Showing papers in "Geological Society of America Bulletin in 1977"

Evolution of the Norwegian-Greenland Sea

Abstract: Geological and geophysical data collected aboard R/V Vema during five summer cruises in the period 1966 to 1973 have been used to investigate the geological history and evolution of the Norwegian-Greenland Sea. These data were combined with earlier data to establish the location of spreading axes (active as well as extinct), the age of the ocean floor from magnetic anomalies, and the locations and azimuths of fracture zones. The details of the spreading history are then established quantitatively in terms of poles and rates of rotation. Reconstructions have been made to locate the relative positions of Norway and Greenland at various times since the opening, and the implications of these reconstructions are discussed here.

Late Paleozoic strike-slip faulting in southern Europe and northern Africa: Result of a right-lateral shear zone between the Appalachians and the Urals

Abstract: Late Paleozoic wrench faulting in southern Europe and northern Africa is interpreted as a right-lateral shear zone induced by the relative motion of two plates – a northern one that includes the Canadian Shield, Greenland, and stable Europe and a southern one that includes the African Shield plus an unknown eastern extension. The relative movement of these two plates was transformed into shortening at both ends of the shear zone and led to the formation of late Paleozoic mountain belts: the Urals to the east and the southern Appalachians to the west. Theoretical and experimental models of the dynamics of faulting may account for the arrangement of the fractures in the shear zone and for the observed displacements.

Sediment transport and deposition at river mouths: A synthesis

Abstract: River-mouth process studies and comparisons of river-mouth forms from contrasting environments suggest that sediment dispersal and accumulation patterns are governed by three basic effluent forces and by tide- or wave-induced processes. Neglecting modifications by tides or waves, effluent behavior and consequent depositional patterns depend on the relative dominance of (1) outflow inertia, (2) turbulent bed friction seaward of the mouth, and (3) outflow buoyancy. Inertia-dominated effluents are characterized by fully turbulent jet diffusion, exhibit low lateral spreading angles and progressive lateral and longitudinal deceleration, and produce narrow river-mouth bars. Under most natural circumstances, inertial effects are equaled or exceeded by either turbulent bed friction or effluent buoyancy. Shallow depths immediately basinward of a river mouth enhance the effects of bed friction, causing more rapid deceleration and lateral expansion. Triangular “middle-ground” bars and frequent channel bifurcation result. Low tidal ranges, fine-grained sediment loads, and deep outlets favor strong density stratification within the lower reaches of the channels. Under such circumstances, effluents are dominated by the effects of buoyancy for at least part of the year. Buoyant effluents produce narrow distributary mouth bars, elongate distributaries with parallel banks, and few bifurcations. In macrotidal environments where tidal currents are stronger than river flow, bidirectional currents redistribute river sediments, producing sand-filled, funnel-shaped distributaries and causing linear tidal ridges to replace the distributary mouth bar. Powerful waves promote rapid effluent diffusion and decleration and produce constricted or deflected river mouths.

Specification of fabric shapes using an eigenvalue method

Abstract: Eigenvalues, derived from the “orientation tensor” method for analyzing directional data, are useful indicators of fabric shape. Three possible methods of graphing these eigenvalues are discussed. These provide a convenient visualization of fabric shapes and strengths. Examples are given of the use of such graphs for representing field data and for tracing progressive deformation of fabrics.

Transport phases of transition metals in the Amazon and Yukon Rivers

Abstract: Samples representing yearly averages of material transported by the Amazon and Yukon Rivers were analyzed to separate the transition metals (Cr, Mn, Fe, Co, Ni, and Cu) into the following transport phases: (1) crystalline particles, (2) metal hydroxide coatings, (3) solid organic material, (4) sorbed material, and (5) those in solution. The major transport phases are crystalline particles and metal hydroxide coatings, which, combined, carry 65% to 92% of the transition metals transported. Solid organic material, the next most important phase, transports between 5% and 19% of the total transported. Material carried in solution transports 0.6% to 17% of the total transported. Sorbed transition metals account for between 0.02% and 8% of the total transported. Metal hydroxide coatings represent the major transporting mechanism potentially available to organisms, since, for the Amazon and Yukon Rivers, respectively, 87% and 78% of the Fe, 69% and 73% of the Mn, and 71% and 69% of the Ni are transported in this form. Comparing the concentrations of transition metals carried to the oceans with the concentrations on the continents, a high Cu ratio (5 to 7) indicates continental depletion or river output enrichment; a moderate ratio (1.1 to 1.7) for Ni, Co, and Cr indicates intermediate depletion or enrichment, and a near-unity ratio for Fe and Mn indicates little depletion or enrichment. The sediments transport >97% of the total mass of transition metals to the world9s oceans.

Mechanism of burial metamorphism of argillaceous sediments: 3. O-isotope evidence

Abstract: O-isotope analysis of shales sampled from wells drilled through sedimentary deposits in the Gulf of Mexico region indicates that the sediments and rocks are not isotopically equilibrated systems — even those that have been buried to depths where temperatures are as high as 170 °C. In comparison with the coarser fractions, the finer fractions of both clay minerals and quartz are almost always richer in O 18 . O-isotope disequilibrium among the clay fractions becomes less marked as burial temperature increases. O-isotope exchange between clay and pore water become more extensive at higher temperatures; this corresponds to more extensive diagenetic alteration of mixed-layer illite-smectite. There is no evidence for O-isotope exchange between detrital quartz and pore water. However, quartz that forms diagenetically as an accompaniment to the conversion of smectite to illite layers in the mixed-layer clay forms in equilibrium with the pore water. The usefulness of O-isotope geothermometry for determination of the maximum temperatures to which shales have been heated during burial was investigated. Temperatures were calculated from the O-isotope fractionations between coexisting fine-grained quartz and clay from three wells; these calculated temperatures progressively approached the measured well (logged) temperatures as depth of burial and temperature increased. In one well, good agreement between calculated and measured temperatures was obtained for measured temperatures between 100 and 180 °C. In two other wells, satisfactory agreement was approached but not obtained at measured temperatures as high as 120 °C. Temperatures calculated from the O-isotope fractionations of quartz and calcite or calcite and clay were not reasonable. This probably reflects isotope exchange between calcite and pore water after the silicates attained their measured isotope ratios. Consequently, calcite is not a suitable mineral for use in isotope geothermometry of diagenetically altered shales.

Stream-channel response to floods, with examples from central Texas

Abstract: The principle that most geomorphic work is accomplished by relatively frequent events of moderate intensity requires modification for application to stream channels in certain climatic and physiographic settings. Small drainage basins in regions of highly variable flood magnitudes appear to have a high potential for catastrophic response. Flash-flood potential for small basins can be regionally mapped by computing the standard deviation of the logarithms of the annual flood peaks. Highly right-skewed flood-frequency distributions indicate that a high potential exists in certain arid regions of the southwestern United States and in the seasonal subtropical-to-steppe climate region of central Texas. High-magnitude flood response is also promoted by physiographic factors, such as hillslope morphology, soils, rock type, and drainage density. The relative importance of overland flow, which produces intense flood peaks, versus interflow and ground-water flow, which produce more uniform streamflow, appears to integrate both the climatic and the physiographic influences on the potential for catastrophic floods. Another factor in realizing the climatic-hydrologic potential for catastrophic stream-channel response is the resistance of the channel itself to scour. Small limestone streams in central Texas show significant channel modification only during the rare high-magnitude floods characteristic of that region. This is mainly because of the high response threshold required to scour bouldery alluvium and dense valley-bottom vegetation. Effects of especially intense floods on such streams include the following: entrainment of jointed bed rock and boulders as much as 3 m in diameter, uprooting of trees that usually bind coarse-grained point bars, macroturbulent transport of boulders even over divides into adjacent drainages, local scour of chutes on meander bends, and passive boulder deposition on other preflood valley-bottom surfaces.

Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of Basin and Range faulting

Abstract: In the Yerington district, western Nevada, pre-Tertiary rocks are overlain by an Oligocene ignimbrite sequence and Miocene andesites. Basin and Range normal faulting began in Miocene time, as andesitic volcanism died out (17 to 18 m.y. ago), and has continued to the present. The faults dip east and are curved, concave upward, with net displacements in a nearly east-west direction. Movement on the curved faults has resulted in steep westward tilting of the Miocene andesites and of all older rocks. Alluvium and 8- to 11-m.y.-old basalt flows deposited during the period of faulting are tilted gently west. The oldest faults, which dipped steeply east when they were active, are now inactive and dip gently eastward as a result of westward tilting on other faults. Younger faults dip more steeply east, and the youngest faults, those responsible for present Basin and Range topography, are the steepest. More than 100 percent of east-west extension has taken place across the district because of normal faulting. The rate of extension was most rapid between 17 and 11 m.y. ago and was slower after 11 m.y. ago. The extension is deep seated rather than thin skinned and apparently involves thinning of the crust. Several theories of origin for Basin and Range structure can be rejected because of the field data at Yerington, and the theory that Basin and Range structure was caused by a continental spreading axis best fits the data. Basin and Range spreading seems to have been most active between the projections of the Mendocino and Murray fractures. It may have first started south of the Great Basin, when these fractures were farther south relative to the continent and when the oceanic spreading axis that had been between these fractures was interacting with the continent.

Late Quaternary deposition of ice-rafted sand in the subpolar North Atlantic (lat 40° to 65°N)

Abstract: A major change in the North Atlantic pattern of ice-rafting deposition, during the last interglacial-glacial cycle, occurred approximately 75,000 B.P. Prior to this time, deposition for a period of almost 50,000 yr during isotopic stage 5 was greatest in the northwest near Greenland and Newfoundland. The main glacial pattern was very different; the main depositional axis shifted abruptly to a zonal axis along lat 46° to 50°N, reflecting the passage of ice farther from the pole before reaching water warm enough in which to melt. This pattern remained essentially unchanged for 65,000 yr during the main Wurm glaciation. The peak interglacial depositional pattern can best be explained by analogy with the modern oceanic flow, except for the addition of a concentrated eastward component along lat 50°N. The glacial pattern is also best explained by counterclockwise flow. Laurentide and Greenlandic ice entering the western North Atlantic from the Labrador Sea moved to the east and southeast directly into the glacial depositional maximum. Scandinavian ice dropped part of its bed load near Norway, looped to the southwest into the North Atlantic in a counterclockwise passage south of Iceland, and finally melted along the primary depositional maximum. Total input rates of ice-rafted sediment to the Atlantic and Norwegian Sea increased slightly at 115,000 B.P. (the glacial inception), rose markedly at 75,000 B.P. (the major glacial transition), and continued to rise late in the Wurm toward the late-glacial maximum. North Atlantic ice-rafting deposition is thus positively correlated with ice-sheet size. During Quaternary time, roughly 70% of ice-rafted deposition of continental detritus in the world9s oceans has occurred in the subpolar North Atlantic south of Iceland. During the past 3 m.y., a mass of wet unconsolidated drift estimated at 200,000 km 3 has been moved from the continents to the deep Atlantic by ice-rafting alone. This is equivalent to a layer of drift 16 m thick over all parts of the continents thought to have supplied ice-rafted detritus to the North Atlantic.

Tectonic evolution of the Cocos-Nazca spreading center

Abstract: Magnetic and bathymetric data from the eastern Pacific have been analyzed and a model for the evolution of the Galapagos region developed. The Farallon plate appears to have broken apart along a pre-existing Pacific-Farallon fracture zone, possibly the Marquesas fracture zone, at about 25 m.y. B.P. to form the Cocos and Nazca plates. This break is marked on the Nazca plate topographically by the Grijalva scarp and magnetically by a rough-smooth boundary coincident with the scarp. The oldest Cocos-Nazca magnetic anomalies parallel this boundary, implying that the early Cocos-Nazca spreading center trended east-northeast. This system soon reorganized into an approximately east-west rise–north-south transform configuration, which has persisted until the present, and the Pacific-Cocos-Nazca triple junction has since migrated north from its original location near lat 5°S. If correct, the combination of these simple geometric constraints produced the “enigmatic” east-trending anomalies south of the Carnegie Ridge. The axes of the Cocos-Nazca spreading center and the Carnegie Ridge are essentially parallel; this can lead to paradoxical conclusions about interpretation of the Cocos and Carnegie Ridges as hotspot tracks. Hey and others (1977) have shown that recent accretion on the Cocos-Nazca spreading center has been asymmetric, resulting at least in part from small discrete jumps of the rise axis. I show here that the geometric objections to both the “hot-spot” and “ancestral-ridge” hypotheses on the origin of the Cocos and Carnegie Ridges can be resolved with an asymmetric-accretion model. However, all forms of the ancestral-ridge hypothesis encounter more severe geometric difficulties, and these results support the hotspot hypothesis. After further elaboration of the hotspot hypothesis by Johnson and Lowrie (1972) and Hey and others (1973), Sclater and Klitgord (1973) examined both the hotspot and ancestral-ridge hypotheses and decided that both should be rejected, concluding that the Cocos and Carnegie Ridges “are not tectonically related” (p. 6973). The difficulty in the Galapagos area arises primarily because the older magnetic anomalies have proven extremely difficult to correlate, surprisingly so considering the high data density and the ease with which the very young anomalies are correlated. An important problem is the reason for this difficulty in correlating older anomalies — either clearly recognizable anomalies were never formed here, or some mechanism has acted in this area to destroy them after they were formed. On the basis of all available evidence, including new data presented here, I conclude that a model based on the hotspot hypothesis, with the modification of asymmetric accretion resulting at least in part from discrete jumps of the rise axis as discussed by Hey and others (1973) and demonstrated by Hey and others (1977), successfully meets the objection of Sclater and Klitgord (1973) and allows us to outline the history of the area from the break-up of the Farallon plate and birth of the Cocos-Nazca spreading center to the present. The “instantaneous” (a term Hey and others, 1977, have examined) configuration of plate boundaries and motions (Fig. 1) has generated a wedge of crust spread from the Cocos-Nazca spreading center, which is characterized by a slow spreading rate, rough topography, and strong magnetic anomalies. This wedge, termed the Galapagos gore by Holden and Dietz (1972) and discussed in detail by Hey and others (1977), is surrounded by crust spread from the Pacific-Cocos and Pacific-Nazca spreading centers which has the smooth morphology common to fast-spreading rises and low-amplitude magnetic anomalies, as both these segments of the East Pacific Rise are oriented nearly parallel to the Earth's magnetic field vector. My model explains the location and orientation of the magnetic and bathymetric rough-smooth boundaries that thus bound the gore and implies that there are two genetically different magnetic and bathymetric boundaries in the area.

Profiles and ages of young fault scarps, north-central Nevada

Abstract: The geomorphic characteristics of young fault scarps can be used as a key to the ages of fault displacements. The principal features of scarps younger than a few thousand years are a steep free face, a debris slope standing at about 35°, and a sharp break in slope at the crest of the scarp. The principal slope of older scarps declines with age, so that scarps of about 12,000 yr of age have maximum slope angles of 20° to 25°, and slopes as low as 8° to 9° represent ages much older than about 12,000 yr. The crestal break in slope broadens with age. The material in the scarp face, whether loose fanglomerate or indurated bedrock, controls to a large extent the rate of scarp degradation. Where more than one displacement has occurred along a fault, a composite or multiple scarp develops. Composite or multiple scarps suggest mean recurrence intervals on individual faults measured in thousands of years.

Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near lat 36°49′N

Abstract: Fifty acoustically positioned samples of fresh basalt were collected by the submersible Alvin from the median valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (FAMOUS) in the summer of 1974. The samples show regular compositional variations from the center of the rift valley (central lava flows) out to the rift valley walls (flank lava flows). The central lava samples show higher ratios of olivine relative to clinopyroxene and plagioclase phenocrysts and contain chrome spinel. Glasses of the flank lava samples are enriched in SiO 2 , TiO 2 , K 2 O, H 2 O, and FeO/MgO relative to central lava samples. Studies of the thickness of palagonite and manganese crusts indicate that the flank lava flows are considerably younger than the inferred spreading age of the crust on which they occur. Flank lavas are generally older than central lavas, but notable exceptions occur. The composition of the flank lava glass can be derived by the removal of approximately 29 wt percent of analyzed phenocrysts (in the ratio 5.7 plagioclase, 2.5 olivine, 1.8 clinopyroxene) from the central lava glass. In addition, other processes (possibly involving volatile transfer) must enrich the flank lavas in K 2 O, TiO 2 , and H 2 O. A model is proposed whereby this crystal fractionation occurs in a shallow, narrow (6-km-wide) magma chamber underlying the median valley. The chamber is compositionally zoned, and central lavas are fed from dikes tapping its hotter axial zone, whereas flank lavas are fed from the cooler, differentiated melt on the margins. The nature of the chemical variations in the lavas permits an estimate of the composition and thickness of the cumulates forming at the base of the chamber.

Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington, and Idaho

Abstract: Mesozoic orogeny and magmatism began in the northwestern United States soon after deposition of Permian strata, but no rocks have yet been dated from the Permian-Triassic orogenic period. Middle Triassic to Late Jurassic sediment sequences include major unconformities and evidence of several episodes of igneous activity. An early culmination of magmatism occurred in Late Triassic and Early Jurassic time (200–217 m.y. ago) in eugeosynclinal parts of far western Idaho. A widespread and intense culmination in Late Jurassic time was the final major orogenic event in the Oregon eugeosyncline. The Bald Mountain (147 to 158 m.y. old), Wallowa (probably 143 to 160 m.y. old but affected by Cretaceous metamorphism), Deep Creek (at least 137 m.y. old), and many other plutons in the Blue and Klamath Mountain regions in Oregon and in western Idaho were emplaced shortly before the end of Jurassic. The bulk of the Idaho batholith was emplaced during a Cretaceous culmination of igneous activity — the southern (Atlanta) lobe about 75 to 100 m.y. ago and the northern (Bitterroot) lobe about 70 to 80 m.y. ago. Much of the batholith was affected by Eocene magmatism which resulted in widespread resetting of isotopic dates for older rocks to values of 50 m.y. or less. Between 55 and 70 m.y. ago, there was a lull in igneous activity in the northwestern United States. Sr isotope initial ratios change abruptly across a boundary in western Idaho from ∼0.7040 or less, to the west, to ∼0.7060 or greater, to the east. This change marks the boundary between Precambrian crust and Phanerozoic eugeosyncline. The geologic setting of the observed transition and its time independence suggest that it is due to contamination and assimilation processes involving magmas from the mantle and enclosing crustal rocks. Contamination of magmas with radiogenic Sr renders the Sr whole-rock isochron technique useless in dating the Idaho batholith and other intrusive rocks in central and eastern Idaho, areas underlain by Precambrian basement.

Formation of folds, boudinage, and mullions in non-Newtonian materials

Abstract: Previous work has suggested that the formation of folds, boudins, and mullions by creep is caused by the same general type of instability. The results of Newtonian flow models of this process compare poorly with observation, however. The study reported here extends the analysis to include a general class of non-Newtonian materials restricted only to being incompressible, anelastic (that is, without memory), isotropic, and homogeneous. Although the material is isotropic, the perturbation flow associated with growing structure is found to obey an anisotropic type of flow law. In a strain-rate softening material undergoing pure shear, resistance to additional normal straining is significantly reduced from the background level, whereas resistance to tangential straining is unchanged. This has a profound effect on the formation of geologic structures, increasing the growth rates and altering the dominant wavelengths. Non-Newtonian behavior turns out to be necessary for the formation of boudins and mullions and plays an important role, but not a necessary one, in the formation of folds.

Morphology and tectonics of the inner rift valley at lat 36°50′N on the Mid-Atlantic Ridge

Abstract: A segment of the inner rift valley of the Mid-Atlantic Ridge was investigated in detail from the American submersible Alvin . Fifteen traverses were made across the floor and up the first major fault scarps in the valley walls. The asymmetric morphology of the inner floor is found to be the primary result of volcanic activity modified by tectonic activity. Analysis of the tectonic features revealed that the rift is evolving within a single stress field that has its least principal strain axis (the compressional axis) aligned with the valley axis of N20°E. This is in contrast to the direction normal to plate divergence (N0°E). The tectonic elements in the inner floor are primarily vertically dipping tension fractures, whereas the fault scarps of the flanking walls are closer to a 60° dip and reflect a component of downdip shear. The information base obtained from Alvin was broadened with information collected in the area with more conventional techniques. Through an analysis of this information, primarily the topography, it was possible to extrapolate the detailed observations obtained from the submersible to intervening areas to produce a comprehensive geological interpretation of the study area. An evolutionary model was developed which suggests that the inner rift is a product of axial volcanic activity. Shortly after formation, the original volcanic edifice is modified by vertical collapse, which leads to a reduction of the bottom relief. This process is reversed in the outer portions of the valley as uplift begins. Tensional extension changes into vertical shear as the volcanic blocks are incorporated into the walls and elevated. During the various stages of uplift, readjustment takes place on the terraces, which results in the preservation of the original volcanoes as recognizable units. This model, which spans 180,000 yr of inferred time, is examined in detail in an attempt to identify its weaknesses as well as to delineate the specific factual constraints upon which it is built. Alternate interpretations are proposed and tested in a similar fashion; the result is the identification of key problems that need to be solved.

Geochemistry and origin of phosphorite deposits from off Peru and Chile

Abstract: Holocene phosphorite nodules sampled from the sea floor off the coasts of Peru and Chile have been investigated in order to characterize the deposits geochemically and to evaluate their mode of formation. Bulk chemical and mineralogical compositions of the phosphorite nodules reflect varying degrees of dilution of the phosphatic material, fluorapatite, by other authigenic minerals and various allogenic components. Examination with the scanning electron microscope of freshly fractured surfaces of nodules and small pellets separated from associated diatomaceous ooze suggests that the apatite formed authigenically as a direct chemical precipitate rather than by replacement. Surfaces of siliceous biogenic materials as well as some inorganic phases appear to be favored sites for apatite nucleation. The proposed model of phosphorite formation involves inorganic precipitation of apatite within pore waters of anoxic sediments and subsequent concentration of the apatite by physical processes. Oxidation of organic materials (mainly diatoms) during SO 2− 4 reduction is the main source of dissolved phosphate. Apatite precipitation is favored within the sediments by the high phosphate concentration in the interstitial waters, by the availability of suitable nucleation sites, and by diagenetic reactions that remove interfering Mg 2+ ions from the pore solutions. Concentration of apatite into indurated phosphate nodules is brought about by winnowing and reworking processes, possibly in response to a change in the sedimentary environment caused by eustatic sea-level fluctuations or tectonism.

Seismic structure of the Transverse Ranges, California

Abstract: Travel-time data obtained from both natural and artificial events occurring in southern California indicate a major, lateral crustal transition within the Transverse Range Province The eastern crust is very similar to the adjacent Mojave region, where a crustal velocity of 62 km/sec is typically observed The western ranges are dominated by an extensive 67 km/sec layer P_n velocity beneath the western Mojave, Transverse Ranges, and northern Peninsular Ranges is 78 km/sec The crustal thickness of these provinces is 30 to 35 km The Transverse Ranges do not have a distinct crustal root Unlike other provinces within southern California, the Transverse Ranges are underlain at a depth of 40 km by a refractor with a P-velocity of 83 km/sec P-delays from a vertically incident, well-recorded teleseism suggest that this velocity anomaly extends to a depth of 100 km These data indicate that this high-velocity, ridge-like structure is coincident with much of the areal extent of the geomorphic Transverse Ranges and is not offset by the San Andreas fault Four hypotheses are advanced to explain the continuity of this feature across the plate boundary: (1) dynamic phase change; (2) a coincidental alignment of crust or mantle anomalies; (3) the litho-sphere is restricted to the crust; (4) the plate boundary at depth is displaced from the San Andreas fault at the surface Within the context of the last model, we suggest the plate boundary at depth is at the eastern end of the velocity anomaly, in the vicinity of the active Helendale-Lenwood-Camprock faults The regionally observed 78 km/sec layer is suggested as a zone of decoupling necessary to accommodate the horizontal shear that must result from the divergence of the crust and upper mantle plate boundaries The geomorphic Transverse Ranges are viewed as crustal buckling caused by the enhanced coupling between the crust and upper mantle which is suggested by the locally thin, 78 km/sec layer

Volcanism of Afar: Small-scale plate tectonics implications

Abstract: Structures analogous to oceanic spreading ridges, transform faults, and “leaky” fracture zones have been identified in Afar. They were formed during the past 3 to 4 m.y. and permit identification of the present plate boundaries within Afar and determination of the amount of spreading in this interval. At least two microplates are required in the zone of the junction of the African, Arabian, and Somalian plates. Variations of the spreading rate along single ridges or from one ridge to another, ridge jumping, migration of spreading with time, and counterclockwise rotation of the microplates are inferred by comparing volcanological, chronological, and geochemical data with aeromagnetic data. Deformation affects both accreting and transform plate boundaries within zones similar in width to the microplates, whose interiors are also systematically affected by regional faulting and locally by volcanism and transverse faulting. This indicates that although plate tectonics can explain most of the Afar features it fails when applied at such a small scale.

Upper Cretaceous–Paleocene magnetic stratigraphy at Gubbio, Italy V. Type section for the Late Cretaceous-Paleocene geomagnetic reversal time scale

Abstract: A biostratigraphically complete and well-exposed sequence of Upper Cretaceous-Paleocene pelagic limestones at Gubbio, Italy, has provided a record of geomagnetic polarity reversals that closely matches the sequence inferred from marine magnetic anomalies. Abundant foraminifera permit accurate dating of the sequence. Because of these favorable circumstances, the Gubbio locality is formally proposed as the magnetostratigraphic type section for the Upper Cretaceous and Paleocene.

Genesis and transformation of metalliferous sediments from the East Pacific Rise, Bauer Deep, and Central Basin, northwest Nazca plate

Abstract: Analytical data for northwest Nazca plate sediments can be described in terms of a mixture of hydrothermal, detrital, hydrogenous, and biogenous material. Fe, Mn, Cu, Zn, Ni, Ba, Si, and Al are more than 50 percent hydrothermal in East Pacific Rise samples from lat 10° to 25 °S. The first four elements are dominantly hydrothermal in the Bauer Deep and Central Basin as well. Seventy to 80 percent of the Ni, 60 to 80 percent of the Ba, and 30 to 60 percent of the Cu and Zn in Bauer Deep and Central Basin sediments are hydrogenous. Si, Ba, and Zn are dominantly biogenous on the northern East Pacific Rise crest, where more than one-third of the Cu also is derived from this source. Detrital Al and Si are dominant away from the rise crest, particularly in the Central Basin, where about 40 percent of the Fe and 15 percent of the Zn may also be detrital. Much of the hydrothermal Fe and biogenous Si have been transformed to an iron-rich smectite. The proportion of total Fe bound in this phase varies from less than 20 percent on the southern rise crest to about 40 percent in the Bauer Deep. The distribution of each element is governed by (1) supply from the four basic sources; (2) lateral transport by bottom currents moving east and then south across the northern East Pacific Rise and Bauer Deep to the Central Basin and moving west from the Peru Basin to the Central Basin; and (3) transformation of the unstable metalliferous hydroxides into more stable smectite and ferromanganese oxyhydroxides.

Sedimentation and climatic patterns in the Santa Barbara Basin during the 19th and 20th centuries

Abstract: The thickness of annual sediment laminations in the Santa Barbara Basin is compared to southern California drought-resistant tree growth and to regional indices of rainfall and temperature. The rate of sedimentation was found to be independent of temperature, but it is highly correlated with rainfall and tree growth. We suggest that sedimentation, like tree growth, is a function of the amount of rainfall in the prior seasons as well as the current season. The natural filter displayed by the sedimentation and tree-growth records can be described by a simple mathematical model which, in the case of sedimentation, can be related to upstream aggradation or to distributional processes on the shelf. The pair of laminae that constitute a single year9s sediment accumulation are directly related. This suggests that the process of detrital sediment delay and redistribution operates primarily in the marine environment. The density difference that distinguishes “winter” laminae and “summer” laminae is ascribed to the interaction of the seasonal rate of deposition and the growth of a mat-forming organism endemic to the surface sediment of the Santa Barbara Basin.

Deep-tow studies of the structure of the Mid-Atlantic Ridge crest near lat 37°N

Abstract: A detailed study of the structure of the Mid-Atlantic Ridge median valley and rift mountains near lat 37°N (FAMOUS) was conducted using a deep-tow instrument package. The median valley may have either a very narrow inner floor (1 to 4 km) and well-developed terraces or a wide inner floor (10 to 14 km) and narrow or no terraces. The terraces appear to be non–steady-state features of the rift valley. The entire depth and gross morphology of the median valley may be accounted for by normal faulting, while volcanic relief contributes to the short-wavelength topography (<2 km). Most faults dip toward the valley axis an average of 50°, and the blocks are tilted back 2° to 3°. Fault dip is asymmetric about the valley axis. Active crustal extension in the inner floor and inner walls has the same sense of asymmetry as the local spreading rates, reaching a maximum of 18 percent. Thus, asymmetric spreading appears to be accomplished by asymmetric crustal extension on a fine scale as well as by asymmetric crustal accretion. Spreading is 17° oblique to the transform faults and shows no indication of readjusting to an orthogonal system, even on a fine scale. Eighty percent of the decay or transformation of median-valley relief into rift-mountain topography is accomplished by normal faults that dip away from the valley axis. Most of the outward-facing faulting occurs near the median-valley–rift-mountain boundary. Tilting of crustal blocks accounts for only 20 percent of the decay of median-valley relief. Most long-wavelength topography in the rift mountains has a faulted origin. As in the median valley, volcanic relief is short wavelength (<2 km) and appears to be fossil, originating in the median-valley inner floor. Bending of large faulted blocks toward nearby fracture zones suggests that spreading-center tectonics is affected by fracture-zone tectonics throughout the length of the rift in the FAMOUS area. Both the crustal accretion zone and transform fault zone are narrow, only 1 to 2 km wide, over short periods of time. In the course of millions of years, however, they apparently migrate over a zone 10 to 20 km wide.

Composition and phase chemistry of sulfide globules in basalt from the Mid-Atlantic Ridge rift valley near 37°N lat

Abstract: The electron microprobe was used to determine the bulk composition of immiscible sulfide globules trapped in the glass phase of 25 fresh submarine basalt samples from the Mid-Atlantic Ridge. Twenty-three samples represent a spectrum of primitive through differentiated tholeiites from the FAMOUS dive area; two are differentiated basalts from the Reykjanes Ridge. The analyzed globules range in diameter from 11 to 233 µm. On the average, they constitute only 0.0022 volume percent of the rocks and contain less than 1.5 percent of the sulfur. Compositions of the globules change with differentiation as measured by Fe/(Fe+Mg) or TiO 2 content of the host glass. Globules in glass containing 0.66 to 1.0 wt percent TiO 2 typically contain 20 to 26 wt percent Ni + Cu and have an average atomic Ni/Cu of 1.6. With differentiation toward 1.6 wt percent TiO 2 , Ni + Cu content of the globules falls to less than 10 wt percent and atomic Ni/Cu falls to 0.4. Sulfur content of the host glasses shows a strong correlation with FeO content, increasing from 840 ppm to 1,370 ppm as FeO content increases from 8.0 to 12.6 wt percent. Reference to experimental studies shows that this relationship is consistent with sulfur saturation of the host glass at liquidus temperatures. Crystal fractionation is considered to be the dominant factor in keeping the differentiating melt at sulfur saturation. The sulfide globules may have persisted in the basaltic melt from its place of formation by partial melting in the mantle, or they may have exsolved from the melt as it became sulfur-saturated in a high-level magma chamber. Globule abundance and composition indicate adjustment to the composition of the melt in which they were trapped. Material balance calculations suggest that one-third of the Cu and commensurate amounts of S, Ni, and Fe have settled from the magma as immiscible globules. The sulfide globules contain less than 4 wt percent magnetite, compatible with low f o 2 in the magma. Three sulfide phases coexisted in the globules at about 600 °C: monosulfide solid solution, intermediate solid solution, and pentlandite. At lower temperatures, the intermediate solid solution has broken down, and the monosulfide solid solution has exsolved a second generation of pentlandite.

Upper Cretaceous–Paleocene magnetic stratigraphy at Gubbio, Italy III. Upper Cretaceous magnetic stratigraphy

Abstract: Paleomagnetic study of the Upper Cretaceous part of the Scaglia Rossa pelagic limestone in the section at Gubbio, Italy, yielded a sequence of magnetic polarity zones that corresponds precisely with the polarity sequence inferred from marine magnetic anomaly profiles. This study shows the existence of a long normal polarity zone corresponding to the Cretaceous Quiet Zone, establishes the presence of the controversial interval of reversed polarity between marine magnetic anomalies 33 and 34, and places the Cretaceous-Tertiary boundary near the top of the reversed polarity interval immediately preceding anomaly 29.

Timing of mid-Tertiary volcanism in the Sierra Madre Occidental between Durango City and Mazatlan, Mexico

Abstract: The relatively flat-lying rocks of the upper volcanic supergroup exposed between Durango City and Mazatlan in western Mexico consist of two distinct sequences. Near Durango about 800 m of ignimbrites and associated lavas and tuffs were emplaced between 32.1 and 28.3 m.y. ago (23 K-Ar dates). About 100 to 200 km to the west, at least 1,000 m of ignimbrites, lavas, and tuffs were emplaced 23.3 ± 0.3 m.y. ago, as indicated by 8 K-Ar dates. The two calc-alkalic sequences have significant but slight chemical and mineralogic differences, the older and eastern one being more alkalic. In the Durango vicinity, distinctive alkali basalts were erupted during normal faulting about 12 m.y. ago. Tholeiitic basalt of Quaternary age is also widespread there. Neither the normal faulting nor the basalts are prominent to the west, except near the Gulf of California. The upper volcanic supergroup commonly rests upon an older volcanic-intrusive complex approximately 100 to 45 m.y. old. Between 45 and 32 m.y. ago little or no magmatism occurred in the Sierra Madre Occidental, but major rift-related alkalic volcanic rocks were erupted then in Trans-Pecos Texas. Voluminous rhyolitic activity occurred between 32 and 23 m.y. ago in western Mexico. Intense volcanism began in southern Baja California 20 m.y. ago. The shifting position, nature, and amount of volcanism may correspond to changes in lithospheric plate motions in the east Pacific region, and 45 m.y., 32 m.y., and 23 m.y. ago appear to be key times of plate-boundary reorganization.

Recent plate motions in the Galapagos area

Abstract: Recent accretion on the Cocos-Nazca spreading center has been asymmetric, with more material (along most of the rise) added to the Cocos than to the Nazca plate. There is evidence in the magnetic record that some of this asymmetric accretion has resulted from small discrete jumps of the rise axis to the south, forming and destroying transform faults. Relative and absolute models of instantaneous plate motions derived here provide an accurate representation of recent motions in the east Pacific. The existence of a self-consistent model that fits all the relative-motion data provides strong support for the hypothesis that plates behave rigidly. In addition, the exceptional agreement between the relative-motion and absolute-motion models provides strong support that the Wilson-Morgan hotspot hypothesis holds for the recent past. In particular, the agreement of the predicted instantaneous azimuths of the Cocos and Carnegie Ridges with their observed azimuths strongly suggests that at least the young parts of both aseismic ridges were formed by the motion of the Cocos and Nazca plates over a Galapagos hotspot.

Near-bottom magnetic anomalies, asymmetric spreading, oblique spreading, and tectonics of the Mid-Atlantic Ridge near lat 37°N

Abstract: A detailed study of the magnetic anomalies of the Mid-Atlantic Ridge crest near lat 37 °N (FAMOUS) was conducted using a deeply towed instrument package. The most recent expression of the accreting plate boundary in rift valley 2 is an alternating series of linear central volcanoes and depressions that are marked by a sharp maximum in crustal magnetization only 2 to 3 km wide. Spreading in the FAMOUS area is highly asymmetric, with rates of 13.4 mm/yr to the east and 7.0 mm/yr to the west. At 1.7 m.y. B.P., the sense of asymmetry reverses in direction, with spreading faster to the west; this results in a gross symmetry when averaged through time. The change in spreading asymmetry occurred in less than 0.15 m.y. Spreading in the FAMOUS area is 17° oblique. Even on a fine scale there is no indication of readjustment to an orthogonal plate-boundary system. Spreading has been stably oblique for at least 8 m.y., even through a change in spreading direction. The presence of negative polarity crust within the Brunhes normal epoch in the inner floor has been observed and may be due to old crust left behind or to recording of a geomagnetic field event. Crustal magnetization decays to 1/e its initial value in less than 0.6 m.y. The rapid decay may be facilitated by very intense crustal fracturing observed in the inner floor. Crustal magnetic sources may be approximated (mathematically) by a uniformly magnetized layer 700 m thick. Magnetic studies indicate that over 90 percent of the extrusive volcanism occurs within the rift inner floor and is extremely rare in the rift mountains. Magnetic anomaly transition widths vary from 1 to 8 km with time and appear to reflect a time-varying median-valley structure. The valley has either a wide inner floor and narrow terraces, in which case the volcanic zone is wide and magnetic anomalies are poorly recorded (wide transition widths), or it has a narrow inner floor and wide terraces — the volcanic zone is then narrow and anomalies are clearly recorded (narrow transition widths). The median valley of any ridge segment varies between these two structures with time. At present rift valley 2 has a narrow inner floor and volcanic zone (1 to 3 km), whereas rift valley 3 is at the opposite end of the cycle with a wide inner floor and volcanic zone (9 to 11 km).

Geometry of plate accretion

Abstract: A steady-state model for plate accretion at oceanic ridges, developed using constraints mainly from western Newfoundland ophiolite complexes, involves four main rheological components: (1) a convectively cooling lid accelerating from the ridge axis across a zone of decreasing dike-injection rate and thickening by the addition of extrusive basalts above and gabbro underplating beneath; (2) a wedge-shaped magma chamber with a flat floor; (3) a differentially subsiding wedge of cumulates; and (4) a narrow axial partially melting Iherzolite-derived diapir from which basalts are liberated over a narrow axial welt into the magma chamber and from which residual harzburgites are plated near the axis below the base of the subsiding cumulates. The axial depth of the magma chamber determines the thickness of plated gabbro. The basal width of the magma chamber and the height of the partially melted welt control the thickness of cumulates and the final attitude of cumulate banding.

Nucleation and growth of plagioclase, Makaopuhi and Alae lava lakes, Kilauea Volcano, Hawaii

Abstract: The Hawaiian lava lakes offer an unparalleled opportunity to study the processes that occur during the crystallization of basaltic magma. This paper presents estimates of the rates of nucleation and growth of plagioclase in the Kilauea lava lakes, Makaopuhi and Alae, and a discussion of the processes that control the nucleation and growth. The observed growth rates perpendicular to (010) vary from 1,7 to 11.0 × 10 −10 cm sec −1 . The nucleation rates vary from 6.8 × 10 −3 to 2.0 cm −3 sec −1 . In general the rates increase with increasing crystallization at any point, decrease with increasing distance from the surface, and are higher in the shallower lake, Alae. For the most part, nucleation appears to occur heterogeneously on previously existing crystals. The growth appears to be controlled by the interface attachment kinetics and not by diffusion in the melt. The observed results are in qualitative agreement with theoretical predictions.

Magnetic study of basalts from the Mid-Atlantic Ridge, lat 37°N

Abstract: An intensive paleomagnetic and rock magnetic study has been carried out on the basalt samples from the FAMOUS area of the Mid-Atlantic Ridge at lat 37 °N. In addition to the samples obtained by the submersible Alvin from the floor and walls of the central rift valley, dredge and rock-drill samples were taken on a line perpendicular to the axis of the ridge out to a distance of 30 km from the central valley. Although the rocks from the floor of the central valley were roughly uniformly magnetized, a strong decrease in remanence intensity occurred at the valley walls. This progressive decrease continued until the remanent intensity was reduced by a factor of 5 at a distance 10 km and farther from the ridge axis. The other magnetic parameters of weak-field susceptibility, median demagnetizing field, Q , and Curie temperature were also roughly constant in samples from the valley floor. However, the rise in Curie temperature, which began at the valley walls, was well correlated with the decrease in remanent intensity and Q , an increase in the median demagnetizing field, and a sharp increase and then slow decrease in susceptibility; this rise can be attributed to the low-temperature oxidation of titanomagnetite to titanomaghemite. Of the 9 vertically oriented samples that were obtained from the valley floor and inner west wall, seven were normally magnetized and two were reversely magnetized. The two reversely magnetized units from the valley floor and east wall may be from blocks of older, pre-Brunhes crust that have not yet moved out of the rift valley. Comparison of the magnetic results of the surface rocks with models of the associated magnetic anomalies suggests that the oceanic magnetic layer can be represented by a permeable zone of pillow basalts roughly one km thick and that the oxidation state of these pillows progressively increases with time.