On the Low-angle Faults and Strike-slip Faults in the Coal Fields of Japan

Abstract

1.The Suihei-Danso (Horizontal fault, literally translated), as conventionally named by geologists and engineers, occurs in many coal fields of Japan. It comprises two different sorts of fault-low-angle fault and strike-slip fault.
Low-angle faults may be grouped into the following two types:
A.Thrust fault and gravitational gliding which are originally horizontal or low-dipping (Fig.1).
B.Normal or reverse faults which originated as high-angle faults but turned low-angle through. the later crustal movement (Fig.2, 3, 4).
2.Strike-slip or wrench fault in which horizontal component is larger than the vertical one, is not uncommon, as shown by many examples in the Ishikari, Kushiro and Chikuho coal fields (Fig.7, 9-12, 14-18). It is highly probable that many more similar faults may be discovered if sufficient attention. be paid in studies of faults, because a shearing stress or a couple of stresses producing strike-slip faults is believed to have influenced the structural development of the coal fields in Japan during the late Tertiary.
3.As a reasonable suggestion for the condition of a coal seam in an area beyond a fault encountered is one of the important tasks of a coal mine geologist, it must be necessary to know the direction of shifting, at least, whether vertical or horizontal component is bigger than the other, before reaching a conclusion.
4.The following features may be taken up as important keys for detection of strike-slip faults:
(1) Occurrence of low-angle striae on the fault plane.
It must be noticed that the observed striae indicate, in most cases, only the direction of the last movement of the block concerned, and the fault movement is usually very complicated.
(2) Occurrence of auxiliary faults.
They are usually branching from the main fault with high angle, more than 60 degrees, sometimes at right angle to the strike of the main fault, and most of them are running parallel to each other. Their extensions are much shorter than that of the master fault.
They are always normal faults, their dip being not necessarily in the same direction, and the amount of vertical displacement becomes larger as they approach the main fault.
(3) Occurrence of horizontal drag.
In case of steeply dipping strata, the shape of horizontal drag which is expressed by a change of strike, immediately suggests a horizontal movement. But this cannot be taken as a good key in an area where the strata are gently dipping.
(4) Non-coincidence of geology between two blocks dislocated by a fault.
When geologic features of one block of a fault, irrespective to stratigraphic or structural natures, are corresponding but are discontinuous to those of the other block, that is, when those features of one block are same in order of aerial distribution but is being shifted in the same direction relative to the other one, they are probably regarded as evidences of a horizontal movement, if they cannot be interpreted by a rotational or a hinge fault.