摘要:

Five gravity anomaly profiles across the Sea of Okhotsk and the Kuril island arc and trench and two profiles from Fiji to Tonga are presented and studied. The four profiles across the Kuril arc are similar and differ from the profile extending off southern Kamchatka. The contributions to gravity anomalies from the crust and mantle are investigated by means of continuous seismic refraction measurements along tracks coincident with the gravity profiles. The mantle gravity anomalies computed from various density models are positive over the island arc, trench, and adjacent Pacific basin and amount to at least +50 mGal. Gravity anomalies due to the mantle suggest the existence of a denser slab 50–60 km thick parallel to the seismic zone. The positive gravity effect of the slab appears to be balanced over the Okhotsk basin by the negative effect of the heated, less dense mantle. The local minimum of Bouguer anomalies occurring over the shoreward flank of the Kuril trench is rather insensitive to the density contrast adopted. The sediments measured seismically account for this Bouguer minimum. The gravity field over the Kuril and Tonga ridges supports the idea of reduced density under the volcanic chain. The local minimum of gravity anomalies associated with the northwestern boundary of the Okhotsk basin is probably related to the fault separating the Academy of Sciences rise from the basin. The ocean‐to‐continent trend of the free air anomalies in the Kuril area is unlike that in Tonga, which might be related to features of heat flow anomalies in the K

ISSN:0148-0227    

DOI:10.1029/JB080i011p01381

年代:1975

数据来源: WILEY

摘要:

Simple models for the flexure of the lithosphere caused by the load of the Great Meteor seamount have been determined for different assumed values of the effective flexural rigidity of the lithosphere. The models utilize a new method for determining the flexure of the lithosphere caused by a three‐dimensional load. The gravity effect of the models has been computed and compared with observed free‐air anomalies in the vicinity of the seamount. Computations show that the observed free‐air anomalies can be most satisfactorily explained for an assumed effective flexural rigidity of the lithosphere of about 6×1029dyn cm. This value, which is similar to other values determined for loads of different ages, suggests that the oceanic lithosphere is rigid enough to support applied loads for periods of time of at least several tens of millions of

ISSN:0148-0227    

DOI:10.1029/JB080i011p01391

年代:1975

数据来源: WILEY

摘要:

Transform faults offset the pipe‐shaped region of partial melting and magma generation below the Midoceanic ridge. Hence if there is flow along the pipe, it will be blocked or at least impeded at major transform faults. There is evidence on the Reykjanes ridge that minor transform faults, with offsets of a few tens of kilometers, may be converted to oblique spreading axes by asthenosphere flow. Quantitative estimates of the extent of blocking are derived from a Parker‐Oldenburg law of plate thickness increasing as the square root of crustal age. There are several kinds of evidence that the subcrustal partial melts generally moving away from long‐wavelength topographic and gravity highs on the Midoceanic ridge (hot spots) are partly blocked at transform faults: (1) elevations of particular isochrons, including the present spreading axis, frequently jump discontinuously across major transform faults (the best examples of this are in the northeast Atlantic); (2) morphology and seismicity change abruptly across the Blanco fracture zone, which separates a ridge influenced by a hot spot (the Juan de Fuca ridge) from one not so influenced (the Gorda ridge); (3) a zone of relatively high magnetic amplitudes is associated with the hot spot influenced Juan de Fuca and central Galapagos ridges (this zone may delineate how much crust was produced by the Fe/Ti‐rich melts of hot spot origin (whether due to distinctive source composition or subsequent fractionation); on both ridges the high‐amplitude magnetic zones are terminated on both ends at transform faults, again suggesting blockage of the hot spot melts); (4) prominent ridges, such as the Mendocino and Charlie Gibbs ridges, may form on the ‘upstream’ side of some fracture zones at certain times during their evolution (such fracture ridges seem to be constructional volcanic piles formed continuously at or near the fracture‐ridge crest junction; they are here interpreted as being due to excess basalt melts, produced from the partial melts ponded on the upstream side of the transform fault; fracture ridges are proposed to record the past intensity of pipe flow and hence hot spot activity); and (5) recent studies by Solomon (1973) show a region of highSwave attenuation and lowQat the southern end of the Reykjanes ridge, below the prominent fracture ridge just north of the Charlie Gibbs fracture zone. Taken together, the various observations support the existence of pipelike flow at shallow depths (up to a few tens of kilometers) below the Midoceanic ridge. The interaction between mantle hot spots and nearby plate boundaries discussed in this paper may produce features, such as the Ninetyeast and Broken ridges, that exhibit a strong plate tectonic fabric (structures parallel or perpendicular to transform faults) but nevertheless attest to special convective processe

ISSN:0148-0227    

DOI:10.1029/JB080i011p01399

年代:1975

数据来源: WILEY

摘要:

Focal mechanisms are presented for 46 earthquakes that occurred in the South Atlantic Ocean, in the Scotia Sea, and in southern Chile during the period 1963–1973. The slip vectors of shallow earthquakes indicate that the South American plate is moving directly west with respect to the Antarctic plate at the ridge‐fault‐fault triple junction in the South Atlantic. The directions of motion of Africa with respect to the South American (SA) and Antarctic (ANT) plates at the triple junction are N70°E and N47°E, respectively. The SA‐ANT relative motion between the triple junction and the South Sandwich trench is best described by a pole of rotation at 80°S, 166°W, with an angular rotation rate of 0.24 deg/m.y. Shallow earthquakes along the South Sandwich trench indicate that the oceanic portion of the South American plate is being thrust under the South Sandwich arc in an east‐west direction. Most of the earthquakes at the northern end of the arc are due to hinge faulting or bending stresses within the underthrust oceanic plate. The focal mechanisms of intermediate depth events beneath the arc indicate down‐dip extension in the northern end of the downgoing slab and down‐dip compression in the southern end. This change in the stress pattern may be caused by reduced negative buoyancy forces in the younger, southern half of the subducted plate. The seismicity and focal mechanisms suggest that the SA‐ANT relative motion in the Scotia Sea region is taken up on both the north and the south Scotia ridges. However, the plate boundaries in this area and in southern Chile are not well defined. There does appear to be a consistent pattern of horizontal compressive stress directed ENE‐WSW throughout the Scotia Sea region, probably induced by the convergence of the South American and Antarctic plates. The SA‐ANT relative motion observed in this study is not consistent with the motion predicted from the summation of motions observed on other plate boundaries. This discrepancy may be due to (1) systematic errors in the data, (2) a recent change in plate motions, or (3) minor nonrigid plate behavior. The third explanation is preferred because internal deformation of the plates at very slow strain rates can explain other examples of seemingly inconsistent plate motions and may also account for the existence of diffuse intraplate seismicity. The apparent relative drift of hot spots may also be due to internal deformation of the plates after the seam

ISSN:0148-0227    

DOI:10.1029/JB080i011p01429

年代:1975

数据来源: WILEY

摘要:

The interpretation of the seismic low‐velocity zone as a region of partially molten rock is extended to explain the transient displacements following the 1946 Nankaido earthquake. Three partial melt models are considered to account for the observed time constant of 3–5 years: large‐scale diffusion of melt through a porous matrix can decay over thousands to billions of years and is much too slow. Simple shearing in ‘penny‐shaped’ cracks happens on a seismic time scale and is much too rapid. Interconnected penny‐shaped cracks at different orientations with respect to the principal stresses respond on an intermediate time scale by short‐range melt squirt from one crack to another, providing a reasonable mechanism to account for the transient deformation at Nankaido, while components of shear parallel to each individual crack relax quickly according to the better‐known mechanism for seismic attenuation in the

ISSN:0148-0227    

DOI:10.1029/JB080i011p01444

年代:1975

数据来源: WILEY

摘要:

Gravity models of oceanic trenches computed prior to the advent of plate tectonic concepts fell into two classes of solutions: (1) if a homogeneous mantle density was assumed, then the gravity models required an abnormally thin oceanic crust in the first 100 km seaward to the trench axis; or (2) if the oceanic crust was not thinned, then high‐density mantle was included beneath the Moho near the trench axis. In contrast to the gravity models, however, seismic refraction studies near trench axes and on the seaward trench slope have generally observed normal oceanic crustal thicknesses and normal mantle velocities. This apparent conflict between the refraction data and the gravity interpretations can be resolved by taking into account density anomalies in the descending lithosphere. A theoretical density model for the downgoing slab was computed from thermal and petrologic data and was then compared with the observed gravity data of Hayes (1966) and the refraction data of Fisher and Raitt (1962) over the Chile trench at 23°S. Considerations of pressure‐temperature data suggest that the oceanic crust and the lithosphere may transform to eclogite (3.55 g/cm3) and garnet peridotite (3.38 g/cm3), respectively, at depths as shallow as 30 km in the descending slab. This model predicts density anomalies of +0.08 to +0.28 g/cm3at depths between 30 and 80 km and +0.04 to +0.024 g/cm3between 80 and 150 km and predicts a mean density anomaly of +0.05 g/cm3at depths between 150 and 300 km. Incorporation of these high density mantle zones into a two‐dimensional gravity model allows a gravity solution that is in much better agreement with the refraction data seaward of the Chile trench than earlier gravity models, which assumed a homogeneous

ISSN:0148-0227    

DOI:10.1029/JB080i011p01449

年代:1975

数据来源: WILEY

摘要:

Supplement is available with entire article on microfiche. Order from American Geonhvsical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J75‐002; $1.00. Payment must accompany order.Rare‐earth (RE) abundance patterns in 26 basalts occurring across normal segments of the Reykjanes ridge, 60°–53°N; the mid‐Atlantic ridge, 29°S (Deep‐Sea Drilling Project leg 3); and the East Pacific rise, 2°–19°S, including the Nazca plate, are reported. Without exception, all the RE patterns were found to be characteristically light RE depleted, reflecting to a first‐order approximation, a mantle source with uniquely defined RE content. Computer multiphase and eutectic, equilibrium‐partial melting models show that the reconstituted RE pattern of the mantle underlying these regions must be more depleted in light RE than the basalts generated. This is so whether a lherzolite (spinel free or bearing), garnet peridotite, or plagioclase peridotite is considered for the primary mantle source. The source is thought to be the low velocity layer apparently depleted in light RE and other large ionic lithophile elements. A lherzolite phase assemblage is found to be preferable for the depleted low‐velocity layer source under normal midocean ridge segments at a depth greater than 30–38 km. Above 30–38 km, equilibration of normal ridge basalts with a plagioclase peridotite cannot be ruled out. Rare‐earth results across these three normal ridge segments further emphasize (1) the spatial and temporal RE uniformity of the depleted low‐velocity layer source, (2) the widespread geographic coverage of this mantle source, (3) its RE composition contrast with mantle source regions, such as those beneath Iceland, the Azores, and the Afar, which have been assumed to represent rising plumes (or blobs), and finally (4) the passive nature of volcanism along normal ridge segments occurring in response to the spreading

ISSN:0148-0227    

DOI:10.1029/JB080i011p01459

年代:1975

数据来源: WILEY

摘要:

Nearly 200 travel times of the phaseScShave been measured at World Wide Standardized Seismographic stations for 10 globally distributed deep‐focus earthquakes. By comparing these times we have obtained estimates of the differences in one‐way vertical shear wave travel times beneath various tectonic provinces of the earth. In particular, vertical shear wave travel times beneath normal ocean basins average about 5 s greater than times beneath continents, confirming the hypothesis advanced by Jordan and Anderson (1974) to explain the discrepancy between travel time base lines derived from free oscillation studies and those derived from body wave studies. The observed difference in vertical travel times, together with pure path surface wave dispersion data, implies that the differences between oceanic and continental structure extend to depths exceeding 400 km.ScStimes for a station on the Galapagos Islands are smaller than those observed for normal ocean basins, suggesting that the velocities beneath the are higher than those beneath typical ocean basins. This observation is inconsistent with the existence of a simple thermal plume beneath the Galapa

ISSN:0148-0227    

DOI:10.1029/JB080i011p01474

年代:1975

数据来源: WILEY

摘要:

The Ruwenzori Mountain is a fault‐bounded, uplifted block of Precambrian rocks situated immediately north of the equator within the western branch of the rift system of East Africa. Understanding the structure and development of the Ruwenzori Mountain may yield additional insight into the evolution of the East African rift system as a whole. During June through mid‐September 1973 four portable seismographs were used in a microearthquake survey of the area around the Ruwenzori Mountain. A microearthquake reconnaissance of various districts of Uganda during the latter part of September 1973 showed the Ruwenzori region to be the most seismically active area in Uganda. In the Ruwenzori region the spatial trends of the microearthquakes are correlated with the major rift faults and volcanic zones. The seismicity is shown to extend beneath and transverse to the Ruwenzori Mountain block, where there is an indication of subcrustal seismicity (25–40 km). Elsewhere in the region studied, hypocentral depths ranged between 0 and 25 km. Composite focal mechanism solutions for events associated with the Bwamba, Ruimi‐Wasa, and Nyamwamba faults show dip slip motion along steeply dipping planes. The extensional axes trend east‐west. The motion tends to raise the mountain block relative to the surrounding country. A focal mechanism determined at the junction of the northern and southern Ruwenzori indicates that the northern portion is being uplifted with respect to the south

ISSN:0148-0227    

DOI:10.1029/JB080i011p01485

年代:1975

数据来源: WILEY

摘要:

The time variation of crustal velocities in tectonic regions is most reasonably attributed to stress induced variations in crack porosity. The decrease inVp/Vsbefore earthquakes is due primarily to a large decrease inVp. This supports the Nur dilatancy hypothesis but not the effective stress hypothesis. New data from the San Fernando region verify theVpdrop, show that this drop cannot be entirely due to source depth effects, and give strong support to the explanation of material property, or path effect, rather than source effect variations. Calculations show that the crack‐widening model works even for mid crustal depths in saturated rock. Narrow cracks of low aspect ratio are required to satisfy the velocity and uplift constraints. The recovery of velocity prior to fracture can be due to fluid flow or crack closure. Thet∼L2relation does not require diffusion. Diffusion of groundwater or crack closure leads to increased pore pressure and rock weakening. Observations of gravity, conductivity, and crustal distortions along with velocities should narrow the choice of models. The crust in regions of thrust tectonics is probably always dilatant to some degree. The aftershock region is smaller than the anomalous velocity region, which in turn must be smaller than the dilatant region. A simple relationship is derived for the relative sizes of the anomalous and aftershock regi

ISSN:0148-0227    

DOI:10.1029/JB080i011p01497

年代:1975

数据来源: WILEY