摘要:

West of the Tonga‐Kermadec trench is a series of ridges and basins including, from east to west, the Tonga‐Kermadec ridge (frontal arc), the Lau‐Havre trough (interarc basin) and the Lau‐Colville ridge (third arc). The frontal and third arcs have asymmetric cross sections with scarps on the flanks facing the interarc basin and substantial accumulations of sediment on the outer ridge flanks. Geology of exposed islands indicates that the Tonga‐Kermadec ridge has been a frontal arc since the early Tertiary; the third arc is probably at least as old. A thin cover of rapidly deposited sediment suggests that the morphology of the interarc basin, which consists of linear ridges and troughs with approximate relief of 1000 meters, is no older than latest Tertiary. The interarc basin is underlain by a crust of oceanic character and is the locus of high heat flow. The regional morphology and distribution of sediment must be ascribed to tectonic activity, involving either creation of new oceanic crust or modification of older oceanic crust. An origin of the interarc basin by extensional rifting within an older frontal arc best satisfies the available data and suggests that the extension is related to intensification of island arc tectonism at the close of the Tertiary. Additional studies indicate that the South Fiji basin and at least several other marginal basins on the concave sides of western Pacific trenches have had a similar mode of origin at an earlier time. The result of such activity would be the migration of the trench‐frontal arc complexes away from the Asian continent, with creation of new basins with oceanic crust on the convex sides of the f

ISSN:0148-0227    

DOI:10.1029/JB075i002p00239

年代:1970

数据来源: WILEY

摘要:

Titanium‐rich clinohumite and layered structure minerals are observed in kimberlite and as inclusions in pyropic garnets from the Moses Rock dike, a kimberlite‐bearing breccia dike in San Juan County, Utah. Associated clinopyroxenes observed as inclusions within similar pyropes and also in kimberlite are estimated to have equilibrated at depths ranging from about 50 to 150 km at modest temperatures, generally less than 1000°C. The presence of titanoclinohumite, a high‐density hydrous phase, is of considerable interest as a possible site for volatiles in the earth's upper mantle. The dehydration of hydrous phases such as titanoclinohumite within the upper mantle (1) may provide water as a free phase, (2) could be important in the genesis of kimberlite and alkali‐basalt magma, and (3) may be one means of producing a low‐velocity zone in the up

ISSN:0148-0227    

DOI:10.1029/JB075i002p00255

年代:1970

数据来源: WILEY

摘要:

A mechanism for the creation of lunar mascons is proposed that requires no abnormal density materials or major density inversions. The mascons are produced by mantle plugs upwelling into giant impact basins punched through the lunar crust followed by volcanic filling of the remainder of the crater above the plug. It is explicitly shown that continued volcanic filling is not inhibited by the attainment of isostatic equilibrium. If the density contrast between the lunar crust and mantle is 0.5 g/cm3, the minimum depth of the crust is 45 km in the Imbrium region. The strength of the lunar mantle must be somewhat greater than the strength of the earth's mantle; however, this is not inconsistent with a composition essentially similar to the composition of the earth. The approximately linear relationship between the peak gravity anomaly and the mare basin diameter suggests that for craters less than 200 km in diameter, the strength of the moon coupled with its crustal thickness are adequate to preclude significant mantle plug upwelling and mascon formation. With better density data the mascons may well be seen to result from combination of several effects requiring some modification of present geometry based on plug and topographic effects alone. Nevertheless, given a lunar (or Martian) crust and. mantle capable of healing giant impact basins by normal isostatic and volcanic processes, then mantle plug mechanics should make a significant contribution to the geologic evolution of the basin and to the associated gravity anomaly.

ISSN:0148-0227    

DOI:10.1029/JB075i002p00261

年代:1970

数据来源: WILEY

摘要:

Volcanism, heat flow, gravity, morphology, and tectonism in the North Island of New Zealand are examined in the light of seismic evidence for inhomogeneity in the upper mantle. The high‐frequency attenuating/transmitting boundary underlies the junction between volcanic and nonvolcanic provinces, and the now active andesite volcanoes lie above this boundary. Heat flow is higher than normal over the attenuating zone and lower than normal over the transmitting zone. Gravity anomalies are positive over the attenuating zone and negative over the transmitting zone; a density model is proposed in which the density variations are envelopes of seismicity in the upper mantle. The Hikurangi trench curves in a similar manner to the attenuating/transmitting boundary and shoals in a similar manner to the seismicity. Similar data are examined for northern Japan, and there is good agreement between the two areas. The special importance of New Zealand in testing the Oliver‐Isacks model of an active margin is demonstra

ISSN:0148-0227    

DOI:10.1029/JB075i002p00269

年代:1970

数据来源: WILEY

摘要:

Seismic wave forms recorded in New Zealand from nearby deep earthquakes show significant frequency differences, which can be correlated with ray paths in the upper mantle. Stations on the east side of the North Island (Gisborne, Wellington, and Tuai) record frequencies that are dominantly 3 cps or greater from deep shocks that are both nearby and up to 10° north toward the Kermadec Islands. Stations on the west side of the North Island (Tarata and Karapiro) record frequencies that are dominantly 2 cps or lower from the same earthquakes. Closer examination of the pattern shows two exceptions. First, shocks southward from 39½°S tend to show high frequencies, not low, at the western stations. Second, shocks on the easterly side of the seismic zone north of New Zealand tend to show low frequencies, not high, at the eastern stations. Evidence is presented to show that these observations can be explained only in terms of inhomogeneities within the upper mantle. Alternative explanations that can be rejected include: depth, distance, or magnitude effects; source mechanism; crustal effects; instrument or station peculiarities.We interpret the results to indicate the existence of a low‐Qregion within the upper mantle under part of the North Island of New Zealand and the surrounding oceans. Where this region exists, seismic waves traversing it lose a significant fraction of their higher‐frequency components. Where the region does not exist, high frequencies can be propagated to relatively large distances. The region appears to lie northward from 39½°S and north‐westward from a NE‐SW line through the center of the North Island. In depth, we prefer an interpretation in terms of a horizontal layer lying between 75 and 125 km, although the data could be reconciled with a dipping layer lying above the zone of seismicity. The low‐Qregion appears to be absent along a continuous zone, which is possibly narrow, extending along the eastern side of the North Island and north to 32°S and probably beyond. Secondary evidence from travel‐time residuals and isoseismals adds support to this interpretation. Estimates forQwithin the low‐Qregion yield values i

ISSN:0148-0227    

DOI:10.1029/JB075i002p00285

年代:1970

数据来源: WILEY

摘要:

The available shock‐wave data for solid a quartz in the stishovite pressure regime are reduced to a 25°C isotherm and an adiabat, centered at standard conditions, using recent standard density, enthalpy, and coefficient of thermal expansion data. The calculated isothermal bulk modulus, 3 Mb, as determined from the Birch‐Murnaghan equation, depends critically on the value of (dK/dP)Tat zero pressure and to a yet unknown extent on the form of the equation of state. The high‐temperature value of Grüneisen's ratio (0.8 to 0.9) along the a quartz (stishovite regime) Hugoniot was obtained from the pressure offsets of the fused quartz and porous quartz Hugoniot. The high value for γ obtained from thermochemical data at standard conditions (1.5±0.3) suggests that a marked decrease in the value of γ to 0.8 occurs with increasing t

ISSN:0148-0227    

DOI:10.1029/JB075i002p00310

年代:1970

数据来源: WILEY

摘要:

Gravity fields in the continental interior reveal little relation to late Paleozoic structure and often show an inverse relation to early Paleozoic structure. Because of the inversion, displacements have been interpreted by several investigators as being caused by an isostatic mechanism; however, structural development recorded by geologic evidence indicates a more complex history than simple isostasy would allow. Detailed gravity surveys covering large areas in Illinois are used to determine mechanisms of crustal movements more definitively. From these detailed studies interpretations are extended to other areas having similar geologic‐gravity relationships within the midcontinent region. In the interior, a mosaic pattern of gravity highs and lows outlines mass concentrations that formed during the evolution of the craton. The midcontinent gravity high, the central anomaly lying within a linear, symmetrical belt of gravity anomalies, is superimposed over the mosaic pattern and trends through the Lake Superior‐Keweenawan rift system. Similar anomalies, although of smaller intensity, trend through centers of intracratonic basins. From the relation between linear, symmetrical anomalies and structure displayed by the Lake Superior rift system, intracratonic basins are interpreted as representing the terminal stage of more subdued rifting. Initial depression of basins is believed to be caused by the collapse of incipient rift systems. The presence of numerous ancient rift systems in the craton suggests that mantle convection played an important part in the deformation of the youthful continental crust. Erosion from uplifts and sedimentation in depressions resulted in further deflections, but of an isostatic nature, with subsequent faulting and plutonism. Subcrustal convection continued throughout Paleozoic time, but its effects on crustal bending became less pronounced. Examples of these structures and related gravity fields are presen

ISSN:0148-0227    

DOI:10.1029/JB075i002p00317

年代:1970

数据来源: WILEY

摘要:

Twelve successful heat‐flow measurements, ranging from 0.33 to 1.43 μcal/cm2sec, with a mean of 0.98 μcal/cm2sec were observed during a survey of a 20‐ by 20‐km area centered at 24°25'N, 157°40'W, near the crest of Hawaiian arch. The normal and subnormal values suggest that there is no widespread youthful volcanism and that in general the area is in thermal equilibrium. As this precludes a credible explanation based on the effects of recent volcanism, the scatter in the heat flow casts doubt upon the validity of interpreting the surface field from widely separated observations. A bathymetric, side‐looking sonar, magnetic, and airgun survey conducted by the Marine Physical Laboratory deeply towed instrument package provided detailed information of the local environment. It has been suggested that much of the local scatter in heat‐flow observations may be caused by previously undetected relief on the ocean floor. A statistical determination along 65.8 km of deep‐tow track of the effect of the relief on the surface field demonstrates that irregular topography cannot account for the scatter in the observations. The scatter can be explained, however, by the refraction of the heat flux by irregularities in the sediment cover and the effect of recent slumps from a nearby sedimen

ISSN:0148-0227    

DOI:10.1029/JB075i002p00333

年代:1970

数据来源: WILEY

摘要:

Critical compositional plots chosen to enable a distinction to be made between rocks formed by igneous processes, sedimentary processes, and vapor fractionation processes are presented. Niggli numbers are used in order to circumvent apparent correlations resulting from the swamping influence of major variations in silica. None of these processes seems to explain the Australasian microtektite compositional trends at present.

ISSN:0148-0227    

DOI:10.1029/JB075i002p00349

年代:1970

数据来源: WILEY

摘要:

A measurement method of Rn220soil emanation is presented. Some measurements are made by means of an experimental model in the laboratory in order to determine the influence of certain parameters on the emanation rate. The seasonal and daily variations of the Rn220flux are also studied. It is shown that the seasonal variations are related to soil moisture variations, whereas some daily variations are due to the temperature difference between the air and the soil.

ISSN:0148-0227    

DOI:10.1029/JB075i002p00357

年代:1970

数据来源: WILEY