(First published in New Concepts in Global Tectonics Newsletter,
no. 21, p. 10-24, December 2001)
Space-geodetic techniques, such as satellite laser ranging (SLR), very long baseline interferometry (VLBI), and the Global Positioning System (GPS), provide valuable data on the relative motion between sites on the earth’s surface up to 12,000 km apart. Measurements to date are said to be in generally good agreement with the motions predicted by plate tectonics, and are widely regarded as having confirmed seafloor spreading, subduction, and continental drift.
Paul Lowman (1995, 2001) denies that space geodesy has proved the movement of large continents, such as the Americas and Eurasia. He states that although transatlantic SLR and VLBI baselines show increases close to those predicted by plate-tectonic theory, there are not enough intracontinental baselines to demonstrate plate rigidity; in fact some baselines within North America and western Europe show significant deformation. Lowman contends that the very deep roots of continental cratons, which seismotomography has shown to extend to depths of 400 to 600 km, make it impossible for very large continents to drift. He discounts palaeomagnetic data that seem to support drift on the grounds that they are unreliable. He nevertheless contends that seafloor spreading is taking place in the Atlantic, but argues that subduction is occurring under passive Atlantic margins – though he appears to be virtually alone in this view.
Lowman claims that space-geodesy methods have confirmed plate movements in and around the Pacific: islands on the Pacific plate are moving towards Japan at several centimetres per year; the plate is internally rigid; and movement of Australia also appears to have been demonstrated, though with much less certainty. Lowman believes that this confirms seafloor spreading and subduction in the Pacific. However, these conclusions are open to question.
Space geodesy provides a useful guide to crustal stresses and strains. But a far more extensive network of sites would be required to determine to what extent the movements detected are local, regional, continent-wide, ocean-wide, or “plate”-wide. The number and distribution of SLR and VLBI sites worldwide are limited, owing to the high cost and nonportability of the systems; most are located on the North American, Pacific, and Eurasian plates. The GPS network has a better global distribution but coverage is still rather sparse. Another limitation is that space-geodetic surveys using benchmarks on the seafloor are far more difficult and expensive than such surveys on the continents. An additional problem is that measurements of vertical movements are significantly less accurate than measurements of horizontal movements.
Average plate angular velocities deduced from VLBI and SLR are said to be about 6% slower than those determined by the NUVEL-1 plate-motion model, which is based on the magnetic-anomaly timescale for the past 3 million years. A revision of this timescale in the early 1990s led to the development of NUVEL-1A, in which average angular velocities are 4.4% slower than in NUVEL-1. This reduces the average discrepancy to under 2%, though some workers find that VLBI data are in better agreement with NUVEL-1 than with NUVEL-1A (DeMets et al., 1994). However, these percentages, based on averaged, “best-fit” angular velocities, are misleading and conceal major anomalies; there are “significant differences between the models and the measurements at most plate boundaries, and in some cases at considerable distances from the boundary” (Smith and Baltuck, 1993, p. 1).
The main discrepancies arising from VLBI data are shown in Figure 1. SLR data for the Pacific are shown in Figure 2. It can be seen that the NUVEL-1 velocity for the relative motion between Arequipa and Easter Island is 61% higher than the measured velocity. Further notable discrepancies are shown in Figure 3.
Figure 1. Differences between VLBI velocities and NUVEL-1A plate-model velocities. Velocity residuals less than 2 mm/yr are not displayed. (Goddard Space Flight Center, NASA, http://lupus.gsfc.nasa.gov/global/velocity.html)
Figure 2. Selected SLR spherical rates for lines crossing the Pacific Basin. NUVEL-1 model rates are shown in brackets for lines crossing at least one plate boundary. All rates are in mm/yr. (Smith et al., 1994, fig. 4)
Figure 3. Selected SLR rates across the Atlantic and Pacific. The rates predicted by the AM0-2 and NUVEL NNR-1 models are given in brackets. All rates are in mm/yr. (Murata, 1993, fig. 8)
Plate rigidity is a central tenet of plate tectonics. However, it is
recognized to be only an approximation: intraplate deformation is demonstrated by earthquakes in
stable plate interiors, subsidence in midcontinental basins, and uplift of the surface over hotspot
swells in the oceans. Deformation in the stable interior of the North American plate is said to be no
more than a few millimetres per year, but is greater in the Basin and Range province and other
regions in the west of the US, which are regarded as part of the plate boundary zone. It is
acknowledged that “the modelled assumption of rigid plates frequently fails not only at plate
boundaries but at considerable distances from boundaries” (Smith and Baltuck, 1993, p. 2). For
instance, predicted motion in the mid-Pacific is in error by 6 to 8 mm/yr (Ryan et al., 1993).
Whenever discrepancies are found between measured and predicted motions, explanations are sought within the context of plate tectonics. For instance, the NUVEL-1 Africa-North America pole of rotation is said to lie “surprisingly far” (14°) from that of the best-fitting angular velocity; it is speculated that this misfit may be due to “systematic errors or significant plate nonrigidity” (Gordon, 1995).
On the basis of VLBI/GPS data for three sites in the Caribbean, the angular velocity of the relative motion between the Caribbean and North American plates was computed to have a rate of 0.23 ± 0.08°/Myr, twice the NUVEL-1A rate of 0.11 ± 0.03°/Myr; the direction was 62.2°N and -93.3°E, as against 74.3°N and 153.9°E. Furthermore, the estimated geodetic angular velocity failed to explain all the observed motion at the three sites. Three possible explanations were given: (1) the three sites are not all attached to a common rigid plate, (2) there are systematic measurement errors, or (3) the measurement uncertainties have been underestimated (MacMillan and Ma, 1999).
The pole of rotation derived from GPS data for the Pacific plate was found to lie 11.5° west of the NUVEL-1A pole, with an angular speed 10% faster. The suggested explanation was that the motion of the Pacific plate over the last 5 years did not agree with its motion over the last 3 Myr. The GPS velocity of Baltra Island on the Nazca plate is almost 50% slower (20±5 mm/yr) than the predicted value. The investigators stated that they could not account for the entire discrepancy, but thought that the plate might be deforming internally (Larson et al., 1997). Angermann et al. (1999) found that GPS velocities of four Nazca plate sites, relative to the South American plate, are about 20% slower than the NUVEL-1A plate model velocities and earlier geodetic measurements. Convergence rates for the two plates from several studies vary considerably.
The observed motion of Arequipa in the western Andes relative to North America is 13±1.5 mm/yr with an azimuth of 55°. According to model predictions, however, it should be moving at 10 mm/yr with an azimuth at 293° (Robaudo and Harrison, 1993). This major difference in the direction of movement is said to be due to a portion of the subduction motion being transferred into a portion of the overriding plate; some 25% of Nazca-South America plate motion is allegedly taken up by seismic and shortening mechanisms in the Andes (MacMillan and Ma, 1999). Similar discrepancies have been found in other backarc regions and interpreted in the same way.
The reigning plate-tectonic paradigm has biased the interpretation of space-geodetic data with its assumption that entire “plates” must be moving as more or less rigid units. The literature is riddled with anomalies, inconsistencies, and ad hoc explanations. Motion is certainly occurring in seismotectonic zones but, as Oard (2000a) remarks, “It is possible that in some areas the motion is in the opposite direction from that inferred by PT, and may be caused by vertical tectonics, instead of by underthrusting of one plate below another” (p. 43). He cites various pieces of geodetic evidence: the Tonga arc is moving eastward relative to the Pacific plate, which can be considered stationary (Bevis et al., 1995); the 1994 Shikotan earthquake caused GPS benchmarks on eastern Hokkaido, Japan, to move eastward up to 42 cm and subside up to 60 cm (Kikuchi and Kanamori, 1995); the magnitude 8.0 Antofagasta earthquake of 1995 moved the coast of Chile almost one metre westward relative to the Pacific plate (Klotz et al., 1999); and the Timor trough (eastern Java trench) appears to be inactive while the southern Banda arc is undergoing north-south extension (Genrich et al., 1996).
A brief review of some of the other problems facing plate tectonics provides further grounds for questioning PT interpretations of space-geodetic data.
Many geology textbooks contain colourful pictures of uniformly thin (~150 km) plates moving over a continuous, global asthenosphere. Such pictures are far removed from reality. After reviewing evidence for 400-km-thick roots beneath stable cratons, Lerner-Lam (1988) concluded: “Evidently, the earth has flunked the seismological test of the thin-plate theory” (p. 51-53). He might equally well have said that plate tectonics has flunked the seismological test.
Geophysical data show that, far from the asthenosphere being a continuous layer, it is made up of disconnected lenses, which are observed only in regions of tectonic activation and high heat flow. The asthenosphere is essentially absent beneath ancient continental nuclei. Although averaged surface-wave observations suggested that the asthenosphere was universally present beneath the oceans, detailed seismic studies indicate that here, too, there are only asthenospheric lenses. Several low-velocity zones occur in the oceanic mantle, but it is difficult to establish any regularity between the depth of the zones and their distance from the midocean ridge. The very concept of a “lithospheric plate” is therefore ambiguous (Pavlenkova, 1990, 1996).
This means that labelling major seismic and volcanic belts and deformation zones “plate boundaries” is also questionable. Moreover, earthquake activity has not always been confined to the same places, and 500 years ago some plate boundaries would probably have been defined differently. For instance, some of the present plate boundaries in Asia Minor appear to have been quiescent centuries ago, while earthquake activity ran right through the middle of what is today’s plate (James, 1994).
Originally, about ten “plates” were recognized, but they now number over 100, due to the addition of numerous microplates (including exotic terranes) to accommodate discrepant palaeopole positions or other anomalous data. However, the boundaries of the main plates are sometimes ill defined or nonexistent (Oard, 2000a). For instance, there is no consensus on the location of the northwest Pacific boundary of the Pacific, North American, and Eurasian plates. The boundary between the North and South American plates from the Caribbean plate east to the Mid-Atlantic Ridge is ill defined. There are problems locating the southern boundary between the Caribbean plate and the South American plate, as well as the triple junction with the Nazca plate. The boundary between the South American and Antarctic plates, including the Scotian plate is problematic. The South Atlantic Ridge and Antarctic Ridge do not connect to the Mid-Indian Ridge. There is no separation between the Philippine and Pacific plates at the southeast edge of the Philippine plate. The boundary between the Pacific plate and the Australian plate is ill defined between the Tonga and New Hebrides trenches. There is no boundary for 20 km between the Cocos and Nazca plates, just east of the East Pacific Rise.
The theory that the earth’s surface is divided into separate lithospheric “plates” is clearly artificial. It is further undermined by the growing need to invoke “diffuse plate boundaries”, especially where a distinct boundary is missing. Whereas in initial plate-tectonic theory, “plate boundaries” were considered to be fairly narrow, it is now believed that their width can range from a few hundred metres to thousands of kilometres. Gordon and Stein (1992) state that plate tectonics “does not usefully describe the kinematics of many active deformation zones”, and argue that these zones are better described as wide plate boundary zones than as intraplate deformation zones. Diffuse plate boundaries are said to cover about 15% of the earth’s surface area, though some of them are said to be speculative (Figure 4). It is acknowledged that “there may be no sharp contrast, but a gradation, in behavior between intraplate deformation and diffuse plate boundaries” (Gordon, 1995, p. 24373).
Figure 4. Map showing idealized narrow plate boundaries, supposed velocities between plates, and regions of deforming lithosphere (stippled), which are regarded as “diffuse plate boundaries”. Plate velocities are shown by arrows; their length indicates the displacement expected in a period of 25 million years. (Gordon, 1995, fig. 1)
Lowman believes that Australia is able to drift because it is a
relatively small continent carried on a mainly oceanic plate. However, the thickest
seismically-defined cratons are the Western Australian shield, the Canadian shield, and the
Baltic-Ukrainian shield (Anderson et al., 1992). In addition to the problem of identifying the forces
that are supposed to move the Australian plate together with Australia’s continental root, the fact that some
Precambrian lineaments in Australia extend into the surrounding seafloor and can be traced across
plate boundaries casts severe doubt on Australia’s supposed drift (Choi, 2001). An orthogonal
fracture pattern is well developed throughout the Pacific, and connects with major Precambrian
structural trends in continents, and this rules out any large-scale plate movements (Smoot, 2001).
Lowman accepts the orthodox view that oceanic islands and seamounts in the Pacific are the result of the Pacific plate moving over hotspots. This should give rise to a systematic age progression along hotspot trails, but good age progressions are very rare, and a large majority show little or no age progression. The Cook-Austral and Marquesas chains, for example, are marked by gross violations of a simple age-distance relationship and by extreme variations of isotopic signature, inconsistent with a single volcanic source. The Hawaiian-Emperor chain provides a more consistent age sequence, but there is no systematic variation of heat flow across the Hawaiian swell, contradicting the simple hotspot model (Keith, 1993).
Hotspots are commonly attributed to “mantle plumes” rising from the core-mantle boundary. Sheth (1999) showed that plume explanations are ad hoc, artificial, and inadequate, and that plumes are not required by any geological evidence. A mantle plume from a deep hotspot would broaden upward as a result of drag forces, and would attain a surface width of several hundred kilometres, far beyond oceanic island dimensions. It is therefore claimed that hotspot tracks are produced by plume tails – but the problem of what has happened to ancient and modern plume heads remains unsolved.
It is significant that many ocean island chains are found along fracture zones, and flood basalt provinces are at orthogonal intersections of the fracture zones (Smoot, 1997). A credible alternative explanation is that hotspot tracks are produced by propagating rifts, and delineate the stress field, not the displacement field, of the lithosphere (Sheth, 1999). In surge tectonics, linear volcanic chains are believed to be produced by magma surge channels in the lithosphere (Meyerhoff et al., 1996).
There is a major controversy among plate tectonicists as to how fast hotspots move relative to one another; one group believes that hotspots move at 3 mm/yr or less, whereas the other believes that hotspots move at 10-20 mm/yr or more. These differences are partly the result of “plate reconstruction” uncertainties (Gordon, 1995; Baksi, 1999).
Geology textbooks generally claim that seafloor spreading is proven by the fact that no rocks older than 200 million years (Jurassic) have ever been found in the oceans. Those who make such statements must be either ignorant or dishonest. Literally thousands of rocks of Paleozoic and Precambrian ages have been found in the world’s oceans (see Pratt, 2000; Sánchez Cela, 2000). For instance, the ultramafic rocks forming the St. Peter and Paul islands near the crest of the Mid-Atlantic Ridge gave ages of 350, 450, 835 and 2000 Myr, compared with a seafloor-spreading age of 35 Myr.
Attempts are occasionally made to explain such anomalies away on an ad hoc basis, e.g. as glacial erratics or as “nonspreading blocks” left behind during rifting. Lowman thinks that the idea of spreading axes and transform faults skipping from place to place is plausible, while others consider it mechanically questionable and contrived. Some ancient rocks in the oceans may have been dropped by icebergs, but this cannot account for large areas of continental crust such as Bald Mountain at 45°N on the Mid-Atlantic Ridge, which has an estimated volume of 80³ km and appears to be of Proterozoic age. There is growing evidence that there used to be even larger (now submerged) continental landmasses in the present oceans (Dickins et al., 1992; Choi et al., 1992; Choi, 1999, 2001).
A major effort should be made to drill the ocean floor to much greater depths to see whether there are more ancient sediments beneath the basalt layer that is currently – and conveniently – labelled “basement”. That older sediments may well be found is shown by the fact that some basalts had baked contacts with the overlying sediments, had chilled margins, alternated with sediments, or showed other characteristics indicative of intrusives (dykes and sills), or extrusives on the seafloor (e.g. pillow structure) (Meyerhoff et al., 1992; Choi, 2001). The basalts appear to be magma floods which cover the real “oceanic” basement underneath. This was clearly shown at drill site 10 on the Mid-Atlantic Ridge, where the lowermost sediments are Cretaceous (about 80 Myr) and the underlying basaltic sill, erroneously termed “basement”, had a fission-track age of only 15.9 Myr (Macdougall, 1971).
According to the seafloor-spreading hypothesis, basaltic magma intrudes into tensional cracks within the rift valley on the ridge crest and cools to form dikes. Extension produced by the moving seafloor then cracks a dike neatly into two halves which are carried away in opposite directions down the flanks of the ridge. It is not clear how a broad mantle plume can yield a narrow axial accretion zone, less than 5 km wide in some models, and be subject to “knife-edge” offset at major transform faults (Keith, 1993).
Axial igneous activity ought to be proportional to spreading rate, but that does not seem to be the case. The ridge near Iceland, one of the most active volcanic areas in the world, has a calculated spreading rate of only 1 cm per year, whereas volcanically inactive ridges – such as the East Pacific Rise and the nearly aseismic ridge south of Australia – are assigned spreading rates of 4.6 cm to 10 cm per year on the basis of magnetic stripes (Keith, 1972).
Side-scanning radar images show that ocean ridges are cut by thousands of long, linear, ridge-parallel fissures, fractures, and faults, extending along strike for thousands of kilometres. This strongly suggests that they are underlain at shallow depth by interconnected magma channels, in which semi-fluid magma moves horizontally and parallel with the ridges. The fault pattern observed is therefore inconsistent with the plate-tectonic assumption of ridge-orthogonal flow, and has largely been ignored.
Plate tectonicists who recognize the existence of ridge-parallel flow generally argue that a mantle diapir wells up beneath each ocean ridge segment, and that at the crest of each diapir, radial horizontal flow takes place, with a significant component parallel to the strike of the ridge and in opposite directions. However, this should produce a pattern of radial fissures, fractures, and faults near the centre of each diapir, whereas no radial or even annular pattern is observed on any ridge segment for which sonographs are available. Also, pressure ridges and similar compressive structures should be visible in the depressions between adjacent ridge segments, but they are not (Meyerhoff et al., 1992).
The spatial distribution of shallow-water sediments in the present oceans and their vertical arrangement in some of the drilled sections are not consistent with seafloor spreading (Ruditch, 1990). Younger shallow-water sediments are often located farther from the axial zones of the ridges than older ones, and some areas of the oceans appear to have undergone alternating subsidence and elevation.
Ocean ridges are supposed to be dominated by tension, but earthquake data show that they are characterized by widespread compression (Zoback et al., 1989). A zone of thrust faults, 300-400 km wide, has been discovered flanking the Mid-Atlantic Ridge over a length of 1000 km, produced under conditions of compression (Antipov et al., 1990). In Iceland, the largest landmass astride the Mid-Atlantic Ridge, the predominant stresses in the axial zone are compressive rather than extensional. Furthermore, the rift valley floor in Iceland is sinking and the bordering plateau is also sinking, at rates that increase toward the axis, and this is difficult to reconcile with the PT hypothesis of axial upwelling, single-stage accretion, and spreading (Keith, 1993).
Geodetic surveys across “rift” zones in Iceland and East Africa have failed to detect any consistent and systematic widening as postulated by plate tectonics. After finding no tensile motions associated with the East African rift, the investigators felt forced to postulate that plate motions there must be episodic (Asfaw et al., 1992). Earthquake data show that the African and Middle East rift systems and the Rhine Graben are associated with compressional activity (Zoback et al., 1989). Measurements across the San Andreas “transform and spreading” system show widely variable shear motion but no spreading (Keith, 1993).
Lowman believes that “ridge push” transmits compressive forces over vast distances. This is questionable because, aside from the dubious status of seafloor spreading, evidence for the long-term weakness of large rock masses casts doubt on the idea that edge forces can be transmitted from one margin of a “plate” to its interior or opposite margin (Keith, 1993).
Seafloor spreading, in combination with global magnetic reversals, is supposed to produce the alternating bands of slightly higher and lower magnetic intensity on either side of ocean ridges. However, linear magnetic anomalies are known from only 70% of the seismically active midocean ridges, and the diagrams of symmetrical, parallel, linear bands of anomalies displayed in many plate-tectonics publications bear little resemblance to reality (Figure 5). The anomalies are symmetrical to the ridge axis in less than 50% of the ridge system where they are present, and in about 21% of it they are oblique to the trend of the ridge. Linear anomalies are sometimes present where a ridge system is completely absent, and not all the charted anomalies are formed of oceanic crustal materials. A complicating feature is that each magnetic lineation consists in detail of numerous, narrow, high-amplitude anomalies.
Figure 5. Two views of marine magnetic anomalies. Top: a textbook cartoon. Bottom: magnetic anomaly patterns in the North Atlantic (Meyerhoff and Meyerhoff, 1972, fig. 5).
Correlations between magnetic stripes on either side of a ridge or in
different parts of the ocean have been largely qualitative and subjective, and are therefore highly
suspect. The data are subject to significant manipulation and smoothing, and virtually no effort has
been made to test correlations quantitatively by transforming them to the pole (i.e. recalculating each
magnetic profile to a common latitude). The magnetic anomalies of the Reykjanes Ridge are
supposed to be a classic example of ridge-parallel symmetry, but Agocs et al. (1992) concluded from
a detailed, quantitative study that the correlations were very poor; the correlation coefficient along
strike averaged 0.31 and that across the ridge 0.17, with limits of +1 to -1. Correlations between the
anomalies and bottom topography, on the other hand, averaged 0.42.
The distance between bands of magnetic anomalies is not proportional in most cases to the length of the geomagnetic epochs. The lengths of the Brunhes, Matuyama, and Gauss epochs have the ratio 1.0 : 2.4 : 1.6. However, on the Reykjanes Ridge, the distances between the anomalies closest to its axis have the ratio 1.0 : 0.5 : 0.4. An equally great breach of proportionality is observed on the East Pacific Rise, which can be explained from the angle of plate tectonics only by invoking considerable changes in the spreading rate over the past 3 million years (Beloussov, 1980). Gordon and Stein (1992), on the other hand, maintain that “Space geodetic data show that plate velocities averaged over a few years are similar to velocities averaged over millions of years” (p. 338). Asymmetric seafloor spreading frequently has to be invoked as well, and is attributed to ridge migration or jumps resulting from interaction with mantle plumes (Müller et al., 1998).
A complex seafloor-spreading history has been proposed for the Pacific basins to explain the complicated patterns of magnetic anomalies observed there. In the western Pacific, for example, it has been suggested that the different anomaly patterns must have been generated by a system of five spreading centres joined at two triple points. The investigators stated that all correlations among the magnetic profiles were established “by eye”. In contrast, if the anomalies were produced by once-active surge channels, a coherent and internally consistent flow pattern emerges (Meyerhoff et al., 1996, fig. 4.4).
On the Mid-Atlantic Ridge, two magnetic anomalies which, according to the “fossilized geomagnetic timescale”, should be 8 million years old, are traceable onto Iceland, where they correspond to manifestations of Pleistocene and Holocene volcanism of a considerably younger age (Beloussov, 1980). Magnetic-anomaly bands strike into the continents in at least 15 places and “dive” beneath Proterozoic or younger rocks; they are also approximately concentric around Archaean continental shields. This suggests that they are the sites of ancient fractures, which partly formed during the Proterozoic and have been rejuvenated since (Meyerhoff and Meyerhoff, 1974). The stripe pattern is better explained by fault-related bands of rock of different magnetic susceptibilities (Agocs et al., 1992; Choi et al., 1992).
The initial, highly simplistic seafloor-spreading model for the origin of ocean magnetic anomalies has been disproven by ocean drilling (Hall and Robinson, 1979; Pratsch, 1986). First, the hypothesis that the anomalies are produced in the upper 500 metres of oceanic crust has had to be abandoned. Magnetic intensities, general polarization directions, and often the existence of different polarity zones at different depths suggest that the source of magnetic anomalies lies in deeper levels of ocean crust not yet drilled or dated. Second, the vertically alternating layers of opposing magnetic polarization directions disprove the theory that the oceanic crust was magnetized entirely as it spread laterally from the magmatic centre, and strongly indicate that oceanic crustal sequences represent longer geologic times than is now believed. This is consistent with the numerous rock age “anomalies” in the oceans. Given that the magnetic-stripe timescale is probably fictional, any correspondence between plate-motion estimates derived from it and space-geodetic data is likely to be either coincidental or the result of biased interpretation.
Interpreting Wadati-Benioff zones as “subduction zones” is fraught with difficulties; the main problems are summarized below (see Pratt, 2000; Oard, 2000b; and references therein).
The volume of crust generated at ocean ridges is supposed to be equalled by the volume subducted. But the ocean ridge system is allegedly producing new crust along a total length of 2 x 74,000 km, whereas there are about 43,500 km of trenches and 9000 km of “collision zones” – or a third of the amount of “spreading centres”.
How ocean crust can be thrust down into the denser mantle has never been satisfactorily explained. An analysis of the mechanics of subduction suggests that it could probably never have started, let alone continued (James, 2000).
Ocean trenches were initially expected to contain thick, deformed sediment accumulated during millions of years of convergence. Instead, 44% of trenches are empty of sediments. The rest do contain “accretionary wedges” along the landward slope, but smaller than expected. Accretionary wedges were expected to grow and uplift with time, but it is now known that some have subsided several kilometres. Moreover, the sediment in them is usually horizontally layered and undisturbed, and is mainly derived from the land rather than being offscraped oceanic sediment. Plate tectonicists have had to resort to the far-fetched notion that soft ocean sediment can slide smoothly into a subduction zone without leaving any significant trace. The finding of old sedimentary rocks along the inner trench slope has led to the belief that younger rocks from the continental margin must have been eroded and subducted as well. Offscraped seamounts and seamount fragments should also be piled up in ocean trenches, but they are not.
The original prediction that subduction zones would show abundant and obvious compressional features has also proven false. Extension is ubiquitous on the oceanward trench slope and in backarc basins, and has unexpectedly been found on island arcs. The trench itself has the cross-sectional appearance of a graben. Extension even predominates on the middle and upper parts of the landward or arcward slope. Compression is now relegated to the lower trench slope, but this area is better interpreted as the toe of a large slump or debris flow.
Choi (2000) argues that plate-tectonic interpretations of seismic profiles across Pacific trenches lack geological integrity and are clearly model driven. Several profiles appear to show that the Precambrian lower crust is present under both the ocean floor and continental slope and passes across the trench without any subduction. Landward-prograding sediments together with geophysical and dredging data indicate that continental landmasses once existed in the present Pacific where there are now deep abyssal plains and trenches.
Depictions of subduction zones in PT textbooks are highly stylised. Wadati-Benioff zones actually have a highly variable and complex structure, with transverse as well as vertical discontinuities and segmentation. They frequently consist of two separate sections: the upper segment tends to have a shallower dip than the lower segment, and the two sections may be offset by up to 350 km. Deep earthquakes are disconnected from shallow ones and there are very few intermediate earthquakes (Figure 6). The very low level of seismicity within about 50 km of the trench axis, and the lack of a large thrust fault at the base of the continental slope contradict the alleged presence of a downgoing slab.
Figure 6. Earthquake distribution perpendicular to the Andes (15-30°S)
(Teisseyre et al.,
1974,
fig. 8). The outlined “subducting slab” appears to be based largely on wishful
thinking.
Lowman maintains that other earthquake data support subduction. It
is true that most deep earthquakes display first-motion compressional forces that are vertical dip slip.
But contrary to PT predictions, intermediate-depth earthquakes have a first-motion force that is
highly variable but generally downdip tensional. The variability in focal mechanisms can be
extreme even over a short distance in the same Wadati-Benioff zone, and this is difficult to explain if
seismicity is driven by a uniform stress field.
Most large earthquake foci are believed to occur at the plate interface in a Wadati-Benioff zone, but deep and intermediate quakes occasionally occur well outside this zone. For instance, four strong deep-focus earthquakes have occurred 75 to 200 km west of the Kurile-Kamchatka subduction zone, and were attributed to a detached piece of subducted lithosphere that is still deforming. This was seen as evidence that the slab does not continue below 700 km, though this is the very subduction zone in which seismic tomography first gave rise to the theory that slabs penetrate into the lower mantle. It is puzzling that many earthquakes occur within the “subducting slab” rather than along the plate interface, where the stress is supposed to be highest. Another disconcerting development for PT is that the slip motion of earthquakes rarely occurs in the direction of the dip of the Wadati-Benioff zone; it is commonly oblique to the “subduction” plane (Oard, 2000b; Suzuki, 2001b). The magnitude 8.3, deep-focus earthquake in Bolivia in 1994 appeared to slip on a horizontal plane cutting across and through the supposed steeply dipping subduction slab.
Instead of oceanic lithosphere being underthrust beneath the island arcs and backarc basins at Wadati-Benioff zones, an alternative view is that the continental cores of the mountain belts and island arc-trench system are being overthrust toward the foreland or the ocean basins by vertical-tectonic processes (e.g. Krebs, 1975; Wezel, 1986; Dickins and Choi, 2000; Oard, 2000b; Suzuki, 2001a). The Wadati-Benioff zone would represent the deformation interface between the uplifting island arc/continental region and the subsiding ocean crust and mantle. This would explain why earthquake motions are more chaotic than assumed in plate tectonics. This hypothesis would also explain the high heat flow under the island arc and backarc basin, together with arc volcanism, backarc extension (and collapse), and the strong positive gravity anomaly parallel to the island arc – none of which are readily explained by plate tectonics. PT mechanisms also fail miserably to account for mountain-building around the Pacific Rim and elsewhere (Ollier and Pain, 2000).
Plate tectonics faces many severe and apparently fatal problems, some of which have been outlined above. There is an urgent need for further investigation of the composition and age of what is currently labelled “ocean crust”, as the findings could seal the theory’s fate once and for all.
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