The geotectonic stress field and crustal movements

Tectonophysics, 71 (1981) 217--226

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Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

4. Interpretation o f recent crustal movements THE GEOTECTONIC STRESS FIELD AND CRUSTAL MOVEMENTS

ADRIAN E. SCHEIDEGGER

Institute of Geophysics, Technical University, Gusshaus St. 27--29, A-1040 Vienna (Austria) (Received July 1, 1980)

ABSTRACT Scheidegger, A.E., 1981. The geotectonic stress field and crustal movements. In: P. Vysko~il, R. Green and H. MS_Izer (Editors), Recent Crustal Movements, 1979. Tectonophysics, 71 : 217--226. It is shown that there is a direct connection between the geotectonic stress field and crustal displacements. The determination of the stress field is based on in-situ measurements and on observations of orientations of joints, the knowledge of the crustal displacements stems from geodetic measurements as well as from geological-geomorphological considerations. In this fashion, it was possible to correlate plate tectonic motions as well as vertical motions with the neotectonic stress field in various parts of the world, such as the Alps, the Himalaya and North America. In all these areas, a good conformity between the indicated phenomena was found.

INTRODUCTION

The surface of the Earth is in a state of motion. Vertical and horizontal displacements occur everywhere. Thus, in most mountain-areas of the world, vertical movements of the order of millimeters per year have been observed. Indeed, displacements of this order of magnitude are necessary to counteract the effects of erosion. Measurements of the sediment-transport in major rivers, interpreted in terms of erosive lowering of their drainage areas, have also yielded values of the same order of magnitude. This means that the present
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year for ocean--ocean plate boundaries and 15 m m / y e a r for ocean--continent plate boundaries (Kaula, 1975). One will, therefore, have to explain the origin of the observed displacements in geomechanical terms. It will be the thesis of the present paper that the cause of the displacements is the neotectonic stress field, and that its patt e m fits well together with the observed motions. Of course, the origin of the stress field itself must be sought in thermomechanical processes inside the Earth. The nature of the latter is still subject to some controversy. THE GLOBAL NEOTECTONIC STRESS FIELD General remarhs

The present-day stress field manifests itself in a variety of phenomena. First of all, there are direct effects in tunnels and mines. Secondly, such tectonic effects as earthquakes are manifestly due to the action of tectonic stresses. Thirdly, there are geological effects: Faults and joints are caused by stresses, as well as other phenomena in rock texture. Finally, many geomorphic features, such as the directions of valley trends, are t h o u g h t to be due to the action of the geotectonic stress field. All of the listed phenomena are due to the acting stresses which, in turn, can be inferred and deduced from a study and interpretation of these phenomena. In-situ m e a s u r e m e n t s

Turning first to in-situ stress measurements, we note that the most common methods of this type are stress-relief methods. In these methods, the differential strains experienced by a sample of material (rock) when it is removed from its natural surroundings (where it is stressed)into an unstressed state, are measured. A knowledge of the stress-strain behavior of the sample enables one to calculate the stresses from the observed strains. An other direct in-situ stress determination procedure is based on the analysis of hydrafracturing data. This has particularly been applied by Haimson (1978) to the eastern part of North America. The above methods have been applied to various parts of the world. Of particular interest in the present context are these areas in which comparisons can be made with stress-effects observed by other means. Thus, the cited article of Haimson (1978) is especially important. Figure 1 shows the orientation of the m a x i m u m compression as based on a compilation from insitu measurement data made by Haimson (1978) for Ontario. Similarly, Fig. 2 shows an analogous compilation made by the writer (Scheidegger, 1980a) for Western Europe.

219 J

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~ANc R~'~~

N

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0

50

100

.AM.L,O"

. , . O ~ . . E.L"

Fig. 1. Orientation of maximum in-situ stress directions in Ontario, based on data collected by Haimson (1978).

Joints It is possible to deduce the orientation of the principal tectonic stress directions from a measurement of joint orientations. In the case of Mohrtype fracture, the bisectrix of the smaller angle should be the greatest compression. However, inasmuch as, as noted, the angle between steeply dipping conjugate joint sets is usually close to 90 ° , it is often not possible to distinguish reliably between the largest and smallest principal stress direction. The determination of the preferred joint orientations in an area has to be carried out by a statistical procedure. For this purpose, a computational m e t h o d was developed by Kohlbeck and Scheidegger (1977). In that method, two statistical probability distributions of the type exp(k cos20) about a mean direction are fitted to the data; the 2 best-fitting mean directions are determined by computer using a function-minimization procedure. The com-

Z'20 2O

0

Fig. 2. Average orientation of m a x i m u m in-situ stress directions in Western Europe collected by the writer.

putational procedure is nothing b u t a development of the older m e t h o d of drawing density
20

joints

-x/

~J

/\

4C

3O

Fig. 3. Principal stress directions in Europe as calculated f r o m joint orientation measure ments.

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~A~A~IA fALLS

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M~ R S T a U * G

Fig. 4. Principal stress directions in North America as calculated from joint orientation measurements.

even possible to identify which is the largest (P) and which is the smallest (T) compression direction. In most instances, however, this angle is close to 90 ° so that the identification of P and T is uncertain. Various areas of the world have been analysed regarding the jointing patterns. Scheidegger has made a summary of the results for Europe (Scheidegger, 1980a) and for North America (Scheidegger, 1978). The reader is referred to the original papers for the details; we give here a qualitative summary picture for the regions in question in Fig. 3 and Fig. 4. As noted above, the identification of P and T is uncertain, so that the sign of the arrows in the above figures must be considered as not significant.

Valley trends The stress field has been found to express itself also in geomorphological features. The most important of these are the orientations of the valley trends in an area. The assumption that valley trends are predetermined b y geotectonic phenomena stands in contrast to the assumption of valleys being solely caused by exogenic agents (i.e. wind, water and ice). In fact, there is a certain controversy a b o u t this matter to this day, b u t the evidence in favor

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of s o m e geotectonic control of the valley orientations is building up steadily. Thus, it has been shown that the main characteristic of exogenic agents is their randomness (Gerber and Scheidegger, 1973). Valleys caused by erosion alone should, therefore, be randomly oriented. Evidence from statistical analyses of valley directions shows that the latter are not random. Furthermore, the often large vertical displacement rates in mountain areas suggest that the surface features are of very recent origin. This makes it difficult to believe that modern river-nets should have been determined by some ancient drainage pattern which is retained to this day. In order to study their orientations, the valley directions have to be "rectified" (i.e., straightened) by considering them as edges in a graph. This may be a somewhat " b r u t a l " procedure, but it is at least independent of the bias of the researcher. Otherwise, the fitting of straight (and therefore measurable) segments to the "wiggly line" representing the river course on a map, would be extremely arbitrary. However, in the described fashion, the distribution of valley orientations can be represented numerically in a unique fashion and, in consequence, can be analyzed statistically. The determination of the best fitting orientation to the histogram is best performed by the same procedure as with joints. For this purpose, a valley is

÷" ........

~. . . . . . .

.........

Fig. 5. Principal stress directions in Ontario calculated from valley orientations.

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regarded, so to speak, as a "vertical joint". Then, a series of superposed distributions of the type exp(k ~ cos20) are again fitted to the data by the usual function minimization program. When this is done for various regions of the world, one finds a good correspondence between the valley trends and the joint-orientations in the Alps (Scheidegger, 1980a), in the Himalaya (Scheidegger, 1979b) and in North America (Scheidegger, 1978}. One can again calculate the hypothetical stress field in which the valleys would be shear lines. This stress field coincides with that which is postulated from the orientation of joints. Figure 5 gives the results for Ontario; the correspondence with Fig. 4 is startling.

Summary The previous compilation leads to the general impression that there is a general correspondence in any one area between in-situ measurements, joint orientations and valley trends. There is no question, therefore, that the neotectonic stress field can be ascertained in this fashion. CRUSTAL DISPLACEMENTS

We have mentioned several means of determining crustal displacements. The most elegant idea proposed to deduce regional strains is based upon a comparison of geodetic nets at subsequent times. This method has been applied successfully by Thurm et al. (1977) to a region of Saxony. In principle, the shift of the coordinates of "fixed" points leads directly to the strain tensor built up between the two surveys. In Saxony, two very accurate surveys, lying about 80 years apart, are available. From these surveys the principal strain directions were calculated, at least in plan. Assuming an isotropic material, the principal stress directions must coincide with these. It turned out that the maximum-compression axis has an azimuth of 140 ° . It will be noted that the thus determined azimuth agrees closely with the principal neotectonic stress direction found in Europe from the analysis of joints. The large~scale horizontal displacements occurring in the European tectonic plate agree, therefore, exactly with those that a neotectonic stress system fitting the orientation of joints would be expected to produce. Otherwise, it is difficult to measure horizontal displacements. We have mentioned the work of Kaula (1975} who obtained 15--20 mm/year from paleomagnetic extrapolations. Direct geodetic measurements on the San Andreas fault (Lensen, 1971) yielded 10 mm/year, on other faults in California up to 20 m m / y e a r (Scholz and Fitch, 1970). Similar rates were also observed at other great faults of the globe, with the Alpine fault in New Zealand (Pavoni, 1971) attaining a maximum of 70 mm/year. The motion on these faults fits exactly with plate-tectonic theory and the orientation of the neotectonic stress field.

224

3

f

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Vectors:

o• 10

Locetion: li

0

I

500

|

metres

1000

20 30 40 mrn

50 |

1 500

Fig. 6. Orientation of displacement vectors on a slope in the Lesach valley (after Hauswirth et al., 1979).

On a smaller scale, we have made measurements on unstable mountainslopes in the Alps. Thus, a geomechanical investigation has been made of a slide area in the Felber Valley (Salzburg Province) in Austria (Carniel et al., 1975). For this purpose, tachymetric, seismic, geomorphological and geological studies were carried out. It was found that slow mass movements occur from the l e d g e a t o p the valley down to the very b o t t o m . Atop the ledge the movements present the aspect of a " m o u n t a i n fracture", in the middle of the slope the aspect of soil-creep, and at the b o t t o m the aspect of

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an actual slide. It was found that the orientation of the "mountain fractures", of the joints in the rocks bounding the slide area and of the features on the creeping slope all fit into one single geophysical stress pattern, viz., into one with a maximum horizontal compression in the N--S and a minim u m compression in the E--W direction. This is somewhat turned with regard to the general " E u r o p e a n " intraplate stress orientation (see above), but, owing to the internal consistency of the phenomena mentioned, probably indicates a local anomaly in the latter. Similar results were obtained from an investigation of an unstable area near Bad Gastein. Many houses of this resort-town show traces of movement (cracks in walls and foundations). Data from the Austrian Geodetic Survey were used for a determination of the displacement patterns (Hauswirth and Scheidegger, 1980). Support was again obtained for the thesis that all movements are basically designed by the tectonic stress field, although the individual triggering effect must be sought in exogenic agents. A particularly detailed study by geodetic means was made on a slope at the Lesach Ledge in Eastern Tyrol, Austria (Hauswirth et al., 1979) at which markers were established, exact geodetic measurements were made which were repeated in later years. Mass movements of the order of centimeters were found; the pattern of the motion fits into the scheme of a "rotational slump" envisaged by Terzaghi (1943). Fig. 6 shows the results. Evidently the displacement vectors correspond to those of a slump and are of the order of millimeters/year. The mean displacement direction coincides closely with the direction of the smallest principal stress of the neotectonic stress field in Europe. Thus, the motions are predesigned by this stress field; the actual triggering, of course, m a y again be caused by exogenic agents. SYNTHESIS

The discussion given in this paper establishes the thesis that there is a direct correlation between geodynamic and geodetic phenomena. The connecting link is the neotectonic stress field. On a large scale, the geodetic phenomena reflect the motion of the tectonic plates with regard to each other. On a smaller scale, such as in "mountain fracture" and slumping of valley sides, the geotectonic stress field designs the basic pattern of the events; the triggering is, o f course, conditioned by exogenic agents. REFERENCES Carniel, P., Hauswirth, E.K., Roch, K.H. and Scheidegger, A.E., 1975. Geomechanische Untersuchungen in einem Rutschungsgebiet im Felbertal in C}sterreich. Verh. Geol. B.-A. Wien, 1975 (4): 305--330. Gerber, E. and Scheidegger, A.E., 1973. Erosional and stress-induced features on steep slopes. Z. Geomorphoh Suppl., 18: 38--49. Glen, W., 1974. Continental Drift and Plate Tectonics. Merrill, Columbus, Ohio, 188 pp. Haimson, B.C., 1978. Crustal stress in the Michigan Basin. J. Geophys. Res., 83: 5857-5863.

226 Hauswirth, E.K., Pirkl, H., Roch, K.H. and Scheidegger, A.E., 1979. Untersuchungen eines Talzuschubes bei Lesach (Kals, Osttirol). Verh. Geol. B.-A. Wien, 1979 (2): 73. Hauswirth, E.K. and Scheidegger, A.E., 1980. Tektonische Vorzeichnung von Hangbewegungen im Raume von Bad Gastein. Interpraevent 1980. Kaula, W.M., 1975. Absolute plate motions by boundary velocity minimizations. J. Geophys. Res., 80: 244--247. Kohlbeck, F. and Scheidegger, A.E., 1977. On the theory of the evaluation of joint orientation measurements. Rock Mech., 9: 9--25. Lensen, G.J., 1971. Phases, nature and rates of earth deformation. R. Soc. N.Z. Bull., 9: 97--105. Pavoni, N., 1971. Recent and late Cenozoic movements of the Earth's crust. R. Soc. N.Z. Bull., 9: 7--17. Scheidegger, A.E., 1978. Joints in Eastern North America and their geotectonic significance. Arch. Met. Geoph. Biokl., A27: 375--380. Scheidegger, A.E., 1979a. The principle of antagonism in the Earth's evolution. Tectonophysics, 55: T7--T10. Scheidegger, A.E., 1979b. On the tectonics of the Western Himalaya. Arch. Met. Geoph. Biokl., A28: 89--106. Scheidegger, A.E., 1979c. Orientationsstruktur der Talanlagen in der Schweiz. Geogr. Helv., 34: 9--15. Scheidegger, A.E. (Editor), 1980a. Tectonic Stresses in the Alpine-Mediterranean Region. Rock Mech. Suppl., 9, 255 pp. Scheidegger, A.E., 1980b. The orientation of valley trends in Ontario. Z. Geomorph., in press. Scholz, C.H. and Fitch, T.J., 1970. Strain and creep in Central California. J. Geophys. Res., 75: 4447--4453. Terzaghi, K., 1943. Theoretical Soil Mechanics. Wiley, New York, N.Y., 510 pp. Thurm, H., Bankwitz, E. and Harnisch, G., 1977. Rezente horizontale Deformationen im Siidostteil der DDR. Petermann's Geograph. Mitt., 1977(4): 281--304.