Deformation):

    • Occurs when rocks break under conditions of brittle strain.

  • Folding (Ductile Deformation):

    • Involves bending of rocks under ductile strain.

Faulting

  • Normal Fault:

    • The hanging wall moves down relative to the footwall; typically results from tensional forces and is common at divergent boundaries.

  • Reverse Fault:

    • The hanging wall moves up relative to the footwall; occurs due to compressional forces and is prevalent at convergent boundaries.

  • Strike-Slip Fault:

    • Blocks of rock move horizontally past one another; caused by shear stress and commonly found at transform boundaries.

  • Right vs. Left Lateral:

    • Observing from one side of the fault:

    • If the opposite block moves to the right, it’s classified as right-lateral.

    • If the opposite block moves to the left, it’s left-lateral.

  • San Andreas Fault:

    • A classic example of a right-lateral strike-slip fault.

  • Dip-Slip vs. Strike-Slip:

    • Dip-Slip Faults: Involve vertical motions (either normal or reverse).

    • Strike-Slip Faults: Involve horizontal motion.

Folding

  • Anticline:

    • An arch-shaped fold where limbs dip away from the fold axis; the oldest rocks are found in the center.

  • Syncline:

    • A trough-shaped fold where limbs dip toward the fold axis; the youngest rocks are located in the center.

  • Dip:

    • The angle at which a rock bed is tilted from the horizontal position.

  • Strike:

    • The compass direction of a horizontal line on the tilted bed.

  • Fold Axis (Axial Plane):

    • The imaginary line that divides the fold into two symmetrical halves.

  • Map View Clues:

    • Anticline: Dip symbols on geological maps point away from the axis and older rocks are positioned in the center.

    • Syncline: Dip symbols point towards the axis, and younger rocks lie in the center.

Symbols on Maps

  • Horizontal Beds: Indicated by a cross mark “+”.

  • Fold Axes: Shown with dashed lines.

Topography and Erosion

  • Fold axes do not always align with the surface features like hills and valleys.

  • Erosion can obscure the visible signs of fold structures.

Domes and Basins

  • Domes:

    • Upwarped geological structures where the oldest rocks are exposed in the center, with progressively younger rocks outward.

  • Basins:

    • Downwarped geological structures where the youngest rocks are in the center, surrounded by older rocks.

  • Map View Appearance:

    • Both domes and basins resemble bullseyes: domes have older rocks at the center while basins have younger rocks.

  • Plunging Folds:

    • Characterized by a fold axis that tilts into the ground, often leading to “V” shaped patterns due to erosion.

    • Plunging Anticline: “V” points in the direction of the plunge.

    • Plunging Syncline: “V” points opposite to the direction of the plunge.

  • Rock Ages Along Plunge:

    • Rock ages change progressively along the “V” pattern, providing cues to identify the type of fold.