Pacific Ridge Research Articles

Break in the Indian summer monsoon

The changes in circulation patterns over Eurasia during break monsoon condition over India are studied in comparison to the active monsoon condition. Break monsoon condition seems to set in over the India Sub-Continent in association with eastward movement of middle latitude westerly trough at 500 mb, having large amplitude extending into west Pakistan and northern India. Simultaneously the subtropical anticyclonic ridge over Arabia protrudes into central and Peninsular India. The high latitude blocking high over East Siberia retrogrades and the East Asiatic trough deepens and moves eastwards. The west Pacific subtropical ridge recedes eastward from the China continent. During this period the monsoon trough shifts, from its normal position over Gangetic plains, northwards to the foot of the Himalayas and the monsoon westerlies in the lower troposphere extends right upto the rim of the Tibetan Plateau. The sub-tropical ridge line in the upper troposphere shifts northwards during break and lies approximately above the lower monsoon trough. This seems to provide an effective process of removing ascending air in the lower monsoon trough causing exceptionally heavy rainfall over Assam and along the foot of the Himalayas.

Tectonic lineaments on the cocos plate

Mapping of the Siqueiros Fracture Zone shows that it extends at least 850 km east of the crest of the East Pacific Ridge with an overall azimuth of 082°. The bearing of its younger portion is 084°. The relatively constant trend of the fracture zone indicates a constant direction of motion at that plate boundary location for the past 20 my. However, comparison of the observed trend of the fracture zone with a synthetic trend generated from the Cocos-Pacific pole of rotation at 40°N, 110°W shows that the pole has remained in this location for only the past 5–7 my. Prior to that time, the pole must have been close to the Pacific-Farallon pole of rotation at 78°N, 136°W. One minor change in the bearing and character of the Siqueiros Fracture Zone may be interpreted in terms of plate reorganization in the central eastern Pacific, favoring a date of 10 mybp for this episode. A band of east-west oriented fracture zone topography in the Guatemala Basin is interpreted to be the Clipperton Fracture Zone. The change in plate geometry at 10 mybp eliminated this fracture zone offset of the ridge crest. The Tehuantepec Ridge is not the continuation of the Siqueiros Fracture Zone. Instead, the Tehuantepec Ridge may be a hinge fault produced by the Cocos plate underthrusting the North American plate on the northwest and the Caribbean plate on the southeast.

Subduction and Aseismic Ridges

WHETHER or not aseismic ridges and seamounts on the ocean floor are due to stationary hot spots1–7 or some other process, one may ask what happens when an aseismic ridge is forced into a subduction zone. Here I consider one aspect of the problem, namely whether or not the plan view of island arcs and their associated trenches has been influenced by aseismic ridges being subducted. Island arc type subduction occurs primarily in the western Pacific and the Indian Oceans. As indicated in Fig. 1, there are a small number of aseismic ridges in the vicinity of the north and western Pacific and Indian trenches. The bathymetric charts recently published by Chase et al.8 were used to obtain the general outlines of the Pacific aseismic ridges. The two most massive Pacific ridges, the Emperor and Caroline, intersect the trench system near prominent cusps, where one arc gives way to the next. In addition, the Marcus–Necker ridge intersects the northern edge of the arcuate portion of the Marianas trench; the Gulf of Alaska seamounts, presumably generated by the Juan de Fuca and/or its neighbour, the Yellowstone plume5–7, intersect the Aleutians only slightly west of the cuspate eastern end of the Aleutian arc. In the Indian Ocean, the two principal cusps in the boundary between Asian and Indo–Australian plates are now intra-continental. It may reasonably be supposed, from its arcuate character and continuity with the Indonesian arc, that the Himalayan arc replicates the shape of an extinct island arc system lying in the Tethyan Ocean, that lay between India and central Asia until it was closed by subduction in later Tertiary times. Whether or not the Himalayan arc preserves the shape of an old island arc, the Indian Ocean's two prominent aseismic ridges9 extend, with comparatively minor extrapolation, into the two major cusps of the Himalayan tectonic arc.

Geological interpretation of the gravity data for a profile across the Pacific Ocean

This paper presents a model of geological interpretation of gravity observations along the profile across the Pacific Ocean from the Japan trench across the Hawaiian ridge to the Middle America trench crossing eleven geological regions (the total length of the profile is about 14,000 km). The interpretation was made by comparing the gravitational attractions of the combined layer of the hydrosphere and the crust and upper mantle from sea level to the depth of isostatic compensation by using the fundamental principles of the theory of isostasy. The density of the crust and mantle beneath the M discontinuity was determined from the results of deep seismic sounding (DSS) and the correlation between compressional velocities and densities of rocks. A statistical analysis was made of the data obtained by many scientists to improve the correlation for different rocks at pressures of 0.001–10 kb. Density distribution curves are presented for different rocks and different velocity intervals. The lateral density inhomogeneities in the lower part of the combined layer from a depth of 50 km to the depth of isostatic compensation are supposed to cause regional gravity anomalies of the first order determined from satellite observations, whereas density inhomogeneities of the combined layer of the hydrosphere and the crust and upper mantle to a depth of 50 km account for residual gravity anomalies obtained from free-air anomalies (the normal gravity formula derived by Helmert (1901–1909)) after the regional first-order anomalies are subtracted from the satellite data. For the profile across the Pacific Ocean we obtained curves of regional gravity anomalies of the first order, residual gravity anomalies, ocean-floor topography, variations of the apparent density of the combined layer of the crust and mantle to a depth of 50 km below sea level, variations of the apparent density of the upper-mantle layer to the same depth, variations of heat flow, and a model of a density section to a depth of 50 km below sea level. The density of the crustal rocks varies from 2.7 to 3.0 g/cm3. The upper-mantle layer has its highest density (3.42 g/cm3) directly beneath the M discontinuity under the Hawaiian ridge (from DSS) and under the Japan trench (under the continental slope). Compact rocks lie beneath the northwest Pacific basin and the mid-Pacific mountains and also in fracture zones. The upper-mantle rocks of the lowest density (3.13 g/cm3) lie beneath the east Pacific ridge, the Hawaiian ridge (at a greater depth), and in the form of isolated horizontal lenses beneath the mountains of the Darwin uplift. Anomalously high values of the heat-flow intensity were found. A suggestion is made that the upper-mantle rocks with a density of 3.13 g/cm3 west of the Hawaiian ridge are genetically related to the rocks of about the same density lying at the base of the third crustal layer (density obtained from DSS data). This interesting region requires geophysical investigation and deep drilling to reach the rocks with a 3.13-g/cm3 density at a depth of about 3 km. These studies will throw light on the processes of transformation of rocks in the crust and upper mantle.

Two Small Crustal Plates in the South Pacific near Easter Island

THE evolution of the South Pacific during the Cainozoic involved several patterns of crustal plates in relative motion for various periods of time1. The present East Pacific Ridge, which now outlines the Pacific-Nasca Plate boundary, has been the only major spreading centre north of 36° S for the past 9 m.y. Before then, a fossil ridge, continuous with the still active Chile Ridge2,3 was a locus of spreading in what is now the middle of the Nasca Plate; between 9 and 20 m.y. ago, both the East Pacific and the Fossil Ridges were active, suggesting that the Antarctic Plate, which now occupies the area south of the Chile Fracture Zone between the Pacific-Antarctic and Chile Ridges, extended almost to the equator only 10 m.y. ago. The apparent ease with which the northern boundary of the Antarctic Plate has moved during the past 20 m.y. suggests the triple junction between the Antarctic, Pacific and Nasca Plates has been unstable.

Sea-Floor Spreading and the Cenozoic History of the East-Central Pacific

The east-central Pacific between 20° N. and 45° S. records several patterns of sea-floor spreading. Three distinct spreading centers are now active in the area, and total spreading rates across one of them, the East Pacific Ridge, are the fastest observed anywhere. The present East Pacific Ridge between 20° N. and 45° S. has maintained its present configuration only during the last 9 m.y. Prior to 10 m.y. ago, a north-northwest-trending ridge was active throughout the east-central Pacific, and the fossil crest of this ridge can be identified in the basin east of the present ridge axis between 10° S. and 30° S. and west of the ridge axis between 20° N. and the equator. Between the equator and 20° N., the north-northwest-trending ridge system has been continuously active since at least the Late Cretaceous, but 5 to 10 m.y. ago the position of the axis jumped to the east. South of the Chile Fracture Zone at 36° S., the north-northwest-trending Chile Ridge appears to be a still active segment of this old ridge system. Spreading on the north-northeast-trending East Pacific Ridge began more than 50 m.y. ago at 55° S. and grew northward so that opening at 35° S. began about 20 m.y. b.p. Between 10° N. and 10° S., spreading about the present axis was initiated only during the last 10 m.y. Analysis of magnetic anomalies associated with north-trending ridges near the equator is of limited value because of the extremely low amplitudes of the anomalies generated by the ridge. However, although identification of individual anomalies is tenuous at best, comparison of relative amplitudes and shapes with computed models offers limited support of the hypothesis of Francheteau and others (1970) that the Pacific Plate has migrated northward since the Late Cretaceous.

Oceanic basalts: Chemistry versus distance from oceanic ridges

The major elements chemistry, as expressed by the normative composition, and the degree of silica saturation of basalts erupted at various distances from the axis of the East Pacific Ridge do not differ systematically from the same parameters of basalts erupted at the axis of the Ridge. These results suggest that the conditions of magma generation are similar beneath Ridges and away from them.

Heat flow from the Mid-Ocean Ridges and sea-floor spreading

Observations of heat flow, topographic elevation and topographic slope are examined in the light of the hypothesis of ridge formation by ocean-floor spreading. By normalizing the distance of observations from the ridge axis in terms of the spreading rate, a large amount of data from all parts of the ridge system can be used to derive the pattern of heat flow with greater confidence than here-to-fore obtained. This analysis shows that the shape of the normalized anomaly of high heat flow over the ridge is independent of spreading rate, however, the amplitude of this anomaly over the fast-spreading Pacific ridges (3–6 cm/year) is generally 0.5 μcal/cm 2sec larger than that over the Atlantic and Indian Ocean Ridge which are spreading at a slower rate (1–2 cm/year). More than 60% of the heat lost over the ridges in excess of the adjacent basin heat loss takes place in a narrow zone near the axis, corresponding to the part of the ridge that has been created in the last 6 million years. There is also a nearly linear relation between spreading rate and ridge topographic slope. The normalized slope, however, decreases slightly with increasing spreading rate. Simple mechanical models of ocean-floor spreading such as a thick plate cooling as it moves away from the axis or a thin plate moving over a shallow, near isothermal zone do not adequately explain the pattern of heat flow and topographic slopes derived in this paper. A complex model such as a thin crustal layer moving over a broad stagnation zone could fit the observed data. However, more definitive geophysical data are required to determine the details of this type of model.

Intraoceanic islands, east pacific ridge, island arcs: Volcanism and upper mantle

Two classes of volcanic rocks can be distinguished with respect to petrochemistry, namely oceanic and continental classes. Lavas of island arcs belong to the continental type. Lavas of oceanic ridges are close not to the oceanic, but to the continental type. On petrochemical and geophysical bases the author has drawn the following conclusions: 1. (1) the foci of volcano feeding lie beyond the limits of the earth's crust—in the upper mantle. 2. (2) the composition of the upper mantle under continents, oceans, island arcs and oceanic ridges is somewhat different. 3. (3) the role of assimilation on the passage of magma to the surface is very limited as a rule. According to these conclusions, volcanism may be considered as a certain indicator of the composition and state of the upper mantle matter. A scheme for the evolution of volcanism is suggested on this basis (as a reflection of the evolution of the upper mantle) which is in an agreement with geological and geophysical facts.

Evidence from ocean islands suggesting movement in the Earth

Oceanic islands increase in age from the mid-ocean ridges towards continents and the andesite line reaching a maximum known age of Upper Jurassic. The Seychelles appear to be a continental fragment. Several pairs of lateral aseismic ridges extend from islands on the mid-ocean ridge to adjacent continents. Their continental junctions mark points on opposite coasts which would also fit if the continents were reassembled according to the criteria used by Wegener. As Holmes has shown each pair of ridges tends to have distinctive chemical characteristics. One possible explanation is that convection currents in the mantle rising along the mid-ocean ridges and sinking beneath trenches have carried the crust apart across the Atlantic, India and East Pacific Oceans. The lateral ridges may be approximately streamlines. Although Darwin showed that most volcanic islands sink, a few have been uplifted. Most of these lie a few hundred kilometres in front of deep trenches, suggesting that they may be on the crest of a standing wave in front of the trenches and that the crust is rigid. Of eleven straight chains of young islands in the Pacific ten get older away from the East Pacific Ridge. They could also be streamlines, fed by lava rising from deep within convection cells with stagnant cores. The regularity of ridges suggests non-turbulent flow.

Submarine Fracture Zones, Aseismic Ridges and the International Council of Scientific Unions Line: Proposed Western Margin of the East Pacific Ridge

Submarine Fracture Zones, Aseismic Ridges and the International Council of Scientific Unions Line: Proposed Western Margin of the East Pacific Ridge

Exploration Activity and Oil and Gas Development in Utah during 1962: ABSTRACT

Exploration activity continued at a moderate pace in Utah during 1962. The Paradox basin, which has been very active since the discovery of Aneth in 1956, had considerably less activity during the year. The Uinta basin, on the other hand, experienced a considerably accelerated rate of exploration, due largely to the completion of the Mountain Fuel Supply Company pipeline connecting many of the gas fields of the basin to the Wasatch Front markets. Two rather insignificant new-field discoveries were made during the year in the southern end of the Paradox basin. Drilling of a few infield development wells and field extension wells continued in the greater Aneth area. The Ismay field received considerable drilling, extending the field south and southeast. The northern Paradox basin experienced some deep wildcat drilling in search of the Mississippian and deeper oil. All were unsuccessful. Three new discoveries were completed in the Paradox salt section in the general Big Flat area, indicating possible greater production from this stratigraphic interval. The Lisbon, Salt Wash, and Grassy Trail fields received some development drilling. The Uinta Basin Uncompahgre area had at least ten discoveries or extensions of significance, five predominantly gas producers, and five oil. Two of these, the Phillips Flat Rock well No. 2 in sec. 30, T. 14 S., R. 20 E., and Pacific Natural Gas Exploration Moon Ridge No. 1, sec. 15, T. 16 S., R. 21 E., indicated possible important field discoveries. Formations from Tertiary Green River to Cretaceous Dakota shared in the discoveries. Development in the Red Wash, Chapita Wells, Ute Trail, and Rock House fields continued at a fair rate. The emphasis of surface field work and geophysical work generally shifted west to the Wasatch Plateau, and Basin-and-Range areas, from the Paradox Basin, which has received most of the attention for the past few years. Considerable leasing took place in the Wasatch Plateau from the Clear Creek field south to the general Fish Lake area. Areas in the Basin-and-Range province, notably Box Elder and Iron Counties, experienced considerable leasing. Oil production in 1962 was 31,104,169 barrels, a decline of approximately 813,000 barrels from 1961. Gas production totaled 72,516,409 MCF, an increase of nearly 14,000,000 MCF over 1961. End_of_Article - Last_Page 119------------