Rickard M [PDF]

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M. J. RICKARD Geology Department, Australian National University, Canberra, Australia



Fault Classification: Discussion



The modified fault classification by Gill (1971) considerably reduces the confused state of fault nomenclature. However, two additions should be made to his comments. First, directions of net slip, which provide the main classification categories, should be precisely defined. Billings (1954) divided movements into two groups—dip slips for which the pitch of net-slip lines could vary from 45° to 90°, and strike slips with 0° to 45° pitch. Gill (1941) proposed three groups with a wide zone of oblique slips, whereas Hills (1963) suggested that oblique slip should be used only where dip and strike components are approximately equal (ca. 45° pitch of net slip). Following the practice with fold-hinge orientations (Fleuty, 1964), it is suggested that the special cases of dip and strike slip should be restricted to pitches of 80° to 90° and 0° to 10°, respectively. The second point concerns separations. In describing the separation of marker beds, it would be useful to use a simple unique separation line rather than dip and strike separations which depend on chance exposure for their measurement. For this purpose "separation" should be defined as "the perpendicular distance between the traces of a displaced marker, measured in the fault plane." Distances measured in other specified directions (for example, dip and strike directions) are then components of the separation; it is only these that have previously been defined (Reid, 1913). Since the net-slip line and the dip of the fault must be known to use a kinematic classification, it should be possible to describe translational faults exactly. For this purpose it is convenient to plot the essential geometric attributes on a modified version of the DPP (Dip, Pitch, Plunge) diagram recently proposed for the classification of fold orientations (Rickard, 1971). The pitch of the determined net-slip line is plotted on a special triangular diagram along the line representing the dip of the fault (Fig. la). Four triangles are necessary



to allow representation of normal, reverse, and left-handed and right-handed movements (Fig. lb). In this way any translational fault can be plotted. In addition to the category names, the fault designation could be made more precise by assigning an index symbol that can also express geographical orientation; for example, right-normal-slip fault (D60/270:P60N)—dip and dip direction of fault 60/270, pitch of net-slip line 60° to the north. This fault is plotted on Figure lb. The term "slip" has been added to the fault names in Figure lb following Hill (1959) and Kupfer (1960); if the separation classification is dropped as advocated by Gill (1971), then normal and reverse should only apply to movements and the term slip can be dropped from the fault names, except in the case of strike-slip faults. This point is not made clear in Gill's (1971) revised classification. The four combined triangles make a useful diagram for teaching fault classification. In addition the diagram can be used for tectonic analysis; by plotting faults of different ages, localities, or orientations with different symbols their geometries can be compared. To counter the possible criticism that such a classification is too precise, it must be pointed out that loose classification and intuitive assignment to one of Anderson's (1951) three stress-based categories leads to false dynamic conclusions. Faults are too often named on the basis of an assumed dip, often that of a faultline scarp; movements are often assigned intuitively; and principle-stress orientations may be erroneously inferred by considering noncontemporaneous diagonal faults as conjugate pairs. The acceptance of a precise classification for those (perhaps rare) instances where fault geometries can be well established would clarify this situation so that a tectonic analyst would know what reliance to place on published data.



Geological Society of America Bulletin, v. 83, p. 2545-2546, 1 fig., August 1972 2545



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2546



M. J. RICKARD



RIGHT



Figure 1. Classification diagram for translational faults, a, Dip-pitch triangular grid; pitch (rake) is measured along line representing the dip of the fault plane, b, Fault categories on dip-pitch triangles. Dip-slip categories are ruled; faults dipping less than 45° may be called thrusts if movement is reverse, lags (Marr, 1906) if movement is normal. Strike-slip categories are dotted; oblique-slip categories are blank; *f is example of right-normal-slip fault (D60:P60N).



REFERENCES CITED Anderson, E. M., 1951, The dynamics of faulting and dyke formation with application to Britain (2d ed.): London, Oliver & Boyd, p. 206. Billings, M. P., 1954, Structural geology (2d ed.): New York, Prentice-Hall. Fleuty, M. J., 1964, The description of folds: Geol. Assoc. London Proc., v. 75, p. 461-492. Gill, J. E., 1941, Fault nomenclature: Royal Soc. Canada Trans., 3d ser., v. 35, p. 71-85. 1971, Continued confusion in the classification of faults: Geol. Soc. America Bull., v. 82, p. 1389-1392. Hill, M. L., 1959, Dual classification of faults: Am. Assoc. Petroleum Geologists Bull., v. 43, p. 217-221.



Hills, E. S., 1963, Elements of structural geology: London, Methuen, p. 483. Kupfer, D. H., 1960, Problems of fault nomenclature: Am. Assoc. Petroleum Geologists Bull., v. 44, p. 501-505. Marr, J. E., 1906, The influence of the geological structure of English lakeland upon its present features—a study in physiography: Geol. Soc. London Quart. Jour., v. 62, p. Ixxvi. Reid, H. F., 1913, Report of the committee on the nomenclature of faults: Geol. Soc. America Bull., v. 24, p. 163-183. Rickard, M. J., 1971, A classification diagram for fold orientations: Geol. Mag., v. 108, p. 23-26. MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 17, 1972



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Geological Society of America Bulletin Fault Classification: Discussion M. J RICKARD Geological Society of America Bulletin 1972;83, no. 8;2545-2546 doi: 10.1130/0016-7606(1972)83[2545:FCD]2.0.CO;2



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