摘要:

Tom Crough's friends remember him as warm, shy, inspiring. He was slow to discover his flair for geophysics. He graduated in economics in 1969 from the Woodrow Wilson School at Princeton. After graduation, he and Faith, a zoology graduate of the University of Ohio, were married; Tom described meeting Faith as the best thing that ever happened to him.

ISSN:0148-0227    

DOI:10.1029/JB089iB12p09869

年代:1984

数据来源: WILEY

摘要:

This paper presents a new age‐depth relation and a comprehensive depth anomaly map for the Pacific Ocean. Age, depth, and sediment thickness were averaged for each 1°×1° area. Bathymetry was corrected for isostatic effects due to sediment and water load and for the variation of sound velocity in water. Seafloor existing at extreme distance from hot spot tracks is found to be anomalously deep. Hot spots were treated as thermal anomalies, and data within 800 km of them were eliminated from further analysis. An age‐depth equation was determined by fitting a least squares line to the remaining data. The resulting depth versus age distribution confirms the linear dependence of depth on √age up to 80 Ma. All older seafloor is within 800 km of hot spots or hot spot tracks and is shallower than predicted. This “flattening” of the age‐depth distribution requires that a separate function be fit to data older than 80 Ma. As a result, data representing the deepest seafloor (older than 80 Ma) were selected for curve fitting, and the age‐depth distribution was approximated by an exponential function. It is impossible to distinguish between the infinite half space and cooling plate models of the lithopshere using the age‐depth data from the Pacific Ocean. A depth anomaly map prepared by comparing observed depth with the depth predicted by the new age‐depth formula shows several newly recognized features, including swells over the Louisville Ridge in the South Pacific and in the Gulf

ISSN:0148-0227    

DOI:10.1029/JB089iB12p09873

年代:1984

数据来源: WILEY

摘要:

A geological evaluation of geoid anomalies over the North Atlantic is presented with emphasis on short (40–300 km) wavelength undulations mapped by Seasat radar altimetry. Long (≥3000 km) and intermediate (300–3000 km) wavelength undulations constrain subcrustal processes and are briefly reviewed. Portions of three Seasat revolutions (revs) totaling 9300 km were followed by research ship, which measured gravity, magnetics, topography, and for a 800‐km segment also seismic reflection data, to form a ground truth data set for comparison to the altimetry. The intermediate‐wavelength geoid, the residual upon removal of a (14,14) long‐wavelength surface, correlates with plate age (slope ∼10 cm/m.y.) over Cenozoic crust, and major fracture zones appear as geoid steps. A 3.5‐ to 5‐m high over the Bermuda Rise may reflect thermal rejuvenation created within the plate 40–50 Ma. Short‐wavelength geoid anomalies, typically ±10–50 cm in the open ocean, were analyzed by high‐pass (200‐ or 400‐km cutoff) along‐track filtering and smoothing. The vertical deflection computed from the Seasat geoid compares well with one based on seismic reflection. Shipborne free air gravity, except for a higher‐frequency cutoff, correlates closely with geoid anomalies. Except near seamounts, the principal contribution to geoid anomalies is fracture zone topography. All major and many minor fracture zones (FZ's) appear as geoid lineations, particularly between the 15°20′and Kane FZ, from the ridge axis to anomaly 34. The WSW trend of FZ's between anomaly 21 and 34 is clearly expressed in the altimetry, as is the smooth geoid over mid‐Cretaceous smooth basement formed by fast spreading. Crustal ribbons, 50–150 km wide, of higher or lower geoid, parallel FZ trends. Some geoid lineations are oblique to both isochron and FZ trends; east of the Puerto Rico Trench such anomalies may reflect compressiona

ISSN:0148-0227    

DOI:10.1029/JB089iB12p09885

年代:1984

数据来源: WILEY

摘要:

The relations of geoid data from radar altimeter measurements (GEOS 3) to water depth and geochemical variations along the Mid‐Atlantic Ridge are investigated. Eight geophysical lines across the ridge in the Atlantic Ocean were constructed approximately normal to the local ridge trend. Slopes of geoid per kilometer of topographic relief show an asymmetry between the two sides of the ridge that generally is not matched by topography, which has better symmetry between the two flanks. Subtracting a regional field, based on previous work, produces residual geoid anomalies whose source must be in the earth's outer 600 km. The residual geoid anomalies of the North Atlantic show striking correlation with the axis of the Mid‐Atlantic Ridge, but those in the South Atlantic do not. Instead, major positive residual geoid anomalies appear over the Rio Grande and Walvis ridges. Determination of slopeS, residual geoid per million years crustal age, reveals thatSincreases with greater residual geoid anomaly and with higher (shallower) crestal ridge topography. The slope of 22 cm/m.y. west of the Azores high (near 38°N) suggests that the anomalously high crestal residual geoid and depth anomalies may not be entirely of thermal origin, which may normally produce slopes only of about 8 cm/m.y. as found near 28°N away from shallow ridge topography. To account for this difference in slopes simply by a variation in mantle temperature would require a difference of about 940°C between the two sites, which appears excessive. Also, the decrease in topographic height with crustal age away from the Azores high is much greater than predicted by theoretical cooling curves. Correlation of residual geoid anomalies with depth, geochemical variations along the length of the Mid‐Atlantic Ridge, and the chemistry of the Rio Grande Rise and Walvis Ridge suggest that many regions of anomalous residual geoid and crestal depth values may be in large part a function of crustal thickness and density variations induced by plume intrusion and volcanism in addition to their transient thermal

ISSN:0148-0227    

DOI:10.1029/JB089iB12p09905

年代:1984

数据来源: WILEY

摘要:

Hot spots have an irregular distribution over the earth's surface. Part of this irregularity is obviously due to the finite number (40–100) of hot spots, but the distribution does not appear to be completely random. In this paper, statistical analyses are compared for two published hot spot data sets, one minimal set of 42 and another larger catalog of 117. Three approaches are taken: (1) chi‐square tests of equal area boxes, (2) cumulative distributions about principal axes, and (3) construction of a density function. These methods all indicate that hot spots have a nonuniform distribution, even when statistical fluctuations are considered. To the first order, hot spots are concentrated on one half of the surface area of the earth; within that portion, the distribution is consistent with a uniform distribution. A prediction of hot spot density is made based on an empirical model relating number of hot spots to plate speed. Comparison of this predicted density function to the constructed hot spot density functions for the two data sets yields some similarities; however, the observed hot spot densities for neither data set are explained solely by plate sp

ISSN:0148-0227    

DOI:10.1029/JB089iB12p09919

年代:1984

数据来源: WILEY

摘要:

We examined the early Tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early Tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole. In our reconstruction, the velocities of the Pacific and African plates were inferred directly from their hot spot tracks, and the velocities of other plates with respect to the hot spots were calculated from the motion of the Pacific or African plate and by relative motions estimated from seafloor spreading data. The relative motion between assumed‐separate North Pacific and South Pacific (Chatham Rise) plates was estimated by assuming the hot spots in the Atlantic Ocean basin have been fixed with respect to those in the Pacific Ocean basin, thereby avoiding the use of a poorly defined relative motion circuit. We found that the characteristics of early Tertiary plate motions with respect to the hot spots resemble those at present. The root‐mean‐square velocity of every major continental plate with respect to the hot spots during the early Tertiary exceeds its present velocity but (with the exception of the Indian plate) is less than the root‐mean‐square velocity of every early Tertiary oceanic plate. Equatorial lithosphere moved faster than polar lithosphere during the early Tertiary, but the difference is less than at present. Over the interval 64 to 56 Ma, the no net‐torque absolute velocities differ from the assumed fixed‐hot spot velocities by only 0.04°/Ma, which is insignificant. Thus two dissimilar approaches, fixed hot spots and no net torque of the lithosphere, when applied to identical early Tertiary relative plate velocities, yield similar absolute plate velocities. We attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than

ISSN:0148-0227    

DOI:10.1029/JB089iB12p09927

年代:1984

数据来源: WILEY

摘要:

The results of paleomagnetic, petrographic, and radiometric studies of the Eastern Caroline Islands in the western Pacific indicate that the islands were formed by a hot spot located near the paleoequator between 1 and 11 Ma. The islands show a linear progression of mean ages from 1 Ma in the east (Kusaie) to 11 Ma in the west (Truk). The results of volumetric measurements and geochemical studies suggest that the hot spot source is waning and perhaps was slowly dying during the time Truk, Ponape, and Kusaie were being formed. The dominant shield‐building magmas in the Caroline Islands are part of a differentiated alkalic series. The posterosional lavas are highly silica undersaturated and trace element enriched nephelinites. The latter were erupted subsequent to the cessation of the main shield phase of volcanism. The petrography and geochemical evolution of Truk are strongly reminiscent of that of the Hawaiian chain; however, the shield‐building lavas are compositionally similar to the alkalic lavas that typically form only thin, late‐stage caps on many Hawaiian volcanos. No tholeiitic rocks were found despite sampling deep within the eroded volcanic structure of the islands. This absence of tholeiitic lavas and dominance of alkalic lavas stand in contrast with Hawaii, where tholeiitic volcanism dominates and alkalic lavas form only a minor component of the exposed lavas. The absence of tholeiitic lavas in the main shield‐building phase of construction, however, is not unique to the Caroline Islands. Dominant alkalic volcanism appears characteristic of other seamounts in the Pacific, including the Samoan, Austral‐Cook, and Lin

ISSN:0148-0227    

DOI:10.1029/JB089iB12p09937

年代:1984

数据来源: WILEY

摘要:

The proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot. This produces more basalt and thicker crust at these locations, thus forming a plateau over time. The geometric constraints of such a model predict a unique orientation, location, and age progression for a plateau formed by this mechanism. The hotspot will channel material to the closest part of the rise; therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis. The plateau will be symmetric with respect to the location of the rise axis at the time of formation. Also, the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor, just with thicker crust. A set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the Norwegian‐Greenland Sea as a means of evaluating the model's predictions. By locating the Iceland hotspot, reconstructing the relative positions of the Greenland and European plates, and then assuming material would be channeled from the hotspot to the closest section of the rise crest, we can trace the tectonic evolution of the Greenland‐Faeroe and Vøring plateaus. The model is able to locate the plateaus, explain their orientations, and predict an age progression that satisfies observed age determinations. The analysis demonstrates that both plateaus could have been formed by the Iceland hotspot with the Greenland‐Faeroe Plateau being in effect a continuation of the Vøring Plateau, which was cut off due to transform motion between the northern and southern spreading

ISSN:0148-0227    

DOI:10.1029/JB089iB12p09949

年代:1984

数据来源: WILEY

摘要:

The relief of Cretaceous guyots indicates that an enormous region of the Pacific plate was 3300–;4800 m deep about 100 Ma ago. The plate, then 13–64 Ma old, had depth anomalies of −170 to −1600 m. It was drifting over a group of overlapping midplate swells that comprised a bumpy Darwin Rise of smaller extent than originally believed. In 100 Ma much of the region has subsided only 1000–1200 m instead of the 2000–3000 m predicted by thermal models. A post‐Cretaceous sea level lowering of 200–500 m accounts for some of the difference. The remainder appears to require repeated thermal rejuvenation such as occurred in Tertiary time in the central South Pacific. The Jurassic and Early Cretaceous crust is not anomalously shallow just because it is old. It was shallow wh

ISSN:0148-0227    

DOI:10.1029/JB089iB12p09960

年代:1984

数据来源: WILEY

摘要:

Morphologic and magnetic data suggest that the Mid‐Pacific Mountains formed during Early Cretaceous time as a broad ENE trending double chain of midplate island seamounts over a mantle hot spot as the Pacific plate moved westward and slightly southward. Dredge, drill core, and reflection seismic data indicate that coral‐rudistid reefs grew on the subsiding seamounts and evolved to atolls and banks, largely burying the volcanic foundations. Magnetic data indicate that by late Aptian time, about 110 Ma, the seamounts were located at about 20°–25°S, which we suggest was near the fringes of the latitudinal zone of vigorous reef growth, where upward growth rates could just keep up with subsidence. A broad uplift probably related to the widespread regional emplacement of Aptian volcanics as oceanic plateaus, seamounts, and deep‐water flows and sills raised the Mid‐Pacific Mountain reefs out of the water, and both reflection seismic and isotopic data suggest that a karstic topography developed on many of the emergent reefs. As subsidence recommenced, the reefs could hot grow upward apace with subsidence. Renewed volcanism in Late Cretaceous time in the easternmost Mid‐Pacific Mountains maintained islands for a while, as at Horizon Guyot, but probably without large reefs. Elsewhere, pelagic conditions have prevailed as the guyots sank to their present‐day depths. These depths are systematically related to the inferred age of the volcanic foundations, being greatest on the you

ISSN:0148-0227    

DOI:10.1029/JB089iB12p09969

年代:1984

数据来源: WILEY