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Back-arc spreading: Trench migration, continental pull or induced convection?
Tectonophysics, 74 (1981) 89-98 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
89
BACK-ARC SPREADING: TRENCH MIGRATION, CONTINENTAL PULL OR INDUCED CONVECTION?
ALBERT T. HSUI * and M. NAFI TOKSGZ Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 (U.S.A.) (Received October 24,198O)
ABSTRACT Hsui, A.T. and Toksoz, M.N., 1981. Back-arc spreading: Trench migration, continental pull or induced convection? In: S.C. Solomon, R. Van der Voo and M.A. Chinnery (Editors), Quantitative Methods of Assessing Plate Motions. Tectonophysics, 74: 8998.
Mechanisms of the opening of back-arc systems are analyzed. Limited focal mechanisms of intraplate earthquakes are used to determine the stress regime of an overriding plate. Preliminary analyses show that compressive deviatoric stresses exist in the plate except near the spreading center. Based on this observation “trench suction” does not appear to be the primary force that drives back-arc spreading, since it will result in tensional deviat&c stresses within the overriding plate. Even though “continental pull” is able to satisfy the stress requirements, it does not appear to be a likely mechanism either because of the initiation and subsequent symmetric spreading difficulty associated with such a mechanism. The mechanism we favor is the one that involves the induced convective current in the mantle wedge immediately above the slab. Calculations show that the induced flow is able to generate sufficient stress to break up the overriding lithosphere if the tectonic stresses of the region are favorable. Both trench suction and continental pull may help to provide such a favorable tectonic stress regime.
INTRODUCTION
The large-scale lithospheric plate motions have been studied extensively (Morgan, 1972; McElhinny, 1973; van Andel, 1974; Jurdy and Van der Voo, 1974; Solomon and Sleep, 1974; Lliboutry, 1974; Minster et al., 1974; Solomon etal., 1975,1977; KauIa, 1975; Minster and Jordan, 1978). Even though these studies have used different mantle rheologies, different geophysical constraints and different frames of reference, the relative motions between * Present address: Department of Geology, Urbana, IL 61801 (U.S.A.). 0040-1951/81/0000-0000/$02.50
University of Illinois at Urbana-Champaign,
@ 1981 Elsevier Scientific Publishing Company
90
plates determined from these calculations are reasonably consistent. Plate motions thus derived are able to explain many geophysical and geological processes such as earthquake distribution, earthquake source mechanisms, the building of mountain ranges and the formation of island arcs and oceanic trenches. Although global plate motions are widely accepted, the details of many small-scale motions at plate boundaries are not yet fully understood. One example of this is the relative motions between plates in trench-c-backarc systems. In this paper, we shall first examine the surface motions in trench-arcback-arc environments. We shall use stress patterns as revealed by earthquake mechanisms of these areas to study the inter-relationships among these motions. TRENCH
MIGRATION
AND BACK-ARC
SPREADING
Trenches in the Western Pacific have been recognized to migrate towards the ocean in geological time scale (Chase, 1978). The phenomenon may be associated with back-arc spreading. Simply based on geometrical arguments, trenches and continents must move away from each other if the back-arc basins are actively spreading. It has been suggested (Elsasser, 1971) that old oceanic lithosphere is gravitationally unstable, such that it will collapse under its own weight. As it collapses and sinks downward, the trench will appear to migrate towards the ocean. This motion will generate a suction force which may be responsible for back-arc opening (Forsyth and Uyeda, 1975; Molnar and Atwater, 1978). Alternatively, trench migration can be a consequence of back-arc opening. Since the overriding plate in back-arc regions does not descend into the mantle, spreading will widen this plate. As a result, the trench will be pushed away from the inter-arc spreading center. It is apparent then, that both processes seem to be able to explain the relative surface motion of oceanic trenches with respect to backarc spreading centers. The question is whether one can determine which process is the more likely one that is taking place in a trench-arc-back-arc environment. In order to determine the plausible mechanism, the tectonic stress regime within the overriding plate could be helpful. Using a simplified geometry and assuming plate uniformity, we shall examine the resultant stress regime within the overriding plate caused by the two different geological processes described above. Figure 1A shows a schematic diagram of a trench--arcback-arc system. This diagram shows the relative positions of every major element that composes the system. If “trench suction” is the dominating process taking place in these areas, it will imply that the oceanic trench will retreat faster than the overriding plate can catch up. Hence a low-pressure region will form such that a suction force will be generated and it will act on the overriding plate so as to pull it towards the ocean (Fig. 1B). In this case, deviator& stresses within the overriding plate would be tensional. On the
91
‘ONTINENT
ISLAND ARC BACK-ARC SPREAUNS CENTER VOLCANO
SUBOUCTlffi
TRENCH
SUCTION
LITHOSPMRE
I TENSION I
m COLLAPSE
SACK-ARC
RIDQE PUSH
OF SLAB
t COMPRESSION 1
=%=e?=
Fig. 1. A. Schematic diagram showing the simplified vertical cross-section of a trench-arcback-arc system. B. Schematic diagram showing the working principle of trench suction and the resultant stress regime within the overriding plate. C. Schematic diagram showing the stress regime within an overriding plate if trench migration is caused by back-arc spreading.
other hand, if trench migration is caused by back-arc opening, the overriding plate will be pushed towards the ocean by back-arc spreading (Fig. 1C). In this case, the stress regime within the overriding plate will be compressional. Although the two different processes can explain the surface relative motion equally well, they differ in such a way that the resultant stress regimes within the overriding plate are completely opposite. Therefore, the stress regime within this plate becomes a natural criterion to discriminate the more plausible geological process taking place in these areas. A tensional stress regime within the overriding plate will imply that trench suction is a more important process, while a compressional stress regime will indicate that it is not. STRESSES
IN TRENCH-ARC-BACK-ARC
SYSTEMS
Stresses in convergent plate boundaries are primarily compressive. This is implied by both seismic studies (Katsumata and Sykes, 1969; Johnson and Molnar, 1972; Chung and Kanamori, 1978; Pascal et al., 1978) and by the geological observations of heavy folding in the accretionary prism area (Karig, 1974). However, it is more difficult to determine the stress regime of the whole overlying plate. Although the spreading ridges can be located behind island arcs, it is difficult to establish if normal faulting is actually taking
92
place in these areas. This is mainly because the distribution of seismic stations surrouding these areas is sparse and the earthquake magnitudes are generally small. Despite the lack of focal mechanism data, the existence of a spreading center in back-arc basins is generally accepted because of other geophysical and geological observations. These include bathymetry and heat flow data (Sclater, 1972; Sclater et al., 1972a, b; Anderson, 1975; Toksoz and Bird, 1977; Watanable et al., 1977), magnetic anomaly measurements (Barker, 1972; Watts et al., 1977; S. Uyeda, pers. commun., 1979) as well as seismic reflection studies (Karig, 1974). Therefore, it is apparent that the stress regime in the subduction zone does change from trench axis to back-arc spreading center. The transition of stresses in plate convergent boundaries has been discussed by McKenzie (1978) and Nakamura and Uyeda (1980). Based on surface geological observations of the distribution of dikes and faults, Nakamura and Uyeda (1980) inferred that the stress regime within this plate transforms from compressional to strike-slip and then extensional as one moves from the trench axis towards the back-arc basins. The best way to study present stresses within a plate is to look at the focal mechanisms of intraplate earthquakes. Unfortunately, intraplate earthquakes within these regions are scarce and small in magnitude. Not too many reliable data are available. In New Hebrides, Chung and Kanamori (1978) identified three possible intraplate earthquakes which are indicated as events 8, 12, and 18 in Fig. 2. These events are located in the overriding lithosphere and away from other events in the Benioff zone. Their fault-plane solutions are given in the same diagram. They are all thrust events, and their compressive P-axes are approximately horizontal. These indicate that the stresses within the overriding plate in New Hebrides are probably compressional. Two intraplate events in the Lau Basin have been investigated by Sleep (1972) and are shown in Fig. 3. The earthquake near the volcanic arc is of thrusting type. The compressive P-axis is approximately horizontal. Thus, the stress system in the Lau Basin is also compressional. The focal mechanism of the other earthquake close to the spreading center is of normal faulting
Fig. 2. Focal mechanisms and the locations of the three intraplate earthquakes in New Hebrides. The fault-plane solutions represent the lower hemisphere projection of the focal sphere. (Data taken from Chung and Kanamori, 1978).
93 BASC
V
LAU l I
300
I 200
I
I 100
BASIN I
I I 0
DISTANCE IN Km.
Fig. 3. Focal mechanism and the location of the intraplate earthquake which occurred on July 20, 1965 in the Lau Basin. Also shown is the earthquake which occurred at the backarc spreading center on October 8, 1966. Epicenters of these two earthquakes are 20.9’S, 175.7Z”W, and 16.54’S, 177.45’W, respectively. The fault-plane solution is the lower hemisphere projection of the focal sphere. (Data taken from Sleep, 1972).
type. Here we see an example of tensile deviatoric stress at the back-arc spreading center. The studies mentioned above indicate that the overriding plate is very likely to be under compression except at the spreading center. Therefore, it appears that back-arc spreading is probably not a result of trench migration. Furthermore, if trench suction forces do exist as demonstrated in Fig. lB, it appears likely that hot and less viscous asthenosphere materials will be sucked up to fill the low-pressure gap, instead of breaking the more rigid overriding plate to close the gap. Consequently, magma should extrude from the trench axes. However, this phenomenon has not been observed. SUBDUCTION INDUCED CONVECTION AND BACK-ARC SPREADING
The problem to be examined next is what drives back-arc spreading. The driving mechanisms of back-arc opening have been proposed by many investigators. They can be grouped into three different categories. First is the induced flow mechanism as first proposed by McKenzie (1969) and Sleep and Tokdz. (1971). Details of this mechanism will be discussed later. The second mechanism is the trench suction mechanism (Forsyth and Uyeda, 1975; Molnar and Atwater, 1978). The possibility of having this mechanism work is perhaps small in light of the analysis given in the previous section. The third mechanism is that proposed by Uyeda and Kanamori (1979). They hypothesize that a subducting oceanic lithosphere will anchor in a fixed mantle. Back-arc spreading is mainly due to the retreating of the continent
94
with respect to a fixed slab. This mechanism is able to satisfy the presentday compressional stress requirement within the overriding plate. However, the initiation of this mechan~m is difficult to visualize. When a continent first starts to retreat from a fixed trench, it appears more likely that the two plates should separate at the shear zone between the overriding plate and the subducting oceanic lithosphere. Therefore, it is difficult to perceive why the overriding plate consistently breaks at a place about 150-250 km behind the volcanic arc and forms a spreading center there. Addition~ly, retreating of a continent represents an asymmetric driving mechanism for inter-arc spreading. Based on studies of wax models (Oldenburg and Brune, 1972), asymmetric driving forces will result in asymmetric spread~g. However, available magnetic anomalies in the back-arc basins (Barker, 1972) seem to suggest that spreading in these areas is fairly symmetric. Consequently, even though the model is able to solve certain problems, many questions remain. Finally, the induced flow mechanism will be examined. As pointed out by McKenzie (1969) and Sleep and Toksiiz (1971), slab subduction will naturally generate an induced flow in the mantle wedge above the slab. Associated
TOPOGRAPHY
E-1
A
bkm
100
bar 500 DISTANCE
(km)
SCHEMATIC DIAGRAM OF INTER -ARC SPREADING
Fig. 4. Calculated topography and vertically averaged horizontal deviatoric compressive stresses within the lithosphere are plotted as a function of the distance from the wedge corner. Bottom diagram is a vertically exaggerated schematic sketch of the inter-arc spreading center and its relationship to the induced flow beneath. It also shows the direction of compression forces within the lithosphere.
95
with this induced flow, there will be an upwelling current. This current brings hot mantle materials at depth towards the surface, resulting in thermal erosion at the base of the overriding lithosphere and upward normal stresses to elevate the topography. Based on our previous theoretical study (ToksSz and Hsui, 1978), these two combined effects are able to generate more than 100 bars of tensional stresses (Fig. 4). This is believed to be sufficient to break the lithosphere and initiate back-arc spreading if the tectonic stresses in the area are favorable. We also showed that the location of the upwelling current, hence the location of the spreading center, is strongly dependent on the viscosity structure within the mantle. Using a viscosity model similar to that determined based on glacial uplifting data (Cathles, 1975; Peltier, 1976; Peltier and Andrews, 1976), it is shown that the upwelling will consistently take place about 150-250 km behind the island arc (Fig. 4). Based on these calculations, the induced flow mechanism is probably the best available model at present to explain the initiation of back-arc spreading, and the subsequent formation of marginal basins. It should be pointed out that we are suggesting that back-arc spreading may contribute to trench migration. However, it may not be the only force that drives trench migration. Furthermore, we are by no means advocating that the subduction of oceanic lithosphere is due to back-arc opening. In fact, according to the induced flow mechanism, plate subduction must have already been taking place for a certain period of time (e.g., approximately 5 million years after the initiation of the subduction process) before back-arc opening becomes effective (ToksSz and Bird, 1977; Toksijz and Hsui, 1978). Therefore, it must be emphasized that it is plate subduction that indirectly generates back-arc spreading. Back-arc opening by itself cannot initiate plate subduction. DISCUSSION
In this paper, we have examined three proposed mechanisms that may cause back-arc opening. According to our study, the mechanism that involves the induced flow appears to be the most likely mechanism. However, it should be noted that this mechanism will not unconditionally generate backarc spreading centers. In order to have this mechanism be effective, the overall deviatoric stresses normal to the arc must be tensional or slightly compressional with a magnitude less than about 100 bars. If the ambient compressive stress is higher than about 100 bars, the stresses generated by the induced flow will not be sufficient to break the lithosphere and initiate spreading. The continental migration mechanism as proposed by Uyeda and Kanamori (1979) may help the initiation of spreading. However, their model by itself is not likely to generate symmetric spreading in the back-arc basins as observed. The trench suction mechanism by itself is probably the least likely mechanism to explain the stress regime of the overriding plate. The limited earthquake focal mechanisms presented are not consistent with this model.
96
It should be noted, however, that this mechanism may also contribute to generating a favorable stress regime for the induced flow to initiate back-arc spreading. Finally, it should be pointed out that throughout the discussion of this paper, the effect of the existence of island-arc volcanoes has been ignored. The presence of island arcs will undoubtedly affect the stress regime within the overriding plate. Because of the topographic elevation above the surrounding basins, gravitational sliding will act on the surrounding lithosphere (Hales, 1969; McKenzie, 1972; Artyushkov, 1973). The elevated arc and volcanoes will exert compressive stresses within the plate on both sides of the arc as if to push them away from the axis of the volcanic line. The magnitude of stresses thus generated is directly proportional to the elevation and size of the island arc. As these island-arc volcanoes grow in size, a stress level will be reached such that the induced flow is no longer able to break the lithosphere and generate back-arc opening. This may eventually contribute to the cessation of active back-arc spreading as observed in the Sea of Japan. Since the purpose of this paper is to point out the problem as well as a plausible approach to identify a realistic process taking place in these areas, our analysis is carried out based on a highly idealized model which considers only the most important factors. In order to achieve a more detailed analysis of the overall problem, the contribution of other effects such as that of the island-arc volcanoes upon the regional stress regime should be taken into account. In summary, plate subduction will naturally generate an induced flow above the slab. The upwelling of this induced current will generate tensile stresses in the back-arc basins. If ambient stresses are favorable, the tensional stresses generated by the induced flow will be able to break the lithosphere and initiate back-arc opening. However, if the ambient stress regime is strongly compressive, the induced flow will only heat the medium and may not generate back-arc spreading centers. In order to better understand the back-arc spreading problem, more detailed analyses of seismic data and more improved theoretical calculations of the thermal mechanical conditions of the trench--arc-back-arc system are required. ACKNOWLEDGEMENT
We thank Dr. Wai-Ying Chung for many helpful discussions about the data on earthquake focal mechanisms in plate convergent areas. This research was supported by NASA Grants NSG-7081 and NAG 5-41. REFERENCES Anderson, R.N., 1975. Heat flow in the Mariana marginal basin. J. Geophys. Res., 80: 4043-4048. Artyushkov, E.V., 1973. Stresses in the lithosphere caused by crustal thickness inhomogeneities. J. Geophys. Res, 78: 7675-7708.
97 Barker, P.F., 1972. A spreading center in the east Scotia Sea. Earth Planet. Sci. Lett., 15: 123-132. Cathles, L.M. III, 1975. The Viscosity of the Earth’s Mantle. Princeton University Press, Princeton, N.J., 386 pp. Chase, C.G., 1978. Extension behind island arcs and motions relative to hotspots, J. Geophys. Res., 83: 5385-5387. Chung, W.Y. and Kanamori, H., 1978. Subduction process of a fracture zone and aseismic ridges-the focal mechanism and source characteristics of the New Hebrides earthquake of 1969 January 19 and some related events. Geophys. J.R. A&on. Sot., 54: 221-240. Elsasser, W.M., 1971. Sea-floor spreading as thermal convection. J. Geophys. Res., 76: 1101-1112. Forsyth, D.W. and Uyeda, S., 1975. On the relative importance of driving forces of plate motion. Geophys. J.R. Astron. Sot., 43: 163-200. Hales, A.L., 1969. Gravitational sliding and continental drift. Earth Planet. Sci. Lett., 6: 31-34. Johnson, T. and Molnar, P., 1972. Focal mechanisms and plate tectonics of southwest Pacific. J. Geophys. Res., 77: 5000-5031. Jurdy, D.M. and Van der Voo, R., 1974. A method for the separation of true polar wander and continental drift, including results for the last 55 m.y. J. Geophys. Res., 79: 2946-2952. Karig, D.E., 1974. Evolution of arc systems in the western Pacific. Annu. Rev. Earth Planet. Sci., 2: 51-75. Katsumata, M. and Sykes, L.R., 1969. Seismicity and tectonics of the western Pacific: Izu-Mariana-Caroline and Rynkyn-Taiwan regions. J. Geophys. Res., 74: 5923-5948. Kaula, W.M., 1976. Absolute plate motions by boundary velocity minimizations. J. Geophys. Res., 80: 244-248. Lliboutry, L., 1974. Plate movement relative to rigid lower mantle. Nature, 250: 298300. McElhinny, M.W., 1973. Mantle plumes, paleomagnetization and polar wandering. Nature, 241: 523-524. McKenzie, D.P., 1969. Speculations on the consequences and causes of plate motions. Geophys. J.R. Astron. Sot., 18: l-32. McKenzie, D.P., 1972. Plate tectonics. In: E.C. Robertson (Editor), The Nature of the Solid Earth. McGraw-Hill, New York, N.Y., pp. 323-360. McKenzie, D.P., 1978. Active tectonics of the Alpine-Himalayan belt: the Aegean Sea and surrounding regions. Geophys. J.R. Astron. Sot., 55: 217-254. Minster, J.B. and Jordan, T.H., 1978. Present-day plate motions. J. Geophys. Res., 83: 5331-5354. Minster, J.B., Jordan, T.H., Molnar, P. and Haines, E., 1974. Numerical modeling of instantaneous plate tectonics. Geophys. J.R. Astron. SOC., 36: 541-576. Molnar, P., and Atwater, T., 1978. Interarc spreading and Cordilleran as alternates related to the age of subducted oceanic lithosphere. Earth Planet. Sci. Lett., 41: 330-340. Morgan, W.J., 1972. Deep mantle convection plumes and plate motions. Am. Assoc. Pet. Geol. Bull., 56: 203-213. Nakamura, K. and Uyeda, S., 1980. Stress gradient in arc-back arc regions and plate subduction. J. Geophys. Res., 85: 6419-6428. Oldenburg, D.W. and Brune, J.N., 1972. Ridge transform fault spreading pattern in freezing wax. Science, 178: 301-304. Pascal, G., Isacks, B.L., Barazangi, M. and Dubois, J., 1978. Precise relocations of earthquakes and seismotectonics of the New Hebrides Island Arc. J. Geophys. Res., 83: 4957-4973. Peltier, W.R., 1976. Glacial-isostatic adjustment, II. The inverse problem. Geophys. J.R. Astron. Sot., 46: 669-705.
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Peltier, W.R. and Andrews, J.T., 1976. Glacial-isostatic adjustment, I, The forward prohlem. Geophys. J.R. Astron. Sot., 46: 625-668. Sclater, J.G., 1972. Heat flow and elevation of the marginal basins of the western Pacific. J. Geophys. Res., 77: 5705-5719. Sclater, J.G., Hawkins, J.W., Mammerickx, J. and Chase, C.G., 1972a. Crustal extension between the Tonga and Lau Ridges: Petrologic and geophysical evidence. Geol. Sot. Am. Bull., 83: 505-518. Sclater, J.G., Ritter, U.G. and Dixon, F.S., 1972b. Heat flow in the southwestern Pacific. J. Geophys. Res., 77: 5697-5704. Sleep, N.H., 1972. Deep Structure and Geophysical Processes beneath Island Arcs. Ph.D. Thesis, M.I.T., Cambridge, MA. Sleep, N.H., and Toksiiz,M.N., 1971. Evolution of marginal basins. Nature, 33: 548-550. Solomon, S.C. and Sleep, N.H., 1974. Some simple physical models for absolute plate motions. J. Geophys. Res., 79: 2557-2567. Solomon, S.C., Sleep, N.H. and Richardson, R.M., 1975. On the forces driving plate tectonics: Inferences from absolute plate velocities and intraplate stress. Geophys. J.R. Astron. Sot., 42: 769-801. Solomon, SC., Sleep, N.H. and Richardson, R.M., 1977. Implications of absolute plate motions and intraplate stress for mantle rheology. Tectonophysics, 37: 219-231. Toksiiz, M.N. and Bird, P., 1977. Formation and evolution of marginal basins and continental plateaus. In: M. Talwani and W.C. Pitman (Editors), Island Arcs, Deep Sea Trenches and Back-Arc Basins. Maurice Ewing Series, Vol. 1, American Geophys. Union, Washington, D.C., pp. 379-393. Toksiiz, M.N. and Hsui, A.T., 1978. Numerical studies of back-arc convection and the formation of marginal basins. Tectonophysics, 50: 177-196. Uyeda, S., 1978. The New View of the Earth -Moving Continents and Moving Oceans. W.H. Freeman and Co., San Francisco, Calif. Uyeda, S. and Kanamori, H., 1979. Back-arc opening and the mode of subduction. J. Geophys. Res., 84: 1049-1061. Van Andel, T.H., 1974. Cenozoic migration of the Pacific plate, northward shift of the axis of deposition, and paleobathymetry of the central equatorial Pacific. Geology, 2: 507-510. Watanabe, T., Langseth, M. and Anderson, R.N., 1977. Heat flow in back-arc basins of the western Pacific. In: M. Talwani and W.C. Pitman (Editors), Island Arcs, Deep Sea Trenches and Back-Arc Basins. Maurice Ewing Series, Vol. 1, Am. Geophys. Union, Washington, D.C., pp. 137-161. Watts, A.B., Weissel, J.K. and Davey, F.J., 1977. Tectonic evolution of the South Fiji marginal basin. In: M. Talwani and W.C. Pitman (Editors), Island Arcs, Deep Sea Trenches and Back-Arc Basins. Maurice Ewing Series, Vol. 1, Am. Geophys. Union, Washington, D.C., pp. 419-427.
89
BACK-ARC SPREADING: TRENCH MIGRATION, CONTINENTAL PULL OR INDUCED CONVECTION?
ALBERT T. HSUI * and M. NAFI TOKSGZ Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 (U.S.A.) (Received October 24,198O)
ABSTRACT Hsui, A.T. and Toksoz, M.N., 1981. Back-arc spreading: Trench migration, continental pull or induced convection? In: S.C. Solomon, R. Van der Voo and M.A. Chinnery (Editors), Quantitative Methods of Assessing Plate Motions. Tectonophysics, 74: 8998.
Mechanisms of the opening of back-arc systems are analyzed. Limited focal mechanisms of intraplate earthquakes are used to determine the stress regime of an overriding plate. Preliminary analyses show that compressive deviatoric stresses exist in the plate except near the spreading center. Based on this observation “trench suction” does not appear to be the primary force that drives back-arc spreading, since it will result in tensional deviat&c stresses within the overriding plate. Even though “continental pull” is able to satisfy the stress requirements, it does not appear to be a likely mechanism either because of the initiation and subsequent symmetric spreading difficulty associated with such a mechanism. The mechanism we favor is the one that involves the induced convective current in the mantle wedge immediately above the slab. Calculations show that the induced flow is able to generate sufficient stress to break up the overriding lithosphere if the tectonic stresses of the region are favorable. Both trench suction and continental pull may help to provide such a favorable tectonic stress regime.
INTRODUCTION
The large-scale lithospheric plate motions have been studied extensively (Morgan, 1972; McElhinny, 1973; van Andel, 1974; Jurdy and Van der Voo, 1974; Solomon and Sleep, 1974; Lliboutry, 1974; Minster et al., 1974; Solomon etal., 1975,1977; KauIa, 1975; Minster and Jordan, 1978). Even though these studies have used different mantle rheologies, different geophysical constraints and different frames of reference, the relative motions between * Present address: Department of Geology, Urbana, IL 61801 (U.S.A.). 0040-1951/81/0000-0000/$02.50
University of Illinois at Urbana-Champaign,
@ 1981 Elsevier Scientific Publishing Company
90
plates determined from these calculations are reasonably consistent. Plate motions thus derived are able to explain many geophysical and geological processes such as earthquake distribution, earthquake source mechanisms, the building of mountain ranges and the formation of island arcs and oceanic trenches. Although global plate motions are widely accepted, the details of many small-scale motions at plate boundaries are not yet fully understood. One example of this is the relative motions between plates in trench-c-backarc systems. In this paper, we shall first examine the surface motions in trench-arcback-arc environments. We shall use stress patterns as revealed by earthquake mechanisms of these areas to study the inter-relationships among these motions. TRENCH
MIGRATION
AND BACK-ARC
SPREADING
Trenches in the Western Pacific have been recognized to migrate towards the ocean in geological time scale (Chase, 1978). The phenomenon may be associated with back-arc spreading. Simply based on geometrical arguments, trenches and continents must move away from each other if the back-arc basins are actively spreading. It has been suggested (Elsasser, 1971) that old oceanic lithosphere is gravitationally unstable, such that it will collapse under its own weight. As it collapses and sinks downward, the trench will appear to migrate towards the ocean. This motion will generate a suction force which may be responsible for back-arc opening (Forsyth and Uyeda, 1975; Molnar and Atwater, 1978). Alternatively, trench migration can be a consequence of back-arc opening. Since the overriding plate in back-arc regions does not descend into the mantle, spreading will widen this plate. As a result, the trench will be pushed away from the inter-arc spreading center. It is apparent then, that both processes seem to be able to explain the relative surface motion of oceanic trenches with respect to backarc spreading centers. The question is whether one can determine which process is the more likely one that is taking place in a trench-arc-back-arc environment. In order to determine the plausible mechanism, the tectonic stress regime within the overriding plate could be helpful. Using a simplified geometry and assuming plate uniformity, we shall examine the resultant stress regime within the overriding plate caused by the two different geological processes described above. Figure 1A shows a schematic diagram of a trench--arcback-arc system. This diagram shows the relative positions of every major element that composes the system. If “trench suction” is the dominating process taking place in these areas, it will imply that the oceanic trench will retreat faster than the overriding plate can catch up. Hence a low-pressure region will form such that a suction force will be generated and it will act on the overriding plate so as to pull it towards the ocean (Fig. 1B). In this case, deviator& stresses within the overriding plate would be tensional. On the
91
‘ONTINENT
ISLAND ARC BACK-ARC SPREAUNS CENTER VOLCANO
SUBOUCTlffi
TRENCH
SUCTION
LITHOSPMRE
I TENSION I
m COLLAPSE
SACK-ARC
RIDQE PUSH
OF SLAB
t COMPRESSION 1
=%=e?=
Fig. 1. A. Schematic diagram showing the simplified vertical cross-section of a trench-arcback-arc system. B. Schematic diagram showing the working principle of trench suction and the resultant stress regime within the overriding plate. C. Schematic diagram showing the stress regime within an overriding plate if trench migration is caused by back-arc spreading.
other hand, if trench migration is caused by back-arc opening, the overriding plate will be pushed towards the ocean by back-arc spreading (Fig. 1C). In this case, the stress regime within the overriding plate will be compressional. Although the two different processes can explain the surface relative motion equally well, they differ in such a way that the resultant stress regimes within the overriding plate are completely opposite. Therefore, the stress regime within this plate becomes a natural criterion to discriminate the more plausible geological process taking place in these areas. A tensional stress regime within the overriding plate will imply that trench suction is a more important process, while a compressional stress regime will indicate that it is not. STRESSES
IN TRENCH-ARC-BACK-ARC
SYSTEMS
Stresses in convergent plate boundaries are primarily compressive. This is implied by both seismic studies (Katsumata and Sykes, 1969; Johnson and Molnar, 1972; Chung and Kanamori, 1978; Pascal et al., 1978) and by the geological observations of heavy folding in the accretionary prism area (Karig, 1974). However, it is more difficult to determine the stress regime of the whole overlying plate. Although the spreading ridges can be located behind island arcs, it is difficult to establish if normal faulting is actually taking
92
place in these areas. This is mainly because the distribution of seismic stations surrouding these areas is sparse and the earthquake magnitudes are generally small. Despite the lack of focal mechanism data, the existence of a spreading center in back-arc basins is generally accepted because of other geophysical and geological observations. These include bathymetry and heat flow data (Sclater, 1972; Sclater et al., 1972a, b; Anderson, 1975; Toksoz and Bird, 1977; Watanable et al., 1977), magnetic anomaly measurements (Barker, 1972; Watts et al., 1977; S. Uyeda, pers. commun., 1979) as well as seismic reflection studies (Karig, 1974). Therefore, it is apparent that the stress regime in the subduction zone does change from trench axis to back-arc spreading center. The transition of stresses in plate convergent boundaries has been discussed by McKenzie (1978) and Nakamura and Uyeda (1980). Based on surface geological observations of the distribution of dikes and faults, Nakamura and Uyeda (1980) inferred that the stress regime within this plate transforms from compressional to strike-slip and then extensional as one moves from the trench axis towards the back-arc basins. The best way to study present stresses within a plate is to look at the focal mechanisms of intraplate earthquakes. Unfortunately, intraplate earthquakes within these regions are scarce and small in magnitude. Not too many reliable data are available. In New Hebrides, Chung and Kanamori (1978) identified three possible intraplate earthquakes which are indicated as events 8, 12, and 18 in Fig. 2. These events are located in the overriding lithosphere and away from other events in the Benioff zone. Their fault-plane solutions are given in the same diagram. They are all thrust events, and their compressive P-axes are approximately horizontal. These indicate that the stresses within the overriding plate in New Hebrides are probably compressional. Two intraplate events in the Lau Basin have been investigated by Sleep (1972) and are shown in Fig. 3. The earthquake near the volcanic arc is of thrusting type. The compressive P-axis is approximately horizontal. Thus, the stress system in the Lau Basin is also compressional. The focal mechanism of the other earthquake close to the spreading center is of normal faulting
Fig. 2. Focal mechanisms and the locations of the three intraplate earthquakes in New Hebrides. The fault-plane solutions represent the lower hemisphere projection of the focal sphere. (Data taken from Chung and Kanamori, 1978).
93 BASC
V
LAU l I
300
I 200
I
I 100
BASIN I
I I 0
DISTANCE IN Km.
Fig. 3. Focal mechanism and the location of the intraplate earthquake which occurred on July 20, 1965 in the Lau Basin. Also shown is the earthquake which occurred at the backarc spreading center on October 8, 1966. Epicenters of these two earthquakes are 20.9’S, 175.7Z”W, and 16.54’S, 177.45’W, respectively. The fault-plane solution is the lower hemisphere projection of the focal sphere. (Data taken from Sleep, 1972).
type. Here we see an example of tensile deviatoric stress at the back-arc spreading center. The studies mentioned above indicate that the overriding plate is very likely to be under compression except at the spreading center. Therefore, it appears that back-arc spreading is probably not a result of trench migration. Furthermore, if trench suction forces do exist as demonstrated in Fig. lB, it appears likely that hot and less viscous asthenosphere materials will be sucked up to fill the low-pressure gap, instead of breaking the more rigid overriding plate to close the gap. Consequently, magma should extrude from the trench axes. However, this phenomenon has not been observed. SUBDUCTION INDUCED CONVECTION AND BACK-ARC SPREADING
The problem to be examined next is what drives back-arc spreading. The driving mechanisms of back-arc opening have been proposed by many investigators. They can be grouped into three different categories. First is the induced flow mechanism as first proposed by McKenzie (1969) and Sleep and Tokdz. (1971). Details of this mechanism will be discussed later. The second mechanism is the trench suction mechanism (Forsyth and Uyeda, 1975; Molnar and Atwater, 1978). The possibility of having this mechanism work is perhaps small in light of the analysis given in the previous section. The third mechanism is that proposed by Uyeda and Kanamori (1979). They hypothesize that a subducting oceanic lithosphere will anchor in a fixed mantle. Back-arc spreading is mainly due to the retreating of the continent
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with respect to a fixed slab. This mechanism is able to satisfy the presentday compressional stress requirement within the overriding plate. However, the initiation of this mechan~m is difficult to visualize. When a continent first starts to retreat from a fixed trench, it appears more likely that the two plates should separate at the shear zone between the overriding plate and the subducting oceanic lithosphere. Therefore, it is difficult to perceive why the overriding plate consistently breaks at a place about 150-250 km behind the volcanic arc and forms a spreading center there. Addition~ly, retreating of a continent represents an asymmetric driving mechanism for inter-arc spreading. Based on studies of wax models (Oldenburg and Brune, 1972), asymmetric driving forces will result in asymmetric spread~g. However, available magnetic anomalies in the back-arc basins (Barker, 1972) seem to suggest that spreading in these areas is fairly symmetric. Consequently, even though the model is able to solve certain problems, many questions remain. Finally, the induced flow mechanism will be examined. As pointed out by McKenzie (1969) and Sleep and Toksiiz (1971), slab subduction will naturally generate an induced flow in the mantle wedge above the slab. Associated
TOPOGRAPHY
E-1
A
bkm
100
bar 500 DISTANCE
(km)
SCHEMATIC DIAGRAM OF INTER -ARC SPREADING
Fig. 4. Calculated topography and vertically averaged horizontal deviatoric compressive stresses within the lithosphere are plotted as a function of the distance from the wedge corner. Bottom diagram is a vertically exaggerated schematic sketch of the inter-arc spreading center and its relationship to the induced flow beneath. It also shows the direction of compression forces within the lithosphere.
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with this induced flow, there will be an upwelling current. This current brings hot mantle materials at depth towards the surface, resulting in thermal erosion at the base of the overriding lithosphere and upward normal stresses to elevate the topography. Based on our previous theoretical study (ToksSz and Hsui, 1978), these two combined effects are able to generate more than 100 bars of tensional stresses (Fig. 4). This is believed to be sufficient to break the lithosphere and initiate back-arc spreading if the tectonic stresses in the area are favorable. We also showed that the location of the upwelling current, hence the location of the spreading center, is strongly dependent on the viscosity structure within the mantle. Using a viscosity model similar to that determined based on glacial uplifting data (Cathles, 1975; Peltier, 1976; Peltier and Andrews, 1976), it is shown that the upwelling will consistently take place about 150-250 km behind the island arc (Fig. 4). Based on these calculations, the induced flow mechanism is probably the best available model at present to explain the initiation of back-arc spreading, and the subsequent formation of marginal basins. It should be pointed out that we are suggesting that back-arc spreading may contribute to trench migration. However, it may not be the only force that drives trench migration. Furthermore, we are by no means advocating that the subduction of oceanic lithosphere is due to back-arc opening. In fact, according to the induced flow mechanism, plate subduction must have already been taking place for a certain period of time (e.g., approximately 5 million years after the initiation of the subduction process) before back-arc opening becomes effective (ToksSz and Bird, 1977; Toksijz and Hsui, 1978). Therefore, it must be emphasized that it is plate subduction that indirectly generates back-arc spreading. Back-arc opening by itself cannot initiate plate subduction. DISCUSSION
In this paper, we have examined three proposed mechanisms that may cause back-arc opening. According to our study, the mechanism that involves the induced flow appears to be the most likely mechanism. However, it should be noted that this mechanism will not unconditionally generate backarc spreading centers. In order to have this mechanism be effective, the overall deviatoric stresses normal to the arc must be tensional or slightly compressional with a magnitude less than about 100 bars. If the ambient compressive stress is higher than about 100 bars, the stresses generated by the induced flow will not be sufficient to break the lithosphere and initiate spreading. The continental migration mechanism as proposed by Uyeda and Kanamori (1979) may help the initiation of spreading. However, their model by itself is not likely to generate symmetric spreading in the back-arc basins as observed. The trench suction mechanism by itself is probably the least likely mechanism to explain the stress regime of the overriding plate. The limited earthquake focal mechanisms presented are not consistent with this model.
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It should be noted, however, that this mechanism may also contribute to generating a favorable stress regime for the induced flow to initiate back-arc spreading. Finally, it should be pointed out that throughout the discussion of this paper, the effect of the existence of island-arc volcanoes has been ignored. The presence of island arcs will undoubtedly affect the stress regime within the overriding plate. Because of the topographic elevation above the surrounding basins, gravitational sliding will act on the surrounding lithosphere (Hales, 1969; McKenzie, 1972; Artyushkov, 1973). The elevated arc and volcanoes will exert compressive stresses within the plate on both sides of the arc as if to push them away from the axis of the volcanic line. The magnitude of stresses thus generated is directly proportional to the elevation and size of the island arc. As these island-arc volcanoes grow in size, a stress level will be reached such that the induced flow is no longer able to break the lithosphere and generate back-arc opening. This may eventually contribute to the cessation of active back-arc spreading as observed in the Sea of Japan. Since the purpose of this paper is to point out the problem as well as a plausible approach to identify a realistic process taking place in these areas, our analysis is carried out based on a highly idealized model which considers only the most important factors. In order to achieve a more detailed analysis of the overall problem, the contribution of other effects such as that of the island-arc volcanoes upon the regional stress regime should be taken into account. In summary, plate subduction will naturally generate an induced flow above the slab. The upwelling of this induced current will generate tensile stresses in the back-arc basins. If ambient stresses are favorable, the tensional stresses generated by the induced flow will be able to break the lithosphere and initiate back-arc opening. However, if the ambient stress regime is strongly compressive, the induced flow will only heat the medium and may not generate back-arc spreading centers. In order to better understand the back-arc spreading problem, more detailed analyses of seismic data and more improved theoretical calculations of the thermal mechanical conditions of the trench--arc-back-arc system are required. ACKNOWLEDGEMENT
We thank Dr. Wai-Ying Chung for many helpful discussions about the data on earthquake focal mechanisms in plate convergent areas. This research was supported by NASA Grants NSG-7081 and NAG 5-41. REFERENCES Anderson, R.N., 1975. Heat flow in the Mariana marginal basin. J. Geophys. Res., 80: 4043-4048. Artyushkov, E.V., 1973. Stresses in the lithosphere caused by crustal thickness inhomogeneities. J. Geophys. Res, 78: 7675-7708.
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