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Structural Geology

GLY 4400 Lecture 1

Introduction

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What is Structural Geology?

Structural Geology is the branch of geologythat deals with the form, arrangement, and

internal structure of the rocks, andespecially with the description,representation, and analysis of structures,chiefly on a moderate to small scale

Similar to tectonics, but the latter term isused for broader regional or historicalstructures

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Geologic Structure

A geometric feature in rock whose shape,

form, and distribution can be described

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Geometrical Classification

A. Planar to subplanar

B. Curviplanar

C. Linear

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Geological Significance

Classification A. Primary - Formed by the formation of the rock itself

B. Local gravity-driven - formed due to slip down aninclined surface - Includes: Slumping, at any scale, driven

by gravitational potential which exceeds friction

C. Local density-inversion driven - formed because of locallateral rock-density variations, causing local buoyancyforces

D. Fluid-driven pressure: Injection of unconsolidatedmaterial due to sudden release of pressure

E. Tectonic - Formed due to interactions betweenlithospheric plates, or regional interactions between theasthenosphere and the lithosphere. The forces aregravitational, trying to achieve isostatic equilibrium, atcrustal to lithospheric scales

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Time of Formation Classification

A. Syn-formational - Structure forms at the same

time as the material that forms the rock

B. Penecontemporaneous - Formed afterdeposition, but before full lithification

C. Post-formational - Structure forms after the

rock has fully formed, as a consequence of forces

not related to the formation of the rock

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Debris Flow

Flowing mixture is dominantly sediment

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Formation Process Classification

A. Fracturing - related to development orcoalescence of fractures in rock

B. Frictional sliding - The slippage of one rock

past another, or of grains past each other, resistedby frictional forces

C. Plasticity - Deformation by the internal flow ofcrystals w/o loss of cohesion, or non-frictional

sliding of crystals past each other D. Diffusion - material transport either in the

solid-state or by dissolution in a fluid

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Mesoscopic Cohesion Classification

A. Brittle - loss of cohesion across a

mesoscopically discrete surface

B. Ductile - w/o loss of cohesion across a

mesoscopically discrete surface

C. Brittle/ductile - some combination of the

two behaviors

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Significance of Strain Classification

A. Contractional - Shortening of a region

B. Extensional - Lengthening (extension) of

a region

C. Strike-slip - movement w/o dimensional

change

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Distribution of Deformation

Classification A. Continuous - deformation throughout the rock

body, on all scales

B. Penetrative - At the scale of observation, occurs

throughout the rock body - at finer scales, theremay be gaps between structures

C. Localized - Continuous or penetrativethroughout a definable subregion of the overall

structure D. Discrete - a structure which occurs as an

isolated feature

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Plate Tectonic Features

Figure 1.4 in text