Since we have described the general condition of mechanical instability of the earth's surface in the preceding chapter, we are now in a position to discuss some of the specific results of this

STRUCTURAL GEOLOGY

Types of Structures

Since we have described the general condition of mechanical instability of the earth's surface in the preceding chapter, we are now in a position to discuss some of the specific results of this instability. Movement of bodies of rock relative to each other is generally given the term deformation. The results of such movement can commonly be described in terms of the development of either faults or folds, both of which terms have been mentioned briefly before. A fault is simply a surface, or zone of roughly parallel surfaces, along which relative movement has occurred. The fault is a fracture surface in the sense chat the rocks on opposite sides have moved past each other, and previously coinciding points in the opposite blocks are no longer together. A fold is a bend in rocks. In a fold, the orientation of features such as bedding planes changes from place to place. In a simple fold, surfaces parallel to folded material must slip past each other (as in bending a deck of cards), but the slippage between any two adjacent layers is extremely small and no actual fracturing occurs.

Before describing various structures more accurately, it is first necessary for us to define the terms strike and dip. All planar features such as sedimentary beds have an orientation, or attitude, with respect to the earth's surface. Thus, when beds are parallel to the surface (indeed, they are generally deposited very near to that attitude), such beds are called "horizontal". Other beds may be inclined at some angle to the earth's surface. The orientation of a plane may be determined by the orientation of any two lines on it which are not parallel to each other. In geologic terminology, these two lines are termed the strike and the dip. The strike of a bed or other surface is the orientation of a line along the bed parallel to the earth's surface. The orientation of this horizontal line is generally given in terms of degrees to the east or west of north. Thus a bed may strike north 40° east, signifying that the orientation of a horizontal line upon it has a bearing 40° to the east of true north. The strike can be readily visualized as the line formed by the rock plane and the surface of an imaginary lake. The term "dip" refers to the angle with the horizontal surface made by a line in the bed perpendicular to the strike. This dip angle is obviously equal to the angle made by the bed or surface with the horizontal. The angle of dip of a surface is generally added to the notation for the strike. Thus a symbol N 40 E 30 W refers to a bed whose strike is 40 degrees to the east of north and which is dipping to the west at an angle of 30 degrees to the horizontal. The actual direction of dip, of course, is north 50° west. Figure shows the determination of strike and dip diagrammatically.

We are now in a position to describe the various common types of faults and folds. Faults can be subdivided into three major categories, depending upon the direction of movement and its relation to the dip of the fault surface. These three types of faults are diagrammatically indicated in Fig.10-4. A normal fault is one in which the downthrown block lies on the top of the fault surface, and normal faulting thus results in an increase of the distance between two points on opposite sides of the fault plane. A reverse fault is simply the opposite of a normal fault and results in a shortening of the distance between two points. Reverse faults with extremely low angles of dip are commonly called thrusts. A strike-slip fault is one in which the movement is parallel to the horizontal, that is, parallel to the strike of the fault surface. Both normal and reverse faults may be grouped as dip-slip faults. For most faults the movement is neither purely strike-slip nor purely dip-slip, and the terms applied to any particular fault simply refer to the dominant type of movement.

Fault surfaces are interesting features which bear close study where found, although actual exposure of the surfaces of most faults is comparatively rare. A number of features can be used to recognize fault surfaces and to determine something about the direction of displacement on the fault. The observable direction of offsetting of beds or other features is the principal type of evidence that can be used to reconstruct movement on a fault. Such displacements are not always

easily observed, however, particularly if the movement on the fault has been large and the blocks opposite each other are completely dissimilar. Geologists have used, with some hesitation, a feature called slickensides to determine the direction of movement. When two blocks of rock are moved past each other along a fracture surface, they rub against each other and tend to form a highly polished and commonly grooved surface. The grooves on the surface are caused by small, particularly resistant, projections from the sides of each block being dragged along the surface of the other block, and the grooves obviously are parallel to the direction of movement. The difficulty with the assignment of directions of movement from a study of slickensides is that they can give only an indication of the last direction of movement on a fault, and if movements have been repeated and complex, then the major direction of a movement may be quite different from the last one. Furthermore, slickensides on, for example, a fault striking north-south may indicate that the movement was lateral, but they do not indicate whether the eastern or western block moved to the north or south.

Another feature which may be used to indicate direction of movement on fault surfaces is the tendency of beds near the fault to be dragged along by the movement. This feature is appropriately termed drag, and the result of the dragging is that the beds are folded slightly in the direction of movement. Figure shows a typical example of drag along a small fault. In addition to slickensides and drag, many faults are marked by the presence of a large amount of ground-up rock flour, commonly termed gouge. This material has been formed by the grinding action of the two blocks as they moved past each other, and the zones of gouge may be many feet wide on some faults.

Folds may be classified into two main groups, anticlines and synclines. An anticline is simply an upward arching of beds and a syncline is a downward arching. These two types of folds are shown in Fig. 10-7. Also shown in Fig. 10-7 is a fold which is not easily classifiable as either anticline or syncline and is referred to as a monocline. A line along the high point of an anticline or the low point of a syncline is referred to as the axis of the fold. Fold axes may be horizontal or they may dip, a non-horizontal fold is said to plunge.

In addition to folds and faults, we must mention two other types of commonly developed structures. A joint (Figure) is a fracture surface along which there has been little or no movement. Joints are common in all types of rocks. Many granites have extensive sets of joints, particularly near their margins where the intruded melt had crystallized before all movement had stopped. The resulting movement of the still-fluid components in the core of the granite batholith then caused stretching and deformation of the margin and the consequent development of large joint blocks. Joints are also present in both sedimentary and metamorphic rocks, where they are presumed to be incipient fracture surfaces caused by regionally developed forces. The term cleavage, when applied to rocks, has a meaning identical to that of cleavage in minerals; that is, rock cleavage signifies a tendency for the rock to split along some set, or perhaps several sets, of parallel planes. In some rocks cleavage may simply be regarded as an exceptionally closely spaced jointing. In other rocks cleavage occurs because of some mineralogical orientation, as in slates, even though visible joint surface are not present.

Normal fault

Reverse fault (thrust if dip angle is very low)

Strike-slip fault

Fig. 10-4. Diagrammatic representation of the three types of faults.

Anticline Syncline Monocline

Fig. 10-7. Diagrammatic representation of three major types of folds.

Дата добавления: 2015-11-05