Showing papers in "Geological Society of America Bulletin in 1971"

Elevation-Relief Ratio, Hypsometric Integral, and Geomorphic Area-Altitude Analysis

Abstract: Mathematical proof establishes identity of hypsometric integral and elevation-relief ratio, two quantitative topographic descriptors developed independently of one another for entirely different purposes. Operationally, values of both measures are in excellent agreement for arbitrarily bounded topographic samples, as well as for low-order fluvial watersheds. By using a point-sampling technique rather than planimetry, elevation-relief ratio (defined as mean elevation minus minimum elevation divided by relief) is calculated manually in about a third of the time required for the hypsometric integral.

Growth of the Bengal Deep-Sea Fan and Denudation in the Himalayas

Abstract: A geological and geophysical survey in 1968 has shown that the Bengal Deep-Sea Fan is almost 3000 km long, and 1000 km wide. We estimate that it may exceed 12 km in thickness. The sediments of the fan have been transported by turbidity currents from the Ganges-Brahmaputra River delta, through the “Swatch of No Ground” submarine canyon and into an extensive, complex, meandering, and braided net of fan valleys. Present rate of sediment influx suggests a regional rate of denudation in the Himalayan source area of over 70 cm/10 3 years. The sediment section in reflection profiles of the fan has been subdivided into three units separated by prominent unconformities. Volumes of the upper two units compared with the sediment influx rate extrapolated into the past suggest that the unconformities may be late Miocene and earliest Pleistocene. These times correspond to periods of orogeny in the Himalayas and suggest contemporaneity between plate-edge orogeny and mid-plate tectonic activity.

Plate Tectonic Model for the Evolution of the Central Andes

Abstract: Data on the geophysics and geology of the central Andes are interpreted in terms of plate theory and a model for Andean evolution is presented. Analysis of upper mantle structure and seismicity shows that the underthrusting Pacific plate is now about 50 km thick and the overriding South American plate 200 to 300 km thick. Underthrusting of the Pacific plate probably began in Triassic time and has continued without substantial change to the present. Prior to underthrusting, the west coast of South America was quiescent, and great thicknesses of Paleozoic continental shelf deposits were laid down in an area east of the present volcanic arc. In Late Triassic or Early Jurassic time, an incipient arc began to form at or west of the present coast of South America. Igneous activity has since migrated eastward, culminating in the Pliocene-Pleistocene volcanic episode. The crust beneath the volcanic cordillera is more than 70 km thick and probably consists largely of rocks compositionally equivalent to those of the volcano-plutonic suites observed at the surface. Increase in crustal volume of the volcanic arc between Cretaceous time and the present implies either that the mantle above the under-thrust plate has undergone 18 to 36 percent partial melting or that 1 to 2 km of rock has been melted from the underthrusting plate. The intrusion of melt into the crust beneath the volcanic cordillera and the resultant crustal dilatation produced continentward compression of the Paleozoic sedimentary rocks which form an easterly belt of thrust and fold mountains. Here crustal shortening has produced crustal thicknesses of 50 to 55 km. Few deposits of the type normally termed eugeosynclinal, and no ophiolites, are observed between trench and volcanic arc; only in the intermontane foredeep behind the arc has a clastic wedge of geosynclinal proportions formed.

Structural History of the Mariana Island Arc System

Abstract: Results from a series of surveys across the Mariana island arc system clarify the geologic processes by which arc systems develop and test the hypothesis that the marginal basins behind the trenches are extensional in origin. Two surveys were made to investigate the sediment-covered upper slope on the eastern flank of the frontal arc and a mid-slope basement high which separates the upper and lower sections of the trench-facing slope. The upper slope sedimentary wedge probably spans the Tertiary history of the arc system, although a Deep Sea Drilling Project hole penetrated only to lower Miocene strata. The survey of a submarine canyon which traverses the upper slope and mid-slope basement high demonstrates recent folding, faulting, and uplift of the basement high. Pillow basalts, diabases, and metabasalts, were dredged from the ridges within the inter-arc basin (Mariana Trough). Sampling indicated that the basin fill is almost all Quaternary in age and that crustal extension and basalt extrusion has occurred along an axial high. Late Miocene coralline limestone and late Pliocene dacite pumice were dredged from the third arc, west of the inter-arc basin, indicating more than 1 km Quaternary subsidence of the ridge. Slightly west of the third arc is a chain of seamounts interpreted as andesitic volcanoes which fed the large sediment apron to the west. This apron, covering the eastern Parece Vela Basin, consists mostly of lower and middle Miocene volcaniclastics and overlies a basaltic basement similar to that of the inter-arc basin. Tectonic activity in the island arc system appears to be discontinuous. A typical pulse is visualized as beginning with trench formation, followed by volcanism and later by extensional basin development. In addition to the Quaternary pulse, the Philippine Sea was affected by an early Miocene tectonic event which opened the Parece Vela Basin and probably a late Eocene event which opened the basin west of the Palau-Kyushu Ridge.

Alpine Deformation and the Oceanic Areas of the Tethys, Mediterranean, and Atlantic

Abstract: A preliminary least-squares fit of the 1,000-m contour of the continental fragments around the Mediterranean is presented It extends the Atlantic reassembly of Bullard and others to the Balearic Islands, Corsica, Sardinia, Italy, Yugoslavia, Greece, and Turkey Geological and geophysical data suggest that the reconstruction applies to Permo-Triassic time The large gap between southern Eurasia and the northern and eastern parts of the reassembled fragments shows the probable shape of the Tethyan ocean of this period During Permo-Triassic time the Mediterranean is thought not to have existed, and therefore it should not be regarded as a remnant of the Tethys As in several other syntheses, the formation of the Mediterranean, together with that of the Alpine orogenic belts around it, are attributed principally to the relative movements between Africa and Eurasia Unlike these syntheses, the nature of this motion has been estimated much more precisely by constructing a simple sea-floor spreading model of the central and northern Atlantic Ocean The model permits the relative movement hypothesis to be tested more rigorously than previously Available spreading data show that at least three phases of relative movement must have taken place Relative to Eurasia, the opening model shows the earliest phase consists of an eastward movement of Africa in Lower Jurassic to early Upper Cretaceous time, followed by a westward movement in later Upper Cretaceous to late Eocene time, finally terminating with latest Eocene to present-day northward movement Anomalous Early Permian and Cretaceous paleomagnetic data, the contrasting age differences between ophiolite-chert sequences in the eastern and western Mediterranean region, and the positions of the Alpine orogenic belts are all shown to be consistent with general features of the reassembly and the opening model The reassembly and the model also permit a self-consistent plate tectonic interpretation to be made of Lower Jurassic geology, which suggests the existence of a single, irregular plate margin between Africa and Eurasia, whose rotation pole was close to the best-fit pole of Africa and North America on the Atlantic reassembly It is believed to link such diverse features as central Atlantic opening, sea-floor spreading in Yugoslavia and Albania, transcurrent faulting in the eastern Alps, and compressional features in the Crimea and the Great Caucasus Although Lower Jurassic geology is consistent with a single irregular plate margin, it is shown that, for the reassembly and opening model adopted, all the larger fragments (Spain, Corsica-Sardinia, Italy, and Yugoslavia-Greece-Turkey) must at times have belonged to a plate or plates independent of Africa and Eurasia Because the Atlantic spreading pattern has been grossly simplified, and also because verifiable reassemblies of later times cannot yet be made, plate tectonic interpretations of such periods are premature, although partial interpretations are still possible A possible interpretation is given of a phase of movement of the south Atlas fault and of the relationship of the ophiolite-chert sequences in the Oman Mountains and the Zagros Range of Iran to Atlantic sea-floor spreading A number of fundamental problems emerge, which include the necessity to postulate different mechanisms for eliminating oceans at compressive plate margins, and the conclusion that present interpretations of structures within orogenic belts provide poor estimates of the relative displacement vectors that have produced them

Sedimentary Facies Patterns and Geologic History of a Holocene Marine Transgression

Abstract: Studies of Holocene sediments in coastal Delaware show complex sediment distribution patterns resulting from lateral and vertical movement of successive environments of deposition over a Pleistocene unconformity. These sediments are infilling a drowned topography with a local relief of 70 ft and possibly up to 125 ft eroded on highly variable Pleistocene sediments. Identification of the Pleistocene surface remains a problem. However, it may be recognizable at the unconformity as a soil zone or intermixture of firm marsh clay-silts with Pleistocene sands, as well as on the basis of radiocarbon dates. Larger depositional features forming around eroding Pleistocene headlands and infilling the estuaries include characteristic shoreline environments, such as spits, dunes, baymouth barriers, an intermeshing network of tidal deltas, nearshore marine erosional-depositional sands and gravels, and lagoons or estuaries with fringing Spartina, Distichlis, and Phragmites marshes, which form the westernmost edge of the transgressive units. The thickness and areal extent of the sedimentary bodies are to a large degree controlled by the morphology of the Pleistocene unconformity. A large portion of the Holocene sedimentary units is being eroded by the transgressing Atlantic Ocean. Cores of sediment under the shallow lagoons, such as Rehoboth, Indian River, and Assawoman Bays, and in the fringing marsh environment, show that the depositional units are thin, highly irregular in areal extent, extremely variable in thickness, and difficult to project. Sedimentary processes active in the shallow bays include shoreline marsh erosion and the formation of thin, possibly ephemeral, beach-dune washover complexes consisting of clean, well-sorted sand, with typical beach and washover sedimentary structures. These wash-over beaches are an anomaly completely surrounded by Spartina marshes on the landward side and extremely muddy sands grading into dark gray lagoonal muds on the bay side. It appears that distinctive sedimentary structures and sediment size-sorting relationships, such as those that characterize the larger, more common sedimentary units of the coastal area, may be formed in miniature at the very thin edge of transgression and may lead to considerable confusion in the interpretation of sediments of this type in the geologic record.

Tectonic History of the Fiji Plateau

Abstract: The Fiji Plateau is a high, hot area of young oceanic crust. It is bounded on the north by a Cretaceous Pacific archipelago, and to the east and west by the Tonga and New Hebrides island arcs which go back to the Eocene. The Fiji Islands are an Eocene and younger continental mass formed within the ocean basin. Plate tectonics provides the key for understanding the area. Marine geological and geophysical data from Scripps Institution of Oceanography expeditions, especially Nova, and published seismic, gravity, and island geologic information provide the basis for the interpretation. Fiji is now flanked by three active sea-floor spreading centers which are part of a very complicated transform linking the Tonga and New Hebrides crustal consumption zones. Extension in the Lau Basin is also taking place. Magnetic anomalies and seismicity permit six small blocks and the large Pacific and Australian plates to be distinguished, and some idea of their relative motions to be gained. From published magnetic anomaly and fracture zone data, a detailed history of the Tertiary motions of the Pacific and Australian plates with respect to Antarctica has been deduced. By a suitable choice of plate boundaries, horizontal movements of the larger tectonic units of the Fiji Plateau region can be worked out for the entire Tertiary. This reconstruction successfully accounts for many hitherto unexplained bathymetric and geologic features of the area. The history proposed for the Fiji area is probably unique among the world9s oceans.

Areal Sorting of Bed-Load Material: The Hypothesis of Velocity Reversal

Abstract: The hypothesis of velocity reversal seems adequate to explain the areal sorting of channel material, that is, relatively large material in riffles and finer material in pools. The hypothesis is based primarily on the measured observations that, with increasing discharge, the average bottom velocity of a pool increases faster than that of a riffle until at relatively high flow the average bottom velocity of the pool exceeds that of a riffle. The areal sorting produced by the velocity reversal occurs at flows of moderate frequency.

Hydraulic Jumps in Turbidity Currents

Abstract: If uniform flow conditions are approached, turbidity currents should be supercritical (Froude number greater than unity) while passing through submarine canyons, and subcritical (Froude number less than unity) in the upper submarine fan channel. The change in flow regime requires that the current pass through a hydraulic jump in the vicinity of the canyon mouth. During the jump, the velocity of the flow would be reduced and its thickness markedly increased. The aspects of such hydraulic jumps are examined. The quantitative analysis of hydraulic jumps in turbidity currents is based upon solutions of the momentum flux equation and the equation resulting from the assumption of a continuity of sediment flux. It is found that for the average submarine canyon-channel system, the flow would more than double in thickness and its velocity would be halved during a hydraulic jump. The entrainment of water through the interface of the flow during the jump is significant in reducing the density of the current. The higher the initial Froude number, the greater the expected density reduction. However, the density reduction due to the jump is probably not sufficient to convert a high-density slide into a low-density turbidity current. There must be an initial density reduction by entrainment while the slide is passing through the canyon at high Froude number. Turbulence generated within the hydraulic jump would stir the flow and help to insure suspension of its sediment.

Geology of Eastern Papua

Abstract: Eastern Papua is a southeasterly trending mountainous peninsula with islands to the east and southeast, between 7° to 12° S. and 146° to 155° E. The peninsula and islands consist of a linear core of Mesozoic sialic metamorphic rocks flanked by Mesozoic and younger mafic rocks, and partly concealed by still younger sedimentary and volcanic rocks. The Mesozoic sialic rocks are exposed in a belt 900 km long and up to 60 km wide. They consist of Cretaceous (and possibly older) sediments which were metamorphosed during the Paleocene or Eocene. Metamorphic grade is generally greenschist facies, but is higher in the D9Entrecasteaux Islands and on part of Misima Island; lawsonite occurs mainly within a few kilometers of the Owen Stanley fault which bounds the mainland metamorphics on the north and northeast. Mesozoic mafic rocks include peridotite, gabbro and basalt of the Papuan ultramafic belt, metabasalt exposed in the Suckling-Dayman mountain block and on Normanby and the Deboyne Islands, and unmetamorphosed basalt elsewhere on the mainland. These rocks are, at least in part, Cretaceous. Younger mafic rocks include Eocene basalt on the southeastern mainland, and upper Oligocene and lower and middle Miocene tuff and lava at scattered localities. Eocene sediments consist of chert and deep-water limestone along the south coast of the mainland at Port Moresby and Magarida, and Eocene clastic sediments inland from Port Moresby and near Tapini. No lower or middle Oligocene rocks are known. Upper Oligocene, Miocene and Pliocene sediments in the Aure Trough (145° to 146° E.) have a maximum thickness of about 12,000 m; the sediments are mainly alternating mudstone and graywacke, and are probably turbidites. Middle Miocene reef limestone on the eastern hinge line of the trough at 146° E. is 1,000 m thick. Middle Miocene and younger clastic sediments in the Cape Vogel Basin (150° E.) are at least 4,000 m thick. Pliocene and Quaternary volcanic rocks range in composition from basalt to rhyolite and include some potash-rich rocks. Intrusive rocks include Eocene tonalite in the ultramafic belt, Oligocene? gabbro near Port Moresby, middle Miocene granodiorite west of Salamaua, middle Miocene potash-rich intrusives on the southeastern mainland, Pliocene andesitic porphyries near Wau, and Pliocene granodiorite in the D9Entrecasteaux Islands. The Cretaceous (and possibly older) sialic sediments are thought to have been metamorphosed during the early Eocene at the time of emplacement of the Papuan ultramafic belt. The ultramafic belt is thought to be part of a thrust slice of oceanic mantle and crust which rode over or was underridden by the Cretaceous sediments on a low-angle fault (the Owen Stanley fault). The metamorphic rocks were partly exposed during the middle or late Eocene but there was no major land mass in the area until the upper Oligocene. During the upper Oligocene, Miocene and Pliocene the sialic metamorphic rocks emerged, perhaps isostatically. Erosion of the resulting mountain block and contemporary volcanism and limestone development contributed great volumes of sediment to the Aure Trough and lesser volumes to the Cape Vogel basin. The history of the area can be interpreted in terms of interaction between Australian and Pacific lithospheric plates and opening of the Coral Sea by rifting.

Pattern of Triassic-Jurassic Diabase Dikes around the North Atlantic in the Context of Predrift Position of the Continents

Abstract: Parallel sets of Late Triassic to Early Jurassic diabase (dolerite) dikes in eastern North America, West Africa, and northeastern South America form a radial pattern if the continents are reassembled into their relative positions in Triassic time. The pattern is roughly convergent on the Blake plateau, the Bahama platform, and the western Senegal basin. The dike pattern can be interpreted as being parallel to lines of tension in a net of principal stress trajectories. This net describes a stress field imposed on the continental crust by movements in the upper mantle at the onset of North Atlantic sea-floor spreading.

Experimentally Produced Devitrification Textures in Natural Rhyolitic Glass

Abstract: Experimental devitrification of natural rhyolitic glass was undertaken in an attempt to produce, under known conditions, analogs of naturally devitrified rocks. The results should facilitate the recognition and interpretation of devitrification textures. Previous studies have shown that alkali-rich aqueous solutions increase devitrification rates sufficiently to produce solid pieces of devitrified obsidian in the laboratory, but none of these has provided a systematic study of experimentally produced devitrification textures. Cylinders of natural volcanic glass, 4.6 mm in diameter and 10 to 20 mm long, were sealed in 5-mm-diameter gold capsules, along with either pure water or alkali-rich solutions. Runs were made in externally heated pressure vessels in the temperature range 240° to 700°C and in the pressure range 0.5 to 4 kb. Textures recognized in the products of the runs closely resembled hydration and devitrification texture s in some rhyolitic rocks. Although hydration fronts and strain birefringence were developed during some experimental runs, no perlitic fractures were observed. Devitrified products contained spherulites, micropoikilitic quartz, orb texture, axiolites, and miarolitic cavities. Two stages of devitrification are distinguished. The glassy stage is characterized by glassy or felsitic textures with isolated spherulites, and the spherulitic stage by spherulitic textures and micropoikilitic quartz. A hypothetical third stage in this succession, not represented in products from this study, most likely has a granophyric or granitic texture, with no evidence of glassy precursors.

Transverse Bars and Braiding in the Lower Platte River, Nebraska

Abstract: The Platte is a wide, shallow river which flows eastward from the Rocky Mountains across the Great Plains of Nebraska. Its lower reaches carry a dominantly sandy load and during intermediate and low discharges display a pronounced braided character accomplished primarily through dissection of tabular, flat-topped transverse bars. Transverse bars form by sediment aggrading to a profile of equilibrium (Jopling, 1966) and grow by downcurrent extensions of avalanche faces. Depth, velocity, and grain size tend to decrease on active bar surfaces from their upstream mouths to the downstream and lateral margins. Active surfaces are covered with small-scale bed forms whose distributions are controlled by the flow characteristics. A typical mouth-to-margin bed form progression is dunes to diminished dunes to ripples, reflecting downcurrent reduction of stream power. Water-surface slopes over active bars tend to be greater than those of the channel segments which feed them. Under ideal conditions, transverse bars are essentially lobate; however, most bars, especially during low discharges, assume irregular or asymmetrical patterns due to any of several factors that include bar-mouth cross-sectional geometry, proximity to exposed banks, adjacent currents, steadiness of flow, and basin depth distribution. Braiding (bar dissection) begins during decreasing discharges when the flow passing through the bar mouth becomes unable to sustain active sediment transport over the entire bar surface. A single bar, examined closely over a five-day period of gradually decreasing discharge, documents the evolution from wholly active to dissected states.

A Sedimentary Model for Determining Paleotidal Range

Abstract: Prograding clastic tidal coastlines in embayed areas, lagoons, and open coasts generate a graded, fining-upward sequence of sediments. This graded sequence is produced by a distinct zonation of sediment transport processes across a clastic intertidal flat. These transport zones are (1) tidal bedload transport (producing subtidal sand), (2) combination of tidal bedload transport and emergence runoff prior to exposure (producing low tidal flat sand), (3) alternation of bedload and suspension sedimentation (producing midflat interbedded sands and muds), and (4) suspension sedimentation (producing high tidal flat muds and clays). The distinction between subtidal and low intertidal flat sands is achieved by identifying sedimentary structures indicating late-stage emergence runoff prior to exposure in the low tidal flat sands. The contact between these two sands, one with and the other without emergence runoff sedimentary structures, coincides with the postion of mean low water. The top of the graded, fining-upward sequence consists of clay-sized sediment and coincides with the level of mean high tide. Such prograded fining-upward sequences contain within them a preserved record of tidal range. In Holocene sediments, the thickness of the interval representing low tidal flat, midflat and high tidal flat sediments coincides with mean tidal range . In fossil equivalents, the thickness of similar sequences gives a quantitative measurement of paleotidal range.

Petrologic and Geophysical Nature of Serpentinites

Abstract: Mineralogically, serpentinites consist predominantly of lizardite, clinochrysotile, and antigorite. Recent work has shown that these minerals are not polymorphs. Chrysotile is the only mineral recognized as a synthetic product in experimental studies of the system MgO-SiO2-H2O. Antigorite seems to be stable at higher temperatures than lizardite or chrysotile. The density of individual serpentine species is dependent on their morphology; the low-density serpentinites (<2.55g/cc) consist predominantly of clino-chrysotile. Seismic velocities and magnetic susceptibilities of serpentinites are related to the degree of serpentinization. The transition of massive serpentinites from ductile to brittle behavior in laboratory experiments at high confining pressures and temperatures above 300°C has been related to dehydration which may provide a mechanism for developing deep-focus earthquakes along Benioff zones. Serpentinite is formed by direct hydration of ultramafic protolith in the crust. The most common ultramafic protoliths are harzburgite, dunite, and Iherzolite. The assemblage generally developed from these is lizardite + chrysotile + brucite + magnetite. In areas of high-grade metamorphism, antigorite is the predominant serpentine mineral. The common, large, alpine-type serpentinized ultramafic masses contain brucite and have MgO/SiO2 ratios similar to those of their protolith, resulting in volume increase during serpentinization. Metamorphic serpentinites and some highly sheared alpine-type serpentinites have lower MgO/SiO2 ratios than their protolith, lack brucite, and appear t o have been formed by volume-for-volume replacement with concomitant loss of magnesium or addition of silica. Many large, young masses of peridotite appear to be slabs of oceanic mantle over-thrust onto continental edges. Subsequent sedimentation, serpentinization, and tectonism have greatly modified these original slabs so that their recognition in older orogenic zones is equivocal. The concept of the tectonic evolution of ultramafic rocks from oceanic crust-mantle slabs invading continental margins and being incrementally serpentinized and moved by later tectonic events provides a working hypothesis that allows a better explanation of the many peculiar and varied occurrences of serpentinite. The evidence does not support Hess' suggestion that the third layer of the oceanic crust consists of partly serpentinized mantle peridotite.

South Atlantic, Benue Trough, and Gulf of Guinea Cretaceous Triple Junction

Abstract: During the early separation of Africa from South America, the South Atlantic and the transform faults of the Gulf of Guinea were joined by a short-lived line of lithosphere spreading running northeast from the Niger delta, under the Benue trough. The separation of the continents was established by Aptian times, and the spreading under the Benue trough lasted from the Albian to the Santonian. The early separation of West Africa from northern Brazil was characterized by lateral movements of the two continents against each other along the St. Paul9s and the Romanche oceanic transform faults. Some 20 m.y. of transform fault motion may have elapsed before West Africa was completely separated by oceanic crust from northern Brazil. The South Atlantic, Benue trough, and the Gulf of Guinea formed an unstable RRF triple junction, which may have caused internal strain in the African plate. It may also have resulted in possible dilation of the Gulf of Guinea transform faults which, together with the short intervening ridge segments, served to localize the Cretaceous volcanicity thought to be responsible for the recently discovered North Brazilian Ridge.

Evolution of the Central Indian Ridge, Western Indian Ocean

Abstract: Topographic, magnetic, and earthquake epicenter data from the wholly submerged Central Indian Ridge were interpreted, using the Theory of Plate Tectonics. The pole of relative motion between the Indian and Somalian plates, lying at 16.0° N., 48.3° E. and with opening at 6.2 × 10−7 deg/yr, was obtained from the strike of fracture zones taken as transform faults and the spreading rates based on magnetic anomaly patterns. Since this pole appears to have moved little since the Miocene, the plate positions at that past time can be obtained by finite rotation about the present rotation pole. Such a reconstruction shows that the complicated nature of the present plate margins results from Miocene to Recent opening along a north-south fracture zone that existed in this area during an interval of rapid spreading in the late Cretaceous and early Tertiary.

Distribution and Age of High-Grade Blueschists, Associated Eclogites, and Amphibolites from Oregon and California

Abstract: Isolated blocks of high-grade blueschist and amphibolite facies metamorphic rocks occur within the Jurassic and Cretaceous eugeosynclinal deposits of the Coast Ranges of southwestern Oregon and California. The blocks range in size from individual rock masses commonly 5 to 1,000 ft in diameter to a few larger masses as much as 7 mi long and 2 mi wide. The high-grade blocks are predominantly basaltic in composition and include glaucophane schists, eclogites, and gneissic rocks of the amphibolite facies. Field relationships indicate that the blocks are closely associated with serpentine, that high-grade blueschist and amphibolite blocks, lower grade blueschists, volcanic rocks, and cherts occupy disturbed zones that may be related to thrusting, and that there is no exposed in situ provenance for the high-grade blueschists, eclogites, and amphibolites. Potassium-argon mineral ages of white mica and actinolite from the blueschists and of hornblende from the amphibolites indicate that these minerals crystallized approximately 150 m.y. ago, but the ages measured on glaucophane from the blueschist blocks are commonly younger. These data suggest that the high-grade blue-schist and amphibolite blocks represent fragments of a cryptic metamorphic terrane of pre-Tithonian age that have been tectonically mixed with younger rocks of the Franciscan Formation in California and Otter Point Formation in Oregon. The younger ages for glaucophane probably reflect metamorphic episodes in which lower grade in situ blueschist facies mineral assemblages were developed in the blocks after their emplacement within the Franciscan Formation. This pre-Tithonian cryptic metamorphic terrane probably developed as a result of interaction between oceanic and continental plates. The occurrence of tectonic blocks of this terrane within melange zones in Oregon and California may be related to later plate interaction.

Thin Skin Distension in Tertiary Rocks of Southeastern Nevada

Abstract: Volcanic rocks of late Tertiary age, aggregating about 17,000 ft, accumulated on a surface of low relief cut on Precambrian rocks in the Basin and Range province south of Lake Mead, in Nevada and Arizona. They consist mostly of lava and flow breccia of intermediate composition with minor ash-flow tuff, bedded tuff, and lava of rhyolitic composition. The last of three main phases of volcanism was accompanied by widespread epizonal plutonism and intense faulting. All or parts of six similarly but separately fault-deformed structural units are recognized in a 92-sq-mi mapped area. The structural units are highly distended by a system of closely spaced north-to northwest-striking shingling normal faults (many of which are low angle) that displace younger over older rocks in a west to west-southwest direction. Cumulative amounts of distension approximate the breadth of the structural units and are as much as 20,000 ft, whereas cumulative vertical displacements are much less and in some places are minimal. The structural units are floored at or near the present level of exposure by complex low-angle zones of detachment or decollement into which the numerous shingling normal faults merge. Where the units abut along their strike, they are separated by complex zones of transcurrent faults that appear to merge with the detachment structures and thus mark the ultimate limits of the structural units. Displacement on the detachment structures has the same sense as, but in some places is much greater than, that of the cumulative offset on the shingling faults, thus indicating low-angle movement of the structural units as platelike or lobate masses. These relationships indicate remarkably thin-skinned, large-scale, fault-related tectonism of a type which is present in a broad belt south of Lake Mead and in numerous other areas in the Basin and Range province. The best exposed structural units exhibit a serial eastward progression from broad areas of steeply dipping strata, low-angle faults, and deep denudation to gently dipping strata, high-angle faults, and little denudation. Reverse-drag flexing, a volume-compensating mechanism for movement on concave-upward faults, is inferred to have produced the gentle to moderate dips of the strata, whereas the nearly vertical dips in the western parts of the units probably resulted from a combination of reverse-drag flexing and rotation related to uplift. Evidence of compression-related folding is absent. The extreme distension is viewed as a surficial feature of a crustal belt that was subjected to a brief episode of tensional rifting. Rifting at subjacent levels along the belt was compensated for by emplacement of plutons. The surficial rocks were stretched and thinned over the plutons.

Basin and Range Structure: A System of Horsts and Grabens Produced by Deep-Seated Extension

Abstract: Basin and Range structure can be interpreted as a system of horsts and grabens produced by the fragmentation of a crustal slab above a plastically extending substratum. According to this view, the extension of the substratum causes the basal part of the slab to be pulled apart along narrow, systematically spaced zones which in turn cause the downdropping of complex horizontal prisms (grabens) in the brittle upper crust. The grabens form valleys at the surface; the intervening areas are horsts, or tilted horsts. Not all geologists have agreed, however, that Basin and Range structure consists of a system of horsts and grabens. Instead, the structure is commonly considered to consist of tilted blocks in which the upslope part of an individual block forms a mountain and the downslope part a valley. Recent detailed studies, including geophysical work, suggest that the horst and graben model may be more generally applicable. Many of the valleys in the Great Basin are bounded on both sides by faults that drop the valley block down; these faults are exposed at the surface or can be inferred from steep gravity gradients indicative of steep faulted subsurface bedrock slopes. Some areas that were thought to represent a typical series of tilted blocks may be a series of highly asymmetrical grabens in which one side of a valley is marked by a master fault and the other side by valleyward tilt. With present knowledge, most, or perhaps all, of the major valleys in the Great Basin can plausibly be considered to be grabens, and most or all of the mountains can be considered to be horsts or tilted horsts. The grabens, and the underlying inferred deep zones of extension that cause them, are systematically distributed in the Great Basin. They are generally north-trending features spaced 15 to 20 mi apart. Locally, the pattern is more complex, and individual grabens divide and trend away from each other at acute or high angles. In a few places, the pattern may even be roughly polygonal. The distribution pattern of the grabens and the related deep zones of extension resemble crack patterns in small-scale tensional systems, and both patterns may be mechanically related. By analogy with the small-scale systems, the areas of generally north-trending and parallel grabens require east-west extension, whereas the areas with a possible polygonal pattern of grabens must extend radially. The geometry of block faulting related to Basin and Range structure requires sizable east-west extension, estimated at about 1.5 mi on the average for each major valley and at about 30 to 60 mi across the entire Great Basin. Most of this extension has taken place in the last 17 m.y., or perhaps even in the last 7 to 11 m.y., indicating a rate of extension in the range of 0.3 to 1.5 cm/yr.

Tectonic Investigations in Western Colombia and Eastern Panama

Abstract: Precambrian, Paleozoic, and Mesozoic(?) metamorphic rocks form the basement of the central Colombian Andes, which was intruded by Mesozoic quartz diorite batholiths and was the site of Tertiary volcanism. Mesozoic miogeosynclinal rocks are preserved to the east in the Magdalena Valley, and eugeosynclinal rocks are preserved on the west flank of the Cordillera Central. The Cordillera Occidental includes highly deformed rocks of the Mesozoic eugeosyncline, intruded by Tertiary(?) dioritic to granitic plutons and overlain locally by Tertiary volcanic rocks. The fault-bordered Cauca Valley separates these two Andean ranges and was the site of deposition of fluviatile, lacustrine, and volcanic Tertiary rocks. Thick sequences of marine Tertiary sedimentary rocks occupy the Atrato-San Juan basin to the west and the Sinu-Uraba basin at the northern end of the Andes. Pillow basalts, gabbros, and andesites, are the chief rocks forming the basement complex of the Serrania de Baudo along the Pacific coast of Colombia. Bouguer anomalies attain values as high as + 135 mgals over pillow basalts and gabbros of the Serrania de Baudo. A negative anomaly with values as low as −90 mgals is found over the Atrato-San Juan basin. Values increase to +95 mgals over the Sautata arch and then decrease to −45 mgals over the Sinu-Uraba basin. Bouguer anomalies are negative along the Cordillera Occidental southward from Mutata to near lat 5° N., but there the values become positive. These positive values are a southeastern continuation of the gravity high over the Sautata arch, and they persist southward along the Cordillera, despite topographic elevations greater than 3,000 m, which indicates an isostatic imbalance in the crust or upper mantle. This anomaly, the West Colombian gravity high, extends southward into Ecuador. Bouguer anomalies are negative (−20 to −150 mgals) over the Cauca Valley, Cordillera Central, and Magdalena Valley. These regional relations indicate that granitic crust is present under the Cordillera Central but may be thin or absent beneath the Mesozoic eugeosyncline. Mafic, perhaps oceanic, crust serves as floor of the Mesozoic eugeosyncline, the Pacific Lowlands, and the Serrania de Baudo. If an oceanic crust of 16 km is assumed in the Pacific area, west of the Colombian coast, the gravity data indicate an eastward increase in crustal thickness to 30 or 35 km beneath the Cordillera Central and Magdalena Valley. Major tectonic boundaries may be inferred from gravity data as follows: (1) Steep marginal gravity gradients suggest that the Atrato-San Juan basin is like a gigantic graben. (2) Steep gradients on the flanks of the Suatata arch suggest that it is a horst-like feature. (3) Rocks of the Mesozoic-early Tertiary eugeosyncline may locally override the mafic rocks that cause the West Colombian gravity high. (4) Steep gradients on the east side of this high mark the boundary between granitic crust to the east and oceanic crust to the west. South of lat 5° N., this zone coincides with the great Romeral-Cauca fault system. The Mesozoic eugeosyncline is interpreted to be the site of Mesozoic underflow of the Pacific plate beneath nuclear South America; the Romeral fault, with associated serpentinites, is the trace of a Mesozoic Benioff zone. The Atrato-San Juan basin may represent a Tertiary zone of underflow that was an early branch of the Peru-Chile trench. From analysis of regional distribution of earthquakes and focal depths in northern South America and vicinity it is concluded that relative motion between the South American plate and the Pacific plate is partitioned into two types: (1) some underthrusting occurs along the northern extension of the Peru-Chile trench, and (2) right-lateral movement occurs along the Dolores-Bocono-El Pilar megashears. The western Cordillera of northern Ecuador and Colombia and eastern Panama are semi-attached to the Pacific plate and have an east to northeast motion related to eastern spreading from the East Pacific rise and northward spreading along the Galapagos rift zone. Thus, gross relative motion of northern nuclear South America is counterclockwise with respect to the Pacific plate, Panama, and the Caribbean plate.

Tectonics of the Panama Basin, Eastern Equatorial Pacific

Abstract: The Panama Basin includes portions of the Nazca, Cocos and South America lithospheric plates and borders the Caribbean plate. The complex interactions of these units have largely determined the topography, pattern of faulting, sediment distribution, and magnetic character of the basin. Only heat flow data fail to correlate with major structural features related to these units. The topographic basin appears to have been created by rifting of an ancestral Carnegie Ridge. The occurrence of a distinctive smooth acoustic basement and a characteristic overlying evenly stratified sedimentary sequence on virtually all elevated blocks in the basin suggest that they all once formed part of this ancestral ridge. The present Carnegie Ridge is the relatively undeformed southern half of this feature, while the Cocos Ridge is the northern half fragmented by left-lateral north-south transcurrent faulting. As blocks of the Cocos Ridge reach the Middle America Trench, they appear to clog the subduction zone and become welded to the Nazca plate. Thus, the active transform fault at the eastern edge of the Cocos plate has episodically shifted west as segments of the trench were deactivated. Such a shift appears to be occurring at the present time.

Late Cenozoic Evolution of the Great Basin, Western United States, as an Ensialic Interarc Basin

Abstract: The Great Basin of the western United States corresponds closely to a well-defined zone of high heat flow, thin crust, and anomalously high-attenuation low-velocity upper mantle. Seismicity, of predominantly normal faulting type, occurs in two marginal zones above the lateral transitions in the upper mantle, which can be correlated also with the most recent volcanism. Petrological studies have indicated that the region was the site of calc-alkaline andesitic volcanism during the middle to late Cenozoic which changed abruptly to fundamentally basaltic volcanism in the late Cenozoic, accompanied by the beginning of major Basin-Range crustal extension. This is interpreted as a change from the island-arc-type volcanism to active interarc spreading. The latter was triggered by the termination of the early to middle Cenozoic West Coast subduction zone. The release of the compressive stress field of the sub-duction zone is considered necessary for interarc spreading. We suggest that the spreading is caused by a mantle diapir mobilized by the descending lithospheric slab. The diapir is trapped beneath the sialic crust, and flattens and spreads out laterally as it rises. This mechanism can explain the extension and the outward migration of volcanism in the Great Basin and the marginal seismic zones. The anomalous upper mantle now present beneath the Great Basin is interpreted as the remnant of this diapir.

Pleistocene Sedimentation in the Equatorial Atlantic: Stratigraphy and Faunal Paleoclimatology

Abstract: A suite of 15 piston cores up to 23 m long taken in an eastern equatorial Atlantic fracture zone at 8°N. and in the nearby Sierra Leone basin document climatic variations over the last 1.8m.y. Foraminiferal and paleomagnetic stratigraphies were used to correlate the cores and select the most representative pelagic record. "Total fauna" analysis of foraminiferal variations in a suite of shorter cores spanning the last 200,000 yrs substantiates in detail the oxygen-isotope trends over that interval. When applied to two cores containing 1.8 m.y. of equatorial sedimentary history, this analysis pinpoints two prominent, large-scale climatic shifts: (l) at 1.3 m.y. B.P., the mean climatic situation deteriorated, and short but severe cold pulses began to punctuate the previous moderate warmth of the late Matuyama; (2) following 900,000 yrs B.P., the duration of cold intervals increased. Prior to the Jaramillo, no cold pulse exceeded 30,000 yrs; three post-Jaramillo cold intervals ranged in duration from about 50,000 to 150,000 yrs. The shortest and most recent of these correlates with the Wisconsin glaciation. In addition to pelagic carbonates, continental sediment is introduced into these cores by turbidity currents flowing down the axis of the fracture zone and by wind blowing off Saharan and equatorial Africa. Although the absolute input rate of pelagic carbonate to these sediments increases during cold intervals, the glacial carbonate percentages tend to decrease due to even greater influxes of continental detritus. Beginning in the Jaramillo event at roughly 900,000 yrs B.P., this terrigenous dilution depresses carbonate percentages in these cores, often to very low values. Pre-Jaramillo sections are generally calcareous oozes.

New Interpretation of the Geology of Iceland

Abstract: Two active transform faults are identified on land in Iceland. This observation leads to a new interpretation of the tectonics of Iceland that is generally consistent with the available geologic, geomorphic, and geophysical data. This new interpretation provides a framework that can be used to relate detailed geologic and geophysical studies in Iceland to worldwide processes at the crests of mid-ocean ridges. Nearly one-half of Iceland seems to have formed during a period of very slow spreading between about 9 and 20 m.y. B.P. The center of spreading within Iceland apparently shifted from western to eastern Iceland around 7 or 8 m.y. B.P. Iceland, the largest landmass on the mid-ocean ridge system, may have resulted from a change in the stress pattern on a broad fracture zone, allowing large volumes of lava to be erupted while there was little regional spreading.

The Taconic Orogeny

Abstract: The Taconic orogeny of eastern North America was not, as traditionally defined, a single orogenic event that occurred at the end of the Ordovician period, but rather a complex series of orogenic episodes or climaxes spread over the larger part of that period. In most sectors of the northern Appalachians it included at least three of the following: disconformity in an external belt where carbonate was accumulating; severe early deformation in an internal volcanic belt; gravity slides from internal uplifts into the external belt; and widespread deformation, especially in the more external belts. In general, these events did not occur at the same time in the various sectors; each took a considerable time, and they overlapped to some extent. The Taconic orogeny also affected the southern Appalachians and may have been the most important one there, but evidence for this assertion is meager and inconclusive. Detailed analysis of the "fine structure" of the Taconic orogeny combats the dogma that orogenies are sharp, discrete events punctuating the geologic record (separating periods and abruptly terminating geosynclinal sedimentation) and suggests instead that they reflect "random-walk" processes within the Earth, in all likelihood the same as those responsible for sea-floor spreading and the present tectonic state of the Earth.

Hydrothermal Explosion Craters in Yellowstone National Park

Abstract: Hydrothermal explosions are produced when water contained in near-surface rock at temperatures as high as perhaps 250°C flashes to steam and violently disrupts the confining rock. These explosions are due to the same instability and chain reaction mechanism as geyser eruptions but are so violent that a large proportion of solid debris is expelled along with water and steam. Hydrothermal explosions are not a type of volcanic eruption. Although the required energy probably comes from a deep igneous source, this energy is transferred to the surface by circulating meteoric water rather than by magma. The energy is stored as heat in hot water and rock within a few hundred feet of the surface. At least ten hydrothermal explosion craters, ranging in diameter from a few tens of feet to about 5000 ft, have been recognized in Yellowstone National Park. Eight of these craters are in hydrothermally cemented glacial deposits; two are in Pleistocene ash-flow tuff. Each is surrounded by a rim composed of debris derived from the crater. Juvenile volcanic ejecta are absent, and there is no evidence of impact. Geologic relations at the Pocket Basin crater establish that the explosion there took place during the waning stages of early Pinedale Glaciation. This association with ablating ice suggests that an ice-dammed lake existed over a hydrothermal system at the Pocket Basin site and that the hydrothermal explosion was triggered by the abrupt decrease in confining pressure consequent to sudden draining of the lake. Most of the other explosion craters in Yellowstone Park could have been triggered in the same manner. Calculations of energy available in Yellowstone hot-spring systems and of energy required to form craters indicate that the proposed mechanism is reasonable. The sizes of craters expected in various rock types correspond with those observed.

Magnetism of Pillow Basalts and Their Petrology

Abstract: The average intensity of magnetization of a layer of pillow basalt depends on (l) variations in the intensity of the earth's field as the lava is formed, (2) variations in the direction of magnetization within the layer, (3) the percent of nonmagnetic pore space between pillows, and (4) variations in the thermoremanence of the basalt due to variations in composition and in the degree of crystallization. To evaluate (4) we made a centimeter-by-centimeter examination of submarine pillow fragments and found that the remanence increases from almost zero in the glassy crust to high values of .04 emu/cc in the interior. The main control on remanence is the degree of crystallization. In very large pillows the remanence reaches a peak and decreases toward the center, the decrease apparently being due to an increase in grain size. The magnetization of the basalts resides in titanomagnetite grains possessing natural remanences in the range 1 to 2 emu/cc. The variation of natural remanence with grain size in the range 3 to 6 microns suggests pseudo-single domain behavior. Our best estimate of the average magnetization of a layer of submarine pillow basalt is .014 emu/cc. A layer less than 1 km thick with a value of remanence this high is adequate to account for most marine magnetic anomalies.

Deformation of Lee-Side Laminae in Eolian Dunes

Abstract: Processes responsible for structures in sand dunes consist of (l) primary deposition by saltation and creep and by settling from suspension, (2) redeposition accompanying avalanching, and (3) penecontemporaneous erosion. Characteristics of dune structures were examined in the field by introducing marker beds of magnetite at times of sand deposition, thus recording original surfaces and making possible the determination of subsequent changes. Similar structures were examined in the laboratory by testing processes and comparing the resulting structural forms with corresponding natural features. Avalanching in sand is of two types: sand flow and slumping. Deformational structures characteristic of each were recorded in the field and were reproduced in the laboratory. Nine varieties of deformational structures are recognized and described. Analysis of these structures suggests criteria for distinguishing compressional types (lower dune slope) from tensional types (upper dune slope). The analysis of deformational structures also serves to distinguish between forms developed in cohesive sand and those in non-cohesive sand. Since the degree of cohesion is largely a function of the amount of moisture in the sand at the time of avalanching, the deformational structures provide a means for recognizing original dry sand, wet sand, sand crusts, and saturated sand surfaces in ancient deposits. A testing of these criteria was made by comparing laboratory samples with those of dry sand at White Sands, New Mexico, and with those of coastal dunes (probably wet sand) in southern Brazil.

Climate Change in the North Pacific Using Ice-Rafted Detritus as a Climatic Indicator

Abstract: The variations in weight percent of the grain size fraction greater than 250 μ in nine cores from the North Pacific were determined using sampling intervals of 5 to 20 cm. Material in this size fraction is interpreted as transported by icebergs, and fluctuations are attributed to the waxing and waning of glaciers on the surrounding continents. At least eleven periods of increased ice rafting are detected in the cores during the time from 1.2 m.y. ago to the present, whereas only about four are identified from 1.2 m.y. to 2.5 m.y. B.P. The dating and time correlations are based on the magnetic stratigraphy, ash falls, and faunal extinctions. The ice-rafted detritus indicates a cooling beginning about 1.2 m.y. ago and becoming very intense between the Jaramillo event and the Brunhes-Matuyama boundary. This time may correspond to the initiation of mid-latitude glaciations of Europe and North America. At least six zones of ice-rafted sediment are present in the Brunhes normal polarity series. The correlations between these and the carbonate fluctuations of the central Pacific are good. Evidence for a marked interglacial ranging from about 460,000 to 530,000 yrs B.P. occurs within these cores. This interglacial may be worldwide in extent.