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Continental Drift—A Symposium
... However, the mechanisms determining mantle drag and mantle convection remain a matter of debate. The asymmetry of subduction zones (those directed to the west are steeper and faster, e.g., Doglioni et al., 2007;Riguzzi et al., 2010) and plate motion reconstructions relative to the mantle reference frame support a westerly-directed drift of the lithosphere relative to the underlying mantle, also called net rotation (as a mean value) or westward drift (Bostrom, 1971;Carey, 1958;Crespi et al., 2007;Cuffaro and Doglioni, 2007;Gripp and Gordon, 2002;Holmes, 1944;Le Pichon, 1968;Moore, 1973;Ricard et al., 1991;Wegener, 1915). The net rotation or the westward drift of the lithosphere requires both a decoupling at the lithosphere-asthenosphere interface and a mechanism driving this rotation (Doglioni and Anderson, 2015;Doglioni and Panza, 2015;Doglioni et al., 2011). ...
... Proposed mechanisms involve either the negative buoyancy of the lithosphere (Ricard et al., 1991), or the astronomical drag induced by the Earth's rotation combined with tidal friction (Riguzzi et al., 2010). Potentially astronomical forces may interact with mantle convection resulting from the cooling of the Earth (Carey, 1958;Holmes, 1944;Scoppola et al., 2006). ...
Combining geophysical, petrological and structural data on oceanic mantle lithosphere, underlying asthenosphere and oceanic basalts, an alternative oceanic plate spreading model is proposed in the framework of the westward migration of oceanic spreading ridges relative to the underlying asthenosphere. This model suggests that evolution of both the composition and internal structure of oceanic plates and underlying upper mantle strongly depends et al.l scales on plate kinematics. We show that the asymmetric features of lithospheric plates and underlying upper asthenosphere on both sides of oceanic spreading ridges, as shown by geophysical data (seismic velocities, density, thickness, and plate geometry), reflect somewhat different mantle compositions, themselves related to various mantle differentiation processes (incipient to high partial melting degree, percolation/reaction and refertilisation) at different depths (down to 300 km) below and laterally to the ridge axis. The fundamental difference between western and eastern plates is linked to the westward ridge migration inducing continuing mantle refertilisation of the western plate by percolation-reaction with ascending melts, whereas the eastern plate preserves a barely refertilized harzburgitic residue. Plate thickness on both sides of the ridge is controlled both by cooling of the asthenospheric residue and by the instability of pargasitic amphibole producing a sharp depression of the water-undersaturated solidus, its intersection with the geotherm at ~ 90 km, and incipient melt production right underneath the lithosphere-asthenosphere boundary (LAB). Thus the intersection of the geotherm with the water-undersaturated lherzolite solidus explains the existence of a low-velocity zone (LVZ). As oceanic lithosphere is moving westward relative to asthenospheric mantle, this partially molten upper asthenosphere facilitates the decoupling between lower asthenosphere and lithosphere. Thereby the westward drift of the lithosphere is necessarily slowed down, top to down, inducing a progressive decoupling within the mantle lithosphere itself. This intra-mantle decoupling could be at the origin of asymmetric detachment faults allowing mantle exhumation along slow-spreading ridges. Taking into account the asymmetric features of the LVZ, migration of incipient melt fractions and upwelling paths from the lower asthenosphere through the upper asthenosphere are oblique, upward and eastward. MORB are sourced from an eastward and oblique, near-adiabatic mantle upwelling from the lower asthenosphere. This unidirectional mantle transfer is induced by isostatic suction of the migrating spreading ridge.
... The Yucatan block of northeastern Mexico and surrounding platform has traditionally been regarded as of Gondwanan provenance accreted to the southern margin of Laurentia in the Ouachita embayment during the Carboniferous-Permian assembly of Pangea (e.g., Carey 1958;Pindell and Dewey 1982;Sedlock et al. 1993;Dalziel 1997;Dickinson and Lawton 2001). Recent analyses of seafloor spreading in the Atlantic Ocean have resulted in more limited space for the Yucatan block between North and South America (Torsvik et al. 2008;Labails et al. 2010). ...
... Different orientations for the Yucatan in Pangean reconstructions have been proposed (e.g., Carey 1958;Bullard et al. 1965;Pindell and Dewey 1982;Buffler and Sawyer 1985;Ross and Scotese 1988;Seton et al. 2012;. Thus, Seton et al. (2012) rotate the Yucatan block anticlockwise by 40°-50°into an E-W orientation, which results in considerable overlap with southern Laurentia and southern Florida (Fig. 2a). ...
Current reconstructions suggest that the Yucatan block has Gondwanan provenance and orient the Yucatan E–W in the Ouachita embayment where it overlaps southern Laurentia and Florida. Alternatively, if the Yucatan is oriented NE–SW, it fits neatly into the Ouachita embayment with minimal overlap. Furthermore, many of the V-shaped, magnetic anomalies in the Yucatan that are discordant in the E–W reconstruction can be traced across the Yucatan–Laurentian boundary in the NE–SW reconstruction: (a) NW-trending anomalies continue into southern Laurentia where they are associated with Cambrian mafic rocks in the southern Oklahoma and Reelfoot rifts and (b) NE-trending anomalies in the eastern Yucatan are parallel to those over Grenvillian rocks in the western Appalachians. Furthermore, Silurian plutons in the Maya Mountains of Belize that have no counterpart in Texas may be correlated with the Concord–Salisbury plutons in Carolinia, a terrane of Gondwanan provenance in the southern Appalachians. Nd isotopic data from the Chicxulub ejecta in the northern Yucatan block are similar to those in the Llano Grenvillian rocks and differ from those in Oaxaquia. These correlations suggest that much of the Yucatan is of Laurentian provenance and implies that the Laurentia–Gondwana suture crosses the Yucatan west of the Maya Mountains. In this scenario, the Ouachita embayment results from the formation of the Gulf of Mexico during the breakup of Pangea, rather than the Cambrian removal of the Argentine Cuyania terrane. Cambrian (515 Ma) paleomagnetic and faunal data are consistent with Cuyania forming either east of the Yucatan or off eastern Laurentia.
... This prominent feature has become the focus of a debate concerning whether the Strait is the locus of great sinistral strike-slip faulting during the Cretaceous and Tertiary ( e.g. Wegener 1924, Carey 1958, Wilson 1963, 1965, Bullard et al. 1965, Keen et al. 1972, Kristoffersen & Talwani 1977, Newman 1977, Sclater et al. 1977, Newman & Falconer 1978 or not (e.g. Kerr 1967a, b, 1980a, b, 1981, Grant 1975, Le Pichon et al. 1977. ...
The upper Ordovician and lower Silurian sediments in the northern Nares Strait region formed during a series of platform margin collapses and platform subsidences. Facies changes from platform carbonates to slope and trough elastics are intricate, but two almost vertical facies fronts can be correlated across Nares Strait. The northernmost facies front is the carbonate platform margin (associated with a horst) which apparently formed at the fault-bounded edge of the stable craton. At this margin there is a facies boundary (interdigitation) between platform carbonates and elastics of the trough and this feature had a stationary location during the upper Ordovician and lower Silurian. On Ellesmere Island it is traceable from Cañon Fiord to Judge Daly Promontory; on Greenland, directly along strike, available data indicate that the linear coastline of northern Hall Land, and probably Nyeboe Land, coincides with the platform margin. There appears to have been little or no post-depositional offset of this margin along Nares Strait. Mudstones progressively migrated southward in Ellesmere Island during the upper Ordovician and the same facies reached Washington Land and Hall Land, Greenland, in the middle Llandovery (lower Silurian). The line of interdigitation (facies front) between carbonate and mudstone facies was a static, near vertical, feature from the middle Llandovery to upper Silurian in Washington Land and into the lower Devonian in Ellesmere Island. This facies boundary has a slightly curved form when projected across Nares Strait; a shape which is entirely consistent with the meandering nature of facies boundaries. The present distribution and relationship of upper Ordovician to lower Silurian sediments indicate that they were part of a single laterally continuous sedimentary regime. Both vertical facies fronts arc entirely consistent with no post-depositional movement along Nares Strait; however, they cannot rule out left-lateral displacement of as much as 25 km. Displacements greater than this introduce far more geological problems than they solve.
... The most frequently mentioned magnitude of the leftlateral displacement of Greenland relative to Canada is 200-300 km (Wegener 1924, Carey 1958, Clarke 1968, Johnson 1971, Keen & Barrett 1973, Srivastava 1978. Fig. 3 illustrates reconstruction with 250 km of displacement. ...
Nares Strait, part of a 1400km long lineament between Canada and Greenland, may be one arm (along with Lancaster Sound and Baffin Bay) of a triple junction. Study of the geological literature on the area from N Baffin Island to N Greenland revealed a number of possibly important markers and structural features. The most important of these are: 1) the structural boundary between deformed and undeformed rocks at the margin of the Palaeozoic fold belt of N Ellesmere Island and Greenland and 2) the large asymmetrical NW-trending graben systems of N Baffin Island and Greenland, whose fault blocks tilt to the NE and SW respectively. Correlation of these features across Nares Strait suggests that movement along the Strait of a minimum of 200km has occurred since the mid- Palaeozoic, and a maximum of 300km since late Precambrian times. It seems probable that the movement occurred in a number of episodes, beginning with rifting in the Proterozoic and ending in the Tertiary Eurekan orogeny.-Author
... There is a general consensus that the Nares Strait lineament is fault controlled, but little as to the nature of the structure. ls the Strait the site of a transcurrent fault (Taylor 1910, Wegener 1924, du Toit 1937, Wilson 1963, a megashear (Carey 1958, Hilgenberg 1966), a transform fault (Wilson 1965), a rift valley (Kerr 1967a), a subduction zone (see Srivastava 1978), a spreading centre (Miall 1981 ), or even a combination of these structures? ...
The name Nyeboe Land fault zone is proposed for a major dislocation zone in the W part of the N Greenland fold belt, traceable along the outer coast for 300km. Two other names are introduced: 1) the Hand Bugt fault, a reverse fault that juxtaposes strata at least as old as Cambrian to the N, against Silurian rocks to the S, and 2) the Wulff Land anticline, a major fold structure that parallels the fault zone and the outer coast. The Wulff Land anticline is regarded as having formed during the main diastrophism of the Franklinian geosyncline in Devonian time; the Nyeboe Land fault zone is considered an expression of later Tertiary reactivation. Stratigraphic data are presented that define more accurately than hitherto the Franklinian Cambro-Ordovician platform margin. The Nyeboe Land fault zone, the Wulff Land anticline and the platform margin can be correlated across Nares Strait with on-line features on Judge Daly Promontory within the central Ellesmere fold belt. It is concluded that main crustal displacements in the late Phanerozoic probably took place along fracture systems oblique to Nares Strait rather than affecting the separation of Greenland and Ellesmere Island as separate plates.-Author
... Few students of continental drift and plate tectonics of the Arctic have resisted the temptation to accommodate movement between Greenland and North America by transcurrent displacement along the linear channel between Greenland and Ellesmere Island -Nares Strait (Taylor 1910, Wegener 1922, Carey 1958, Wilson I 963, Srivastava 1978. Modern reconstructions, many of which have received wide approval, consider the Labrador Sea and Baffin Bay as oceans formed by sea-floor spreading, with the consequent separation of Greenland and Baffin Island resulting in substantial movement along Nares Strait; in some recent models as much as 300 km or more of sinistral motion has been involved ( e.g. ...
Onshore geological investigations in the Smith Sound region are now so advanced as to allow correlation between Canada and Greenland to be made with confidence. The Precambrian Shield is unconformably overlain by unmetamorphosed Proterozoic strata (Thule Group) that are best preserved in Greenland, where they attain a thickness of at least 4.5km. <1100m are present in SE Ellesmere Island, but the succession is so similar to the lower part of the Greenland succession that unit to unit correlation of both sedimentary and volcanic rocks is possible. This correlation strongly supports the concept of a single intracratonic basin (Thule Basin) spanning the S part of Nares Strait. In Greenland the basin is well defined and its N margin is at approx 78o15'N. In Ellesmere Island paucity of outcrop provides less definition but the N margin lies between Baird Inlet (78o30'N) and Bache Peninsula (79oN). The lithological and thickness correlation of the Proterozoic successions on the opposite shores of the Smith Sound region stongly suggests that any tectonic movement along the Nares Strait lineament has not resulted in major net transcurrent displacement of Greenland and Ellesmere Island.-Authors
... The assembly of Pangaea is not possible on the earth of the present radius, but on a smaller globe, a globe such as is demanded by the orocline analysis, these difficulties vanish. (Carey, 1958) At this time he completely abandoned his equivalent of the Plate Tectonic theory and concentrated on expanding Earth theory, which he considered a more viable solution to what is observed geologically. (Carey called it the expanding Earth but a later Australian geologist, James Maxlow, in 1995 coined the term "expansion tectonics" ...
This thesis investigates, as case studies, three little-known scientific concepts, each put
forth by a well-known, respected scientist. In each case the concept suggested challenges
an established scientific paradigm and, if correct, would significantly alter our current
view of the universe. All three concepts seem to be ripe to invoke a paradigm shift yet
none has yet occurred. This study looks at these cases through the lens of (1) the
rhetorical arguments used by these scientists and their followers to advance their
proposed concepts, and (2) the paradigm shift model as put forth by Thomas Kuhn. A
detailed, largely non-technical, discussion is given for each concept to enable readers to
understand the nature of what is being proposed and the continuing research in each of
these scientific fields. Even though the original three scientists are deceased, their
concepts still have an active, albeit small, following in today’s scientific community.
... Tectonocists have therefore used the geom- etry of bathymetric contours or the edge of the continent-ocean boundary to create "best-fit" rotation poles to account for the crustal extension related to plate break-up (e.g. Carey, 1958;Sproll and Dietz, 1969;Powell et al., 1988), while some have also made an attempt to restore the amount of stretch crust along the margin (e.g. Williams et al., 2011;Veevers, 2012). ...
In recent years several tectonic reconstructions have been presented for Australia-Antarctica break-up, with each putting the Australian plate in a different location with respect to Antarctica. These differences reflect the different datasets and techniques employed to create a particular reconstruction. Here we show that some of the more recent reconstructions proposed for Australia-Antarctica break-up are inconsistent with both our current knowledge of margin evolution as well as the inferred match in basement terranes on the two opposing conjugate margins. We also show how these incorrect reconstructions influence the fit of the Indian plate against Antarctica if its movement is tied to the Australian plate. Such errors can have a major influence on the tectonic models of other parts of the world. In this case, we show how the position of the Australia plate can predetermine the extent of Greater India, which is (rightly or wrongly) used by many as a constraint in determining the timing of India-Asia, or India-Island Arc collisions during the closure of Tethys. We also discuss the timing of Australia-Antarctica break-up, and which linear magnetic features are a product of symmetric sea-floor spreading versus those linear magnetic features that result from rifting of a margin. The 46 Ma to 84 Ma rotational poles previously proposed for Australia-Antarctica break-up, and confined to transitional crust and the continent-ocean transition zone, more likely formed during earlier stages of rifting rather than during symmetric sea-floor spreading of oceanic crust. So rotation poles that have been derived from magnetic anomalies in such regions cannot be used as input in a plate reconstruction. A new reconstruction of the Australia-Antarctica margin is therefore proposed that remains faithful to the best available geological and geophysical data.
... By this time, there had been significant advances in the Earth sciences since the Heroic Era, and the application of this new knowledge to the Earth's only polar continent led to great interest in reports from the region. These advances included a robust geological time scale through radio-isotopic dating (Holmes, 1965), and the different character of the crust beneath oceans and continents recognized from seismology, along with the concept of continental drift from geology (Du Toit, 1937;Carey, 1958), later embodied in the theories of Sea Floor Spreading and Plate Tectonics (Dietz, 1961;Hess, 1962;Wilson, 1965). ...
The scale and antiquity of the Antarctic Ice Sheet was sensed from the time of earliest exploration a century ago. However, significant advances in scientific thinking, along with logistics and technology for gathering data from the continent itself, were required before a clear and consistent framework for ice-sheet history and behaviour could develop and this has emerged only in the last few years. The main features of the present ice sheet were established by over-snow traverses during and following the International Geophysical Year (1957–1958), but the timing and circumstances of its origin remained uncertain. Geological records of post-Jurassic time were largely buried under the ice or the sea floor around the Antarctic margin, though a few radiometric ages from the new K–Ar dating indicated Antarctic glaciation was likely older than the Northern Hemisphere ice ages of the Quaternary Period.
... By this time, there had been significant advances in the Earth sciences since the Heroic Era, and the application of this new knowledge to the Earth's only polar continent led to great interest in reports from the region. These advances included a robust geological time scale through radio-isotopic dating (Holmes, 1965), and the different character of the crust beneath oceans and continents recognized from seismology, along with the concept of continental drift from geology (Du Toit, 1937;Carey, 1958), later embodied in the theories of Sea Floor Spreading and Plate Tectonics (Dietz, 1961;Hess, 1962;Wilson, 1965). ...
This special issue on "Antarctic Climate Evolution — view from the margin" presents results from modelling studies and reports on geoscience data aimed at improving our understanding of the behaviour of the Antarctic ice sheet and the climate of the region. This research field is of interest because of the sensitivity of the polar regions to global warming, and because of the influence of the Antarctic ice sheet on global sea level and climate through most if not all of the Cenozoic Era. The Antarctic ice sheet both responds to and forces changes on global climate and sea level. We need to be aware of the scale and frequency of these changes if we are to understand past patterns of environmental change elsewhere on earth. It was only three decades ago that we discovered from strata drilled in shelf basins on the Antarctic margin that the Antarctic ice sheet had a history that predated the Quaternary ice ages by over 20 million years (Hayes et al., 1975). Later that year the first interpretation of Antarctic glacial history through the Cenozoic Era from oxygen isotopes, recorded in foraminifera from deep-sea sediment cores, was published (Shackleton and Kennett, 1975). Revisions with a more extensive database have modified the story a little (Miller et al., 1987; Zachos et al., 2001), and there have been recent attempts to resolve the temperature–ice volume ambiguity (Lear et al., 2000). However, reports on strata drilled on the Antarctic margin have unambiguously shown the character of this huge ice sheet, which was oscillating in the Oligocene (Barrett et al., 1987; Barrett, 1999) with a period and magnitude comparable with the Northern Hemisphere ice sheets of the Quaternary (Naish et al., 2001a,b). In this issue we present further research on the history of the Antarctic ice sheet from Oligocene to recent times, most of them from the Antarctic margin, but with some on the nature of the deep-sea isotope record, and others using recently developed modeling techniques to investigate the influence of atmosphere, ocean and biosphere on past Antarctic climate. This special issue is the third in three years on the theme of Antarctic Climate Evolution. The first followed a workshop in Erice, Sicily, in 2001 to report on results from ANTOSTRAT, a SCAR-sponsored project for gathering and analysing circum-Antarctic seismic data for planning and promoting offshore drilling for climate history. The introduction to that issue (Florindo et al., 2003) provides a review of the recent history of circum-Antarctic drilling by the Ocean Drilling Program (Legs 113, 114, 119, 120, 177, 178, 188 and 189) and the Cape Roberts Project. For a more comprehensive review of earlier drilling in the Ross Sea region (Deep Sea Drilling Project Leg 28, Dry Valley Drilling Project, McMurdo Sound Sediment and Tectonic Studies, Cenozoic Investigations in the western Ross Sea) see Hambrey and Barrett (1993). The first of these issues (Florindo et al., 2003) featured a global plate reconstruction of the Southern Hemisphere through Cenozoic time with emphasis on evolution of Cenozoic seaways (Lawver and Gahagan, 2003) along with a study of the inception and early evolution of the EAIS using a new coupled global climate (GCM)– dynamic ice sheet model (DeConto and Pollard, 2003b), as well as data from recent drilling around the margin covering time period from Cretaceous to the present. A second special issue on the same theme (Florindo et al., 2005) also featured a mix of modelling and data papers with a focus on the Eocene–Oligocene boundary and the initiation of ice sheet growth, including a pioneering attempt to evaluate the relative influence of fluvial versus glacial processes in shaping the landscape of the Prydz Bay sector of Antarctica (Jamieson et al., 2005). The remainder of the issue comprised further papers on seismic stratigraphy and reports from drilling around the margin. The papers to be found in this special issue, like the previous two, maintain the mix of modelling- and data-oriented papers that reflect the range of this research. Published 1-8 3.8. Geofisica per l'ambiente JCR Journal
Textbooks teach the principles of science. Lyellʼs geology textbooks emphasized vertical crustal movement. He avoided far-fetched continental-drift hypotheses by Hopkins in 1844 and Pepper in 1861. Their notions of drift were supported by fossil and paleoclimate evidence, but their causes were global magnetism and electrochemical crystallization and dissolution. Danaʼs textbooks from 1863 to 1895 taught that the symmetry of North America proved it had always stood alone; thus Americans were conditioned to reject Wegenerʼs concept of a Carboniferous supercontinent. Unaware of Wegenerʼs hypothesis in 1912, Schuchert launched a textbook series that guided American geological opinion from 1915 to the 1960s. His paleogeographic models required Carboniferous land bridges to connect fixed continents. He and coauthors Longwell and Dunbar eventually realized that Wegenerʼs continental-drift hypothesis would disprove land-bridge theory and solve problems of mountain ranges, paleoclimates, and fossil distributions, but they guarded against it in their textbooks. Already in 1927, Holmes proposed that convection with sea-floor spreading drove continental drift, but editor Schuchert would not publish that breakthrough. Geologists Du Toit, Van der Gracht, Holmes, Shand, Bailey, and Grabau showed the merits of continental drift, but their publications had little impact. Willis accepted the invitations of Schuchert in 1932 and Longwell in 1944 to write papers opposing Wegenerʼs hypothesis. Simpson contributed with paleontologic opposition. In 1944 Holmes published a British textbook that showed how continental drift could change geology. It was Holmes, Carey, and Wilson, as much as the Americans Hess and Dietz, who should be credited with instigating the plate-tectonic revolution.
The paleogeography of pre break-up Pangea at the beginning of the Atlantic Spreading has been a subject of debate for the past 50 years. Reconciling this debate involves theoretical corrections that cast doubt on available data and paleomagnetism as an effective tool for performing paleoreconstructions. This 50-year-old debate focuses specifically on magnetic remanence and its ability to correctly record the inclination of the paleomagnetic field. In this paper, a selection of paleopoles was made to find the great circles containing the paleomagnetic pole and the respective sampling site. The true dipole pole (TDP) was then calculated by intersecting these great circles, effectively avoiding non-dipolar contributions and inclination shallowing, in an innovative method. The great circle distance between each of these TDPs and the paleomagnetic means show the accuracy of paleomagnetic determinations in the context of a dominantly geocentric, axial and dipolar geomagnetic field. The TDPs calculated allowed a bootstrap analysis to be performed to further consider the flattening factor that should be applied to the sedimentary derived paleopoles. It is argued that the application of a single theoretical correction factor for clastic sedimentary-derived records could lead to a bias in the paleolatitude calculation and therefore to incorrect paleogeographic reconstructions. The unbiased APWP makes it necessary to slide Laurentia to the West in relation to Gondwana in a B-type Pangea during the Upper Carboniferous, later evolving, during the Early Permian, to reach the final A-type Pangea configuration of the Upper Permian. This article is protected by copyright. All rights reserved.
Two morphologies must be considered for Antarctica, one the bedrock morphology and the other the glacial morphology which is geologically ephemeral (figs. 2 and 3). The continent has been crisscrossed by oversnow traverse parties and aircraft during and since the International Geophysical Year of 1957–58, so that there is now a reconnaissance knowledge of both morphologies, which have been shown on a map of Antarctica, at a scale of 1 to 5 million with contours at intervals of 500 metres, published by the American Geographical Society in 1962. There is now international agreement that the metric scale will be used in Antarctica and that maps will be oriented so that the Prime Meridian (of Greenwich) is at the top of the map.
The Magallanes fold and thrust belt (FTB) presents a large-scale curvature from N-S oriented structures north of 52°S to nearly E-W in Tierra del Fuego Island. We present a paleomagnetic and anisotropy of magnetic susceptibility (AMS) study from 85 sites sampled in Cretaceous to Miocene marine sediments. Magnetic susceptibility is lower than 0.0005 SI for 76 sites and mainly controlled by paramagnetic minerals. AMS results indicate that the sedimentary fabric is preserved in the undeformed areas of Tierra del Fuego and the more external thrust sheets units, where an incipient lineation due to layer parallel shortening is recorded. Prolate AMS ellipsoids, indicating a significant tectonic imprint in the AMS fabric, are observed in the internal units of the belt. AMS results show a good correlation between the orientation of the magnetic lineation and the fold axes. However, in Península Brunswick, the AMS lineations are at ~20° counterclockwise to the strike of the fold axes. Pretectonic stable characteristic remanent magnetizations (ChRM) were determined in seven sites. A counterclockwise rotation (21.2° ± 9.2°) is documented by ChRM data from four sites near the hinge of the belt in Península Brunswick and near Canal Whiteside while there is no evidence of rotation near the nearly E-W oriented Vicuña thrust within Tierra del Fuego. The curved shape of the Cenozoic Magallanes FTB is not related to vertical axis rotation, and thus, the Magallanes FTB can be considered as a primary arc.
Three hypotheses for orocline development including (1) primary plate
boundary shape, (2) arc rotation, and (3) arc overprinting were
evaluated for the formation of the Pennsylvania orocline, Maryland, USA,
using foliation intersection/inflection axes preserved within
porphyroblasts (FIA). The distribution and the timing of sequences of
FIA data measured from garnet porphyroblasts within the Loch Raven
formation are statistically the same across the Pennsylvania orocline,
suggesting that all subregions of the arc have experienced the same
deformation history. This may suggest that the geometry of the
paleocontinental margin controlled the basic shape of this orocline.
However, if the orocline is treated as three separate regions, defined
by the SW-WE-NE trending portions of its slightly staircase-shaped
outline, a much higher proportion of the NNE-SSW trending FIAs (set II)
are preserved in the W-E trending region than those to either side. This
geometric relationship could have resulted from WNW-ESE directed bulk
stress causing a zone of dextral shear along what is now the W-E
trending portion of this orogen. It appears that what could have been an
early formed W-E trending sinistral transform shear zone was
preferentially dextrally reactivated during the Taconian orogeny.
Following the major contributions of Wegener and Argand (Part 1), it was the work of synthesis carried out by R. Staub that represented the major contribution Alpine geology made with respect to that heritage. The research work of young scientists (Gagnebin, Juvet, Wavre, Leuba) who had been influenced by Argand was of lesser importance. Ampferer’s ground breaking contribution, coming along with illuminating graphic illustrations, was all but ignored. Although remaining fairly popular, the theory of continental drift found itself under the heavy fire of criticism from influential geologists in the USA and in Europe. In order to test the validity of the idea, C.E. Wegmann suggested linking geological field work with oceanographic research. He showed that the trajectories of drifting had to be conceived as following the small circles of the sphere. With regard to Alpine geologists of the time, they were renowned for the high quality of their geological mapping. This remained the very special activity in which they excelled, but they focused on topics that were becoming narrower and narrower, and increasingly specialised. The new avenues for research that Holmes and Hess opened up had but little impact on Alpine geologists. In fact, they apparently remained unaware of a note by Holmes written in German and published in a Swiss journal. On the eve of the Second World War, the meeting of the Geologische Vereinigung devoted to the origin of the Atlantic Ocean confirmed that continental drift was being seriously challenged, although a few papers pointed to new developments, e.g. that in Iceland extensional tectonics had been active for the last 5,000 years. Most Alpine geologists were either highly critical of the theory of plate tectonic when it arrived or expressed serious reservations towards the idea. Of the exceptions, first Laubscher and then Bernoulli showed very clearly how important the new theory could be for understanding the evolution of Alpine orogeny. Continental drift and plate tectonics were very much the product of the creative imagination of human minds. Whereas Wegener used a broad range of confirmed results, plate tectonics sprang out of the new research being carried out in the domain of oceans. Graphic illustration was one of the favourite vehicles used to put across these new perspectives. Sometimes their impact remained alive long after their author had withdrawn his backing for the idea (as was the case for Argand’s “embryonic tectonics”); sometimes, even in spite of their very high standard, they were just ignored (which was the case for Ampferer).
The tectonic evolution of Alpine Corsica and surrounding areas is described through a series of plate reconstructions from the onset of the Alpine convergence in the early Cretaceous (~ 120 Ma) to the early Miocene (~ 19 Ma). The southward subduction of Eurasian lithosphere (Alpine Tethys) beneath Adria and Iberia, and the subduction of Ligure–Piemontese Ocean (Iberian plate) beneath the Adria margin, led to the formation of a trench–trench–trench triple junction close to the northern margin of Adria. During the first phase, from the early Aptian (~ 120 Ma) to the Campanian (~ 75 Ma) the northeastern oceanic part of the Iberian plate subducted beneath Adria along an intra‐oceanic subduction zone between Corsica and Adria. This trench was linked to the eastern Iberian convergent margin (having opposite polarity) through a transform fault. Starting from the Campanian and until the Oligocene (~ 34 Ma) oceanic and transitional crust was obducted onto Hercynian Corsica and Eurasia, giving rise to HP metamorphism within the subducted continental crust. From ~ 40 Ma to the early Oligocene (~ 33 Ma) the onset of continental collision in the Alpine area led to thickening of the subducting Eurasian margin through the formation of the European lower crustal wedge. Finally, from the early Oligocene (~ 33 Ma) to the early Miocene (~ 19 Ma), during the rotation of the Corsica and Sardinia blocks, the Pyrenean–Alpine belt lost its continuity in Gulf of Lyon and Provençal area, while the western Adriatic margin (Apennine domain) became a left‐lateral transpressive belt linking the western Alpine chain with the Calabrian Arc.
Aeromagnetic data from parts of the east coast of South America and the west coast of Africa have been compiled and compared. The data available include detailed surveys in Africa over Sierra Leone, the Ivory Coast and Ghana and in South America over Guyana and Surinam. These show a distinct character difference along a boundary between the east and west of the middle of the Ivory Coast. The western pattern is a pronounced, banded pattern of highs and lows trending approximately N 20–40°E, with wavelengths of about 30 km and total amplitudes of 500–800 gammas. To the east, the pattern is quite different as no banding is present and only more local features can be detected.In South America a series of anomalies strike N 45°E, with wavelengths and amplitudes similar to those found in the Ivory Coast.New data from high altitude aeromagnetic surveys have been flown over Africa and South America. These confirm the patterns found in the detailed surveys and show that the Brazilian shield area is quite different magnetically than the Guiana shield. Instead it resembles the eastern part of the African area under study. It therefore appears that the shield in the Ivory Coast in Africa and in the vicinity of the Amazon trough in South America can be divided into two portions. When the two continents are refitted these lines become one continuous ancient geologic “tie line” between the continents. This sort of tie line based on magnetic anomaly character provides yet another constraint in precise pre-drift continental reconstructions.
Plate tectonic theory is used to predict the motion of small plates which remain coupled within discrete regions to adjacent large plates. Taking the Western Cordillera of North America as an example, it is shown that the three oroclines situated between Vancouver and Northern California are very largely removed when that portion of the orogenic belt south of the Klamath mountains is rotated back in time with the Pacific plate while retaining the northern Cordillera as a rigid portion of the American plate. The rotation at the Pacific-America pole is found to be 14.4° which suggests that the deformation began during the Oligocene.
This paper is part of the special publication No.154, Exhumation processes: normal faulting, ductile flow and erosion (eds:U.Ring, M.T. Brandon, G.S. Lister and S.D. Willett). This paper presents a new interpretation of the tectonic evolution of New Caledonia, based on the extrapolation of detailed structural analysis at several different scales. The coherent high-pressure schist belt of northern New Caledonia contains clear evidence for cyclicity in the orogenic process. Two switches from large-scale crustal shortening to extensional tectonism can be recognized. We propose that orogenesis initially resulted in significant crustal thickening, obduction, and high-pressure (15-20 kbar) metamorphism. There is a close temporal link between ophiolite emplacement and high-pressure metamorphism. The high-pressure rocks were exhumed during the first period of extensional tectonism (c.40-36 Ma) as a large coherent terrane (>1500 km 2). During the second period of crustal shortening the New Caledonia orogen was folded into upright megafolds. Basin and Range style normal faulting then took place, bringing the folded extensional terrane and the high-pressure rocks to their present crustal level. Metamorphic grade changes in the high-pressure belt are structurally controlled by these late- stage normal faults. However, movement on these faults is responsible for only the very last stages of exhumation.
Results of palaeomagnetic investigations of the Lower Cretaceous teschenitic rocks in the Silesian unit of the Outer Western Carpathians in Poland bring evidence for pre-folding magnetization of these rocks. The mixed-polarity component reveals inclinations, between 56° and 69°, which might be either of Cretaceous or Tertiary age. Apparently positive results of fold and contact tests in some localities and presence of pyrhotite in the contact aureole suggest that magnetization is primary, although a Neogene or earlier remagnetization cannot be totally excluded since inclination-only test between localities gives `syn-folding' results. Higher palaeoinclinations (66°-69°) correlate with a younger variety of teschenitic rocks dated for 122-120 Ma, while lower inclinations (56°-60°) with an older variety (138-133 Ma). This would support relatively high palaeolatitudes for the southern margin of the Eurasian plate in the late part of the Early Cretaceous and relatively quick northward drift of the plate in this epoch, together with the Silesian basin at its southern margin. Declinations are similar to the Cretaceous-Tertiary palaeodeclinations of stable Europe in the eastern part of the studied area but rotated ca. 14°-70° counter-clockwise in the western part. This indicates, together with older results from Czech and Slovakian sectors of the Silesian unit, a change in the rotation pattern from counter-clockwise to clockwise at the meridian of 19°E. The rotations took place before the final collision of the Outer Carpathians nappe stack with the European foreland.
Thermal analysis of the fossil magnetism of the Silurian Bloomsburg red beds of Pennsylvania indicates that there are two magnetic components; a thermally-discrete component of very great stability which is unchanged in temperatures up to 650 °C and which disappears between 650 and 700 °C, and a thermally-distributed component whose properties are defined by a series of blocking temperatures mostly in the range 300 to 550 °C. The latter is roughly parallel to the supposed direction of the Permian geomagnetic field, and is considered to have been induced by moderate heating during burial at about the time of the Appalachian orogeny. The former has apparently been unaffected by these events and is considered to be of Silurian age. The discrete component has a mean declination 6° W of N diverging by 29° from that observed by Graham (1949) in the Silurian Rosehill Formation which is situated some 300 km to the SW. The divergence is equal to the angular change in tectonic trend between the two areas. This result may be explained by supposing that this part of the Appalachian geo-syncline was straight in Silurian time and that the change in trend is a subsequent strain; that is, the Appalachians at this point are an orocline (Carey 1958). This is not to say that alternative hypotheses will not explain the observations, but it is clear that the question can be decided by further palaeomagnetic studies along this mountain chain.
Physicists believe that known laws should suffice to explain the Earth's behavior, but the complexities of geology have defied simple explanation. Today three developments are helping to solve the problem. First, summary interpretations such as the centennial project of the Geological Society of America are being compiled of the upper 1% (60 km) of the Earth's radius. Second, new methods show the behavior at great depths in the Earth. Third, combining these studies suggests which physical laws apply at various depths. In the mantle the pattern of convection, by viscous fluid flow is becoming-clear; Upwelling plumes start rifts which fragment the lithosphere. Subduction carries plates down. In the strong lithosphere the Coulomb-Navier, laws of brittle failure apply and confine fractures to three types corresponding to plate boundaries. Other laws determine where island arcs form. The idea that some large areas of the surface act independently of the surrounding areas is becoming recognized and simplifies analysis. One large area which behaves independently lies in the southwestern United States, these developments and the observation that most plate boundaries are elevated provide a means of applying laws of physics to classifying ranges, including folded mountains, island arcs, rifts, and ridges, into three classes and many subclasses, illustrated by examples.
Summary An attempt is made to bring into concordance some of the known facts of zoogeography, phytogeography, and geology with respect
toextant geographical dispersal of certain land plant groups (Hepaticae, Coniferales, Angiospermae). Such a synthesis faces innumerable
obstacles, some of which have only recently been removed by proliferation of paleomagnetic and other pertinent geological
data suggesting that the Australian bloc was contiguous to Antarctica until at least the start of the Tertiary and has rapidly
moved northward. As a consequence, concepts of a single “center” of origin of the angiosperms — or any other group — which
encompasses a region including all or part ofboth Southeast Asia (and the adjoining islands that are historically part of the Eurasian bloc) and Australasia, seem untenable
since these two regions have only recently been juxtaposed. It is assumed that this recent approximation offers the most valid
analysis of the significance of “Wallace’s Line” — various versions of which have been almost uniformly accepted by three
generations of zoogeographers.
In the general area “between Assam and Fiji” and between Japan and Tasmania-New Zealand — the Pacific region both Smith (1967,
1970) and Takhtajan (1969) postulate as the “center of origin” of the angiosperms or the “cradle of the angiosperms” — we
have a similar “center of diversity” for other groups, including the Jungermanniae (leafy Hepaticae), Musci, Ferns, and Conifers.
The richness of the fauna of this area has been amply documented. The thesis is presented that the reason for this extraordinary
luxuriousness in types and numbers, in the entire biota, is not necessarily owing to theorigin of the angiosperms or of any other group in such a raonolithically conceived region, but tojuxtaposition of elements of two rich biotas — Laurasian-derived and Gondwanaland-derived. Both elements, furthermore, having probably
developed in areas peripheral to the central land masses of Laurasia and Gondwanaland — in other words in oceanic loci where
climatic parameters are more “favorable” — are assumed to have been intrinsically richer than “continental biotas.” Thus the
extreme diversity which has been noted is presumed to have arisen by juxtaposition and partial fusion of already diversified,
numerically and taxonomically rich biotas. It is assumed that the varying reconstructions of land and ocean areas, as they
are thought to have existed about 150–180 m.y. ago (see, e.g., the reconstruction of Pangaea by Wilson, 1963, or the reconstruction
of Dietz & Holden, 1970), are essentially correct, although all are probably incorrect in some details. If such a concept
of an early Mesozoic configuration juxtaposing the continents in one, or two, major land masses is correct, then the regions
that involved most of Africa, all of eastern North and South America, and most of Europe were once strongly “continental”
in character, prior to the formation of the Atlantic Ocean. In contrast, the entire region from Australasia to northern India,
the “belly” of Asia, and east Asia has been consistently peripheral to what has become the Pacific Ocean — hence has been
highly oceanic. It is assumed, thus, that climatic conditions have been consistently more “fit” for the evolution of a mesophytic
and mesophyllous flora. The perhaps insupportable assumption is made that, since virtually allextant primitive Angiospermae are large leaved and very few are truly microphyllous and sclerophyllous, at least the immediate common
ancestral types probably had a similar morphology — and, hence, similar ecological requirements and restrictions.
In addition, it is postulated that not only was there a Tertiary “rafting” northward of elements of a Gondwanaland flora,via the Australian bloc, but there had been a prior such “rafting”via the Indian bloc during Jurassic-Triassic times. The “Indian raft” was presumably of major significance in enriching the Laurasian
flora, during the first half of the Mesozoic, by moving northward a host of taxa. The fossil Coniferales studied in detail
by Florin (1963) demonstrate nearly conclusively that such “rafting” of various taxa north-ward on the Indian bloc took place.
The presence today in the flora of the Himalayan Uplands of a number of highly disjunct, ancient taxa of Hepaticae such asApotreubia Hatt. et al.,Lophochaete Schust.,Takakia Hatt. & Inoue,Haplomitrium Nees, and others —taxa typical of highly oceanic sites — suggests that, at least in the case of the Bryophyta, such ancient
links have not been wholly obliterated. The role of the “Indian raft” in the dispersal of ancient angiosperms or pre-angiosperms
is conjectural at best, but the timing may have been too early.If the existence of Triassic, simple angiosperms can be demonstrated, however, it is not impossible that,if these existed in Gondwanaland, some of them may have been dispersed to Laurasia in the manner postulated for the dispersal
of certain conifers and Hepaticae.Since the existence of a variety of angiosperms by early Cretaceous times almost requires extrapolation backwards in time to the
Jurassic or Triassic for theorigin of the earliest Angiospermae, the role of the Indian migration in the possible infusion of Gondwanaland angiosperms or pre-angiosperms
into Laurasia cannot be rejected at this time. Furthermore,if recurrent reports of pre-Cretaceous angiosperms can be confirmed,then the horizon on which the angiosperms appeared is pushed back far enough in time so that the Indian migration becomes relevant
to the early dispersal of the angiosperms.
Sanmiguelia, cited (by Axelrod, 1970, and others) as a Triassic angiosperm, is considered by many other botanists to belong to some other
major group of plants. Supposedly pre-Cretaceous plants referred toPalmoxylon (Tidwell et al., 1970) have been a source of controversy and are now widely believed to be of much later age, although Axelrod
(loc. cit., p. 279) claims the fossils occur “insitu, in rocks certainly dated as Jurassic....” Axelrod, indeed, gives an optimistic evaluation of the evidence for pre-Cretaceous
angiosperms, stating (p. 312) that they “were definitely contributing to the fossil record as far back as the middle Jurassic,
if not in the late Triassic....” This evaluation is at variance with that of Scott, Barghorn & Leopold (1960). However,by extrapolation one must almost assume an origin of the angiosperms no later than just before the dawn of the Cretaceous.
It is concluded that existing evidence does not allow us to infer anything definite as to the locus of origin of the angiosperms.
There is no positive evidence that Gondwanaland was — or was not —involved. However, the virtual restriction of a number of
primitive, vesselless angiosperm taxa (Winteraceae, Amborellaceae) to Southern Hemisphere localities that were part of a former
Gondwanaland is suggestive. The presence of primitive Hamamelidae (Trochoden-draceae, Tetracentraceae, Cercidiphyllaceae —
the first two vesselless, the last with fairly primitive wood structure) in Southeastern Asia is equally suggestive. The possibility
must be entertained that some dispersal of the earliest angiosperms took place from Laurasia to Gondwanaland, orvice versa, shortly after origin of the group. Since large land blocs (the Indian bloc early in the Mesozoic; the Australian bloc in
the Tertiary) are now known to have made rapid migrations northward, leaving their former Gondwanaland attachment, there is
an obvious and simple mechanism by which massive transport of whole floras and faunas was possible. As this flow was of necessity
unidirectional, it is likely that the rich diversification of some groups in the Northern Hemisphere was derived in part by
the infusion of elements from the Gondwanaland flora. The luxuriousness, thus, of the southeast Asiatic biota may have had
a triple origin, with partial origin (in the case of organisms originating prior to the Cretaceous) in the “rafting” north
of the Indian bloc, while a second infusion probably late in the Tertiary occurred when the Australian bloc became approximated
to the Asiatic bloc; subsequent expansion in ranges resulted in diffusion of these two elements into the endemic Laurasian-derived
stock. The, at times, seemingly close links between the Laurasian-derived flora of the Indomalayan-East Asiatic region and
the Gondwanaland-derived Australasian floras which are seen in the zone from New Guinea to Taiwan and the Philippines are
probably at least in large part derived by transgression across “Wallace’s Line” (or, earlier, in the Tertiary, across the
wider but rapidly narrowing straits which existed prior to the present narrow channel constituting “Wallace’s Line”). Because
there were two massive transfers of large land areas since the start of the Mesozoic (the Indian bloc in the early Mesozoic;
the Australasian bloc in early and mid-Tertiary times), there must have been, equally, massive transfers of foreign floristic
elements — the infusion of genera and families from Gondwanaland regions to Laurasia. Such mass transfers, at two times and
two loci, if this can be demonstrated to have taken place, so complicates the disentanglement of the early history of several
plant groups — including that of the angiosperms — that it may well prove impossible to ever definitively place the point
oforigin of the angiosperms. If the above analysis is even remotely correct, we must assume that by the start of the Cretaceous primitive
angiosperm units existed in both Laurasia and Gondwanaland; this assumption is not too difficult to accept in view of the
demonstrated occurrence by early Cretaceous times of angiosperms with several distinct pollen types (hence presumably already
somewhat diversified) in areas as remote from the Pacific Basin (and from Gondwanaland) as present-day Maryland and New Jersey.
The assumption that the massive Indian bloc rafted northward, not as a denuded bloc but as a large, physiographically and
biotically complex unit, carries with it one dividend: it explains a paradox which, to this author, has been long irresolvable,
namely the basically “oceanic” and extremely rich and mesophytic nature of much of the Himalayan flora — a flora with repeated
close affinities to that of oceanic regions of the western Pacific. We can find a simple explanation in the assumption that
the “leading edge” of the Indian bloc, folded by resistance to its migration, consisted of a series of mountains which were
a fit environment for the survival (or evolution) of basically oceanic and mesophytic plants whose floristic affinities in
many cases seem to be with those of other sections of Gondwanaland.
Most reconstructions of Pangaea, the early Mesozoic supercontinent,
assume an Earth of modern dimensions. Such reconstructions produce major
geometric and geological fit inconsistencies particularly in areas such
as the Arctic, Caribbean, Mediterranean, and southeast Asia and
Indonesia. The ocean floor spreading history of these regions and the
adjacent oceans indicates that they have grown by areal expansion since
their initiation. In contrast, the various reconstructions of Mesozoic
and Cenozoic stages which assume an Earth of constant dimensions,
require that these regions, either initially or during their
development, should contract in area. The geological evidence from the
continental margins and from the Earth's oceans does not support the
amount of subduction, either in whole or in part, required by the
constant dimension hypothesis. It is shown that an exact fit of the
various continental fragments together to reform Pangaea, which agrees
with the geometric and geological matches, is obtained when the value of
the Earth's surface curvature is increased to the point at which the
diameter of the globe is 80% of its current mean value. This corresponds
in time to the late Triassic-early Jurassic. It is asserted that the
early Upper Jurassic to Recent ocean floor spreading data now available,
displayed here in maps, also demonstrate progressive global expansion
commensurate with an increase in diameter of 20% of the Earth's current
mean value. Series of maps employing a zenithal equidistant projection
are used to illustrate stages in the inferred development of certain
regions during the Mesozoic and Cenozoic according to the ocean floor
spreading data. The global expansion deduced from the geometric
requirements of the spreading data in these maps permits a much more
straightforward reading of the development of ocean basins and
associated displacement of continents; one which accords with the field
evidence. The inconsistencies seen in constant dimensions
reconstructions do not arise. The results are summarized in outline
hemisphere maps for which a new cartographic projection has been
developed.
Reconstruction of the major lithospheric plates 21 and 38 m.y. ago implies that linear island chains and aseismic ridges were not generated by localised sources of volcanism (hot spots) that have been fixed with respect to one another. If the linear volcanic chains are formed over mantle hot spots, the spots move at rates of 0.8 to 2 cm yr-1 with respect to one another.
Evidence which bears on the processes of continental drift, ocean floor spreading and the appearance of mid-oceanic rises is rapidly accumulating. The assumption that the mid-oceanic rises represent a steady state1,2 leads to estimates of the rate of ocean floor spreading and of subsidence which in turn suggest geophysical tests of the theory.
As a geology professor teaching a course in Introductory Geology, it did not make sense to me that Alfred Wegenerʼs theory of continental drift should have been rejected for nearly half a century. From what I knew of his evidence, it seemed convincing enough. Why were geologists so against these ideas? There must have been more to this history than what was commonly known.
I began this project with the feeling that the rejection of continental drift was a scandal for geology and for science. Scientists should not reject a correct interpretation for so long. In more familiar scandals, such as recent ones in finance, politics, sports, and religion, one naturally looks for cover-ups. If there were cover-ups here, what was being hidden and who was being protected? I collected all the important historical literature, and I found what I was looking for.
This is a revisionist history. It is based largely on a type of historical data that has been overlooked by others – the works of leading geology textbook authors. These authors are especially important, because their textbooks teach students the principles of the science. The theory of continental drift involved a new scientific paradigm, of mobile, not fixed, continents. The textbooks used in introductory geology courses defined the fixist paradigm and influenced the likelihood of a paradigm shift. I have thus paid extra attention to what the main English-language textbook authors wrote, and tried to understand in depth how these highly respected scientists thought. I know from long experience that scientists think just the way other people do.
Available geological and geophysical data indicate that the Troodos
massif is a Mesozoic volcanic complex the structure of which is
comparable with that, deduced from geophysical data, for present day
mid-ocean rises. It is suggested that the massif evolved beneath an
oceanic Tethys and may represent a fragment of a mid-Tethyan rise.
Early geologists considered that the Earth's surface is rigid and unchanging. They assumed that the whole Earth is static, except for enough sub‐surface contraction to build mountains. After seismology developed, most geophysicists agreed. A few scientists, notably Wegener, favoured a more mobile Earth. About 1965 fresh evidence showed that both theories were too simple. This evidence explained why neither theory had been able to relate the whole Earth's behaviour to laws of physics. Hence different aspects of geology had only been solved separately which had fragmented Earth science.
This paper proposes a compromise. It is that the rigid lithosphere fractures according to Navier's law of brittle failure which explains the properties and provides methods for classifying faults, plate boundaries and mountains and that the ductile mantle convects by laws of fluid flow in patterns partly controlled by lithospheric fractures. These dual, interacting influences explain tectonic behaviour. The pattern of currents is hidden. At any one time upwelling beneath continents only affects a few limited areas; today some are in southwestern United States, Central Asia, Botswana, Antarctica and rifts in East Africa, Europe and Siberia. Nevertheless recognition of upwelling currents may revolutionize geology because their cumulative effects have been great and neglected.
Best-fitting Euler vectors, closure-fitting Euler vectors, and a new global model (NUVEL-1) describing the geologically current motion between 12 assumed-rigid plates, were determined. The 1122 data from 22 plate boundaries inverted to obtain NUVEL-1 consist of 277 spreading rates, 121 transform fault azimuths, and 724 earthquake slip vectors. The model fits the data well. The strikes of transform faults mapped with GLORIA and Seabeam along the Mid-Atlantic Ridge greatly improve the accuracy of estimates of the direction of plate motion. Data shows that motion about the Azores triple junction is consistent with plate circuit closure, and better resolves motion between North America and South America. Motion of the Caribbean plate relative to North or South America is about 7 mm yr-1 slower than in prior global models. The direction of slip in trench earthquakes tends to be between the direction of plate motion and the normal to the trench strike. -from Authors
The crust of the earth is traversed by a lattice of tectonic lineaments which, since the Precambrian Era, were always and again acting as wrench faults. These shearing movements, independent of the specific crustal structure, are thought to be caused by the wandering of the poles. For the directions and movements of the lineaments correspond to the stress field that was induced by the readjustment of the geoid. The bursting lineaments caused a splitting of the continental masses. The great original continents of the Paleozoic Era disturbed the balance of revolution. There was a trend to separate the disturbing masses and to furnish the surface of the earth as proportionally as possible with continents. Thus, divergent forces started to work on the lineaments. The lineaments were stretched and became rift valleys. When the forces of the continental drift became overwhelming, the rift valleys burst along their median line. A new continent split off. The wandering continents endeavoured and are still endeavouring to reach, in the first place, the Pacific Ocean trying to confine it peripherically. From this trend comes the conformable architecture of the circumpacific tectonics. The drifting masses effected secondary disturbances of the balance of the rotating gyroscope. The poles tried to escape the wandering masses. Once more, lineaments became active, and the process of splitting continued. Consequently, the wandering of the poles and the continental drift are thought to be mutually initiating and controlling processes. The orogenic phases of the folding and the taphrogenic phases of the rift valley forming are in their global coincidence the display of this very process that concerns the entire crust of the earth.
Acceptance of the theory of plate tectonics was accompanied by the rise of the mantle plume/hotspot concept which has come to dominate geodynamics from its use both as an explanation for the origin of intraplate volcanism and as a reference frame for plate motions. However, even with a large degree of flexibility permitted in plume composition, temperature, size, and depth of origin, adoption of any limited number of hotspots means the plume model cannot account for all occurrences of the type of volcanism it was devised to explain. While scientific protocol would normally demand that an alternative explanation be sought, there have been few challenges to “plume theory” on account of a series of intricate controls set up by the plume model which makes plumes seem to be an essential feature of the Earth. The hotspot frame acts not only as a reference but also controls plate tectonics. Accommodating plumes relegates mantle convection to a weak, sluggish effect such that basal drag appears as a minor, resisting force, with plates having to move themselves by boundary forces and continents having to be rifted by plumes. Correspondingly, the geochemical evolution of the mantle is controlled by the requirement to isolate subducted crust into plume sources which limits potential buffers on the composition of the MORB-source to plume- or lower mantle material. Crustal growth and Precambrian tectonics are controlled by interpretations of greenstone belts as oceanic plateaus generated by plumes. Challenges to any aspect of the plume model are thus liable to be dismissed unless a counter explanation is offered across the geodynamic spectrum influenced by “plume theory”. Nonetheless, an alternative synthesis can be made based on longstanding petrological evidence for derivation of intraplate volcanism from volatile-bearing sources (wetspots) in conjunction with concepts dismissed for being incompatible or superfluous to “plume theory”. In the alternative Earth, the sources for intraplate volcanism evolve from the source residues of arc volcanism located along sutures in the continental mantle. Continental rifting and the lateral distribution of intraplate sources in the asthenosphere are controlled by Earth rotation. Shear induced on the base of the asthenosphere from the mesosphere as the Earth rotates is transmitted to the lithosphere as basal drag. Attenuation of the drag due to the low viscosity of the asthenosphere, in conjunction with plate motions from boundary forces, results in a rotation differential of up to 5 cm yr−1 between the lithosphere and mesosphere manifest as westward plate lag/eastward mantle flow. Continental rifting results from basal drag supplemented by local convection induced by lithospheric architecture. Large continental igneous provinces are generated by convective melting, with passive margin volcanic sequences following the axis of rifting and flood basalts overlying the intersection of sutures in the continental mantle. As rifting progresses, the convection cells expand, cycling continental mantle from sutures perpendicular to the rift axis to generate intraplate tracks in the ocean basin. Continental mantle not melted on rifting, or delaminated on continental collision, becomes displaced to the east of the continent by differential rotation, which also sets up a means for tapping the material to give fixed melting anomalies. When plates move counter to the Earth's rotation, as in the example of the Pacific plate, asthenospheric flow is characterised by a counterflow regime with a zero velocity layer at depths within the stability field for volatile-bearing minerals. Intraplate volcanism results when melts are tapped from this stationary layer along lithospheric stress trajectories induced by stressing of the plate from variations in the subduction geometry around the margins of the plate. Plate boundary forces acting in the same direction as Earth rotation, as for the Nazca plate, produce fast plate velocities but not counterflow, though convergent margin geometry may still induce propagating fractures which set up melting anomalies. Lateral migration of asthenospheric domains allows the sources of Pacific intraplate volcanism to be traced back to continental mantle eroded during the breakup of Gondwana and the amalgamation of Asia in the Paleozoic. Intraplate volcanism in the South Pacific therefore has a common Gondwanan origin to intraplate volcanism in the South Atlantic and Indian Oceans, hence the DUPAL anomaly is entirely of shallow origin. Such domains constitute a second order geochemical heterogeneity superimposed on a streaky/marble-cake structure arising from remixing of subducted crust with the convecting mantle. During the Proterozoic and Phanerozoic, remixing of slabs has buffered the evolution of the depleted mantle to a rate of 2.2 εNd units Ga−1, with fractionation of Lu from Hf in the sediment component imparting the large range in relative to observed in MORB. Only the high εNd values of some Archean komatiites are compatible with derivation from unbuffered mantle. The existence of a very depleted reservoir is attributed to stabilisation of a large early continental crust through either obduction tectonics or slab melting regimes which reduced the efficiency of crustal recycling back into the mantle. Generation of komatiite is therefore a consequence of mantle composition, and is permitted in ocean ridge environments and/or under hydrous melting conditions. Correspondingly, as intraplate volcanism depends on survival of volatile-bearing sources, its appearance in the Middle Proterozoic corresponds to the time in the Earth's thermal evolution at which minerals such as phlogopite and amphibole could survive in off-ridge environments in the shallow asthenosphere. The geodynamic evolution of the Earth was thus determined at convergent margins, not by plumes and hotspots, with the decline in thermal regime causing both a reduction in size of crust and continental mantle roots, the latter becoming a source for intraplate volcanism while the crust was incorporated into the convecting mantle.
The acceptance of large scale movements of the lithospheric plates requires an understanding of the forces responsible for such remarkable movements. Large scale thermal convection is to be expected in planetary interiors and in the earth in particular. A fundamental property of convection, that such motions can undergo radical and sudden changes in pattern in response to slow and continuous changes in physical conditions, is used to explain the maxima in world-wide tectonic and magmatic activity at 800 m.y. intervals. Gravity sliding of lithospheric plates as a cause of plate tectonics is shown to be compatible with order of magnitude arguments but fails to explain the initiation of these motions. The mantle plume hypothesis fails as a reasonable model on similar order of magnitude calculations. Earth expansion as a physical cause encounters serious difficulties, especially the implausibly large expansion required in the last 200 m.y. of earth history. Heat flow observations cannot readily be used to infer the major convective patterns in the mantle but the geoid, determined by satellite observations, departs from hydrostatic equilibrium and is thought to be a direct consequence of convective upwelling and downward currents in the mantle: it is the primary evidence for the pattern of upper mantle convection.
The reflection and refraction seismic data collected during the SEISMARMARA Leg 1 survey in the Sea of Marmara provide detailed imaging of sedimentary record and fault activity with deep penetration into its basement. First, a detailed analysis of pre-stack depth-migrated seismic lines crossing the Central Basin enable us to discuss the space and time relations of the large and smaller nested basins of the inner depression, as well as the diversity of style and rate of activity of motion at the diverse basin border faults.Second, forward modeling of OBS refraction arrival times reveals the effect of compaction on the sedimentary pile whereas its layering imaged by MCS as seismic reflectors rather recorded the tectonic evolution. Another major result of the refraction modeling is the identification of the crystalline basement. The latter is imaged about 1 or 2 km deeper than the base of the layered sedimentary sequence imaged on the coincident MCS profiles.This basement exhibits sharp topography across the Central High, the Kumburgaz Basin and the eastern tip of the Cinarcik–North Imrali Basin in an unexpected way with respect to the sea-bottom depressions. We furthermore imaged several large tilted basement blocks, which separate the deep basins as between the Cinarcik and Imrali basins. Despite the varying width of the NMT and the sizes of the tilted blocks, we propose that the imaged finite deformation results from a similar process of partitioning deformation over more than one or even two faults across the NMT that may have changed activity with time and space.
A quantitative morphological fit of Australia and Antarctica which is also geologically permissible.
Between 10° and 19°N the North Atlantic Ocean has been covered by four east-west crossings and one north-south section at 60°W, using a continuous seismic reflection recorder (air gun). The northernmost section extends to the Canary Islands. The region comprises a great variety of phenomena: mid-oceanic ridge, fracture zones, oceanic basins, volcanic islands, continental rises and part of a zone of negative gravity anomalies (the Vening Meinesz zone), running from the Puerto Rico Trench over Barbados into Trinidad. A central zone of the Mid-Atlantic Ridge appears to be void of sediment. In the fracture zones (grabenlike depressions that off-set the axis of the ridge) sedimentary thicknesses of the order of 1 km have been found. Evidence was found for the existence of current-influenced sedimentation other than from turbidity currents, and for the occurrence of erosion at depths of more than 5,000 m (the Vidal Channel). In the oceanic basins sedimentary thicknesses occur of maximum 2,000 m in the Cape Verde/Madeira Abyssal Plain and more than 1,400 m (no basement found) in the Demerara Abyssal Plain. The continuity of sedimentation from the continental rise into the abyssal plains proves that turbidites can be deposited on slopes with an inclination of 12′. Locally deposition of turbidites occurs on slopes with a much higher inclination. The occurrence of horizon A at a distance of 700 km from the axis of the Mid-Atlantic Ridge gives, after a correction for probable tectonic movements, a maximum of 1.2 cm/year possible spreading since the Upper Cretaceous. The uppermost layer of sediments in the Vening Meinesz zone of about 1 km thickness is only little deformed. This is also true for the Barbados Ridge, which implies that the sediments must be younger than the deformed Tertiary series found at Barbados. In discussing the distribution of sediments on the Mid-Atlantic Ridge and the configuration of the ocean basins to both sides, the possibility is considered that both the negative gravity zones and the ridge are secondary and relatively young features with regard to continental drift and ocean spreading. This interpretation would be alternative to the hypothesis that the distribution of sediments should be explained by large variations with time of the rate of spreading (or the rate of sedimentation respectively). The Late Cretaceous and the Miocene orogenies might represent principally different phases of the Alpine orogeny and the Mid-Atlantic Ridge as a topographic feature might be broadening constantly.
An 'incipient' spreading centre east of (and orthogonal to) the East Pacific Rise at 2 degrees 40' N has been identified as forming a portion of the northern boundary of the Galapagos microplate. This spreading centre was described as a slowly diverging, westward propagating rift, tapering towards the East Pacific Rise. Here we present evidence that the 'incipient rift' has also rifted towards the east and opens anticlockwise about a pivot at its eastern end. The 'incipient rift' then bounds a second microplate, north of the clockwise-rotating Galapagos microplate. The Galapagos triple junction region, in the eastern equatorial Pacific Ocean, thus consists of two counter-rotating microplates partly separated by the Hess Deep rift. Our kinematic solution for microplate motion relative to the major plates indicates that the two counter-rotating microplates may be treated as rigid blocks driven by drag on the microplates' edges3.
ANALYSES of seismic and continuous gravity measurements over the northern Tasman Sea1 have confirmed the existence of ``quasi-continental'' crust (20-26 km thick) beneath the parallel ridges-Norfolk Ridge, Lord Howe Rise and Dampier Ridge-that extend north-westward from New Zealand (Fig. 1). The intervening major trough, New Caledonia Basin, is shown to have a floor of true oceanic thickness (9 km).
DE GEER1 in 1926 first referred to the existence of a
physiographic lineament following the western continental slope of the
Barents Shelf from Norway to Svalbard, then running along the
north-eastern continental slope of Greenland and Canada. Subsequent
authors2,3 suggested that extensive dextral strike slip
movements had occurred along this line during the continental drift of
Greenland and North America away from Eurasia.
THE question of the contraction, stability or expansion of the Earth during geological time is one of the basic problems of the geosciences.
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