A scheme of deep mantle convection is proposed in which narrow plumes of deep material rise and then spread out radially in the asthenosphere. These vertical plumes spreading outward in the asthenosphere produce stresses on the bottoms of the lithospheric plates, causing them to move and thus providing the driving mechanism for continental drift. One such plume is beneath Iceland, and the outpouring of unusual lava at this spot produced the submarine ridge between Greenland and Great Britain as the Atlantic opened up. It is concluded that all the aseismic ridges, for example, the Walvis Ridge, the Ninetyeast Ridge, the Tuamotu Archipelago, and so on, were produced in this manner, and thus their strikes show the direction the plates were moving as they were formed. Another plume is beneath Hawaii (perhaps of lesser strength, as it has not torn the Pacific plate apart), and the Hawaiian Islands and Emperor Seamount Chain were formed as the Pacific plate passed over this “hot spot.” Three studies are presented to support the above conclusion. (1) The Hawaiian-Emperor, Tuamotu-Line, and Austral-Gilbert-Marshall island chains show a remarkable parallelism and all three can be generated by the same motion of the Pacific plate over three fixed hot spots. The accuracy of the fit shows that the hot spots have remained practically fixed relative to one another in this 100 m.y. period, thus implying a deep source below the asthenosphere. (2) The above motion of the Pacific plate agrees with the paleo-reconstruction based on magnetic studies of Pacific seamounts. The paleomotion of the African plate was deduced from the Walvis Ridge and trends from Bouvet, Reunion, and Ascension Islands. This motion did not agree well with the paleomagnetic studies of the orientation of Africa since the Cretaceous; however, better agreement with the paleomagnetic studies of Africa and of seamounts in the Pacific can be made if some polar wandering is permitted in addition to the motion of the plates. (3) A system of absolute plate motions was found which agrees with the present day relative plate motions (deduced from fault strikes and spreading rates) and with the present trends of island chains-aseismic ridges away from hot-spots. This shows that the hot spots form a fixed reference frame and that, within allowable errors, the hot spots do not move about in this frame.

Some inexact methods for estimating tectonic rotation poles are described. It is shown geometrically that each compressive plate margin has a “forbidden sector” in which the pole cannot lie. This may help to delimit the pole position, particularly for large orogenic structures. Five present-day compressive zones are examined, and their respective poles lie generally in line with the zone and about 20° to 40° from one end of it. A possible Atlantic-Mediterranean reconstruction is consistent with these findings.

The sea floor inside island arcs characteristically is less deep and has higher heat flow than the ocean floor outside the arc and trench system. Direct evidence from drilling and indirect evidence based on thin sediment cover, interrupted geologic trends, paleomagnetic studies, and fitting of pre-drift continental margins show that the lithosphere behind island arcs is young and commonly did not form on the mid-oceanic ridge system. The slab of dense lithosphere that flexes and sinks spontaneously through the asthenosphere under arcs is shown to sink at an angle that is steeper than the plane of the earthquakes. As a consequence, the trench and arc migrate seaward against the retreating line of flexure of the suboceanic lithosphere. Part of the warm asthenosphere pushed aside by the plunging slab migrates up by creep and as magma, then cools and forms new lithosphere in the extensional region behind the advancing island arc. Extension is favored where the lithospheric plate behind the arc is moving tangentially or away from the plate outside the arc. A series of maps shows the tectonic development of the western Pacific from mid-Eocene to the present. The maps are based on concepts developed from sea-floor spreading and the new global tectonics, and incorporate the postulate that new lithosphere can form behind advancing island arcs. The origin and later deformation of arcs and basins are shown as resulting mainly from the great shear between the northward-moving Australian plate and the northwestward-moving Pacific plate.

Paleocene and Eocene green cherty tuffs and pelagic sediments, in a generally volcanic rock sequence, crop out in Puerto Rico and the Dominican Republic. These rocks are here correlated by lithology and age with oceanic horizons found by reflection profiling and coring: Layer A in the Atlantic Ocean, and Layer A” and the Carib beds in the Caribbean Sea. The generally thin pelagic or airborne sediments in the ocean basins flank thicker clastic, pelagic, and volcanic deposits on the Antillean Ridge. Trenches north and south of the ridge apparently trapped most of the coarser volcanic material, preventing its reaching the ocean basins. Lithification (except that forming chert) and deformation of these sediments was also restricted to the Antillean Ridge. The zone of deformation is about 180 km wide, and the transition between deformed and undeformed rocks takes place over a distance in some places as short as 10 km.

It is postulated that worldwide transgressions (pulsations) and regressions (interpulsations) throughout the course of geologic time are related to the elevation and subsidence of oceanic ridge systems and to sea-floor spreading. During the Mesozoic-Cenozoic interval, for example, the Cretaceous represents a period of worldwide transgression of the seas over the continents. Such a transgression may have been caused by the elevation of the old Mid-Pacific Ridge system, which in turn displaced a considerable amount of sea water from the ocean basins to the continents. Two multiple working hypotheses are proposed to explain major transgressions and regressions and the elevation and subsidence of oceanic ridge systems. One hypothesis interrelates the sea-floor spreading hypothesis to the hypothesis of sub-Mohorovičić serpentinization. The second hypothesis relates the sea-floor spreading hypothesis to a hypothesis involving thermal expansion and contraction.

Mesozoic eugeosynclinal sequences of turbidites with radiolarian cherts, pillowed basalts, and ultramafic rocks, appear to characterize much of the exposed Pacific continental margins and much of the Tethyan tectonic belts. Extremely great stratigraphic thicknesses have been reported for many of these deep-ocean sequences. These eugeosynclinal rocks have had a complex history of penecontemporaneous deformation and subsequent tectonic displacements, and have been uplifted and added to the margins of the continents. Based on studies of the southern continental margin of Alaska, the apparently great thicknesses and the subsequent uplift of these eugeosynclinal sequences seem best explained by deposition in oceanward-migrating trenches and the repeated landward uplift of the sedimentary fill in these successive trenches.

Evidence for freeboard of continents (relative elevation with respect to sea level) as a function of time is evaluated. Eyged’s interpretation of continental emergence with time, based on changing areas of flooding shown on global paleogeographic atlases, seems unfounded on grounds of inherent biases in the original maps, biases associated with changing time segments between successive maps, and by comparison with a freeboard versus time plot for North America compiled from Schuchert’s more detailed atlas. Instead, Hess’s simple assumption of constant average freeboard seems correct. The North American plot is used as a basis for a quantitative estimate of the distribution in time of deviations in freeboard. For over 80 percent of post-Precambrian time, freeboard has remained within ± 60 m of a normal value 20 m above present sea level. A constant freeboard model of the earth is suggested with various feedback mechanisms continually maintaining this fine adjustment between volume of ocean basins and volume of ocean waters. From the model, a number of calculations and implications are drawn for continental and oceanic accretion, as well as for some rate relations in a global tectonic system.

Widespread mid-Cenozoic (25 to 40 m.y. ago) changes in deformation, deposition, and volcanism on continents and island arcs reflect discontinuities in the behavior of lithospheric plates. Varied mid-Cenozoic activity near leading plate edges includes major orogeny and molasse accumulation along most of the western Tethyan belt and the southern Andes (with prevalent volcanism), and a distinct phase of deformation and igneous activity on the Alaska Peninsula, in Central America, and on cratonic Middle Europe. Tectonic stability following early Cenozoic orogeny was established in most of the West Indies and in the eastern ranges of the Rocky Mountains. At the end of mid-Cenozoic time, major orogeny, a new style of deformation or of sedimentation, or a major episode of volcanism occurred in the Red Sea-African rift zone, along most of the Pacific border of the United States and Mexico, in northern South America, in New Zealand and New Guinea, on the large island arcs of the Indian and western Pacific basins, and along the eastern Tethyan belt (with only minor volcanism). On trailing margins of continents most of the marine embayments underwent extensive regression during mid-Cenozoic time while interior continental Eurasia was flooded by a broad sea.

Geologic studies of present ocean basins and continental rift systems suggest a four-stage sedimentary model for evolution of rifted continental margins. Each stage produces a diagnostic sedimentary record, and examples of each can be seen today. (1) Uplift, volcanism, rifting, and nonmarine sedimentation patterns characterize the rift valley stage. (2) In the Red Sea stage and later, restricted size and circulation of the proto-ocean create a reducing environment; chemical precipitates (halides, gypsum, and metallic sulfides) and saprophitic muds are deposited. (3) Gravity-induced processes dominate the sedimentary regime during the turbidite-fill stage, and flat-lying beds of coarse-grained, highly reflective sediments are formed. (4) Strong thermohaline contrasts in the mature ocean basin indicate geostrophic deep-water circulation. This deep-ocean current stage results in current-controlled sediment deposition and redistribution.

The basic tectonic features of the western Mediterranean are explained by a model invoking the activity of a toroidal convection cell in the upper mantle, uncoupled from the base of the lithosphere (a situation different from the usual plate tectonic conditions). The key feature of this model is that the current is visualized as eroding the base of the lithosphere in a manner analogous to the erosion of sand by running water or wind, producing a pattern of inverted subcrustal ripples concentric about the rising mantle column. Isostatic subsidence of zones thinned in this manner would have given rise to the geosynclinal furrows which were present throughout most of the Mesozoic and the Tertiary. Continued removal of lithosphere beneath such a furrow would eventually bring it to the point where it would no longer be capable of resisting compressive stresses applied by the thick zone behind it, which would be pressing outward as a result of the drag of the mantle current. At this point the furrow would be crushed between the two adjacent thick zones, and its sedimentary fill would be bulldozed out and slide away as great gravity nappes. Subcrustal erosion and the episodic outward movements as furrows collapsed would lead to the removal of continental crust from above the rising mantle column. When convection ceased, new oceanic crust in the central area would subside to its normal isostatic level, while the surrounding regions would rise in compensation, producing the pattern seen today.

A new method for predicting rotational poles from biological diversity gradients consists of fitting a fourth degree polynomial in latitude to the data for each of a grid of trial “north poles” and measuring the “goodness of fit” for each tested pole. A confidence area for the predicted pole can then be contoured on the array of trial north poles. This method was tested against modern diversity gradients, and when applied to Stehli’s Permian brachiopod data, it showed many advantages over the spherical harmonics method. A hypothetical Permian continental reconstruction and the present continental arrangement were tested, but the paleoclimatic data used were unable to discriminate adequately between the two.

Paleomagnetic measurements (including alternating field demagnetization) on 16 oriented samples from five sites in the Precambrian anorthosite massif at Egersund, southern Norway, revealed a stable, consistent reversed remanent magnetization (after 150 oersted AF demagnetization, D = 315°, I = −82°, k = 92, α 95 = 8°). The calculated paleomagnetic (south) pole lies at 42° N., 20° E. and is more than 45° from the pole (39° N., 40° W.) previously obtained from anorthosite of similar age at Allard Lake, Quebec. If Eurasia is fitted to North America according to the model of Bullard and others (1965), the rotated Egersund pole is at 42° N., 18° W. Allowing for the uncertainties in the data, this is not significantly different at the 5-percent level from the Allard Lake pole, and suggests that about 1,000 m.y. ago North America and Europe were contiguous as in Laurasian reconstructions.

The Hanson Lake area, Saskatchewan, lies within the Churchill structural province of the Canadian Shield, about 150 mi west of the Churchill-Superior boundary. On the Tectonic Map of Canada (1969) its rocks are designated as “mainly Archaean, folded during the Kenoran and refolded during the Hudsonian (orogenies).” Detailed geological studies indicate that these ideas need some revision. The area is underlain by Kisseynew-type felsic gneisses that are overlain with no evident disconformity by Amisk-type metavolcanic and metasedimentary rocks, for which an isochron drawn on the basis of Rb/Sr isotope determination indicates an Archaean age of 2,521 ± 60 m.y. The metavolcanic rocks were intruded by four bodies of granite, for one of which a similar Rb/Sr isochron gives an Archaean age of 2,446 ± 16 m.y. The Kisseynew- and Amisk-type rocks were complexly folded during one continuous cycle of deformation, toward the end of which the granites were emplaced. The age of the granites indicates that the deformation took place during the Kenoran orogeny. The rocks were faulted and intruded by unmetamorphosed, discordant pegmatite dikes with an Rb/Sr age of 1,799 ± 2 m.y. However, there is no evidence of penetrative deformation later than the Kenoran and, therefore, no direct evidence of Hudsonian folding in the area.

Three major east-trending structures in Southern Africa are described. They are referred to as the 16°, 23°, and 29° S. lat belts, and they correspond in part to what are commonly referred to in Southern African literature as the Zambesi, Limpopo, and Orange River belts. It is shown that the crustal blocks defined by these belts have displayed a more or less independent mobility at intervals through much of geologic time, and that the belts themselves have acted as loci for periodic volcanism. It is proposed that the belts are analogous to present-day oceanic fracture zones and played a part in the fragmentation of Gondwanaland. An extension of the African Rift System through South West Africa is postulated on the basis of some modern seismic activity and a line of Cretaceous alkaline volcanic centers. Opening of this rift in late Precambrian time provided a site for the accumulation of the Swakop Facies of the Damara System, and the extreme folding and thrusting of these rocks is related to closing in the Paleozoic. Rifting along the present west coast probably occurred simultaneously, leading to the formation of a proto-Atlantic Ocean. Subsequent closure of this proto-ocean is registered by intensely deformed and metamorphosed marginal facies of platform sediments underlying the Damara System. Further, it is suggested that the hydrothermal activity responsible for ore mineralization at Tsumeb is related to these events.

An oceanic trench probably existed off central California in early and middle Cenozoic time, according to recently published interpretations of marine geophysical data. After allowance for late Cenozoic displacement along the San Andreas fault, a comparable part of onshore California was studied to determine if the geologic record there is compatible with these interpretations. Accumulation of mud at abyssal depths, chaotic deformation of sediments, the development of a submarine valley, thrust faulting, and basaltic and andesitic volcanism apparently all took place during late Eocene and early Oligocene time. Thus, onshore data support the concept of a late Eocene-early Oligocene trench. However, none of these phenomena occurred earlier in Cenozoic time in central California except the development of submarine valleys. Therefore, onshore data do not point to the presence of a trench off central California in Paleocene or early and middle Eocene time.

Many ancient ring complexes give evidence of having fed surface flows from rows of circumferential vents around a broad circle many kilometers in diameter. The volcanoes of the Galápagos Islands provide contemporary illustrations of this process. Whenever flows from these vents are viscous or small in volume, the circumferential feeder zone will grow vertically at the expense of the outer flanks. The area within this zone, however, being limited in extent and receiving flows from all sides, will soon fill and grow vertically along with the circumferential zone. The result should be a flat-topped volcano. The Galápagos volcanoes have gently sloping outer flanks which steepen to 35° approaching the circumferential vent zone, but the zone itself is nearly horizontal as the full top appears to have been before the development of central calderas. Filling of calderas in the waning stages of volcano growth may then return the flat-topped morphology. The shapes of several Galápagos volcanoes are strikingly similar to those of many flat-topped seamounts on the ocean floor, and there seem to be no obvious reasons why the processes that built the Galápagos volcanoes could not operate below sea level as well as above. Wave base truncation and subsidence are clearly responsible for many flat-topped seamounts, or guyots, but others may simply exhibit primary shapes controlled by circumferential vents during growth of the volcano.

The model is derived by equating the Troodos igneous massif of southern Cyprus with oceanic crust formed by sea-floor spreading. Comparison of the thicknesses and physical properties of the units of the Troodos massif with those deduced for the oceanic crust by seismic refraction experiments suggests the following correlation: layer 1—sediments; layer 2—pillow lavas and dikes, the lower part being predominantly dikes; layer 3—a layered plutonic complex of gabbros and minor diorites overlain by dikes; layer 4 (upper mantle)—pyroxenites, interpreted as accumulate phases of the gabbroic complex, grading downward into dunites and harzburgites thought to represent depleted mantle. Despite the fact that the pyroxenites and dunites are as strongly magnetized as the unaltered pillow lavas, the record of reversals of the earth’s magnetic field frozen into the oceanic crust at ridge crests is thought to be written largely within the uppermost 500 m or so of unaltered pillow lavas, the lower pillow lavas and the dikes having been subjected to a greenschist facies metamorphism. The total thickness of the dike and gabbroic complexes may well be inadequate to account for the whole of seismic layer 3, leaving the possibility that the lower crust consists of partially serpentinized peridotite and that the Moho is a transition from partially to unserpentinized peridotite as suggested by Hess.