Review Questions and Answers, Crustal Deformation

Deformation describes how the shape and volume of a material change in response to stress. Think of a small, reference cube or sphere embedded in the undeformed rock. With the application of stress, the rock deforms (undergoes strain); changes in volume and dimensions of the reference object record the strain. For example, in response to hydrostatic stress, the radii and volume of the reference sphere will decrease slightly. If the deformation is elastic, the sphere will return to its original size upon relaxation of the applied stress. If the deformation is plastic (opening or closing of pore space, internal material flow or recrystallization), the deformation is permanent and remains after the stresses have dissipated.

2. Explain how confining pressure influences the way rocks deform.

Confining pressure is equivalent to a hydrostatic, compressive stress. The resulting strain involves a decrease in volume and shortening of the reference-sphere radii; internal angles are unchanged so there is no shear strain. In general, rocks are stronger (have higher elastic limits and fracture resistant toughness) in compression than in tension or in an unstressed state. Thus at any given temperature, confining pressure generally increases a rock's resistance to fracturing and plastic deformation and favors plastic flowage over brittle fracturing.

However, confining pressure is almost always determined by depth of burial (equal to the weight of the overlying rock column). As temperatures rise, rocks gradually weaken and are susceptible to plastic deformation (flowage and recrystallization) at lower-magnitude, differential stresses than at lower temperatures. Thus rocks buried deep inside an elevated, overthickened, crustal block will eventually warm, weaken, and deform by vertical shortening and lateral, extensional flowage.

3. In simple terms, what is the difference between brittle and ductile deformation?

Brittle deformation describes material failure by cracking and rupture. Faults and joints in rocks are good examples. Brittle deformation is favored by shallow depths, low rock temperatures, and massive rigid rocks. Ductile deformation describes material failure by internal flowage; recrystallization is usually involved, especially at elevated temperatures. Ductile deformation is enhanced by elevated temperatures and confining pressures. Folding at great depths and elevated temperatures is accomplished by ductile (plastic) flowage without rupture. At shallow depths, layered sedimentary rocks can readily fold (be shortened in a horizontal direction by crumpling and buckling) because the layers bend internally and slide past one another along the bedding surfaces.

4. Contrast compressional and tensional stresses.

Stresses describe the external and internal forces and pressures that cause a material to undergo deformation (strain). Tensional stresses act to pull the rock apart; compressive stresses act to force rock particles inward and closer to one another. Rocks under uniaxial, compressive stress (stresses act parallel to one direction only) are shortened parallel to the stress orientation and elongated vertically (thickened). Stretching may occur in the other, right angle, lateral direction if the compressed rocks are elevated topographically and not laterally confined. Horizontal, tensional stresses oriented at right angles to parallel, vertical planes cause stretching in the direction of normals to the planes, vertical shortening (thinning), and shortening along horizontal lines parallel to the vertical planes. In both cases, predeformational angles change, demonstrating shear strain. In hydrostatic compression or tension, radial distances are shortened or elongated by the same increments; thus volume changes (dilatations) occur without changes of internal angles (no shear is involved).

5. How is elastic deformation different from plastic deformation?

Elastic strains (deformations) involve infinitesimally small, internal stretching, shortening, and bending of rock particles and mineral grains. The strains are recovered when the deforming stresses are relaxed. Plastic strains are larger in magnitude and finite; they are permanent and remain after the deforming stresses have relaxed. In plastic deformation, flowage and recrystallization produce permanent changes in the dimensions and shapes of the rock particles and mineral grains.

6. List three factors that determine how rocks will behave when exposed to stresses that exceed their strength. Briefly explain the role of each.

Rocks fail by brittle and ductile deformation when applied stresses exceed their elastic limit (strength). Temperature, confining pressure, texture, and mineral composition exert important influences on rock strength and rock deformation.

High temperatures and high confining pressures favor plastic deformation over brittle fracturing; brittle cracking and fracturing are favored at low temperatures and low confining pressures. Mineral composition, texture, and other bulk-rock characteristics (stratification, compositional heterogeneity, porosity, cementation, etc.) have important effects on rock strength and on ways that rocks deform. Minerals such as halite and gypsum readily recrystallize and flow at low temperatures; at elevated temperatures, granitic (felsic) rocks recrystallize and flow at lower temperatures than do mafic rocks. Massive rocks such as granite and gabbro will fail by jointing and faulting while sliding and bending can readily accommodate equivalent strains in bedded, sedimentary strata. The effect of mineralogy on rock strength is well illustrated by comparing deformation in different, monomineralic rocks such as limestone (marble) and quartz sandstone (quartzite) under similar conditions. At low temperatures and low confining pressures, both fail by brittle fracturing. Calcite recrystallizes at lower temperatures than quartz; thus, calcite-rich rocks deform by recrystallization and flowage at lower temperatures and confining pressures than do quartzites. At elevated temperatures and confining pressures, both rocks deform by ductile flowage.

In general, rocks are stronger in compression than in tension. Thus rocks can resist certain magnitudes of compressive stress and fail under similar magnitudes of tensional stress.

7. What is an outcrop?

Outcrops are surface exposures of the local, subsurface, lithological material or bedrock. As such, they provide the basic information and data utilized in geologic mapping. Outcrops provide samples of the bedrock and exhibit those structures and features (such as stratification, cross bedding, mineral veinlets, faults, cleavage, etc.) that help a geologist interpret the geologic history of an area. In areas with extensive coverings of soil or other surficial materials (regolith, glacial deposits, landslide debris, sand dunes, etc.), information about the bedrock and subsurface lithologies may be available only through drilling.

8. What two measurements are used to establish the orientation of deformed strata? Distinguish between them.

The measurements are strike and dip (Figs. 15.7 & 15.8); they portray the orientation of geologic surfaces, such as stratification planes, contacts between different rock units, faults, and joints. Strike is the compass direction (with respect to geographic north) of any horizontal line lying in the geologic surface. Dip is the angle between the geologic surface and a horizontal plane; it is visualized and measured in a vertical plane aligned at right angles to the strike line. On geologic maps, a strike and dip symbol is shown as a longer, straight line drawn parallel to the strike; the shorter line of the symbol (drawn at right angles to the strike) points in the dip direction. The numerical value of the dip angle may be printed with the symbol (Fig. 15.8).

9. Distinguish between anticlines and synclines. Domes and basins. Anticlines and domes.

Anticlines are folds with two, well-defined limbs dipping in opposite directions away from a long, linear, fold axis. Strata are raised or buckled upward along the axial part of the fold relative to their elevations farther out on the limbs; thus after erosion, older strata are exposed along the axial part of the fold. Synclines are folds with two, well-defined limbs that dip inward toward a long, linear, fold axis. Strata are lowered or buckled down in the axial region; thus after erosion, younger strata are exposed in the axial portions of synclines.

Domes are more or less circular zones of upraised rocks in which the beds follow the geometry of a dome and dip away in all directions from a high point or apex; unlike an anticline, the dome structure does not have an axis. Geometrically, a basin may be thought of as an inverted dome. The strata dip inward in all directions toward the central, most downbuckled point in the structure. Anticlines and synclines have long, roughly parallel limbs and linear axes. Limbs of domes and basins make circular outcrop patterns, and the crests of domes and the lowest parts of basins are points, not axes. It is very important to distinguish between structural and topographic basins and domes (Figs. 15.15, 15.16 & 15.17).

10. How is a monocline different from an anticline?

Both are types of folds; they typically form in layered, sedimentary strata. Monoclines have only one limb (Fig. 15.14); strata that are steeply inclined in the structure are subhorizontal and relatively undeformed laterally. Monoclines of the Colorado Plateau, such as the Waterpocket fold in Utah, formed in Phanerozoic sedimentary strata above reverse faults in the crystalline, Proterozoic basement rocks. Fault offsets in the crystalline rocks at depth were gradually accommodated upward by bending of the sedimentary strata.

Anticlines and synclines form as layered strata are squeezed and crumpled by unidirectional, horizontal, compressive stresses (Fig. 15.4). In simple anticlines, strata dip in opposite directions (Fig. 15.9), and older strata are elevated in the axial part of the fold. Extensive areas or regions of folded strata (Figs. 15.10 & 20.15), such as the Valley and Ridge province of the Appalachian region in the eastern United States, consist of numerous, laterally connected anticlines and synclines.

11. The Black Hills of South Dakota are a good example of what type of structural feature?

The Black Hills (Fig. 15.16) are a late Cretaceous-early Tertiary, elliptically shaped, domal uplift of crystalline rocks associated in an as yet unknown way with subduction of old, Pacific ocean floor (the Farallon plate) beneath western North America.

They are the easternmost of many such uplifted, crustal blocks in the Rocky Mountain region. In detail, the structure is a doubly plunging anticline with the axis trending almost due north. Proterozoic rocks form the highest terrain along the axial part of the uplift and Mississippian limestones, dipping away from the elevated core of Proterozoic rocks, form an extensive plateau along the western flank of the uplift. The Dakota sandstone (Cretaceous) forms the lowest, prominent hogback and marks the structural transition to relatively flat lying strata surrounding the uplift.

12. Contrast the movements that occur along normal and reverse faults. What type of stress is indicated by each fault?

Both are dip-slip movements in which one block moves up and the other down along the fault surface. Assume that dip-slip faults with vertical dips (the fault surface is vertical) are normal faults. For dip-slip faults with inclinations or dips other than vertical, the hangingwall-footwall designation is very useful. The hangingwall block is the block that is entirely above the fault surface, and the footwall block is entirely below. In normal fault movement, the hangingwall block slides down along the fault surface with respect to the footwall block. Horizontal distances between points in the blocks are increased (stretched) and the stresses are tensional. In reverse fault movement, the hangingwall block slides upward along the fault surface with respect to the footwall block. Horizontal distances between points in the two blocks are decreased (shortened) and the stresses are compressional.

13. Is the fault shown in Figure 15.18 a normal or reverse fault?

Based on the relations shown in the photo (Fig. 15.18), the sense of displacement is normal; the hangingwall block (left) slipped down with respect to the footwall block (right). Note how the stratigraphic sequence shown below can be matched across the fault.

14. Describe a horst and a graben. Explain how a graben valley forms and name one.

A horst (Fig. 15.23) is an uplifted, fault block bounded by two normal faults. A graben (Fig. 15.23) is a downdropped, fault block bounded by two normal faults. A graben valley is the downdropped surface of an active or recently active graben; the valley is bounded by uplifted, fault blocks which may or may not be horsts. Death Valley in southeastern California is a good example of a graben valley.

15. What type of faults are associated with fault-block mountains?

Fault-block mountains (Fig. 15.23) are associated with geologically young, high-angle normal faults that flatten or merge with a regional-scale, low-angle fault at depth. Uplifted blocks form the mountain ranges and downdropped blocks make the valleys. The topography replicates active or recently active fault movements. Long, linear, fault-block ranges and valleys are horsts and grabens.

16. What type of fault is illustrated by Figure 15.25?

The photo shows dark-colored, Cambrian strata juxtaposed over the light-colored Navajo Sandstone of Jurassic age. The contact is subhorizontal; since older strata overlie younger strata, the contact must be a thrust fault. Because of the gently undulating, subhorizontal geometry of the fault, the sense of displacement (indicated by the arrows drawn on the photo) would have to have been determined from regional, geologic studies; it is not evident directly from the photo.

17. How are reverse faults different from thrust faults? In what ways are they the same?

Both are brittle failure, dip-slip faults caused by lateral compression; the hangingwall block moves up and over the footwall block, and overall, horizontal distance perpendicular to the fault trace is shortened. The main distinction is based on the dip angle or inclination of the fault. Reverse faults are high-angle, dip-slip faults and thrusts are low-angle, dip-slip faults. In subhorizontal sedimentary strata, thrusts can propagate along weak bedding plane zones, resulting in extensive, horizontal displacement, crustal shortening, and emplacement of older strata over younger strata.

18. The San Andreas fault is an excellent example of a ____________ (strike-slip) fault.

The San Andreas fault is a well-known, strike-slip (transform) fault (Fig. 15.28) that forms the boundary between the North American and Pacific plates between the head of the Gulf of California and the Mendocino fracture zone north of San Francisco (Box 15.2). Canyons in the hilly terrain to the right of the fault trace are beheaded and offset, suggesting active faulting with an important, strike-slip component. A canyon showing right-lateral displacement is clearly evident in the larger-scale photo (Fig. 15.C). Linear valleys, mangled and pulverized rock of the fault zone, sag ponds, seeps and springs, offset drainages, numerous earthquakes (Table 15.A), and juxtaposition of fundamentally different bedrock assemblages are characteristic features of active, strike-slip faults (Fig. 15.28).

19. With which of the three types of plate boundaries does normal faulting predominate? Reverse faulting? Strike-slip faulting?

Tensional faults (normal faults) dominate at divergent boundaries, and faults due to compression (reverse faults) dominate at convergent margins. Transform (sliding) plate boundaries are strike-slip faults.

20. How are joints different from faults?

Faults and joints are both fractures in rock. Along faults, the fracture-bounded blocks have been displaced (offset) from their unfractured positions; the blocks are not significantly displaced along joints. Joints typically come in sets; a joint set is a group of fractures in a given area that more-or-less exhibit a common orientation (strike and dip). Multiple joint sets may be present in any given area. Joints usually exhibit a strong control over differential weathering and erosion. The fractures, being zones of weakness and accessible to water, weather and erode faster than stronger, unfractured bedrock.