Topic 12: CRACKS AND DENTS IN THE EARTH'S ARMOR: GEOLOGIC STRUCTURE Structural geology is the study of faults, folds and fractures in the Earth. These fault, folds, and fractures were formed by the large stresses caused by plate tectonic processes.
Philosophy - The plate tectonics model is of great importance in appreciating the large-scale aspects of earth "architecture", and of understanding how the various earth processes, involving magmatism, weathering, erosion and landscape evolution, sedimentation, mountain building, seismicity, etc., are tied together in a coherent "system". But it is on a smaller scale that most real-world problems are met and must be solved. Geoenvironmental problems involving the earth's crust, and those of the mining and civil engineer, are met on the scale of the drainage basin, the aquifer, the reservoir area, the mine, the excavation, the building foundation.
On these scales, structural geology cannot be ignored. The real rock mass is NOT made of an isotropic and homogeneous (do you know the meaning of these terms?) material. The rock layers are of different materials, and the folds, faults and rock joints found on the jobsite make these materials weaker in some directions and stronger in others, and more permeable in certain directions to flow of water, petroleum products, methane, and pollutants. Thus the properties of materials can vary substantially over the region of interest. The solution of real problems requires a sound understanding of rock structure, as so-called minor structures may have a major impact on the outcome.
An example is shown in the photo above. On the scales of the outcrop and jobsite, fracture sets (joints) represent defects that reduce rock mass strength to a value far below the strength of the intact rock material. Open fractures also strongly influence the flow of groundwater, petroleum, methane, and pollutants. As clearly shown above, the fractures are preferentially orientated (not random); thus, the permeability and strength are also orientated, and in this way rock masses acquire anisotropic properties. The photograph above shows "edge fringe" cracks propagating downward into a thick shale from a parent joint in a siltstone bed. The parent joint strikes 342 (azimuth), and is typical of early "dip" joints (that is, joint strike is approximately parallel to dip of the mildly-folded beds), that propagated during layer-parallel (horizontal bed) shortening of the Appalachian Plateau detachment sheet in late Paleozoic time. The fringe cracks strike 351, suggesting a clockwise stress field rotation accompanying the Alleghanian Orogeny in both the Appalachian Plateau and Valley and Ridge provinces. An additional fracture set of "strike" joints can be seen in the photo, and the bedding planes represent still another defect or "rock mass discontinuity." So, the rock mass here has 4 discontinuities.
Basic Concepts - Structures are commonly defined by the attitude of beds which are no longer flat but tilted. If beds remain planar but have been tilted, the amount of tilt (as measured from the horizontal reference) is called the dip of the beds. Any plane which has been tilted still has a horizontal line which represents the intersection of the tilted rock layer (i.e., plane) with the horizontal plane. The compass orientation of this line of intersection is called the strike of the tilted bed. If beds become "warped" and are no longer planar, the "local" orientation of the bed at an outcrop can still be characterized by a strike and dip. Of course, the strike and dip will vary with location on a folded bed.
Folds - bends in layered beds of rock. Folded rock layers in mountain ranges such as the Appalachians are similar to layers of rugs which have been squeezed sideways and forced into a series of arches and troughs.
Anticlines - arches of folded rock layers.
Synclines - troughs or down-bends of folded rock.
Fold axis - the hinge line of an anticline or syncline, where the attitude of layered rocks changes dip, from one direction to the opposite direction. The fold axis is the intersection of the "axial plane" with the folded bed.
Limb - the "planar" portion of a folded bed between adjacent anticline and syncline hinges. The limb is the portion between fold axes.
Plunging fold - Although some fold axes are horizontal, most plunge (at least slightly) so that the outcrop expression of a fold often gives the impression that the fold is a cone. Adjacent plunging anticlines and synclines make a zig-zag pattern on a geologic map. The general direction of the plunge can be determined from the relation that rock layers dip from old to young.
Monocline - a local steepening of an otherwise uniformly dipping or flat pile of strata.
Dome - a fold where beds dip away from a center point in all directions.
Structural basin - a fold where beds dip toward a center point.
Fold shapes - the geometry of fold limbs relative to each other.
Open folds - both limbs of folds dip gently away from each other.
Isoclinal folds - both limbs of any fold are parallel to each other.
Overturned folds - strata in one limb have been tilted beyond the vertical.
Recumbent folds - axial planes are horizontal or nearly so.
Fractures - when rock is subject to higher differential stress near the surface of the Earth, it often fails by brittle fracture.
Joints - cracks which typically form when large tensile stresses develop. The joint opens mainly by displacements normal to the crack wall. Joints are commonly filled in later by crystal growth, e.g. calcite or quartz. See the photograph above of joint sets in the Appalachian Plateau.
Shear fractures - fractures which form when the rock is subject to large compressive stresses near the surface of the earth. These fractures form by shear displacement parallel to the walls of the fracture. Shear joints may open with a component of shear displacement, usually less than one centimeter.
Faults - shear fractures with displacements greater than one centimeter. Most faults start as shear fractures, but generally develop into more complex structures involving a number of fractures, crush zones, etc., involving a finite thickness. Often deformation continues over time so that the fault slips a number of times. Large amounts of slip are associated with earthquakes.
Fault Blocks - the fault of usually a planar surface which had a dip. The fault divides rocks above and below the fault into two blocks.
Hanging wall block - The block above the fault plane.
Footwall block - The block below the fault plane.
Normal Faults - the hanging wall moves down relative to the footwall on a fault plan which dips about 60¡.
Graben - a down-dropped block between two uplifted blocks (horsts). This combination of faults leads to the formation of rift zones such as is found in Africa.
Half-graben - a down-dropped block usually found at continental margins where rift zones have filled with ocean crust.
Horst -the uplifted block next to a graben.
Rift - a graben in continental crust which will eventually fill with oceanic crust as continents spread apart.
Reverse Faults - the hanging wall moves up relative to the footwall on a fault plan which dips about 60¡.
Thrust Faults - the hanging wall moves over the foot wall on a fault zone horizontal or dipping at a relatively shallow angle.
Strike-slip faults - a vertical fault along which either wall moves in a horizontal direction relative to the opposite wall.
Transform faults - strike-slip faults at plate boundaries. At mid-ocean ridges, the ridges appear to be offset in one direction but the sense of slip on the fault is actually in the opposite direction, consistent with the generation of new crust at the ridges.