Diastrophism - Folded, Faulted and Tilted Strata.
Introduction
Three types of deformation result from subjecting rock to stress:
1. Folds - bending of rock without breaking (including tilting).
2. Faults - fracture of rock with displacement.
3. Joints - fracture of rock without displacement. Joints affect the resistance
of the rock to erosion by weakening the rock and making it susceptible to
weathering.
Only deformation which includes movement is directly involved in landform development, so the rest of the lecture concentrates on folds, faults and tilted beds.
Behavior of Rock Under
Stress
During tectonic movements, rocks may be subjected to 3 kinds of stress:
COMPRESSIVE, TENSIONAL & SHEAR.
The kind of deformation that results depends primarily on 4 factors:
1. Confining pressure
2. Temperature
3. Strength of rock
4. Time
Rocks under high confining pressure and temperature tend to become DUCTILE and
flow rather than fracture when stress is applied. For this reason, it is
generally true that FOLDING tends to occur at depth in the crust where confining
pressure and temperature are both high.
Many folds in orogenic belts have in fact formed at depth and later been exposed at the surface by erosion of overlying rocks. On the other hand, rocks at or near the surface of the earth are under low confining pressure and low temperature, and tend to be BRITTLE and fracture when subjected to stress.
Strike and Dip
Geologic structures produced by deformation are 3-dimensional (most structures are basically composed of inclined planes - tilted beds alone, the tilted limb of a fold, an inclined fault plane). The geometry of inclined planes is described by the properties of strike and dip.
Most geological information for a region is in the form of 2-dimensional geologic maps - which show the outcrop pattern and, ideally, contain strike, dip and fault symbols. If strike and dip is not included on the map, often this information can be deduced: the outcrop trend gives the strike; "the law of v's" gives the direction of dip and some indication of the dip angle (the larger the V, the lower the angle of dip).
Classification of Folds
Folds = most common type of deformation (most strata is folded/tilted to an
extent, whereas faults are more scattered).
Main types shown below:
Plunging folds dip into the earth at an angle. The resulting outcrop pattern
resembles zig-zag lines.
Basins and domes are special kinds of folds, where all the strata either dip
away (dome) or towards (basin) the center.
Classification of Faults.
