Structural geology And Plate Tectonics

The surface of the Earth is divided into 12 major and several minor plates that are in motion with respect to each other. Plate tectonics describes these relative motions, which are, to a first approximation, rigid body rotations. However, deformation of the plates does occur (primarily in belts tens to hundreds of kilometers in width along the plate boundaries), and in a few places, extends into the plate interiors. structural geology deals with these deformations,

Hypsometric curve showing the distribution of land with different elevations on the planet. Note the bimodal distribution, reflecting two fundamentally different types of crust (oceanic and continental) that have different isostatic compensation levels. Cross sections show a typical continental margin-ocean transition and ocean trench-island arc boundary.

Plains

Mountains

Steppe

Continental shelf

Continental rise

Seamount

Shoreline

Continental slope

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Abyssal plains

Mid-ocean ridge

Highest mountain

Plains

Mountains

Steppe

Continental shelf

Continental rise

Seamount

Shoreline

Continental slope

Abyssal plains

Mid-ocean ridge

Highest mountain

Sea level

Oceanic crust

Sea level

Deepest ocean trench

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Percentage of Earth's surface

Sea level

Oceanic crust

Sea level

Deepest ocean trench

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Percentage of Earth's surface which in turn give clues to the types of plate boundary motions that have occurred and to the tectonic causes of the deformation.

Plate boundaries may be divergent, convergent, or conservative/transform. The most direct evidence for plate tectonics comes from oceanic crust, which has magnetic anomalies or stripes recording plate motions. However, the seafloor magnetic record goes only as far back as 180 million years, the age of the oldest in-place oceanic crust. Any evidence for plate tectonics in the preceding 96 percent of Earth history must come from studying the continents (structural geol°gy).

Highly deformed continental rocks are concentrated in long linear belts called orogens, comparable to those associated with modern plate boundaries. This observation suggests that these belts represent former plate boundaries. The structural geologist examines these orogens, determines the geometry, kinematics, and mechanics of these zones, and makes models for the types of plate boundaries that created them. The types of structures that develop during deformation depend on the orientation and intensity of applied forces, the physical conditions (temperature and pressure) of deformation, and the mechanical properties of rocks.

The most important forces acting on the lithosphere that drive plate tectonics and cause the deformation of rocks are the gravitational "ridge push" down the flanks of oceanic ridges, gravitational "trench pull" of subducting lithosphere caused by its greater density than surrounding asthenosphere, and the resistance of trenches and mountain belts.

At low temperature and pressure and high intensity of applied forces, rocks undergo brittle deformation, forming fractures and faults. At high temperature

Differential stress (MPa) 500

1,000

Differential stress (MPa) 500

1,000

Brittle-plastic transition

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The strength of the lithosphere changes dramatically with pressure and temperature, based on the mechanical properties of the minerals in the rocks at different depths. There are several weak and strong zones at different depths that vary in depth in different parts of the world based on the different rock types in different places.

and pressure and low intensity of applied forces, rocks undergo ductile deformation by flow, coherent changes in shape, folding, stretching, thinning, and many other mechanisms.

Different styles of deformation characterize different types of plate boundaries. For instance, at mid-ocean ridges new material is added to the crust, and relative divergent motion of the plates creates systems of extensional normal faults and ductile thinning at depth. At convergent plate boundaries, one plate is typically subducted beneath another, and material is scraped off the down-going plate in a system of thrust faults and folds. Along transform plate boundaries, systems of strike-slip faults merge downward with zones of ductile deformation with horizontal relative displacements. All types of plate boundaries have small-scale structures in common, so it is necessary to carefully examine regional patterns before making inferences about the nature of ancient plate boundaries.

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Responses

  • Yvonne Schwartz
    How do plate tectonics and structural geology relate?
    6 months ago

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