摘要:

The Shenango and Nemegosenda alkaline complexes of the southern Kapuskasing Structural Zone (KSZ), an uplifted crustal block in northern Ontario, preserve primary thermoremanent magnetizations, with mean directionsD=112°,I=−46° (k=13, α95=13°,N=9 sites) andD=304°,I=+51° (k=31, α95=10°,N=7 sites) for Shenango andD=124°,I=−57° (k=13, α9520°,N=4 sites) andD=312°,I=+54° (k=17, α95=13°,N=7 sites) for Nemegosenda. Their average paleomagnetic pole is 45°N, 189°E (dp=6°,dm=8°), falling on the Keweenawan track of the North American apparent polar wander path (APWP) between 1098–1107 Ma reverse‐polarity (R) and 1087–1098 Ma normal‐polarity (N) poles of Lake Superior lavas and dikes. Unlike the Keweenawan paleopoles, which have a reversal asymmetry of ≈30°, the Shenango and Nemegosenda R and N poles are almost 180° reversed, which was found also by Lewchuk and Symons (1990b). This symmetry of R and N poles casts doubt on the previously accepted predominance of asymmetric reversals of the Earth's field in Keweenawan time. The Archean Shawmere anorthosite is overprinted by a B magnetization with mean directionD=97°,I=−68° (k=12, α95= 10°,N=18 sites). The Shawmere B paleopole at 38°N, 226°E (dp=13°,dm=l6°) falls between the carbonatite poles and ≈1850 Ma cratonic poles, and the B magnetization seems to be a composite of middle and late Proterozoic overprints. Sites in the western part of the Shawmere body close to the “carbonatite line” of faults linking the various alkaline complexes are severely altered, with an abundance of secondary magnetite and hematite. Virtual geomagnetic poles for these sites fall close to the Keweenawan track and have an average pole of 189°, 53°N (dp=15°,dm=20°) suggesting that the B NRM at these sites is an ≈1100 Ma thermochemical overprint caused by hydrothermal fluids at the time of carbonatite intrusion. More easterly sites are less altered. Their VGPs resemble ≈1850 Ma cratonic poles and average to 235°E, 30°N (dp=15°,dm=17°) These B natural remanent magnetizations are probably thermochemical remanent magnetizations impressed by middle Proterozoic hydrothermal activity associated with intrusion of early alkaline bodies, fault movement

ISSN:0148-0227    

DOI:10.1029/92JB02571

年代:1993

数据来源: WILEY

摘要:

Multibeam bathymetry and seismic reflection profiling shows the Cretaceous products of the Hawaiian hotspot to be a ridge of coalesced guyots that prolong the N‐NW trend of the more isolated Paleocene Emperor Seamounts, a 10,000km2plateau (Detroit Plateau), where the trail changes strike to NW, and the Obruchev Rise hotspot ridge, which extends to the Kamchatka Trench. The northernmost guyots were submerged and tilted southeast by the load of new shield volcanoes added to the end of the chain, then secondary volcanism built small cones on their summit platforms and in a gap between two guyots. Dredged rock samples and the distribution of the cones indicates that on one guyot these submarine eruptions were in the alkalic postshield stage of Hawaiian volcanism, and at another were probably in the alkalic rejuvenated stage. Seamount‐building eruptions at Detroit Plateau produced lavas (nepheline melilitites) that belong geochemically to the alkalic rejuvenated stage, and are very similar to Pleistocene lavas in the Hawaiian Islands. However, these eruptions postdated passage off the hotspot plume by a much longer time than the 0.5–2.5 m.y. observed in the Hawaiian Islands, and were probably initiated by different tectonic proc

ISSN:0148-0227    

DOI:10.1029/92JB02452

年代:1993

数据来源: WILEY

摘要:

An aeromagnetic study of the Island of Hawaii provides new insight on magnetic properties of subsurface rock and geologic structure. On a regional scale, spectral‐depth analysis delineates two shallow magnetic zones, each roughly 1.5 km thick, lying at a depth of 1 km. One zone (of unknown origin) lies in the center of the island and correlates with a regional magnetic high. The other zone coincides with pronounced magnetic lows paralleling Kilauea's active east rift zone. These magnetic lows probably depict rocks chemically altered by hydrothermal fluids, in which titanomagnetite has been destroyed. Analysis of magnetic terrain effects indicates that magnetization also decreases with depth within Mauna Kea and Mauna Loa. We estimate that magnetization is reduced by about half at a depth of 1 km. The magnetic method is particularly useful in delineating the lateral extent of local shield structures, such as rifts, summit calderas, pit craters, and vent fissures. Rifts possess characteristic magnetic patterns, primarily long‐wavelength linear magnetic low zones. We propose alteration processes reduce magnetizations along the flanks of rifts. On the other hand, along young rifts (e.g., Kilauea's east rift zone), short‐wavelength magnetic anomalies probably reflect slowly cooled, unaltered intrusions. Altered rock may also produce magnetic lows that help define buried summit cal

ISSN:0148-0227    

DOI:10.1029/92JB02483

年代:1993

数据来源: WILEY

摘要:

The pattern of heat flow from the Barbados Ridge accretionary complex has been derived from marine surveys with heat flow probes, from measurements in drill holes, and from the depths of bottom‐simulating seismic reflectors caused by gas hydrate. The heat flow from the accretionary complex has been simulated using a finite‐difference model to investigate how heat flow responds to changes in the cross‐sectional shape of the complex and the rate of convergence, and to variations in pore‐fluid pressure within the complex and along the decollement at its base. In the south of the complex, heat flow decreases westward from the toe of the wedge, towards the island arc, because the downward movement of subducting lithosphere beneath the wedge and the active thickening of the accreted sedimentary sequence reduce the geothermal gradient more rapidly than thermal diffusion can maintain it. Localised high anomalies, attributed to the flow of warm fluids along fault zones, are superimposed on the conductive heat flow pattern. Farther west, heat flow increases arcward, produced partly by thermal diffusion re‐establishing a steeper thermal gradient where the rate of thickening is decreased, but mainly by the arcward increase of frictional heating along the base of the wedge caused by the increase of slip rate and increased normal stress arising from thickening of the wedge. In the north of the complex, around the ODP Leg 110 drill sites, the conductive model cannot reproduce the anomalously high heat flow measurements, which can only be explained by the advection of warm po

ISSN:0148-0227    

DOI:10.1029/92JB01853

年代:1993

数据来源: WILEY

摘要:

Seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites, yet the characteristics of different spreading centers show great variation. This leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite. We have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust, as implied by recent geophysical studies of spreading centers. Using our model, we have calculated velocity and temperature fields in the crust, including the effects of hydrothermal cooling. We show that oscillations in the temperature field due to episodicities of 200–20000 years have an effect localized to the region within 50–500 m of the intrusion. By assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation, we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux. We have calculated seismic velocities associated with the model temperature field in the crust, and we show that these are consistent with results from both seismic refraction and reflection experiments. The model may be applied to both fast and slow spreading ridges, and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate, hence resolving the paradox. We also show that the implications of this model are consistent with geological observations of layering in ophioli

ISSN:0148-0227    

DOI:10.1029/92JB02661

年代:1993

数据来源: WILEY

摘要:

This paper discusses the geological and geophysical data available on mid‐ocean ridges with outcrops of serpentinized mantle peridotites, with the objective of better constraining the modes of emplacement of these rocks in the seafloor. Ridges with serpentinized peridotites outcrops are in most cases characterized by slow‐spreading rates, and in every case by deep axial valleys. Such deep axial valleys are thought, based on geophysical constraints and on mechanical modelling results, to characterize ridges with a thick axial lithosphere. A predictable effect of a thick axial lithosphere is that it should prevent magmas from pooling at crustal depths in a long‐lasting magma chamber: gabbroïc magmas should instead form shortlived dike or sill‐like intrusions. Samples from axial outcrops of serpentinized peridotites are often cut by dikelets of evolved gabbros which are interpreted as apophyses of such dike and sill‐like intrusions. This observation leads to a discontinuous magmatic crust model, in which mantle‐derived peridotites form screens for numerous gabbroïc intrusions. This discontinuous magmatic crust is expected to form in magma‐poor ridge regions, where there is not enough magma to produce a 4‐to 7‐km‐thick magmatic crust, and where the uppermost kilometers of oceanic lithosphere therefore have to be at least partially made of tectonically uplifted mantle material. Because the dimensions of individual mantle‐derived ultramafic screens may be smaller than seismic experiments detection limits, the discontinuous magmatic crust model discussed in this paper may produce a layer 3‐type seismic signature, even without extensive serpentinization of its ultramafic component. It therefore provides an alternative toHess's[1962] serpentinite layer 3 model, for the geological interpretation of seismic data from oceanic areas with frequent outcrops of deep crustal

ISSN:0148-0227    

DOI:10.1029/92JB02221

年代:1993

数据来源: WILEY

摘要:

Knowledge of the fluid permeability within all parts of submarine hydrothermal systems is needed to understand hydrothermal circulation in ocean crust and the heat budget of mid‐ocean ridges. If hydrothermal circulation extends into the gabbro layer of young ocean crust, and possibly to within meters of a crustal magma chamber, then the fluid permeability close to the interface between the magmatic and hydrothermal parts of the ridge system may originate by the growth of fractures induced by hydrothermal cooling. Similar incremental growth mechanisms for thermal contraction fractures located at the edges of lava flows, dikes, and crustal plutons are suggested by the occurrence of discrete microearthquakes during the solidification of basalt lava lakes and the presence of “joint striations” on the surfaces of some columnar joints in basalt. This similarity of fracture mechanisms approaching magma‐solid interfaces may provide a means of estimating the spacing of thermal contraction fractures in submarine hydrothermal systems. Microearthquakes originating beneath the active hydrothermal vents at 21°N East Pacific Rise have seismic moments of 1017dyn cm and frequencies of 0.1–1.0 d−1. This scale of microearthquake activity is inconsistent with model seismic sources consisting of incrementally growing fractures spaced less than several meters apart and suggests that fluid permeability near the magma chamber boundary is 10−18m2or larger. The lower bound on fracture spacing and fluid permeability computed from the incremental fracture model is independent of any fluid transport model but is consistent with laboratory measurements of permeability and with the assumed permeabilities of recent numerical models of hydrotherm

ISSN:0148-0227    

DOI:10.1029/92JB02046

年代:1993

数据来源: WILEY

摘要:

Both analytical arid numerical models are developed to investigate the effects on alternating field (AF) stability of both microcoercivity associated with pinned walls and magnetic screening by soft walls in multi‐domain (MD) grains. It is shown that due to screening, AF stabilities of remanence in MD grains are usually significantly higher than their microcoercivities, the only exception being for two‐domain grains, in which there is no screening. One important implication is that hard “pseudo‐single‐domain” remanence observed in grains larger than single‐domain size but with AF stabilities higher than the expected maximum microcoercivity associated with domain wall pinning may be simply due to the screening by soft walls in individual grains. Moreover, we predict that MD grains containing a small number of defects do not necessarily show a low AF stability, for the reduction in the AF stability by a small microcoercivity in this case may be outweighed by a large screening due to a relatively large number of soft walls available in the grains. This prediction is supported by the experimental evidence that two types of magnetites (crushed and hydrothermally grown) with very different defect densities have similar values of median dest

ISSN:0148-0227    

DOI:10.1029/92JB02570

年代:1993

数据来源: WILEY

摘要:

We have studied chemical remanent magnetization (CRM) accompanying the phase transformations lepidocrocite (γFeOOH) → maghemite (γFe2O3) → hematite (αFe2O3) in a field of 50 μT at a series of 13 temperatures from 200 to 650°C. CRM intensity peaked after the 250°C run as paramagnetic lepidocrocite transformed to ferrimagnetic maghemite of single‐domain size. After the 275, 300 and 350°C runs, CRM intensity decreased about an order of magnitude, probably because of the formation of antiphase domains with oppositely coupled magnetizations in the maghemite. A second CRM peak after the 400°C run is thought to reflect the growth of hematite on antiphase boundaries, breaking the negative coupling and allowing maghemite antiphase domains to align individually with the field. After runs at 450°C and above, CRM intensities were very low and the CRM consisted of two vectors, sometimes reversed in direction to each other and always at a large angle to the field. The explanation seems to lie in perpendicular coupling of the magnetic sublattices of maghemit

ISSN:0148-0227    

DOI:10.1029/92JB02569

年代:1993

数据来源: WILEY

摘要:

The 3.2‐m.y. whole core magnetic susceptibility record obtained during Ocean Drilling Program leg 117 from the Owen Ridge in the western Arabian Sea represents one of the most convincing demonstrations of the ability of rock magnetic measurements to yield paleoceanographically significant information. The salient features of this record are that (1) it correlates strongly with variations in the concentration and flux of eolian dust; (2) it is driven strongly at Earth orbital periodicities; and (3) there is a significant change in the spectral character at 2.4 m.y., which may reflect the effect of the initiation of major northern hemisphere glaciation on aridity cycles within the eolian source areas. In view of its potential paleoclimatic significance, we have examined the origin of the rock magnetic signal from the Owen Ridge in more detail. We find that despite the strong relationship throughout the record between magnetic susceptibility and percent terrigenous content, there has been significant postdepositional alteration of the magnetic minerals via the process of reductive diagenesis. We ascribe two steplike shifts in magnetic properties in the uppermost part of the section to this process. The first occurs at about 1.5 m depth and is characterized by a loss of fine‐grained ferrimagnetic material. The second stepshift in magnetic properties occurs at a depth of 6–8 m and is characterized by changes in rock magnetic parameters which indicate significant loss of both low coercivity (magnetite) and high coercivity (hematite and/or goethite) magnetic mineral components. Scanning electron microscope and energy dispersive Xray analyses show that the bulk of the coarse‐grained strongly magnetic fraction is detrital titanomagnetite and that there is a significant reduction in the Fe:Ti ratio of these grains across the lower diagenetic front. This suggests that titanomagnetite grains with higher Ti contents may be more resistant to the process of reductive diagenesis, and this may provide a mechanism for the relative stability of the ferrimagnetic fraction below the second diageneti

ISSN:0148-0227    

DOI:10.1029/92JB02914

年代:1993

数据来源: WILEY