Geological Deformation and Earthquakes

Deformation

  • Definition: Deformation changes the character of rocks through displacement, distortion, or stress.

Types of Rocks

  • Undeformed Rocks:

    • Characteristics include horizontal beds, spherical grains, and the absence of folds or faults.

  • Deformed Rocks:

    • Features include tilted beds, metamorphic alteration, folding, and faulting.

Key Concepts of Deformation

  • Displacement:

    • Change in rock location or spatial orientation.

  • Distortion:

    • Change in rock shape, categorized into strain, stretching, shortening, and shearing.

  • Stress:

    • Force exerted per unit area, which causes deformation.

  • Strain:

    • Physical change resulting from stress due to deformation.

Types of Stress

  • Compression:

    • Squeezes material, leading to a shortening and thickening of the crust, drives collision and aids in mountain building.

  • Tension:

    • Pulls material apart, which contributes to processes such as continental rifting.

  • Shear:

    • Occurs when surfaces slide past one another without thickening or shortening the crust.

Types of Deformation

  • Brittle Deformation:

    • Rocks fracture when stress exceeds strength.

  • Ductile Deformation:

    • Rocks flow or fold without breaking, exhibiting plastic behavior.

Depth of Deformation

  • Brittle Deformation:

    • Typically occurs in the shallow crust.

  • Ductile Deformation:

    • Occurs in deeper crust under conditions of higher temperature and pressure.

Effects on Deformation

  • Temperature:

    • Warm Conditions: Lead to plastic deformation.

    • Cold Conditions: Result in brittle deformation.

  • Pressure:

    • High Pressure: Facilitates plastic behavior.

    • Low Pressure: Results in brittle deformation.

  • Deformation Rate:

    • Fast: Leads to brittle behavior.

    • Slow: Typically results in plastic behavior.

  • Composition:

    • Soft Rocks: Tend to deform plastically.

    • Hard Rocks: Generally exhibit brittle behavior.

Structural Geology Terms

  • Strike:

    • Line formed by the intersection of a horizontal plane with a tilted surface.

  • Dip:

    • Angle of a tilted surface down from the horizontal.

  • Dip Direction:

    • Perpendicular to the strike.

  • Plunge:

    • Angle a linear feature makes downward from horizontal.

Fractures and Faults

  • Joint:

    • Planar fracture without offset.

  • Vein:

    • Fracture filled with minerals (e.g., quartz or calcite).

  • Fault:

    • Fracture with displacement.

Displacement Characteristics on Faults

  • Displacement (Faults):

    • Amount of offset along a fault.

  • Types of Faults:

    • Dip-Slip Fault:

      • Blocks move parallel to the dip of the fault.

    • Normal Fault:

      • Hanging wall moves down under tension conditions.

    • Reverse Fault:

      • Hanging wall moves up under compression conditions.

    • Thrust Fault:

      • Low-angle reverse fault (dip < 30°).

    • Strike-Slip Fault:

      • Blocks move horizontally, parallel to the strike.

Lateral Faults

  • Left-Lateral Fault:

    • Opposite block moves to the observer’s left.

  • Right-Lateral Fault:

    • Opposite block moves to the observer’s right.

Products of Brittle Faulting

  • Fault Breccia:

    • Shattered and crushed rock resulting from brittle faulting.

  • Fault Gouge:

    • Pulverized, powdered rock resulting from fault motion.

  • Slickenslides:

    • Grooves or lineations on fault surfaces indicating the direction of slip.

  • Fault Scarp:

    • Surface exposure along a fault trace.

Folds in Structural Geology

  • Fold Hinge:

    • The line of greatest curvature in a fold.

  • Fold Limbs:

    • Less curved sides of a fold.

  • Axial Plane:

    • Connects hinges of successive layers.

  • Syncline:

    • Fold that opens upward, resembling a trough.

  • Anticline:

    • Fold that opens downward, resembling an arch.

  • Dome:

    • Fold with layers dipping away from the center.

  • Basin:

    • Fold with layers dipping toward the center.

  • Flexural-Slip Fold:

    • Layers bend and slip over one another.

  • Passive-Flow Fold:

    • Hot, ductile rocks deform by flowing without breaking.

  • Monocline:

    • Step-like fold caused by movement over a fault.

Tectonic Phenomena

  • Tectonic Foliation:

    • Parallel alignment of minerals formed by compression.

  • Orogenesis:

    • Mountain building caused by compressional tectonics, typically yielding igneous and regional metamorphic rocks.

  • Tectonic Setting of Orogenesis:

    • Mostly occurs at convergent plate boundaries, particularly during continent-continent collisions.

  • Continental Rifting:

    • Process involving normal faulting, which creates fault-block mountains and basins.

  • Decompression Melting:

    • Melting caused by the thinning of the crust in rifting areas, leading to volcanic formations.

  • Delamination:

    • Removal of deep lithosphere that causes uplift.

  • Orogenic Collapse:

    • Phenomenon where mountains collapse under their own weight.

Crustal Structures

  • Craton:

    • A stable portion of continental crust that remains undeformed.

  • Shield:

    • Regions with exposed Precambrian crystalline rock.

  • Cratonic Platform:

    • Layer of sedimentary rocks covering the Precambrian basement rock.

Earthquake Mechanics

  • Causes of Earthquakes:

    • Result from the sudden release of accumulated stress along faults.

  • Seismic Waves:

    • Energy waves that radiate from the hypocenter.

  • Hypocenter:

    • The point of rupture located beneath the surface.

  • Epicenter:

    • The surface location that is directly above the hypocenter.

Stick-Slip Behavior

  • Definition:

    • Describes how rocks resist motion until stress exceeds a critical level, resulting in a sudden slip and energy release.

Earthquake Parameters

  • Recurrence Interval:

    • The average time that transpires between occurrences of similar earthquakes.

  • P-waves:

    • Primary seismic waves that are compressional and travel the fastest.

  • S-waves:

    • Secondary seismic waves that are shear waves and cannot traverse liquid mediums.

  • Surface Waves:

    • Waves that travel along the Earth’s surface and typically cause the most damage.

Seismic Wave Properties

  • Moho:

    • The boundary between the crust and the mantle, detectable by seismic reflection techniques.

  • P-wave Shadow Zone:

    • Area where P-waves are refracted by the core, preventing their detection.

  • S-wave Shadow Zone:

    • Area where S-waves cannot penetrate the liquid outer core.

Seismic Instruments

  • Seismograph:

    • An instrument used to record ground motion during seismic events.

Locating Earthquakes

  • Method:

    • Utilizes three seismic stations along with the arrival times of P and S waves to triangulate the epicenter.

Earthquake Measurement

  • Earthquake Intensity:

    • Relates to the severity of ground shaking and the damage caused.

  • Earthquake Magnitude:

    • Measures the total energy released during an earthquake.

  • Moment Magnitude Scale:

    • The most reliable measurement scale for earthquake size, based on the area of the rupture and the amount of slip.