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Rapid Earth Expansion : An Eclectic View
Gondrvunci Resecrrclr, KI, No. I , pp. 91-94. 0 I997 Interncrtioncrl A.ssociationf?irGondwancr Reseurcli. Japun ISSN: 1342-937X
Rapid Earth Expansion : An Eclectic View Oakley Shields 6506 Jerseydale Road, Mariposa, California 95338, USA (Manuscript received January 14, 1997; accepted March 12, 1997)
Abstract Paleobiogeography, plate evolution, and minor amounts of subduction require a rapidly expanding earth during post-Triassic time, not plate tectonics. An asteroid impact at the PiTr boundary in the Congo Basin ruptured the lithosphere and, together with another impact in the Camian of Arizona, caused Earth’s volume to subsequently expand. The increase in volume was likely due to inner core and lower mantle transformation growth at the expense of the fluid outer core. A modified Pacific reconstruction is proposed that closes up the Pacific Basin in the Triassic and allows continents to cover a 55% radius Earth. Some serious weaknesses in the plate tectonics theory are noted, and tectonics by high-energy impacts is discussed. Key Words: Plate Tectonics, earth expansion, Pacific reconstruction, asteroid impact, Permo-Triassic boundary.
Introduction Currently two hypotheses of Earth history are being debated: plate tectonics and earth expansion. These differ in how we should view earth size and the Pacific region. Problems have been raised against both hypotheses, so which one has the most inherent problems? Assuming that only one is correct, how are we to decide which? Does one hypothesis explain data that are inexplicable by the other? What critical tests will clearly discriminate between the two hypotheses? What are the areas of investigation where critical differences between the two hypotheses will show up and can be discovered? These challenging questions were posed to me in 1973 by Dr. Robert J. Twiss. Here I shall try to answer them objectively and to summarize my current views on earth expansion.
Plate Tectonics Vs. Earth Expansion The reassembly of Pangaea on a constant-sizedEarth creates huge, old oceans (Panthalassa and Tethys) that no longer exist. However, some terrestrial organisms display Triassic transPacific vicariance: the freshwater fish Fukangichthyd Tanaocrossus; the insects Gondwanoperlidium and Diptera; and the plants Lyssoxylon, Williamsonia, Sagenopteris, Dechellyia, Zamites truncatus, and Cynepteris lasiophora. Triassic trans-Tethys vicariance is evident in some terrestrial vertebrates like Lystrosaurus and Kannemeyeria and the insect Triassoblatta. These fossils constrain models since their distributions span Panthalassa or Tethys: unexpected by plate
tectonics since these oceans would have acted as insurmountable barriers to their dispersal, but as expected by rapid earth expansion which provides for the necessary land connections. Similarly,in the Permian the Gigantopteris flora (Pacific) and Glossopteris (Tethys) repeat this pattern. Ribbon cherts are unique to orogenic belts and have never been cored in extant ocean basins. These formed where upwelling and high productivity occurred in rifted continental margins and immature marginal basins. For the Triassic, ribbon cherts are located on some terranes in New Zealand, North Vietnam (Tonkin), Japan, and Alaska to Baja California, and they were deposited almost continuously on the Bridge River terrane in SW British Columbia during Early Permian through Middle Jurassic times (Cordey and Schiarizza, 1993). Thus they support models of orogenic rifting (e.g. Schweickert, 1976) prior to Pacific expansion and refute models of terrane formation near the Tethys, followed by lengthy eastward transport across Panthalassa before finally accreting to the opposite margin (beginning in the Late Triassic). Some other problems with plate tectonics are also readily solved by rapid earth expansion. Examples include ridge-pull (crustal stretching), not ridge-push,predominates at mid-ocean ridges (MOR’s); a tremendous tensional force is required to rift continents apart; the total amount of seafloor generated in the past 165 Ma has been greatly increasing; paleomagnetic polar scatteringincreases back in time, implying greater surface curvature;a continent cannot be moved simultaneouslytoward both spin axis (paleomagnetic)poles; continents are perfectly centered by longitude and latitude despite supposed drift and rotation; and Early Mesozoic rift valleys (precursors to ocean
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basin formation) are present along the cratonic margins of SW North America and E China. Further paleomagnetic problems are discussed at length by Carey (1976, pp. 181-219). In contrast, problems with earth expansion (cf. Hallam, 1984; Harland, 1979) seem to be fewer in number.
Plates Continental roots extend to 400-660 km under shields and lack an asthenosphere, thus acting as obstacles (anchors) to horizontal plate motion and implying stationary continents. Indeed, some paleomagnetic analyses have shown that the lithosphere as a whole has not moved significantly back to at least the Late Cretaceous. Plates cannot move bodily as rigid units since their seafloor is subdivided into a series of faultbounded plate segments each spreading at differing rates and directions. On a rapidly expanding earth, continental dispersal is radially outward, not horizontal, due to seafloor growth between continents as each plate grows in size. Increasing enlargement over time is especially evident for the Pacific, African, and Antarctic plates. As seafloor growth was radially outward from both Africa and Antarctica, their dispersal was by enlargement of the intervening seafloor, not by continental drift. Except in the North Pacific, the oldest seafloor always surrounds continents, while the later seafloor builds radially outward away from continents and is more extensive. The largest continental plate (Eurasia) has increased the least in size while the Southern Hemisphere plates have increased the most, suggesting the Southern Hemisphere has expanded more than the Northern Hemisphere. The nearly exponential increase of ocean crust in all oceans since the Triassic favors earth expansion, MOR’s have elongated by fracture zone displacement relative to continental margin lengths, and MOR lengths (after FZ displacements are subtracted) exceed convergent margin lengths by about 10,000 km. MOR’s are slowly “migrating” away from Africa and Antarctica and toward the Pacific plate, but this is due to slightly asymmetric spreading. Judging by extant seafloor ages, true ocean basins are a Jurassic-Recent phenomenon, decrease in age from north to south, and are pull-apart features. MOR’s are the only sites where lithosphere size increases due to adding seafloor and exhibit tensional FZ’s from stretching over an expanding circumference. The plates show a high degree of planetary symmetry, with the African and Pacific plate centers being precisely antipodal (Kanasewich, 1976). A gradual plate curvature flattening would accompany an earth radius increase over time to produce abundant plate fracturing and continental geosynclines, flanches, and compressional uplifts (orogenies) by stressing the plates, which is exactly what we observe.
Subduction I now favor an expansion model with only minor amounts of Cenozoic subduction (see Yamaoka et al., 1986) since any large amounts would disrupt the global great circles’structural
symmetry (see Shields, 1997). Also, some hotspot motion took place when North America was at a paleomagnetic standstill (Van Fossen and Kent, 1992),which raises serious doubts about large-scale subduction models that depend upon hotspots being fixed. Since hotspots originate in the D” layer, how can they remain stationary for up to 120 Ma if their plumes must travel through a convecting mantle? Radial growth of the Pacific plate MOR’s makes it doubtful that it generated a mirror-image counterpart plate that was mostly subducted. Two southwarddirected, large meteorite impacts at the KIT boundary (65 Ma), located in NW Yucatan and in the Chukchi Sea (cf. Grantz and May, 1981; Hildebrand et al., 1991; Robin et al., 1993; D’Hondt, 1994),could have provided enough energy to initiate the Pacific Basin trench system pattern. Direct evidence of subduction is now fairly extensive. Currently, most North Pacific MOR’s have disappeared, and geodetic measurements show convergence there. Magnetic anomalies strike at oblique angles to many trenches, and magnetic anomalies extend beneath the continental slope in some North Pacific trenches. In some instances, seamounts are entering trenches, aseismic ridges form trench cusps, and deformed turbidite sediments are sometimes accreted to the lower trench wall. High similarity of the MOR hydrothermal vent faunas between NE Pacific/Japan/Marianas/Fiji suggests these regions were interconnected by other MOR’s in the Paleogene before being subducted (Tunnicliffe and Fowler, 1996).
Reconstructions Reconstructions must be congruent not only with the evidence from geology and geophysics but also from paleobiogeography, paleoclimatology, and paleogeography. Unfortunately, geology relies too heavily on geophysics while practically ignoring the other evidence which can test reconstructions. For a rapidly expanding earth in the Triassic, the continents completely covered its surface and had a radius that was 55% of the present radius. There are now a dozen or so such reconstructions. The critical area is the Pacific Ocean closure. I prefer my Pacific closure (Shields, 1979) as a working model but would now fine-tune it by another Caribbean-Gulf of Mexico closure (Ross & Scotese, 1988, Fig. 3), Indonesian closure (Carey, 1976, Fig. 179), Sibumasu-NW Australia closure (Audley-Charles, 1991, Fig. 7), and extreme South Pacific closure (Maxlow, 1996, Fig. 24). A western North America displaced terrane reconstruction is required too due to oroclinal distension (in southern California, the Pacific Northwest, and Alaska) from Cenozoic clockwise rotations and northward displacements along transcurrent faults bordering terranes, but it has not yet been worked out in sufficient detail. This modified Pacific reconstruction rejoins all continents that border the Pacific along latitudinal lines by longitudinalclosure. It seems to agree with Northern Hemisphere paleomagnetic Gondwana Research, KI, No. I ? 1997
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data (Shields, I996), Pacific magnetic anomalies, paleobiogeography, paleoclimatology (Smiley, 1992), paleogeography, Hercynian orogeny alignment, major fault alignments, and paleoequators. It also closes down the 10,000 km of Pacific perimeter lengthening, mostly contributed to by marginal basin growth. Seafloor spreading in the Pacific Basin began i n the lower Pliensbachian (ca. 192 Ma) as radial, outward growth of its spreading centers away from the point of origin located i n the East Mariana-Pigafetta basins, unlike other oceans which spread apart more symmetrically along continental rift lines. Interestingly, continental rocks are reported from some Polynesian islands within the Pacific plate, left over as microcontinental debris plucked off the margins of rifting continents (Shields, 1976). A great circle between Hokkaido and Tierra del Fuego bisects the Pacific into symmetrical halves (Brock, 1972,Fig. 51) and would not permit huge amounts of seafloor to be subducted beneath western North America. Since the Pacific plate displays exponential growth, the Pacific perimeter is widening (not shrinking), and since the other oceans have grown in size, the Earth overall has increased in size since the Triassic. The Gondwanaland reconstruction of Lawver et al. (1992) is in agreement with data from magnetic anomaly patterns, hotspot tracks, paleomagnetism, and continental geology.
Cause of Expansion What caused the Earth to rapidly expand in post-Triassic times, when seafloor spreading and separate APW paths for each continent were initiated? I believe we should look to P/ Tr boundary time (250 Ma) i n Africa (the core of Gondwanaland) for any indication of an unusually large impact with sufficient energy to fragment the continuous continental crust which previously had kept a tight lid on interior inflation. The Congo Basin is a prime candidate, located close to the geographic center of the African plate (cf. Daly et al., 1991; Pavoni, 1992). It is a large, roughly circular basin about 1200 km i n diameter, with an upwarped (buried) rim, dated somewhere between late Permian and earliest Triassic, coeval with rifting, folding, and thrusting events in southern and central Africa. Evidence that the Congo Basin represents a large impact crater includes central uplifts, major faulting, rich diamond and cobalt deposits in places along its margins, great lithospheric thinning along its southern and eastern margins, a huge magnetic anomaly superimposed over a great downwelling mantle flow area in its northern portion and margin, and its precise centralized location within the NW-SE and NE-SW global torsion system (cf. Chadderton et al., 1969; Liu, 1977; O’Driscoll, 1980; Daly et al., 1992; Pavoni, 1992). The asteroid’s energy upon impact was ergs, and it probably had a NW-directed trajectory. The Siberian flood basalts are the oldest (P/Tr boundary) and most extensive continental flood basalts during the past 250 Ma (Renne et al., 1995), erupted over an extremely short time interval (of about Condwtma Resenrcli, K l , No. I ? 1997
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one m.y.), coincide with the great P/Tr faunal mass extinction, and could represent melting by focusing seismic wave energies antipodal to the Congo impact crater on some rapidly expanding earth models (cf. Watts et al., 1991; Rampino and Caldeira, 1992; Boslough et al., 1996). Globally, the P/T boundary is mostly represented by a sedimentary gap (unconformity), although iron and glassy microspherules have been recovered from a thin clay layer in Shansi (Olson, 1980) and an iridium peak of 2 ppb coincides with a spherule-rich layer in Zhejiang (Rampino and Haggerty, 1996). Following this event, the Carnian was the onset time of continental rifting in NW and E Africa, SW, SE and E North America, E China (Dabie Shan), W Madagascar, and NW and W Australia (precursorial to Jurassic seafloor spreading), and a narrow wedge of seafloor briefly opened up propogatively in NW Siberia (cf. Aplonov, 1988; Busby-Spera, 1988; Veevers, 1989; Ames et al., 1993). This extensive rifting event was likely initiated by multiple large impacts in SE Arizona in the early Carnian, dated by associated volcanic formations nearby (cf. Saul, 1978; Stewart et al., 1986; Riggs et al., 1996). Rapid earth expansion necessitates increasing volume and a constant but redistributed mass in the Earth’s interior. Schloessin and Jacobs (1980) hypothesize that the solid inner core and lower mantle have grown at the expense of the fluid outer core since soon after the Earth formed. This mechanism could also explain interior expansion once the lithosphere ruptured, as transformations would have been accelerated with the decrease in pressures. Inner core and lower mantle growth could account for the increased volume and redistributed mass by crystallization of constituents soluble in the fluid part of the core.
Conclusions Earth expansion is superior to plate tectonics in containing fewer inherent problems, i n passing discriminating paleobiogeographic tests, and in explaining data otherwise inexplicable by plate tectonics. Continents radially disperse, total seafloor area increases, and plate curvature decreases over time, while only minor subduction is possible. The Pacific Ocean was closed during the Triassic when continents covered a 55% radius earth. Rapid earth expansion occurred only after the P/Tr boundary when an asteroid impact deeply ruptured the lithospheric shell and allowed the Earth’s interior to expand in volume.
Acknowledgment The author is grateful to Dr. M. Santosh, Centre for Earth Science Studies, Trivandrum, for his kind invitation to communicate this paper, and to Janet Langley for typing the manuscript.
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References Ames, L., Tilton, G.R. and Zhou, G. (1993) Timing of collision of the Sino-Korean and Yangtse cratons: U-Pb zircon dating of coesitebearing eclogites. Geology, v. 21, pp. 339-342. Aplonov, S. (1988) An abortedTriassic Ocean in west Siberia. Tectonics, v. 7, pp. 1103-1 122. Audley-Charles, M.G. (1991) Tectonics of the New Guinea area. Annual Review of Earth and Planetary Sciences, v. 19, pp. 17-41. Boslough, M.B., Chael, E.P., Trucano, T.G., Crawford, D.A. and Campbell, D.L. (1996) Axial focusing of impact energy in the Earth’s interior: a possible link to flood basalts and hotspots. Geol. SOC.of America, Special Paper, v. 307, pp. 541-550. Brock, B.B. (1972) A global approach to geology. A.A. Balkema, Cape Town, 365 p. Busby-Spera, C.J. (1988) Speculative tectonic model for the early Mesozoic arc of thc southwest Cordilleran United States. Geology, V. 16, pp. 1121-1125. Carey, S.W. (1976) The expanding Earth. Elsevier, Amsterdam, 488 p. Chadderton, L.T. Krajenbrink, F.G., Katz, R. and Poveda, A. (1969) Standing waves on the Moon. Nature, v. 223, pp. 259-263. Cordey, F. and Schiarizza, P. (1993) Long-lived Panthalassic remnant: the Bridge River accretionary complex, Canadian Cordillera. Geology, v. 21, pp. 263-266. Daly, M.C., Lawrence, S.R., Diemu-Tshiband, K. and Matouana, B. (1992) Tectonic evolution of the Cuvette Centrale, Zaire. J. Geol. SOC.London, v. 149, pp. 539-546. Daly, M.C., Lawrence, S.R., Kimun’a, D. and Binga, M. (1991) Late Palaeozoic deformation in central Africa: a result of distant collision? Nature, v. 350, pp. 605-607. D’Hondt, S. (1994) The impact of the Cretaceous-Tertiary boundary. Palaios, v. 9, pp. 221-223. Grantz, A. and May, S.D. (1981) Chukchi structure. Lunar and Planetary Institute Contribution 449 (Supplement), pp. 1-2. Hallam, A. (1984) The unlikelihood of an expanding Earth. Geol. Mag., V. 121, pp. 653-655. Harland, W.B. (1979) An expanding Earth? Nature, v. 278, pp. 12-13. Hildebrand,A.R., Penfield, G.T., Kring, D.A.,Pilkington, M., Camargo, A., Jacobsen, S.B. and Boynton, W.V. (1991) Chicxulub Crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico. Geology, v. 19, pp. 867-87 1. Kanasewich, E.R. ( I 976) Plate tectonics and planetary convection. Can. J. Earth Sci., v. 13, pp. 331-340. Lawver, L.A., Gahagan, L.M. and Coffin, M.F. (I 992) The development of paleoseaways around Antarctica. Antarctic Research Series, v. 56, pp. 7-30. Liu, H.-S. (1977) Convection pattern and stress system under the African plate. Physics of the Earth and Planetary Interiors, v. 15, pp. 60-68. Maxlow, J. (1996) Global expansion tectonics: small Earth modeling of an exponentially expanding Earth. Terrella Consultants, Glen Forrest, 59 p. O’Driscoll, E.S.T. (1980) The double helix in global tectonics. Tectonophys., v. 63, pp. 397-417. Olson, E.C. (1980) Problems of Permo-Triassic terrestrial vertebrate extinctions, Historical Biology, v. 2, pp. 17-35. Pavoni, N. (1992) Rifting of Africa and pattern of mantle convection beneath the African plate. Tectonophys., v. 215, pp. 35-53. Rampino, M.R. and Caldeira, K., (1992) Antipodal hotspot pairs on theEarth. Geophys. Res. Lett., v. 19, pp. 2011-2014.
Rampino, M.R. and Haggerty, B.M. (1996) Impact crises and mass extinctions: a working hypothesis. Geol. SOC.of America, Special Paper, v. 307, pp. 11-30. Renne, P.R., Zhang, Z., Richards, M.A., Black, M.T. and Basu, A.R. (1995) Synchrony and causal relations between Permian-Triassic boundary crises and Siberian flood volcanism. Science, v. 269, pp. 1413- 1416. Riggs, N.R.,Lehman,T.M., Gehrels,G.E. and Dickinson, W.R. (1996) Detrital zircon link between headwaters and terminus of the Upper Triassic Chinle-Dockum paleoriver system. Science, v. 273, pp. 97- 100. Robin, E. Froget, L., Jehanno, C. and Rocchia, R. (1993) Evidence for a K/T impact event in the Pacific Ocean. hature, v. 363, pp. 615617. Ross, M.I. and Scotese, C.R. (1988) A hierarchical tectonic model of the Gul: of Mexico and Caribbean region. Tectonophys., v. 155, pp. 139-168. Saul, J.M. (1978) Circular structures of large scale and great age on the Earth’s surface. Nature, v. 27 I , pp. 345-349. Schloessin, H.H. and Jacobs, J.A. (1980) Dynamics of a fluid core with inward growing boundaries. Can. J. Earth Sci., v. 17, pp. 7289. Schweickert, R.A. (1976) Early Mesozoic rifting and fragmentation of the Cordilleran orogen in the western USA. Nature, v. 260, pp. 586-591. Shields, 0.(1976) A summary of the oldest ages for the world’s islands. Papers and Proceedings of the Royal Society of Tasmania, v. 1 10, pp. 35-61. Shields, 0.(1979) Evidence for initial opening of the Pacific Ocean in the Jurassic. Palaeogeography, Palaeoclimatology, Palaeoecology, V. 26, pp. 181-220. Shields, 0. (1996) Plate tectonics or an expanding Earth? J. Geol. SOC.India, v. 47, pp. 399-408. Shields, 0. (1997) Great circles and Earth’s structural design. J. Geol. SOC.India, in review. Smiley, C.J. (1992) Paleofloras, faunas, and continental drift: some problem areas. In: Chatterjee, S. and Hotton, N. (Eds.), New concepts in global tectonics. TexasTech University Press, Lubbock, pp. 241-257. Stewart, J.H., Anderson, T.H., Haxel, G.B., Silver, L.T. and Wright, J.E. (1986) LateTriassic paleogeography of the southern Cordillera: the problem of a source for voluminous volcanic detritus in the Chinle Formation of the Colorado Plateau region. Geology, v. 14, pp. 567-570. Tunnicliffe, V. and Fowler, C.M.R. (1996) Influence of sea-floor spreading on the global hydrothermal vent fauna. Nature, v. 379, pp. 53 1-533. Van Fossen, M.C. and Kent, D.V. (1992) Paleomagnetism of I22 Ma plutons in New England and the Mid-Cretaceous paleomagnetic field in North America: true polar wander or large-scale differential mantle motion? J. Geophys. Res., v. 97, pp. 19,651-19,661. Veevers, J.J. (1989) MiddldLate Triassic (230+5 Ma) singularity in the stratigraphy and magmatic history of the Pangean heat anomaly. Geology, v. 17, pp. 784-787. Watts, A.W., Greeley, R. and Melosh, H.J. (1991) The formation of terrains antipodal to major impacts. Icarus, v. 93, pp. 159-168. Yamaoka, K., Fukao, Y.and Kumazawa, M. (1986) Spherical shell tectonics: effects of sphericity and inextensibility on the geometry of the descending lithosphere. Reviews of Geophysics and Space Physics, v. 24, pp. 27-53.
Gondwnnn Research, V I , No. I , 1997
Rapid Earth Expansion : An Eclectic View Oakley Shields 6506 Jerseydale Road, Mariposa, California 95338, USA (Manuscript received January 14, 1997; accepted March 12, 1997)
Abstract Paleobiogeography, plate evolution, and minor amounts of subduction require a rapidly expanding earth during post-Triassic time, not plate tectonics. An asteroid impact at the PiTr boundary in the Congo Basin ruptured the lithosphere and, together with another impact in the Camian of Arizona, caused Earth’s volume to subsequently expand. The increase in volume was likely due to inner core and lower mantle transformation growth at the expense of the fluid outer core. A modified Pacific reconstruction is proposed that closes up the Pacific Basin in the Triassic and allows continents to cover a 55% radius Earth. Some serious weaknesses in the plate tectonics theory are noted, and tectonics by high-energy impacts is discussed. Key Words: Plate Tectonics, earth expansion, Pacific reconstruction, asteroid impact, Permo-Triassic boundary.
Introduction Currently two hypotheses of Earth history are being debated: plate tectonics and earth expansion. These differ in how we should view earth size and the Pacific region. Problems have been raised against both hypotheses, so which one has the most inherent problems? Assuming that only one is correct, how are we to decide which? Does one hypothesis explain data that are inexplicable by the other? What critical tests will clearly discriminate between the two hypotheses? What are the areas of investigation where critical differences between the two hypotheses will show up and can be discovered? These challenging questions were posed to me in 1973 by Dr. Robert J. Twiss. Here I shall try to answer them objectively and to summarize my current views on earth expansion.
Plate Tectonics Vs. Earth Expansion The reassembly of Pangaea on a constant-sizedEarth creates huge, old oceans (Panthalassa and Tethys) that no longer exist. However, some terrestrial organisms display Triassic transPacific vicariance: the freshwater fish Fukangichthyd Tanaocrossus; the insects Gondwanoperlidium and Diptera; and the plants Lyssoxylon, Williamsonia, Sagenopteris, Dechellyia, Zamites truncatus, and Cynepteris lasiophora. Triassic trans-Tethys vicariance is evident in some terrestrial vertebrates like Lystrosaurus and Kannemeyeria and the insect Triassoblatta. These fossils constrain models since their distributions span Panthalassa or Tethys: unexpected by plate
tectonics since these oceans would have acted as insurmountable barriers to their dispersal, but as expected by rapid earth expansion which provides for the necessary land connections. Similarly,in the Permian the Gigantopteris flora (Pacific) and Glossopteris (Tethys) repeat this pattern. Ribbon cherts are unique to orogenic belts and have never been cored in extant ocean basins. These formed where upwelling and high productivity occurred in rifted continental margins and immature marginal basins. For the Triassic, ribbon cherts are located on some terranes in New Zealand, North Vietnam (Tonkin), Japan, and Alaska to Baja California, and they were deposited almost continuously on the Bridge River terrane in SW British Columbia during Early Permian through Middle Jurassic times (Cordey and Schiarizza, 1993). Thus they support models of orogenic rifting (e.g. Schweickert, 1976) prior to Pacific expansion and refute models of terrane formation near the Tethys, followed by lengthy eastward transport across Panthalassa before finally accreting to the opposite margin (beginning in the Late Triassic). Some other problems with plate tectonics are also readily solved by rapid earth expansion. Examples include ridge-pull (crustal stretching), not ridge-push,predominates at mid-ocean ridges (MOR’s); a tremendous tensional force is required to rift continents apart; the total amount of seafloor generated in the past 165 Ma has been greatly increasing; paleomagnetic polar scatteringincreases back in time, implying greater surface curvature;a continent cannot be moved simultaneouslytoward both spin axis (paleomagnetic)poles; continents are perfectly centered by longitude and latitude despite supposed drift and rotation; and Early Mesozoic rift valleys (precursors to ocean
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basin formation) are present along the cratonic margins of SW North America and E China. Further paleomagnetic problems are discussed at length by Carey (1976, pp. 181-219). In contrast, problems with earth expansion (cf. Hallam, 1984; Harland, 1979) seem to be fewer in number.
Plates Continental roots extend to 400-660 km under shields and lack an asthenosphere, thus acting as obstacles (anchors) to horizontal plate motion and implying stationary continents. Indeed, some paleomagnetic analyses have shown that the lithosphere as a whole has not moved significantly back to at least the Late Cretaceous. Plates cannot move bodily as rigid units since their seafloor is subdivided into a series of faultbounded plate segments each spreading at differing rates and directions. On a rapidly expanding earth, continental dispersal is radially outward, not horizontal, due to seafloor growth between continents as each plate grows in size. Increasing enlargement over time is especially evident for the Pacific, African, and Antarctic plates. As seafloor growth was radially outward from both Africa and Antarctica, their dispersal was by enlargement of the intervening seafloor, not by continental drift. Except in the North Pacific, the oldest seafloor always surrounds continents, while the later seafloor builds radially outward away from continents and is more extensive. The largest continental plate (Eurasia) has increased the least in size while the Southern Hemisphere plates have increased the most, suggesting the Southern Hemisphere has expanded more than the Northern Hemisphere. The nearly exponential increase of ocean crust in all oceans since the Triassic favors earth expansion, MOR’s have elongated by fracture zone displacement relative to continental margin lengths, and MOR lengths (after FZ displacements are subtracted) exceed convergent margin lengths by about 10,000 km. MOR’s are slowly “migrating” away from Africa and Antarctica and toward the Pacific plate, but this is due to slightly asymmetric spreading. Judging by extant seafloor ages, true ocean basins are a Jurassic-Recent phenomenon, decrease in age from north to south, and are pull-apart features. MOR’s are the only sites where lithosphere size increases due to adding seafloor and exhibit tensional FZ’s from stretching over an expanding circumference. The plates show a high degree of planetary symmetry, with the African and Pacific plate centers being precisely antipodal (Kanasewich, 1976). A gradual plate curvature flattening would accompany an earth radius increase over time to produce abundant plate fracturing and continental geosynclines, flanches, and compressional uplifts (orogenies) by stressing the plates, which is exactly what we observe.
Subduction I now favor an expansion model with only minor amounts of Cenozoic subduction (see Yamaoka et al., 1986) since any large amounts would disrupt the global great circles’structural
symmetry (see Shields, 1997). Also, some hotspot motion took place when North America was at a paleomagnetic standstill (Van Fossen and Kent, 1992),which raises serious doubts about large-scale subduction models that depend upon hotspots being fixed. Since hotspots originate in the D” layer, how can they remain stationary for up to 120 Ma if their plumes must travel through a convecting mantle? Radial growth of the Pacific plate MOR’s makes it doubtful that it generated a mirror-image counterpart plate that was mostly subducted. Two southwarddirected, large meteorite impacts at the KIT boundary (65 Ma), located in NW Yucatan and in the Chukchi Sea (cf. Grantz and May, 1981; Hildebrand et al., 1991; Robin et al., 1993; D’Hondt, 1994),could have provided enough energy to initiate the Pacific Basin trench system pattern. Direct evidence of subduction is now fairly extensive. Currently, most North Pacific MOR’s have disappeared, and geodetic measurements show convergence there. Magnetic anomalies strike at oblique angles to many trenches, and magnetic anomalies extend beneath the continental slope in some North Pacific trenches. In some instances, seamounts are entering trenches, aseismic ridges form trench cusps, and deformed turbidite sediments are sometimes accreted to the lower trench wall. High similarity of the MOR hydrothermal vent faunas between NE Pacific/Japan/Marianas/Fiji suggests these regions were interconnected by other MOR’s in the Paleogene before being subducted (Tunnicliffe and Fowler, 1996).
Reconstructions Reconstructions must be congruent not only with the evidence from geology and geophysics but also from paleobiogeography, paleoclimatology, and paleogeography. Unfortunately, geology relies too heavily on geophysics while practically ignoring the other evidence which can test reconstructions. For a rapidly expanding earth in the Triassic, the continents completely covered its surface and had a radius that was 55% of the present radius. There are now a dozen or so such reconstructions. The critical area is the Pacific Ocean closure. I prefer my Pacific closure (Shields, 1979) as a working model but would now fine-tune it by another Caribbean-Gulf of Mexico closure (Ross & Scotese, 1988, Fig. 3), Indonesian closure (Carey, 1976, Fig. 179), Sibumasu-NW Australia closure (Audley-Charles, 1991, Fig. 7), and extreme South Pacific closure (Maxlow, 1996, Fig. 24). A western North America displaced terrane reconstruction is required too due to oroclinal distension (in southern California, the Pacific Northwest, and Alaska) from Cenozoic clockwise rotations and northward displacements along transcurrent faults bordering terranes, but it has not yet been worked out in sufficient detail. This modified Pacific reconstruction rejoins all continents that border the Pacific along latitudinal lines by longitudinalclosure. It seems to agree with Northern Hemisphere paleomagnetic Gondwana Research, KI, No. I ? 1997
RAPID EARTH EXPANSION
data (Shields, I996), Pacific magnetic anomalies, paleobiogeography, paleoclimatology (Smiley, 1992), paleogeography, Hercynian orogeny alignment, major fault alignments, and paleoequators. It also closes down the 10,000 km of Pacific perimeter lengthening, mostly contributed to by marginal basin growth. Seafloor spreading in the Pacific Basin began i n the lower Pliensbachian (ca. 192 Ma) as radial, outward growth of its spreading centers away from the point of origin located i n the East Mariana-Pigafetta basins, unlike other oceans which spread apart more symmetrically along continental rift lines. Interestingly, continental rocks are reported from some Polynesian islands within the Pacific plate, left over as microcontinental debris plucked off the margins of rifting continents (Shields, 1976). A great circle between Hokkaido and Tierra del Fuego bisects the Pacific into symmetrical halves (Brock, 1972,Fig. 51) and would not permit huge amounts of seafloor to be subducted beneath western North America. Since the Pacific plate displays exponential growth, the Pacific perimeter is widening (not shrinking), and since the other oceans have grown in size, the Earth overall has increased in size since the Triassic. The Gondwanaland reconstruction of Lawver et al. (1992) is in agreement with data from magnetic anomaly patterns, hotspot tracks, paleomagnetism, and continental geology.
Cause of Expansion What caused the Earth to rapidly expand in post-Triassic times, when seafloor spreading and separate APW paths for each continent were initiated? I believe we should look to P/ Tr boundary time (250 Ma) i n Africa (the core of Gondwanaland) for any indication of an unusually large impact with sufficient energy to fragment the continuous continental crust which previously had kept a tight lid on interior inflation. The Congo Basin is a prime candidate, located close to the geographic center of the African plate (cf. Daly et al., 1991; Pavoni, 1992). It is a large, roughly circular basin about 1200 km i n diameter, with an upwarped (buried) rim, dated somewhere between late Permian and earliest Triassic, coeval with rifting, folding, and thrusting events in southern and central Africa. Evidence that the Congo Basin represents a large impact crater includes central uplifts, major faulting, rich diamond and cobalt deposits in places along its margins, great lithospheric thinning along its southern and eastern margins, a huge magnetic anomaly superimposed over a great downwelling mantle flow area in its northern portion and margin, and its precise centralized location within the NW-SE and NE-SW global torsion system (cf. Chadderton et al., 1969; Liu, 1977; O’Driscoll, 1980; Daly et al., 1992; Pavoni, 1992). The asteroid’s energy upon impact was ergs, and it probably had a NW-directed trajectory. The Siberian flood basalts are the oldest (P/Tr boundary) and most extensive continental flood basalts during the past 250 Ma (Renne et al., 1995), erupted over an extremely short time interval (of about Condwtma Resenrcli, K l , No. I ? 1997
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one m.y.), coincide with the great P/Tr faunal mass extinction, and could represent melting by focusing seismic wave energies antipodal to the Congo impact crater on some rapidly expanding earth models (cf. Watts et al., 1991; Rampino and Caldeira, 1992; Boslough et al., 1996). Globally, the P/T boundary is mostly represented by a sedimentary gap (unconformity), although iron and glassy microspherules have been recovered from a thin clay layer in Shansi (Olson, 1980) and an iridium peak of 2 ppb coincides with a spherule-rich layer in Zhejiang (Rampino and Haggerty, 1996). Following this event, the Carnian was the onset time of continental rifting in NW and E Africa, SW, SE and E North America, E China (Dabie Shan), W Madagascar, and NW and W Australia (precursorial to Jurassic seafloor spreading), and a narrow wedge of seafloor briefly opened up propogatively in NW Siberia (cf. Aplonov, 1988; Busby-Spera, 1988; Veevers, 1989; Ames et al., 1993). This extensive rifting event was likely initiated by multiple large impacts in SE Arizona in the early Carnian, dated by associated volcanic formations nearby (cf. Saul, 1978; Stewart et al., 1986; Riggs et al., 1996). Rapid earth expansion necessitates increasing volume and a constant but redistributed mass in the Earth’s interior. Schloessin and Jacobs (1980) hypothesize that the solid inner core and lower mantle have grown at the expense of the fluid outer core since soon after the Earth formed. This mechanism could also explain interior expansion once the lithosphere ruptured, as transformations would have been accelerated with the decrease in pressures. Inner core and lower mantle growth could account for the increased volume and redistributed mass by crystallization of constituents soluble in the fluid part of the core.
Conclusions Earth expansion is superior to plate tectonics in containing fewer inherent problems, i n passing discriminating paleobiogeographic tests, and in explaining data otherwise inexplicable by plate tectonics. Continents radially disperse, total seafloor area increases, and plate curvature decreases over time, while only minor subduction is possible. The Pacific Ocean was closed during the Triassic when continents covered a 55% radius earth. Rapid earth expansion occurred only after the P/Tr boundary when an asteroid impact deeply ruptured the lithospheric shell and allowed the Earth’s interior to expand in volume.
Acknowledgment The author is grateful to Dr. M. Santosh, Centre for Earth Science Studies, Trivandrum, for his kind invitation to communicate this paper, and to Janet Langley for typing the manuscript.
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