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Long-wavelength gravity anomalies and intraplate seismicity
Earth and Planetary Science Letters, 37 (1978) 465-475
465
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
LONG-WAVELENGTH GRAVITY ANOMALIES AND INTRAPLATE SEISMICITY PETER R. VOGT Naval Research Laboratory, Washington, DC 20375 (U.S.A.)
Received June 14, 1976 Revised version received August 15, 1977
Anomalously high mid-plate and Mid-Oceanic Ridge seismicity apparently characterizes the flanks of the regional freeair gravity and topographic high roughly centered on the postulated Iceland hot spot; anomalously high mid-plate seismicity also occurs around the deep free-air gravfty low centered just south of India. Although the correlation between gravity and seismicity is still somewhat inconclusive, it seems to suggest active asthenosphere flow associated with the regional gravity anomalies. Added to other stresses in the plate, the stress transmitted to the plate by this flow would encourage its fracture.
1. Gravity anomalies and mantle convection Whether mantle convection takes the form of thin plumes [1] or broad roll-like cells [2], the relatively low-viscosity asthenosphere must take an active part in the flow pattern. Direct evidence for asthenosphere flow is nevertheless difficult to find, since most of the world's surface characteristics are consequences of plate tectonics itself rather than of flow below the plates. I have suggested that the mantle materials brought up by plumes near the Mid-Oceanic Ridge (MOR) are primarily discharged into a pipe-like region of intensive partial melting that lies below the spreading axis [3,4]. V-shaped (diachronous) features of the oceanic basement topography have been delineated near severn hot spots. It was postulated that these "diachrons" could be used to measure the speed of the subaxial flow; rates of the order 5 - 2 0 cm/yr are indicated [3]. Other possible manifestations of pipe flow include fracture ridges (e.g., the Mendocino Ridge) and abrupt changes in age-corrected basement depth, seismicity, or other features of the oceanic plate across faults of large offset. Partial damming of the pipe flow at transform faults was invoked to explain such observations [5]. Geochemical gradients along the MOR away from several hot spots provide independent support for the concept of subaxial flow [6].
If some evidence now exists in favor of pipeconvection from centers of upwelling near the spreading axis, the pattern of asthenosphere flow below the plates at s o m e distance from the MOR is still almost entirely conjectural. Broad anomalies in the free-air gravity field appear to offer the best hope for assessing the flow field [ 7 - 9 ] . A reasonable range of gravity anomaly dimensions associated with asthenosphere flow is 1 2 0 0 - 3 5 0 0 km [8]. Broader features are more likely of deeper origin, while those of shorter wavelength could be supported by the lithosphere [8,9]. At satellite heights short-wavelength anomalies are naturally filtered out. The free-air gravity field computed to the 16th degree spherical harmonics [10], using both satellite and surface observations, shows numerous perturbations in the range +37 to - 7 0 regals in amplitude and several thousand kilometer wavelengths. It can be shown that the mass excesses or deficits implied by these anomalies cannot be supported by the lithospheric plates [11]. At least a significant part of this residual gravity field must reflect density inhomogeneities in the mantle below the plates. Since the plates are incapable of maintaining the density anomalies as static loads, it seems reasonable that the anomalies are dynamic in origin, i.e., they are constantly maintained by motions of the plates and the underlying mantle [ 7 - 9 , 1 2 ] . The next question is whether mantle flow patterns at substantial depth,
466 and of no direct relation to the lithosphere, could be responsible for the long-wavelength gravity anomalies. As Kaula [8,9] has noted, most of the anomalies do correlate with surface features and, therefore, most probably reflect asthenosphere/lithosphere dynamics. Subduction zones are almost invariably represented by gravity highs, whereas deep ocean basins tend to be negative. Roughly equidimensional highs occur over shallow parts of the MOR, such as Iceland, the Azores, Amsterdam, St. Paul, Tristan da Cunha, and Prince Edward Island. (In some cases, for example Iceland and the Azores, the gravity highs are broad enough, compared to their separations, that they fuse together.) Numerical simulation shows that rising convective motion in Newtonian fluids is generally accompanied by topographic and gravity highs [12], at least if a free (fluid) surface is taken as the boundary condition. The sign and magnitude of the observed correlation between topographic and gravity anomalies is in reasonable accord with theory [13]. Vertical motions of Pacific atolls [14] as well as certain heat flow anomalies [15] can be explained by the hypothesis that some regional gravity/topographic highs represent stationary centers of relatively hotter, rising asthenosphere, while gravity/topographic lows represent the converse, i.e., sinking motion below the plates. An example of such a gravity/topographic low is located on the Southeast Indian Ocean Ridge south of Australia [ 16].
2. Gravity and intraplate seismieity Despite the promise of gravity anomalies as indicators of asthenosphere flow, many questions remain. The correlation between surface topography and gravity anomaly is far from perfect, even along the MOR [13]. For example, the gravity high over the Azores is the same value as that over Iceland despite the differences in elevation [7]. A gravity "saddle" corresponding to the pronounced topographic saddle between Iceland and the Azores (Fig. 2A, 2B) is poorly developed. Some recent gravity representations show the Azores and Iceland free-air highs fused into a single broader feature. Extensive gravity lows over Antarctica and north of the Himalayas remain unexplained [8]. The greatest departure from the norm is a low of - 7 0 mgal located south of the
Indian subcontinent (Fig. 3A, 3B); the surface topography gives no hint of this feature. Mantle flow below the asthenosphere and unrelated to the plates might account for some of these discrepancies. Mso, correlations between long-wavelength gravity and magnetic anomalies can only be explained if the density/ flow pattern extends to the base of the mantle [17]. Clearly other kinds of observations are needed to demonstrate that active asthenosphere flow is, in fact, presently occurring under the interiors of lithospheric plates wherever prominent gravity anomalies exist. The thesis of this paper is that intraplate seismicity, by correlating with prominent gravity anomalies in at least two areas (Figs. 1,3), may be an important register of asthenosphere dynamics. In general, this is not a new idea. The connection between intraplate seismicity and gravity is as follows: Runcorn [7] has argued that, with some assumptions, the velocity field of mantle flow could be derived from geoid (and hence gravity anomaly) perturbations. The flow field just below the lithosphere/asthenosphere boundary is then of greatest interest, for the viscous stresses on plate bottoms can be determined, provided viscosity is known. How the plates respond to these stresses depends, of course, on the properties of the plates themselves. In this paper I take the simple, empirical approach: Is there any observable correlation between intraplate seismicity and regional gravity anomalies? If there is, two significant conclusions could be drawn. (1) the gravity anomalies would reflect active asthenosphere flow that extends upward to the base of the lithosphere; and (2) viscous or other stresses generated by this flow must be sufficient to cause brittle failure and resulting seismicity in the overlying plates. Intraplate seismicity has been investigated by Sykes and Sbar in recent years [18-20]. Compared to what was discovered about interplate seismicity, relatively little is known about the phenomenon. There are several reasons for this [21]. First, the level of seismicity in plate interiors is generally much less than along plate boundaries. In a few years observation time the plate boundaries can be outlined by epicenter belts, and this is truely independent of the time period selected. For the relatively small number ofintraplate events accurately located in the last fifteen years, it is often not clear whether the observed "patterns" are real and repeatable or whether
467 they are chance artifacts of observation time. Another problem is that seismograph networks were designed to optimize sensitivity to plate-margin events, such earthquakes being more frequent and of greater concern to society [21]. If the earlier instrumental data (1901-1959) and even older historical information is considered, the picture of intraplate seismicity tends to be strongly biased by population density, while epicenter locations are too poor to resolve possible narrow intraplate fractures. If there is any correlation between intraplate seismicity and regional gravity anomalies, it would most likely appear in areas of (a) intense gravity anomalies and/or (b) good network sensitivity and high population density. If no correlation is observed even in such areas, either of the following could be true: (1) the observed seismicity is due to processes other than viscous stresses, i.e., these stresses are comparatively small or even non-existent; (2) the observation time is insufficient to delineate those earthquake patterns due to viscous stresses; or (3) intraplate seismicity due to viscous stresses consists predominantly of such low-magnitude events that the present seismograph network is inadequate for the purpose even if data are collected over long time periods. Two regions are considered in this paper: (1) the large 3 0 - 4 0 mgal free-air gravity high centered on and evidently associated with the Iceland hot spot (Fig. 1A, B), and (2) the Indian plate, which contains the world's greatest free-air excursions ( - 6 0 to - 7 0 mgal south of India and - 3 0 to - 4 0 mgl south of Australia (Fig. 3A, B)). The gravity contours in Figs. 1A and 3A are reproduced from Kaula [8] and are based on the " S E I I " solution of Gaposchkin and Lambeck [10]. The S E I I field was based on both satellite and surface data. More recent [22,23], presumably more accurate fields, may give somewhat different shapes and extreme values but the overall pattern is not changed unless the number of terms in the spherical harmonic expansion is much smaller. To illustrate this point we show the PGS-110 field, to 12th order and degree [23], Figs. 1B and 3B). This model also incorporates both surface and satellite data. The Iceland area was chosen for four reasons: (1) it is one of the best known and commonly cited examples of a mantle hot spot or plume, (2) the worldwide seismic network is relatively sensitive to
events occurring in this area, (3) the positive gravity anomaly (+30 to 40 mgal) is one of the highest in the world; and (4) the eastern flank of the Iceland gravity high overlaps densely populated western Europe. These factors notwithstanding, there were only a handful of "reliably located" events during the period 1963-1972 [24]. These are events of body wave magnitude >/4.5 and reported by ten or more stations. However, numerous additional events located with lesser reliability occurred in this region; the intraplate seismicity of particular areas has been discussed in several papers [21,25-34]. In order to display seismic activity together with topographic and gravity information, the data of Husebye et al. [25] and Keen et al. [32] were first smoothed. An arbitrary 200 km × 200 km grid was superposed on the intraplate epicenter pattern and the number of events per square contoured. Events within 50 km of the active plate boundary were not counted as "intraplate". Relatively seismic areas [18,21,33,34] in western Norway, England, the Baltic Sea, the MacKenzie River Valley of Canada, and south central Alaska are also shown in schematic fashion. In view of various uncertainties, the depiction of mid-plate seismicity in this vast Arctic region (Fig. 1A, B) is somewhat schematic and undoubtedly biased. Such bias is not thought to alter the thesis of this paper, however. Another feature of seismicity in the greater Iceland area is that the MOR itself is not uniform in seismic activity along its length [4,35,36]. In particular, ridge crest seismicity is exceptionally low within 900 km of Iceland; beyond, two stretches of relatively intense seismicity occur along the MohnsKnipovich and southern Reykjanes Ridges [4]. This is best illustrated by a longitudinal plot of MOR seismicity against plate rotation latitude (Fig. 2). The two zones of high interplate seismicity are also shown, schematically, in Fig. 1A and B. The analyses of Evernden [37] suggest that the variations of seismicity along this part of the MOR are in general not artifacts of variable network sensitivity. When all the seismic activity is shown together, a pattern emerges: The interior or crestal portion of the Iceland gravity high - roughly that region enclosed by the +20 mgal contour - is characterized by relatively low interplate and intraplate seismicity. Surrounding this lies a "halo" of intensified seis-
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Fig. 2. Seismicity (1961-1971), spreading rate, spreading angle, elevation, basalt chemistry along present spreading axis, as a fraction of plate rotation latitude, from the Azores triple junction north to the Arctic Basin. Adapted from Vogt [4]. Cross-hatched bars show seismicity attributed to major fracture zones; black bars are events of magnitude M b >/5.0. RV shows occurrence of a prominent rift valley (heavy black line), which is missing inside seismic halo. Stippled bands show generally higher seismicity roughly where proposed seismic "halo" (Fig. 1A, B) intersects plate boundary.
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Fig. 3. A. Model SE-II free-air gravity contours [8,10], plate boundaries, and reliably located intraplate epicenters in area of Indian lithospheric plate. Cross-hatched belts along plate boundaries approximate zones of interplate seismicity. Note general occurrence of intraplate events around and within deep gravity low centered near Ceylon. Many of remaining intraplate events also tend to lie along belts of high regional gradients in free-air gravity anomaly. Where detailed seismic reflection data are available (parallel-line patterns; [44] and unpublished data), the orientation of young deformational structures is subparalM to the isogals.
micity about 1500 km wide. The seismic halo extends 1000 km further west than east o f Iceland, but its relationship to the regional gravity anomaly is quite distinct: The belt of higher seismicity tends to follow the flanks of the gravity high, i.e., it is essentially confined to the region of maximum gradient. Some seismicity occurs westwards to the McKenzie Delta, southwest towards the Gulf of St. Lawrence, and along the Rhine Graben. In those areas the outer boundary of the seismic "halo" is indistinct and somewhat arbitrary. Nevertheless, a correlation between gravity and seismicity appears too distinctive to be entirely coincidental. One possible explanation is that active outward flow of asthenosphere away from the Iceland mantle plume is transmitting
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Fig. 3 (continued). B. Epicenters and plate boundaries as in Fig. 3A, but with more recent PGS-110-derived field [23], as in Fig. lB. Note basic similarity to earlier gravity pattern (Fig. 3A). Vertical ruling shows zone of postulated enhanced intraplate seismicity associated with gravity low. Question marks show possible plate boundary events. shear stresses to the lithosphere several thousand kilometers from Iceland. Since the gravity negatives over Hudson's Bay and eastern Scandinavia partly reflect recent deglaciation [8], some of the gravity gradients and associated asthenosphere flow may be transient phenomena, unconnected with the Iceland plume. But if the seismic halo crosses oceanic, glaciated, and deglaciated areas alike, the correlation between gravity and seismicity is unlikely to be just an effect of post-glacial adjustment. However, it is true that both gravity gradients and the mid-plate seismicity halo are best developed along zones marginal to Pleistocene ice sheets. Furthermore, there is some concentration of seismicity along the northern flank of a large free-air low centered over Hudson Bay, i.e., approximately along the line of inflection in the contours of the rate of post-glacial uplift [33]. Seismicity elsewhere in the Canadian Arctic as well as in the seismic belt from
472 Svalbard to the North Sea (Fig. 1A, B) does not correlate systematically with local gravity anomalies or with uplift contours. Why should seismicity be greatest around the margins of the gravity high? In general, the flow should cross from high to lower gravity anomaly as observed. However, a large area of reduced seismicity is not what would be expected from a narrow plume. After all, radial outflow from such a plume under Iceland would cause maximum stresses in the immediate Iceland area. Assuming the plume model is valid, we can nevertheless offer several possible explanations: (1) The area of upwelling is rather broad and the stagnation zone of reduced horizontal flow extends more than 1000 km away from Iceland. This possibility certainly does not agree with Morgan's [1] concept of thin plumes. Furthermore, overcoring experiments by Hast [38] indicate that maximum compressive stresses are nearly horizontal and oriented radially about the center of Iceland, as would be expected from tile narrow plume model [20]. (2) Asthenosphere viscosity (r/) is reduced in the central aseismic region, due to higher than normal temperatures and degree of partial melting. Hence, even though flow speeds are high, the viscous traction r~ • 3u/3z, where u is the horizontal component of flow, does not reach maximum until 1000-3000 km from the center of upwelling. (3) The plate, relatively hot and thin, is more likely to respond by plastic deformation near the plume center. (4) The margins of the gravity high represent a broad "binge" belt separating asthenosphere invaded (and thickened) by plume discharge from normal asthenosphere. Such a hinge belt would exhibit the greatest differential vertical motion and the maximum stress gradient caused by vertical motion (horizontal stress gradients caused by viscous traction could peak somewhere else). Of possible significance is that two of the three areas of high seismicity along coastal Norway [27,30] lie just west of the highest mountains. Evidently these local, epeirogenic uplifts are not merely relicts of initial rifting, but are still being formed or modified by whatever process that develops stresses along the coasts. It is inviting to think of outflow from the Iceland plume reaching the continental margin of Norway and the Barents Sea,
and more or less arrested there by the thick, Precambrian to Paleozoic lithosphere. The relative importance of the four factors listed above is unknown; an outward increase in asthenosphere viscosity, plus marginal "hinge line" effects, appear most reasonable. The existence of a broad heat-flow high over the regional Iceland gravity high would support this line of/easoning; unfortunately the existing oceanic measurements, initially interpreted as a regional heat-flow high [39], are now considered within statistical uncertainty of "normal" (G. Zielinski,-unpublished manuscript, 1977). There are few data from the continental parts of the gravity high; furthermore a warming of the sublithospheric mantle ~55 m.y.B.P, would probably have not yet been "felt" in the upper crust. Two other observations pertain to Fig. 1. The Faeroe-Iceland-Greenland and Davis Straits aseismic ridges are virtually devoid of intraplate epicenters, as if the lithosphere under these ridges either does not deform by brittle fracture or else is too thick and too strong. As with most interpretations of intraplate seismicity, this conclusion may be premature. The same applies to the high seismicity of Mohns and southern Reykjanes Ridges [4]. Fault-plane solutions are now needed for these spreading axes to test the hypothesis that the high ridge crest seismicity is caused by a complex stress regime, including tension normal to the ridges as well as compression along their strike. The second area considered here is the Indian plate (Fig. 2). Among the reasons for choosing this area is its equatorial location, which would minimize the deglaciation effects perhaps important in the Iceland case. Only reliable epicenters (1963-1972) and one magnitude-8.l event (1928) are shown in Fig. 2 [24] ; historical data would probably skew the pattern toward India, Indonesia, and the few populated areas of Australia. Again we see some correlation between regional gravity anomalies [8] - in this case largely negative and intraplate seismicity. Compared to others, the Indian plate exhibits both the greatest gravity relief (+37 to - 7 0 regal in the Se-II model, and similar values for subsequent ones) and the highest level of intraplate seismicity. This coincidence can scarcely be fortuitous, and most probably cannot be explained by variable seismic network sensitivity. The intraplate events lie in a loose
473 INTRA- PLATE & MOH SEISMICITY SLIGHTI INCREASE 1 MODER~CREASE, i~;r' ,!: J
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Fig. 4. Schematic illustrating hypothesis of this paper: Intraplate seismicity is enhanced in a halo-like zone of maximum asthenosphere flow (horizontal component). If MOR axis (accretion, spreading) crosses gravity high, ridge axis seismicity is decreased inside halo because upwelling elevates isotherms and thins ridge crest lithosphere. Some asthenosphere flow and intraplate scismicity is assumed to occur outside the stippled zones as well but a correlation may be obscured there by causes of interplate seismicity other than convection in the asthenosphere [ 1 8 - 2 0 , 4 2 ] , or other types of flow, e.g. the postulated longitudinal "rolls" normal to the MOR [45]. Normal gravity high associated with MOR has been ignored.
halo around the - 7 0 mgal gravity low south of India; many of the others tie along zones of maximum gravity gradient that loop through southwestern Australia and along the northeastern flanks of the MOR. Gravity, extrema, saddles, and plains tend to be devoid of intraplate activity. An exception to this may be the belt of historical events that follows the belt of Tertiary volcanism in southeastern Australia
[211. If seismicity southeast of India does relate to the deep gravity low there, this would be the only known correlation between that anomaly and a "surface" feature. Unlike the Iceland high, the south Indian low is not associated with any known topographic or tectonic feature and its source might otherwise be supposed to underlie the asthenosphere. Also in contrast to the Iceland area, any correlation between gravity and seismicity in the Indian plate cannot be ascribed to deglaciation. Sykes [40] suggested that the high seismicity might reflect a nascent island
arc forming as the northward motion of the Indian plate is arrested by collision with Asia. This would not explain the intraplate events northwest and southeast of the gravity low (Fig. 2); however, it is still possible that intensive asthenosphere flow towards the gravity low is capable of developing a new plate boundary, much as the Iceland plume is often visualized as breaking Greenland from Eurasia about 60 m.y.B.P. Thus, the relatively intense intraplate seismicity around the southern margin of the gravity low could reflect nascent break-up of the plate. Fitch et al. [41] have also mentioned a possible connection between gravity anomalies and seismicity within the Indian plate. Although horizontal compression related to resistance to subduction is given as a major reason for the seismicity, these authors also acknowledge the possible role of mantle flow set up in response to the development of the Ganges cone, or for unknown other reasons. Seismic reflection surveys could be used to map geologically young deformation associated with earthquake activity in the Indian plate. In two limited areas where the strikes of young folds and/or faults can be determined, the structures are subparallel to the isogals (Fig. 2). In conclusion, we see some indication that the asthenosphere flow predicted on the basis of regional gravity anomalies does exist and that the viscous stresses associated with this flow may be a significant factor in intraplate seismicity (Fig. 4). If this is true, it may be that improved seismic network coverage and longer observation times -- together with seismic reflection surveys to chart young deformation - will eventually reveal similar relationships in other intraplate areas. However, it is clear already that at least some intraplate seismicity is unrelated to regional gravity patterns; for example, the eastern United States and southeastern Canada are areas of locally significant activity [21 ] but little or no gravity gradient [8]. Also, the patterns of principal stresses deduced from focal mechanism solutions and in-situ measurements do not correspond to those anticipated if radial asthenosphere flow away from Morgan's mantle plumes [1 ] are the sole driving forces behind plate tectonics [18-20]. In the Iceland area (Fig. 1), overcoring experiments do show compression oriented radially about the center of the island [38!.
474 But of the ten focal mechanism solutions in the "seismic halo" (all west of the MOR) five had normal faulting, one had strike-slip, and the remainder thrust faulting as the principal mechanism. Of sixteen solutions in the Indian plate (Fig. 3), seven were thrusting, six strike-slip, and the remaining three - all near the MOR - were normal mechanisms [20]. The Indian Ocean thus differs from the Iceland area in the occurrence there of strike-slip events [20] ; another difference is the relatively high percentage of large earthquakes in the northwest India plate. This may indicate comparatively high stresses [40], perhaps because of exceptionally strong asthenosphere flow and a relatively old, thick plate, further thickened by its location over a site of mantle downwelling, perhaps like those postulated in the eastern Pacific [15] and south of Australia [16]. Evidently the stress fields in both areas are quite complex and locally variable. Local factors such as lines of preexisting weakness probably control the patterns of seismic activity on a more detailed scale. For example, the seismicity in the eastern Canadian Arctic (Fig. 1) seems to be locally influenced [33] by (a) sediment loading on the Arctic continental margin, (b) deformational structures reactivated in Paleozoic and later orogenic phases, or (c) hinge lines, reflected by gravity gradients, where differential postglacial uplift is maximal. Another type of local structure of possible importance to explain the details of mid-plate seismicity and fault plane solutions has been analyzed by Kane and his colleagues [42]. Analyzing certain areas of mid-plate seismicity in the continental United States, these workers have suggested that marie intrusions in the crust act as stress concentrators. The details of failure then depend on the relative strength of the plutons and country rock. Clearly a number of local factors are likely to be important in controlling the details of epicenter distribution within the postulated "seismic halo". To the extent that these factors are important, the finer details of seismicity (Fig. 1 , 2 ; [26,33]) are likely to repeat themselves during a future, independent observation period.
Acknowledgements I thank D. O'Neill, C. Grimstad, and C. Fruik for
assistance. This work was partially sponsored by the Office of Naval Research.
References 1 W.J. Morgan, Deep mantle convection plumes and plate motions, Am. Assoc. Pet. Geol. Bull. 56 (1972) 203213. 2 S.K. Runcorn, Changes in the convection pattern in the earth's mantle and continental drift: evidence for a cold origin of the earth, Philos. Trans. R. Soc. Lond., Ser. A, 258 (1965); 228-251. 3 P.R. Vogt, Asthenosphere motion recorded by the ocean floor south of Iceland, Earth Planet. Sci. Lett. 13 (1971) 153-160. 4 P.R. Vogt, The Iceland phenomenon: imprints of a hot spot on the ocean crust, and implications for flow below the plates, in: Geodynamics of Iceland and the North Atlantic Area, L. Kristjansson, ed. (D. Reidel, Dordrecht, 1974) 105-126. 5 P.R. Vogt and G.L. Johnson, Transform faults and longitudinal flow below the Mid-Oceanic Ridge, J. Geophys. Res. 80 (1975) 1399-1428. 6 J.G. Schilling, Iceland mantle plume: geochemical study of Reykjanes Ridge, Nature 242 (1973) 565-571. 7 S.K. Runcorn, Dynamical processes in the deeper mantle, in: The Upper Mantle, A.R. Ritsema, ed. (Elsevier, Amsterdam, 1972) 623-637. 8 W.M. Kaula, Global gravity and mantle convection, in: The Upper Mantle, A.R. Ritsema, ed. (Elsevier, Amsterdam, 1972) 341-359. 9 W.M. Kaula, Global gravity and tectonics, in: The Nature of the Solid Earth, E.C. Robertson, ed. (McGraw-Hill, New York, N.Y., 1972) 385-405. 10 E.M. Gaposhkin and K. Lambeck, Earth's gravity field to sixteenth degree and station coordinates from satellite and terrestrial data, J. Geophys. Res. 76 (1971) 48554883. 11 K. Lambeck, Gravity anomalies over oceanic ridges, Geophys. J. R. Astron. Soc. 30 (1972) 37 53. 12 D.P. McKenzie, J.M. Roberts and N.O. Weiss, Convection in the earth's mantle: towards a numerical solution, J. Fluid Mech. 62 (1974) 465 --538. 13 R.N. Anderson, D. McKenzie and J.C. Sclater, Gravity, bathymetry, and convection in the earth, Earth Planet. Sci. Lett. 18(1973) 391 407. 14 H.W. Menard, Depth anomalies and the bobbing motion of drifting islands, J. Geophys. Res. 78 (1973) 5128 5137. 15 R.N. Anderson, Cenozoic motion of the Cocos plate relative to the asthenosphere and cold spots, Geol. Soc. Am. Bull. 85 (1974) 175-180. 16 J.K. Weissel and D.F. Hayes, The Australian-Antarctic discordance: new results and implications, J. Geophys. Res. 79 (1974) 2579-2587.
475 17 R.M. Hide and S.R.C. Malin, Novel correlations between global features of the earth's gravitational and magnetic fields, Nature 225 (1970) 605 609. 18 M.L. Sbar and L.R. Sykes, Contemporary compressive stress and seismicity in eastern North America: an example of intra-plate tectonics, Bull. Geol. Soc. Am. 84 (1973) 1861 -1882. 19 L.R Sykes and M.L. Sbar, intraplate earthquakes, lithospheric stresses and the driving mechanisms of plate tectonics, Nature 245 (1973) 298-302. 20 L.R. Sykes and M.L. Sbar, Focal mechanism solutions of intraplate earthquakes and stresses in the lithosphere, in: Geodynamics of Iceland and the North Atlantic Area, L. Kristjansson, ed. (D. Reidel, Dordrecht, 1974) 207 224. 21 J. Oliver, B.L. Isacks and M. Barazangi, Seismicity at continental margins, in : The Geology of Continental Margins (Springer-Verlag, New York, N.Y., 1974) 85-103. 22 M.A. Khan, Dynamic implications of mantle hot spots, Nature 251 (1974) 596 597. 23 B.D. Marsh and J.G. Marsh, On global gravity anomalies and two-scale mantle convection, J. Geophys. Res. 81 (1976) 5267-5280. 24 A.C. Tarr, World Seismicity Map (U.S. Geological Survey, Reston, Va., 1974). 25 E.S. Husebye, H. Gj~bstdal, H. Bungum and O. Eldholm, The seismicity of the Norwegian-Greenland Sea, Tectonophysics 26 (1975) 55 70. 26 E.S. Husebye, H. Bungum, J Fyen and H. Gj~bystdal, Earthquake activity in Fennoscandia between 1497 and 1975 and intraplate tectonics, Nor. Geol. Tidsskr. (1977) in press. 27 M. B~th, Seismicity of Fennoscandia and related problems, Gerlands Beitr. Geophys. 63 (1953) 173 208. 28 M. B~th, Zum Studium der Seismizit~t yon Fennoscandia, Gerlands Beitr. Geophys. 81 (1972) 213 226. 29 J.H. Hodgson, M. B~th, H. Jensen, A. Kvale, N.A. Linden, L.M. Murphy, N.V. Shebalin, E. Tryggvason and E. Vesanen, Seismicity of the Arctic, Ann. Int. Geophys. Year 30 (1965) 33 65. 30 A. Kvale, Earthquakes, in: Geology of Norway, Nor. Geol. Unders. 208 (1960) 490-506. 31 M.J. Keen, B.D. Loncarevic and B.N. Ewing, Continental
32
33
34 35
36 37 38 39
40
41
42
43
44 45
margin of eastern Canada: Georges Bank to Kane Basin, in: The Sea, Vol. 4, Part lI, A.E. Maxwell, ed. (WileyInterscienee, New York, N.Y., 1970) 251 291. C.K. Keen, D.L. Barrett, K.S. Manchester and D.I. Ross, Geophysical studies in Baffin Bay and some tectonic implications, Can. J. Earth Sci. 9 (1972) 239-256. P.W. Basham, D.A. Forsyth and R.J. Wetmiller, The seismicity of northern Canada, Can. J. Earth Sci. 14 (1977) 1646-1667. V. K~rnfk, Seismicity of the European Area, 2 (D. Reidel, Dordrecht, 1971) 218 pp. P.R. Vogt and G.L. Johnson, A longitudinal seismic reflection profile of the Reykjanes Ridge, II. Implications for the mantle hot spot hypothesis, Earth Planet. Sci. Lett. 18 (1973) 49-58. T.J.G. Francis, The seismicity of the Reykjanes Ridge, Earth Planet. Sci. Lett. 18 (1973) 119 124. J.F. Evernden, Location capability of various seismic networks, Bull. Seismol. Soc. Am. 61 (1971) 241 273. N. Hast, The state of stress in the upper part of the crust, Tectonophysics 8 (1972) 169 211. M.G. Langseth and G.W. Zielinski, in: Geodynamics of Iceland and the North Atlantic Area, L. Kristjansson, ed. (D. Reidel, Dordrecht, 1974) 277 296. L.R. Sykes, Seismicity of the Indian Ocean and possible nascent island arc between Ceylon and Australia, J. Geophys. Res. 75 (1970) 5041-5055. T.J. Fitch, M.H. Worthington and I.B. Evcringham, Mechanisms of Australian earthquakes and contemporary stress in the Indian Ocean plate, Earth Planet. Sci. Lett. 18 (1973) 345-356. D.L. Campbell, Implications of Kanc's stress-concentration mechanism for intcrplate earthquakes, EOS, Trans. Am. Geophys. Union 58 (1977) 431. S.C. Solomon, N.H. Sleep and R.M. Richardson, On the forces driving plate tectonics: inferences from absolute plate velocities and intraplate stress, Geophys. J. R. Astron. Soc. 42 (1975) 769-801. S.L. Eittreim and J. Ewing, Mid-plate tectonics in the Indian Ocean, J. Geophys. Res. 77 (1972) 6413 6421. F.M. Richter and B. Parsons, On the interaction of two scales of convection in the mantle, J. Geophys. Res. 80 (1975) 2529 2541.
465
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
LONG-WAVELENGTH GRAVITY ANOMALIES AND INTRAPLATE SEISMICITY PETER R. VOGT Naval Research Laboratory, Washington, DC 20375 (U.S.A.)
Received June 14, 1976 Revised version received August 15, 1977
Anomalously high mid-plate and Mid-Oceanic Ridge seismicity apparently characterizes the flanks of the regional freeair gravity and topographic high roughly centered on the postulated Iceland hot spot; anomalously high mid-plate seismicity also occurs around the deep free-air gravfty low centered just south of India. Although the correlation between gravity and seismicity is still somewhat inconclusive, it seems to suggest active asthenosphere flow associated with the regional gravity anomalies. Added to other stresses in the plate, the stress transmitted to the plate by this flow would encourage its fracture.
1. Gravity anomalies and mantle convection Whether mantle convection takes the form of thin plumes [1] or broad roll-like cells [2], the relatively low-viscosity asthenosphere must take an active part in the flow pattern. Direct evidence for asthenosphere flow is nevertheless difficult to find, since most of the world's surface characteristics are consequences of plate tectonics itself rather than of flow below the plates. I have suggested that the mantle materials brought up by plumes near the Mid-Oceanic Ridge (MOR) are primarily discharged into a pipe-like region of intensive partial melting that lies below the spreading axis [3,4]. V-shaped (diachronous) features of the oceanic basement topography have been delineated near severn hot spots. It was postulated that these "diachrons" could be used to measure the speed of the subaxial flow; rates of the order 5 - 2 0 cm/yr are indicated [3]. Other possible manifestations of pipe flow include fracture ridges (e.g., the Mendocino Ridge) and abrupt changes in age-corrected basement depth, seismicity, or other features of the oceanic plate across faults of large offset. Partial damming of the pipe flow at transform faults was invoked to explain such observations [5]. Geochemical gradients along the MOR away from several hot spots provide independent support for the concept of subaxial flow [6].
If some evidence now exists in favor of pipeconvection from centers of upwelling near the spreading axis, the pattern of asthenosphere flow below the plates at s o m e distance from the MOR is still almost entirely conjectural. Broad anomalies in the free-air gravity field appear to offer the best hope for assessing the flow field [ 7 - 9 ] . A reasonable range of gravity anomaly dimensions associated with asthenosphere flow is 1 2 0 0 - 3 5 0 0 km [8]. Broader features are more likely of deeper origin, while those of shorter wavelength could be supported by the lithosphere [8,9]. At satellite heights short-wavelength anomalies are naturally filtered out. The free-air gravity field computed to the 16th degree spherical harmonics [10], using both satellite and surface observations, shows numerous perturbations in the range +37 to - 7 0 regals in amplitude and several thousand kilometer wavelengths. It can be shown that the mass excesses or deficits implied by these anomalies cannot be supported by the lithospheric plates [11]. At least a significant part of this residual gravity field must reflect density inhomogeneities in the mantle below the plates. Since the plates are incapable of maintaining the density anomalies as static loads, it seems reasonable that the anomalies are dynamic in origin, i.e., they are constantly maintained by motions of the plates and the underlying mantle [ 7 - 9 , 1 2 ] . The next question is whether mantle flow patterns at substantial depth,
466 and of no direct relation to the lithosphere, could be responsible for the long-wavelength gravity anomalies. As Kaula [8,9] has noted, most of the anomalies do correlate with surface features and, therefore, most probably reflect asthenosphere/lithosphere dynamics. Subduction zones are almost invariably represented by gravity highs, whereas deep ocean basins tend to be negative. Roughly equidimensional highs occur over shallow parts of the MOR, such as Iceland, the Azores, Amsterdam, St. Paul, Tristan da Cunha, and Prince Edward Island. (In some cases, for example Iceland and the Azores, the gravity highs are broad enough, compared to their separations, that they fuse together.) Numerical simulation shows that rising convective motion in Newtonian fluids is generally accompanied by topographic and gravity highs [12], at least if a free (fluid) surface is taken as the boundary condition. The sign and magnitude of the observed correlation between topographic and gravity anomalies is in reasonable accord with theory [13]. Vertical motions of Pacific atolls [14] as well as certain heat flow anomalies [15] can be explained by the hypothesis that some regional gravity/topographic highs represent stationary centers of relatively hotter, rising asthenosphere, while gravity/topographic lows represent the converse, i.e., sinking motion below the plates. An example of such a gravity/topographic low is located on the Southeast Indian Ocean Ridge south of Australia [ 16].
2. Gravity and intraplate seismieity Despite the promise of gravity anomalies as indicators of asthenosphere flow, many questions remain. The correlation between surface topography and gravity anomaly is far from perfect, even along the MOR [13]. For example, the gravity high over the Azores is the same value as that over Iceland despite the differences in elevation [7]. A gravity "saddle" corresponding to the pronounced topographic saddle between Iceland and the Azores (Fig. 2A, 2B) is poorly developed. Some recent gravity representations show the Azores and Iceland free-air highs fused into a single broader feature. Extensive gravity lows over Antarctica and north of the Himalayas remain unexplained [8]. The greatest departure from the norm is a low of - 7 0 mgal located south of the
Indian subcontinent (Fig. 3A, 3B); the surface topography gives no hint of this feature. Mantle flow below the asthenosphere and unrelated to the plates might account for some of these discrepancies. Mso, correlations between long-wavelength gravity and magnetic anomalies can only be explained if the density/ flow pattern extends to the base of the mantle [17]. Clearly other kinds of observations are needed to demonstrate that active asthenosphere flow is, in fact, presently occurring under the interiors of lithospheric plates wherever prominent gravity anomalies exist. The thesis of this paper is that intraplate seismicity, by correlating with prominent gravity anomalies in at least two areas (Figs. 1,3), may be an important register of asthenosphere dynamics. In general, this is not a new idea. The connection between intraplate seismicity and gravity is as follows: Runcorn [7] has argued that, with some assumptions, the velocity field of mantle flow could be derived from geoid (and hence gravity anomaly) perturbations. The flow field just below the lithosphere/asthenosphere boundary is then of greatest interest, for the viscous stresses on plate bottoms can be determined, provided viscosity is known. How the plates respond to these stresses depends, of course, on the properties of the plates themselves. In this paper I take the simple, empirical approach: Is there any observable correlation between intraplate seismicity and regional gravity anomalies? If there is, two significant conclusions could be drawn. (1) the gravity anomalies would reflect active asthenosphere flow that extends upward to the base of the lithosphere; and (2) viscous or other stresses generated by this flow must be sufficient to cause brittle failure and resulting seismicity in the overlying plates. Intraplate seismicity has been investigated by Sykes and Sbar in recent years [18-20]. Compared to what was discovered about interplate seismicity, relatively little is known about the phenomenon. There are several reasons for this [21]. First, the level of seismicity in plate interiors is generally much less than along plate boundaries. In a few years observation time the plate boundaries can be outlined by epicenter belts, and this is truely independent of the time period selected. For the relatively small number ofintraplate events accurately located in the last fifteen years, it is often not clear whether the observed "patterns" are real and repeatable or whether
467 they are chance artifacts of observation time. Another problem is that seismograph networks were designed to optimize sensitivity to plate-margin events, such earthquakes being more frequent and of greater concern to society [21]. If the earlier instrumental data (1901-1959) and even older historical information is considered, the picture of intraplate seismicity tends to be strongly biased by population density, while epicenter locations are too poor to resolve possible narrow intraplate fractures. If there is any correlation between intraplate seismicity and regional gravity anomalies, it would most likely appear in areas of (a) intense gravity anomalies and/or (b) good network sensitivity and high population density. If no correlation is observed even in such areas, either of the following could be true: (1) the observed seismicity is due to processes other than viscous stresses, i.e., these stresses are comparatively small or even non-existent; (2) the observation time is insufficient to delineate those earthquake patterns due to viscous stresses; or (3) intraplate seismicity due to viscous stresses consists predominantly of such low-magnitude events that the present seismograph network is inadequate for the purpose even if data are collected over long time periods. Two regions are considered in this paper: (1) the large 3 0 - 4 0 mgal free-air gravity high centered on and evidently associated with the Iceland hot spot (Fig. 1A, B), and (2) the Indian plate, which contains the world's greatest free-air excursions ( - 6 0 to - 7 0 mgal south of India and - 3 0 to - 4 0 mgl south of Australia (Fig. 3A, B)). The gravity contours in Figs. 1A and 3A are reproduced from Kaula [8] and are based on the " S E I I " solution of Gaposchkin and Lambeck [10]. The S E I I field was based on both satellite and surface data. More recent [22,23], presumably more accurate fields, may give somewhat different shapes and extreme values but the overall pattern is not changed unless the number of terms in the spherical harmonic expansion is much smaller. To illustrate this point we show the PGS-110 field, to 12th order and degree [23], Figs. 1B and 3B). This model also incorporates both surface and satellite data. The Iceland area was chosen for four reasons: (1) it is one of the best known and commonly cited examples of a mantle hot spot or plume, (2) the worldwide seismic network is relatively sensitive to
events occurring in this area, (3) the positive gravity anomaly (+30 to 40 mgal) is one of the highest in the world; and (4) the eastern flank of the Iceland gravity high overlaps densely populated western Europe. These factors notwithstanding, there were only a handful of "reliably located" events during the period 1963-1972 [24]. These are events of body wave magnitude >/4.5 and reported by ten or more stations. However, numerous additional events located with lesser reliability occurred in this region; the intraplate seismicity of particular areas has been discussed in several papers [21,25-34]. In order to display seismic activity together with topographic and gravity information, the data of Husebye et al. [25] and Keen et al. [32] were first smoothed. An arbitrary 200 km × 200 km grid was superposed on the intraplate epicenter pattern and the number of events per square contoured. Events within 50 km of the active plate boundary were not counted as "intraplate". Relatively seismic areas [18,21,33,34] in western Norway, England, the Baltic Sea, the MacKenzie River Valley of Canada, and south central Alaska are also shown in schematic fashion. In view of various uncertainties, the depiction of mid-plate seismicity in this vast Arctic region (Fig. 1A, B) is somewhat schematic and undoubtedly biased. Such bias is not thought to alter the thesis of this paper, however. Another feature of seismicity in the greater Iceland area is that the MOR itself is not uniform in seismic activity along its length [4,35,36]. In particular, ridge crest seismicity is exceptionally low within 900 km of Iceland; beyond, two stretches of relatively intense seismicity occur along the MohnsKnipovich and southern Reykjanes Ridges [4]. This is best illustrated by a longitudinal plot of MOR seismicity against plate rotation latitude (Fig. 2). The two zones of high interplate seismicity are also shown, schematically, in Fig. 1A and B. The analyses of Evernden [37] suggest that the variations of seismicity along this part of the MOR are in general not artifacts of variable network sensitivity. When all the seismic activity is shown together, a pattern emerges: The interior or crestal portion of the Iceland gravity high - roughly that region enclosed by the +20 mgal contour - is characterized by relatively low interplate and intraplate seismicity. Surrounding this lies a "halo" of intensified seis-
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Fig. 3. A. Model SE-II free-air gravity contours [8,10], plate boundaries, and reliably located intraplate epicenters in area of Indian lithospheric plate. Cross-hatched belts along plate boundaries approximate zones of interplate seismicity. Note general occurrence of intraplate events around and within deep gravity low centered near Ceylon. Many of remaining intraplate events also tend to lie along belts of high regional gradients in free-air gravity anomaly. Where detailed seismic reflection data are available (parallel-line patterns; [44] and unpublished data), the orientation of young deformational structures is subparalM to the isogals.
micity about 1500 km wide. The seismic halo extends 1000 km further west than east o f Iceland, but its relationship to the regional gravity anomaly is quite distinct: The belt of higher seismicity tends to follow the flanks of the gravity high, i.e., it is essentially confined to the region of maximum gradient. Some seismicity occurs westwards to the McKenzie Delta, southwest towards the Gulf of St. Lawrence, and along the Rhine Graben. In those areas the outer boundary of the seismic "halo" is indistinct and somewhat arbitrary. Nevertheless, a correlation between gravity and seismicity appears too distinctive to be entirely coincidental. One possible explanation is that active outward flow of asthenosphere away from the Iceland mantle plume is transmitting
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Fig. 3 (continued). B. Epicenters and plate boundaries as in Fig. 3A, but with more recent PGS-110-derived field [23], as in Fig. lB. Note basic similarity to earlier gravity pattern (Fig. 3A). Vertical ruling shows zone of postulated enhanced intraplate seismicity associated with gravity low. Question marks show possible plate boundary events. shear stresses to the lithosphere several thousand kilometers from Iceland. Since the gravity negatives over Hudson's Bay and eastern Scandinavia partly reflect recent deglaciation [8], some of the gravity gradients and associated asthenosphere flow may be transient phenomena, unconnected with the Iceland plume. But if the seismic halo crosses oceanic, glaciated, and deglaciated areas alike, the correlation between gravity and seismicity is unlikely to be just an effect of post-glacial adjustment. However, it is true that both gravity gradients and the mid-plate seismicity halo are best developed along zones marginal to Pleistocene ice sheets. Furthermore, there is some concentration of seismicity along the northern flank of a large free-air low centered over Hudson Bay, i.e., approximately along the line of inflection in the contours of the rate of post-glacial uplift [33]. Seismicity elsewhere in the Canadian Arctic as well as in the seismic belt from
472 Svalbard to the North Sea (Fig. 1A, B) does not correlate systematically with local gravity anomalies or with uplift contours. Why should seismicity be greatest around the margins of the gravity high? In general, the flow should cross from high to lower gravity anomaly as observed. However, a large area of reduced seismicity is not what would be expected from a narrow plume. After all, radial outflow from such a plume under Iceland would cause maximum stresses in the immediate Iceland area. Assuming the plume model is valid, we can nevertheless offer several possible explanations: (1) The area of upwelling is rather broad and the stagnation zone of reduced horizontal flow extends more than 1000 km away from Iceland. This possibility certainly does not agree with Morgan's [1] concept of thin plumes. Furthermore, overcoring experiments by Hast [38] indicate that maximum compressive stresses are nearly horizontal and oriented radially about the center of Iceland, as would be expected from tile narrow plume model [20]. (2) Asthenosphere viscosity (r/) is reduced in the central aseismic region, due to higher than normal temperatures and degree of partial melting. Hence, even though flow speeds are high, the viscous traction r~ • 3u/3z, where u is the horizontal component of flow, does not reach maximum until 1000-3000 km from the center of upwelling. (3) The plate, relatively hot and thin, is more likely to respond by plastic deformation near the plume center. (4) The margins of the gravity high represent a broad "binge" belt separating asthenosphere invaded (and thickened) by plume discharge from normal asthenosphere. Such a hinge belt would exhibit the greatest differential vertical motion and the maximum stress gradient caused by vertical motion (horizontal stress gradients caused by viscous traction could peak somewhere else). Of possible significance is that two of the three areas of high seismicity along coastal Norway [27,30] lie just west of the highest mountains. Evidently these local, epeirogenic uplifts are not merely relicts of initial rifting, but are still being formed or modified by whatever process that develops stresses along the coasts. It is inviting to think of outflow from the Iceland plume reaching the continental margin of Norway and the Barents Sea,
and more or less arrested there by the thick, Precambrian to Paleozoic lithosphere. The relative importance of the four factors listed above is unknown; an outward increase in asthenosphere viscosity, plus marginal "hinge line" effects, appear most reasonable. The existence of a broad heat-flow high over the regional Iceland gravity high would support this line of/easoning; unfortunately the existing oceanic measurements, initially interpreted as a regional heat-flow high [39], are now considered within statistical uncertainty of "normal" (G. Zielinski,-unpublished manuscript, 1977). There are few data from the continental parts of the gravity high; furthermore a warming of the sublithospheric mantle ~55 m.y.B.P, would probably have not yet been "felt" in the upper crust. Two other observations pertain to Fig. 1. The Faeroe-Iceland-Greenland and Davis Straits aseismic ridges are virtually devoid of intraplate epicenters, as if the lithosphere under these ridges either does not deform by brittle fracture or else is too thick and too strong. As with most interpretations of intraplate seismicity, this conclusion may be premature. The same applies to the high seismicity of Mohns and southern Reykjanes Ridges [4]. Fault-plane solutions are now needed for these spreading axes to test the hypothesis that the high ridge crest seismicity is caused by a complex stress regime, including tension normal to the ridges as well as compression along their strike. The second area considered here is the Indian plate (Fig. 2). Among the reasons for choosing this area is its equatorial location, which would minimize the deglaciation effects perhaps important in the Iceland case. Only reliable epicenters (1963-1972) and one magnitude-8.l event (1928) are shown in Fig. 2 [24] ; historical data would probably skew the pattern toward India, Indonesia, and the few populated areas of Australia. Again we see some correlation between regional gravity anomalies [8] - in this case largely negative and intraplate seismicity. Compared to others, the Indian plate exhibits both the greatest gravity relief (+37 to - 7 0 regal in the Se-II model, and similar values for subsequent ones) and the highest level of intraplate seismicity. This coincidence can scarcely be fortuitous, and most probably cannot be explained by variable seismic network sensitivity. The intraplate events lie in a loose
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:
GRAVITY (MGALS) 2000
Fig. 4. Schematic illustrating hypothesis of this paper: Intraplate seismicity is enhanced in a halo-like zone of maximum asthenosphere flow (horizontal component). If MOR axis (accretion, spreading) crosses gravity high, ridge axis seismicity is decreased inside halo because upwelling elevates isotherms and thins ridge crest lithosphere. Some asthenosphere flow and intraplate scismicity is assumed to occur outside the stippled zones as well but a correlation may be obscured there by causes of interplate seismicity other than convection in the asthenosphere [ 1 8 - 2 0 , 4 2 ] , or other types of flow, e.g. the postulated longitudinal "rolls" normal to the MOR [45]. Normal gravity high associated with MOR has been ignored.
halo around the - 7 0 mgal gravity low south of India; many of the others tie along zones of maximum gravity gradient that loop through southwestern Australia and along the northeastern flanks of the MOR. Gravity, extrema, saddles, and plains tend to be devoid of intraplate activity. An exception to this may be the belt of historical events that follows the belt of Tertiary volcanism in southeastern Australia
[211. If seismicity southeast of India does relate to the deep gravity low there, this would be the only known correlation between that anomaly and a "surface" feature. Unlike the Iceland high, the south Indian low is not associated with any known topographic or tectonic feature and its source might otherwise be supposed to underlie the asthenosphere. Also in contrast to the Iceland area, any correlation between gravity and seismicity in the Indian plate cannot be ascribed to deglaciation. Sykes [40] suggested that the high seismicity might reflect a nascent island
arc forming as the northward motion of the Indian plate is arrested by collision with Asia. This would not explain the intraplate events northwest and southeast of the gravity low (Fig. 2); however, it is still possible that intensive asthenosphere flow towards the gravity low is capable of developing a new plate boundary, much as the Iceland plume is often visualized as breaking Greenland from Eurasia about 60 m.y.B.P. Thus, the relatively intense intraplate seismicity around the southern margin of the gravity low could reflect nascent break-up of the plate. Fitch et al. [41] have also mentioned a possible connection between gravity anomalies and seismicity within the Indian plate. Although horizontal compression related to resistance to subduction is given as a major reason for the seismicity, these authors also acknowledge the possible role of mantle flow set up in response to the development of the Ganges cone, or for unknown other reasons. Seismic reflection surveys could be used to map geologically young deformation associated with earthquake activity in the Indian plate. In two limited areas where the strikes of young folds and/or faults can be determined, the structures are subparallel to the isogals (Fig. 2). In conclusion, we see some indication that the asthenosphere flow predicted on the basis of regional gravity anomalies does exist and that the viscous stresses associated with this flow may be a significant factor in intraplate seismicity (Fig. 4). If this is true, it may be that improved seismic network coverage and longer observation times -- together with seismic reflection surveys to chart young deformation - will eventually reveal similar relationships in other intraplate areas. However, it is clear already that at least some intraplate seismicity is unrelated to regional gravity patterns; for example, the eastern United States and southeastern Canada are areas of locally significant activity [21 ] but little or no gravity gradient [8]. Also, the patterns of principal stresses deduced from focal mechanism solutions and in-situ measurements do not correspond to those anticipated if radial asthenosphere flow away from Morgan's mantle plumes [1 ] are the sole driving forces behind plate tectonics [18-20]. In the Iceland area (Fig. 1), overcoring experiments do show compression oriented radially about the center of the island [38!.
474 But of the ten focal mechanism solutions in the "seismic halo" (all west of the MOR) five had normal faulting, one had strike-slip, and the remainder thrust faulting as the principal mechanism. Of sixteen solutions in the Indian plate (Fig. 3), seven were thrusting, six strike-slip, and the remaining three - all near the MOR - were normal mechanisms [20]. The Indian Ocean thus differs from the Iceland area in the occurrence there of strike-slip events [20] ; another difference is the relatively high percentage of large earthquakes in the northwest India plate. This may indicate comparatively high stresses [40], perhaps because of exceptionally strong asthenosphere flow and a relatively old, thick plate, further thickened by its location over a site of mantle downwelling, perhaps like those postulated in the eastern Pacific [15] and south of Australia [16]. Evidently the stress fields in both areas are quite complex and locally variable. Local factors such as lines of preexisting weakness probably control the patterns of seismic activity on a more detailed scale. For example, the seismicity in the eastern Canadian Arctic (Fig. 1) seems to be locally influenced [33] by (a) sediment loading on the Arctic continental margin, (b) deformational structures reactivated in Paleozoic and later orogenic phases, or (c) hinge lines, reflected by gravity gradients, where differential postglacial uplift is maximal. Another type of local structure of possible importance to explain the details of mid-plate seismicity and fault plane solutions has been analyzed by Kane and his colleagues [42]. Analyzing certain areas of mid-plate seismicity in the continental United States, these workers have suggested that marie intrusions in the crust act as stress concentrators. The details of failure then depend on the relative strength of the plutons and country rock. Clearly a number of local factors are likely to be important in controlling the details of epicenter distribution within the postulated "seismic halo". To the extent that these factors are important, the finer details of seismicity (Fig. 1 , 2 ; [26,33]) are likely to repeat themselves during a future, independent observation period.
Acknowledgements I thank D. O'Neill, C. Grimstad, and C. Fruik for
assistance. This work was partially sponsored by the Office of Naval Research.
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