Harry Williams Geomorphology 1 Diastrophism Folded Faulted and

Harry Williams, Geomorphology 1

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. FOLD FAULT Harry Williams, Geomorphology 2

EXFOLIATION/UNLOADING JOINTS IN FORMERLY DEEPLY-BURIED ROCK, UNCOVERED BY EROSION. Harry Williams, Geomorphology 3

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CONTRACTION JOINTS IN VOLCANIC ROCK. COOLING LAVA CONTRACTS AND FORMS HEXAGONAL FRACTURES (COLUMNAR JOINTING) Harry Williams, Geomorphology 5

STRESS JOINTS CAUSED BY TECTONIC MOVEMENTS Harry Williams, Geomorphology 6

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 Harry Williams, Geomorphology 7

MARBLE DEFORMATION fracture flow 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. Harry Williams, Geomorphology 8

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. FAULTING FOLDING Harry Williams, Geomorphology 9

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: hills mountains cliffs ridges Harry Williams, Geomorphology 10

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 (see below): Harry Williams, Geomorphology 11

However, many of these maps (especially in the U. S. ) do not usually include many details on underlying geologic structures, including strike and dip symbols. Harry Williams, Geomorphology 12

Some maps, usually smaller scale ones, have cross-sections along selected lines to show the geometry of the underlying strata. S. E. N. W. CRATON Harry Williams, Geomorphology 13

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 - more later on outcrop patterns). Harry Williams, Geomorphology 14

Classification of Folds = most common type of deformation (most strata is folded/tilted to an extent, whereas faults are more scattered). Main types: Harry Williams, Geomorphology 15

Plunging folds dip into the earth at an angle. The resulting outcrop pattern resembles zig -zag lines. Harry Williams, Geomorphology 16

Basins and domes are special kinds of folds, where all the strata either dip away (dome) or towards (basin) the center. Harry Williams, Geomorphology 17

Classification of Faults. Harry Williams, Geomorphology 18

Foot Wall Hanging Wall Harry Williams, Geomorphology 19