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Geosciences LibreTexts

Part II: Strain, Rheology, and Tectonics

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  • Chapters 12-16,19,20 in textbook, covering strain, analysis of folding, lineations, and foliations, and plastic (flow) rheology. The last two chapters cover divergent (rift) and convergent continental tectonics, which we will discuss in the last week of class. The final exam is comprehensive. It covers content from both Part 1 and Part 2.

    • Global Tectonic Processes
      Structural geology alone is not enough to understand the full picture of how and why continents deform. Interactions with oceanic lithosphere fundamentally influence continental deformation.
    • Stress
      Forces in the earth are usually termed as stress. Stress is like a force, but there are important differences: Stress is force per unit area, like pressure. Stress is defined to be at mechanical equilibrium (all forces balanced). Thus stress is represented by a pair of opposing vectors (a single vector is called a traction). Stresses are in general not perpendicular to a plane in the earth, so there are two components, normal stress, and shear stress.
    • Stress Sign Conventions
      General stress is a summation of a normal stress (A or B) and a shear stress (C or D).
    • Structural Models I
    • Structural Models II
      The goal of a kinematic model is a cross-section that is: Balanced: no holes or overlaps when fault slip and folding is restored. Internally consistent: Fault motions are consistent with folding and cross-cutting relationships. Both geometric and kinematic models predict displacement - the relative motion of rocks via formation of structures. Kinematic models do a better job at testing this.

    Thumbnail: Diagram of Wadati-Benioff zone. Image used with permission (Public Domain; <bdi>RyanMikulovsky).</bdi>