As shown in fig. 4-2 1, two parallel lines AB and CD tangent to the upper and lower strata are drawn on the cross-sectional view or photo of the fold perpendicular to the hinge direction, where B and D are the tangent points of the two straight lines and the upper and lower layers of the fold respectively, and the straight line BD is the oblique line of the fold. AB and CD intersect with the vertical line GE of axial trace EF on the section, and the included angle is A. The vertical thickness between the two parallel lines is ta, and the thickness parallel to the axial direction of the fold is ta=Ta cosa. The thickness of the folded strata changes around the hinge of the fold, so ta and ta change with the change of angle α .. At the hinge of the fold, angle A is 0, so ta=Ta. At this time, the two thicknesses are recorded as T0 and t0 respectively. Ta and ta values of the two wings of the fold vary with the type of fold. Ramsay( 1967) divides the folds into three categories and five categories according to the change of ta/t0 and Ta/T0 ratio (Figure 4-22).
Figure 4- Measuring ta and Ta on the Cross Section of 2 1 Fold
(According to J.G. Ramsey 1967)
(1) when ta/t0 > 1, ta/t0 > SECA, the thickness of the folded rock layer at the hub is smaller than that at the wing, and it is a top-thin fold, which is type ia (Figure 4-22A).
(2) When ta/t0= 1 and Ta/T0=seca, the thickness of folded strata is equal everywhere, and the top and bottom surfaces of the strata are parallel to each other, which is an IB-type fold (Figure 4-22B).
(3) When 1 > ta/t0 > COSA, SECA > ta/t0 > 1, it is a parallel fold type that transits to similar folds, that is, I.C. (Figure 4-22C).
(4) If the thickness of folded rock stratum measured in the direction parallel to the axial plane is equal everywhere, that is, Ta/T0= 1, the vertical thickness ta of rock stratum changes with the change of angle a, and this kind of fold is called similar fold and belongs to type II fold (Figure 4-22D).
(5) When ta/t0 > COSA and ta/t0 < 1, the thickness of the folded rock layer at the turning end is greater than that at the wing end, and it is a top-thick fold, which belongs to type III fold (Figure 4-22E).
If angle a is taken as the abscissa and ta/t0 and Ta/T0 as the ordinate respectively, two types of images with thickness ratio varying with angle a can be drawn, and these five types of folds occupy corresponding positions in the image (Figure 4-22 on the right).
Isocline: also known as isocline, it refers to the tangent line connecting the upper and lower fold surfaces of the fold layer with equal inclination angles. Draw the cross section of the fold with a triangular plate and a semicircle instrument (Figure 4-23). The specific steps are as follows:
Figure 4-22 Geometric Features and Parameter Images of Fold Types
(According to J.G. Ramsey 1967)
(1) On the fold profile made according to the dip angle data of the fold hinge, or on the profile or photos perpendicular to the fold hinge, use transparent paper to describe the bending shape of each fold rock layer, and accurately draw the axial trace and the field horizontal line.
(2) On the drawn fold profile, based on the marked horizontal line, on the upper and lower layers of the same fold rock, make a series of tangents with dip angles at certain angular intervals (such as 0, 10, 20, …).
(3) Connect the tangent points with equal dip angles of upper and lower strata with a straight line, that is to say, it becomes an equidip line regularly distributed in folded strata.
As can be seen from the left figure of Figure 4-22, the characteristics and changing rules of equidiagonal lines in Ramsey's (1967) three-type and five-type fold classification scheme.
I-fold: Equal diagonal lines converge to the inner arc, and the curvature of the inner arc is always greater than that of the outer arc. According to the length change and convergence degree of isoclinic line, it can be further divided into three subtypes.
Type ia: Isocline lines converge strongly to the inner arc and fan out in the back shape. The length of each isoclinic line varies greatly. The length of the isoclinic line of the wing is greater than that at the hinge, and the curvature of the inner arc is much greater than that of the outer arc, which is a typical top-thin fold.
Figure 4-23 Drawing Method of Isomorphic Diagonal Lines
(According to J.G. Ramsey 1967)
Type Ⅰ B: Isocline converges to the inner arc and is perpendicular to the folded strata. The length of each isoclinic line is almost equal, the thickness of folded strata remains unchanged, and the curvature of inner arc is greater than that of outer arc, which is a typical parallel fold.
Type Ⅰ C: The equidiagonal line converges slightly to the inner arc, and the equidiagonal line at the hinge is slightly longer than the two wings, reflecting the trend of the thickness thinning of the two wings of the fold, and the curvature of the inner arc is slightly larger than that of the outer arc, which is a transition type from parallel folds to similar folds of Class Ⅱ.
Type II fold: Isocline lines are parallel and equal in length, the curvature of inner arc and outer arc of folded strata are equal, and the dip angles of adjacent folded strata are basically the same, which is a typical similar fold.
Type Ⅲ fold: Isocline lines converge to the outer arc, spread to the inner arc, and are inverted fan-shaped in the anticline. The curvature of the outer arc is greater than that of the inner arc, which is a typical top-thick fold.
In nature, most folds can be classified into the above-mentioned basic types, but there are also complex types of folds, which cannot be simply merged into a certain type. In addition, in folds composed of different lithologic layers, each fold layer often has a different fold shape, which leads to the refraction phenomenon of equal diagonal lines on the fold section (Figure 4-24).
Figure 4-24 Fold Isocline and Its Change in Cretaceous Sandstone and Shale in Wulian, Shandong Province
(According to Zhu Zhicheng, 2005)
The geometric shape of folds can be accurately determined by analyzing the shape of folds with equal diagonal lines. Many fold features that may be neglected or impossible to express by traditional classification methods can be clearly expressed by equal diagonal line method, which can predict the change of fold style from one layer to another and within the fold layer, which is helpful to analyze the genetic mechanism of folds.