Dikes and pods of pseudotachylite up to one meter thick have been found over an area >50 km 2 in the same area as shatter cones and other possible features of shock metamorphism in the Beaverhead and Tendoy Mountains in southwestern Montana, defining the allochthonous remains of the Beaverhead impact structure (see also Hargraves et al., Chapter 19, this volume). They are not associated with any tectonic feature in the area and have several features uncommon in pseudotachylites formed by tectonic processes (large size, vesicles, pseudotachylite clasts within pseudotachylite), but which have been documented in pseudotachylites from other impact structures. Rare single sets of planar deformation features (PDFs) are found in quartz grains in the pseudotachylites with crystallographic orientations similar to those found in shocked quartz from other impact structures. The major and trace element chemistry of the pseudotachylites is similar to their host rocks, but with some enrichments (Al, Mg, Fe, K, volatiles) and depletions (Si, Na) indicating low-grade metasomatic alteration. 40 Ar/ 39 Ar laser microprobe analyses of pseudotachylites from three localities show a wide spectrum of ages, from Precambrian to Tertiary. The distribution of ages suggests two isotopic signatures, one of Precambrian age (although younger than the age of the protoliths), and the other of Cretaceous age. 29 Si Magic angle spinning (MAS) nuclear magnetic resonance (NMR) studies of the pseudotachylite failed to detect the presence of high-pressure polymorphs of quartz. The evidence suggests that these pseudotachylites were formed by the same event that formed the shatter cones. Related work (see Hargraves et al., Chapter 19, this volume) suggests that the original crater was at least 75 km in diameter and was formed in the late Proterozoic to Cambrian, 20 to 150 km to the west of the present location.

Upward-pointing shatter cones in sandstones of uncertain age (Middle Proterozoic? to Lower Cambrian?) and older crystalline basement rocks are exposed over an area of approximately 25 × 8 km in southwestern Montana. These shatter cones, together with pseudotachylites and breccias of various types (particularly in basement gneisses), are inferred to be products of a meteorite or cometary impact. However, Late Cretaceous contraction and Tertiary extension have contributed to the structural complexity of the area, and distinguishing unequivocally the shock brecciation from that due to younger tectonism is difficult. Stratigraphic constraints suggest the structure is Late Proterozoic or Cambrian in age. The shocked rocks are present in the Cabin thrust plate—one of many in the Late Cretaceous Cordilleran Thrust belt—and hence are allochthonous, having been transported tens of kilometers from the west. They are considered to represent only a piece from the central uplift of an original complex crater at least 75 km in diameter. It is speculated that some of the considerable uplift and erosion inferred to have taken place in Late Proterozoic to early Paleozoic time in east-central Idaho (The Lemhi arch) may be related to the postulated impact event. Furthermore, quasi-circular magnetic and regional gravity anomalies (50 to 75 km diameter) centered south-southeast of Challis, Idaho, may mark the concealed scar of the original impact structure.

The mantle reference frame defined by stationary hotspots has been used to determine the positions and motions of continental and oceanic plates surrounding the Caribbean region from late Jurassic time (140 m.y.) to the present. First, the position of the Pacific plate and the Pacific-Farallon spreading ridge has been reconstructed using the ages and geometry of island and seamount chains emanating from Pacific hotspots. Then, by assuming symmetric spreading across the Pacific-Farallon ridge, the motion of the Farallon plate relative to the mantle has been calculated. This shows that the postulated oceanic plateau which may form the core of the present Caribbean plate could have been erupted onto late Jurassic to early Cretaceous oceanic lithosphere as the Farallon plate passed over the Galapagos hotspot, hypothesized to have been initiated in mid- to l ate Cretaceous time (100 to 75 m.y. B.P.). The thickened volcanic plateau collided with the Greater Antilles Arc, then filling the gap between South America and nuclear Central America, in late Cretaceous time (80 to 70 m.y. B.P.) and was not subducted; instead, subduction of the Farallon plate commenced behind the plateau. This buoyant, indigestible piece of oceanic lithosphere drove the Greater Antilles Arc northeastwards, accompanied by subduction of proto-Caribbean crust, until it collided with the Bahama platform in late Eocene time. Concomitantly, the trench and island arc which developed behind (southwest of) the plateau generated what is now a part of Central America. Subsequent westward subduction of Atlantic lithosphere beneath the Lesser Antilles Arc and continuing eastward subduction of oceanic lithosphere beneath Central America, together with transform faulting (left-lateral Cayman transform fault in the north, right-lateral strike-slip motion in and off Venezuela in the south) defined the present boundaries of the Caribbean plate.

The paleomagnetism of 150 oriented hand samples (32 sites) of volcanic, red bed, and dike rocks, constituting the La Quinta Formation in the Perijá Range of Venezuela, has been studied. After demagnetization, consistent vectors from volcanic rocks and red beds are bipolar but show considerable scatter both before and after correction for present dip. A northwest-southeast declination predominates, however, in contrast to the northeast-southwest declination previously recorded in La Quinta strata from the Venezuelan Andes. As the declination predicted from data from the South American craton is northerly, differential rotation betwen the Perijá Range and Mérida Andes, causing opening to the north, is suggested. Six dikes give consistent northerly vectors, indicating emplacement after the differential rotations, probably in Early Cretaceous time.

The mean remanent magnetization vector of six samples from andesite dikes at one locality after 15 mT ODF is D = 309, I = +77, k = 13, α 95 = 20. Five rhyolite samples from each of two sites at a second locality yield: 50 mT ODF, D = 194,1 = +30, k = 166, α 95 = 6; 30 mT ODF D = 158,1 = +46, k = 101, α 95 = 8. These rocks are probably of Mesozoic age and the magnetization appears primary; but the vectors are unlike those for the stable South American craton at any time during the Phanerozoic. These data suggest that complex tectonic rotations have occurred in at least this part of the Santander Massif.

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.