摘要:

The rocks of the Resurrection Peninsula compose an ophiolite (57±1 Ma) within the vast accretionary prism of the Chugach‐Prince William terrane. Paleomagnetic data from pillow basalt and sheeted dikes of the ophiolite yield a mean paleolatitude of 54°±7°, which implies 13±9° of poleward displacement. The characteristic component of the remanence resides in a Ti‐poor titanomagnetite with a blocking temperature close td 580°C. A positive fold test and the petrology of the magnetic minerals lead us to conclude that the characteristic remanent magnetization (ChRM) in the sheeted dikes originated as a primary thermoremanent magnetization (TRM). We hypothesize that the ChRM of the pillow basalt originated as a chemical remanent magnetization (CRM) acquired soon after formation of the rocks during hydrothermal metamorphism. Structural data indicate polyphase deformation with folding about vertical and horizontal axes. The paleomagnetic data pass a fold test only when we incorporate this interpretation in our structural correction. Geologic relationships lead us to interpret the Resurrection Peninsula ophiolite as a fragment of the extinct Kula‐Farallon ridge that accreted to the Chugach‐Prince William terrane soon after formation. The paleomagnetic paleolatitude indicates that the ridge intersected the North American continental margin in the vicinity of northern Washington in late Paleocene time. The paleostrike of the sheeted dikes (N26°E±12°) shows that the ridge trended NNE. According to current plate tectonic models, if the Chugach‐Prince William terrane moved north with the full coastwise displacement of the Kula plate, the Resurrection Peninsula ophiolite would have arrived at its present position at 45 Ma. This age is perhaps slightly later than that of the terranes that lay immediately inboard (Peninsular and Wrangellia terranes), indicating some relative displacement between those terranes and the Chugach‐Prince William terrane. Such displacement, in turn, agrees with studies which imply a Coast plutonicmetamorphic complex provenance for the Chugach

ISSN:0148-0227    

DOI:10.1029/92JB01292

年代:1992

数据来源: WILEY

摘要:

A paleomagnetic study has been undertaken in Jurassic volcanics and Cretaceous intrusions from coastal southern Peru. A secondary magnetization in the Jurassic volcanics was identified at 21 sites from three main localities along the southern coast of Peru (mean virtual geomagnetic pole: 59.5°N, 190.1°E,k=111, α95=3.0). This secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the Peruvian coastal batholith and/or to the first Andean phase of compressional deformation in Late Cretaceous. If we assume that the remagnetization is of Late Cretaceous age, a regional counterclockwise rotation of about 20° is recorded by the forearc of southern Peru from 16°S to 18°S. Primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations. Counterclockwise rotations of 50° to 80° are documented near the localities of Chala (eight sites), Oconà (two sites) and Arequipa (four sites). Preliminary results based on three sites of Late Permian to Early Triassic sediments from northern Bolivia also show a large counterclockwise rotation. The observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization. Perhaps, this first episode of deformation partially structured the Andean chain in the Central Andes and induced an incipient orocline. During the Cenozoic, a differential shortening along the chain gave the present‐day shape of the Bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20° as indicated by the rema

ISSN:0148-0227    

DOI:10.1029/92JB01291

年代:1992

数据来源: WILEY

摘要:

The origin of the main geomagnetic field is related to the current in the fluid core of Earth. A close relationship between the fluctuations of the rotation of Earth and the secular changes of the geomagnetic field is suspected. Two‐channel maximum entropy spectral analyses are applied to the dipole moments of Earth from 1901 to 1980 and the rotational rate fluctuations from 1890 to 1987 in this study. The common spectral peaks are found with periods of 51, 34, 22, 13, and 9.5 years. The values of the coherence are above 0.99, 0.98, 0.95, 0.95, and 0.77 at these peaks. The 51‐ and 34‐year spectral peaks are the dominant ones. The time lags of the magnetic field are 3.1 ± 0.6 years and 22.4 ± 4.5 years, respectively. Two simulated time series are constructed with the obtained time lags. They exhibit features similar to the actual time series of the dipole moment variations and Earth's rotational fluct

ISSN:0148-0227    

DOI:10.1029/92JB01321

年代:1992

数据来源: WILEY

摘要:

Geomagnetic temporal variations can yield valuable information on the electrical conductivity of Earth's mantle and on motion of core fluid. The external‐source signal in first differences of magnetic observatory annual means is primarily due to a degree‐one spherical harmonic closely aligned with Earth's magnetic dipole axis. The transfer function between the electromagnetically induced degree‐one internal Gauss coefficient (Schmidt seminormalized) and the inducing degree‐one external Gauss coefficient is 0.089 ± 0.020 with a phase shift of 0° ± 45° for a 2‐year period. This transfer function is consistent with a nearly insulating mantle and a highly conducting core for which the theoretical transfer function is 0.082 with no phase shift. The temporal power spectrum for noise in first differences of magnetic observatory annual means is approximately white. Third differences of annual means are primarily noise and degree‐one external‐source signal; thus, when the degree‐one external‐source signal is removed from annual means third differences, the root‐mean‐square residuals for a given field component and time interval at a given observatory are a good indicator of noise for the relevant component, observatory, and time interval. These rms residuals were used as weights for construction of spherical harmonic models of geomagnetic secular variation. European secular variation graphs for the 1962–1983 time interval exhibit prominent changes of slope (geomagnetic jerks) in the geomagnetic east component at approximately 1970 and 1978. The jerk of 1970 (but not 1978) is evident on the geomagnetic north and vertical components. The vertical component exhibits additional slope changes at

ISSN:0148-0227    

DOI:10.1029/92JB01847

年代:1992

数据来源: WILEY

摘要:

Goethite was synthesized under controlled field conditions (horizontally directed field of 0.30 mT) at 30° and 55°C with and without the presence of microfiber glass filters. Upon goethite aging from freshly precipitated ferrihydrite an inverse fabric of the magnetic susceptibility develops, more prominently for the 55°C than for the 30°C synthesis. This is compatible with the fact that for well crystalline goethite the minimum susceptibility axis coincides with the long needle axis. Scanning electron microscope (SEM) observation yielded a grain‐size range from ± 0.3 μm to ± 5 μm. For the 55°C synthesis the majority of the grains are at the upper end of this range, whereas for the 30°C synthesis this is less the case. Calculation of the crystallite size from X ray line broadening revealed a range between 20 and 40 nm, indicating that the grains are polycrystalline. The magnetic properties of the resulting grain‐growth chemical remanent magnetization (CRM) which parallels the inducing field, appear to be dependent on the synthesis temperature and the presence of a substrate. Specific values for the CRM range from 1 to 2.5 μA m2/kg in the presence of the microfiber glass filters. Without these filters, values are higher, up to 25 μA m2/kg. Specific values for a thermoremanent magnetization (TRM), also acquired in a 0.30 mT inducing field, are of the same order as the CRM and less than one per mil of the saturation TRM (acquired in a 2 T field) for these samples. All remanences, the 0.30 mT CRM and TRM, as well as the 2 T TRM, showed only marginal decay upon alternating field (AF) demagnetization, as expected for goethite. Thermal decay curves, obtained by routine stepwise thermal demagnetization, appeared to be very similar for all remanence types. For the 30°C synthesis series the maximum blocking temperature of the CRM and the 2 T TRM was 65°C whereas that of the 0.30 mT TRM tends to be slightly higher (∼10°C). For the 55°C synthesis series the maximum blocking temperature of all remanence types was 80–85°C. This indicates a small grain size for the polycrystalline (XRD) synthetic goethites, in agreement with SEM observations. For the 55°C synthesis series without microfiber glass filters the maximum blocking temperature was 95°C, indicating larger goethite grains. The CRM/TRM ratio decreases with increasing grain size as inferred from the maximum blocking temperature. For the 30°C series the average ratio is 1.6 (range 0.9–2.3) and for the 55°C series the average ratio is 0.56 (range 0.44–0.78), whereas a ratio of 0.17 was obtained for the sample synthesized at 55°C without substrate. To test the applicability of the CRM/TRM concept to natural rocks, that is to test whether it was possible to assign a goethite NRM as a CRM with this test, some preliminary results of goethite‐bearing samples from two sites in SE France are presented. Prior to thermal demagnetization, the samples were AF demagnetized up to 100 mT fields to remove a possible low‐coerciyity contribution to the NRM. At one site, BL, in a Jurassic schist, the goethite is formed by recent weathering; the NRM direction corresponds to the present‐day geomagnetic field direction. At the other site, RD, in a Jurassic limestone, a recent NRM component as well as a reversed NRM component were determined, both residing in goethite. Unfortunately, the lack of a fold test hinders establishing a pretilting or posttilting age for the reversed component. The first senario would imply an Eocene age for the reversed component, while the latter would point to a Matuyama age. Also in the pretilting senario, the direction of the reversed component does not correspond with the expected Jurassic direction for stable Europe. Foi BL goetliite, NRM/TRM ratios appear to be up to 1.7, compatible with fine‐grained goethite. For RD goethite, NRM/TRM ratios for die stable goethite range from 0.14 to 0.51, pointing to larger grains or better linked crystallites. A paleomagnetically unstable sample from this site has a much higher NRM/TRM ratio, in agreement with its much finer grain size. The impact of geomagnetic dipole intensity variation on the derived ratios is discussed. Extremely fine‐grained natural goethite shows a remarkable low‐field TRM behavior with a threefold remanence increase after AF demagnetization and storage in a field‐free space for 6 days. The importance of a relatively large number of thermal demagnetization steps below 100°C to detect NRM goethite components is discussed. The observed agreement between the CRM/TRM ratio of experimental data and the NRM/TRM ratio of natural samples is encouraging. This ratio might be shown to be useful for the discrimination between stable and unstable goethite NRMs. More experimental research is necessary before the potential of the CRM/TRM technique can be fully appreciated, also for possible paleointensiy determinations. For example, the possible influence of Al, which may be incorporated in the goetliite lattice, needs to be investigated before the NRM/TRM ratio technique to assess the paleomagnetic stability of g

ISSN:0148-0227    

DOI:10.1029/92JB01026

年代:1992

数据来源: WILEY

摘要:

The discovery of bacteria that precipitate greigite within intracellular organelles (magnetosomes) offers new evidence about the origin of greigite in natural environments. Unlike magnetite, only scarce information is available about the magnetic characteristics of greigite. For this reason, and the present inability to grow these microorganisms in pure culture, it is not known whether or not the magnetosomes in the newly discovered greigite‐precipitating bacteria are of single‐domain (SD) size, as are the magnetosomes from magnetite‐precipitating bacteria. Hie hypothesis of natural selection for magnetotactic behavior predicts that the greigite‐bearing magnetosomes should also be single magnetic domains. Using previously reported magnetic properties and crystallographic features for greigite, we have calculated the size and shape boundaries expected for SD and superparamagnetic (SPM) behavior in this mineral. For further characterization of the greigite crystals, we analyzed the domain state at various length/width ratios assuming crystal shapes of parallelepipeds and prolate spheroids. Magnetite was used as control for the current theories supporting these calculations. We also present a simple algorithm to calculate the upper size limit of single‐domain grains. Our results show that the crystals of bacterial greigite characterized so far are located in the region close to the single‐domain superparamagnetic boundary and should have relatively low coercivity. If these crystals contribute to the magnetization of sediments, remanence produced by bacterial greigite could be mistaken for large, multidomain magnetite in alternating field demagnetizat

ISSN:0148-0227    

DOI:10.1029/92JB01290

年代:1992

数据来源: WILEY

摘要:

The South Atlantic is investigated for evidence of long‐lived (tens of millions of years), tectonic segmentation of the mid‐ocean ridge by examining a variety of geophysical parameters. These parameters include: subsidence rate of the oceanic crust (east flank, west flank, average, asymmetry), zero‐age depth, geoid height decrease with age (geoid rate), and the residual geoid field. The variability in these parameters along‐strike of the mid‐ocean ridge is typically systematic. The ridge can be subdivided into segments (flowline corridors in plan view) within which characteristics are uniform. Eight tectonic corridors have been determined in the area between the Ascension Fracture Zone and the Falkland‐Agulhas Fracture Zone, which serve to define a large scale ridge segmentation on the order of hundreds of kilometers. This segmentation is in addition to the well‐known small‐scale, near‐axis segmentation. Near‐axis geochemical data show boundaries that are generally consistent with the tectonic segmentation. Many of the primary geophysical observations are found to be internally inconsistent with simple thermal conduction models for oceanic crust. Such observations include asymmetric crustal subsidence, large along‐strike variations in subsidence rate and geoid rate, the lack of a systematic correlation between subsidence rate and geoid rate, and the abrupt nature of some of the inferred tectonic boundaries. Thus, the data indicate that in addition to uniform lithospheric cooling, other factors such as variable asthenospheric temperatures are important contributors to the creation and subsequent modification of the oceanic crust and possibly th

ISSN:0148-0227    

DOI:10.1029/92JB01338

年代:1992

数据来源: WILEY

摘要:

The eastern Vema fracture zone intersection with the Mid‐Atlantic Ridge (MAR) axis was surveyed with the French submersibleNautileduring the Vemanaute cruise in 1989. At this ridge‐transform intersection (RTI), an elongated, E‐W ridge, more than 50 km long, is present in the transform valley. This median ridge rises up to 1000 m above the surrounding seafloor. The crest of the median ridge is lower and presents an arcuate shape at the tip of the MAR axis. Dive observations on both the southern and northern flanks of the median ridge as well as sample studies suggest that this morphological feature is not a serpentinized mantle protrusion or a recent volcanic constructional ridge but represents a sliver of uplifted oceanic lithosphere covered by a sedimentary breccia formation. This detrital cover consists of polymictic sedimentary breccias, sandstones and siltstones, composed of basaltic, doleritic, and gabbroic clasts, with less frequent serpentinite and spinel fragments which originated from the disaggregation of shallow to deep levels of tectonically uplifted oceanic crust and upper mantle. Most of these clasts have undergone greenschist facies metamorphism prior to their incorporation in the detrital formation. Disaggregation, mass wasting and rapid emplacement of detrital formations on the valley floor by gravity flows are likely to be related to a major tectonic episode that affected one or both the fracture valley walls. This event could be related to the uplift of the southern wall of the fracture zone (the “transverse ridge”) which took place probably between 10 and 3 Ma ago. Since this uplift episode, the transverse ridge (which is now undergoing subsidence) and the detritus covered transform valley floor, separated by the transform fault zone, have migrated westward and eastward respectively. Vertical tectonics of the median ridge at the approach of the RTI can not be explained solely by the hypothesis of a diapiric intrusion of serpentinite as proposed by earlier authors. A possible interpretation follows the suggestions that the anomalous crust of the fracture valley near the western RTI, is more than 1 km out of isostatic equilibrium. Recent tectonic and magmatic events including subsidence and lava emplacement which occurred at the tip of the MAR axis have been recorded on the southern flank of the median ridge. Several stages in the very recent tectonic‐volcanic history of the eastern RTI, that is, roughly during the last 300,000 years, can thus be defined. The lower elevation and narrow, arcuate shape of the median ridge east of 40°57′W are inferred to have resulted from tectonic extension during the creation of t

ISSN:0148-0227    

DOI:10.1029/92JB01086

年代:1992

数据来源: WILEY

摘要:

The principal deep crustal rock types found at the La Olivina xenolith locality in southeastern Chihuahua, Mexico, are mafic granulites, paragneisses, and intermediate‐ to silicic‐composition orthogneisses. These granulite facies xenoliths are interpreted in terms of two age groups, pre‐Cenozoic and mid‐Tertiary, based on previous ion probe dating of zircons from the xenoliths and on isotopic comparisons of the xenoliths to rocks of known age. The mafic granulites have Pb, Nd, and Sr isotopic compositions identical to those of Oligocene volcanic rocks from the La Olivina region. Compositionally, they are olivine‐normative gabbroic cumulates, and they precipitated from two or more mid‐Tertiary basalt to dacite or rhyolite assimilation/fractional crystallization series. Mineral assemblages in the xenoliths record pressures of ≤7.2 kbar or depths of35 km thick in Oligocene time as inferred from regional tectonic considerations, then the mafic granulites cannot be samples of basaltic magmas underplated near the crust‐mantle boundary. The cumulate protoliths for the mafic granulites probably formed in magma chambers well above the Moho. The mafic granulites are plausibly representative of kilometers of new crust that formed in mid‐Tertiary time beneath the extensive ignimbrite fields of Mexico. Most orthogneiss xenoliths are pre‐Cenozoic, and they are rocks associated with the late Paleozoic Ouachita Orogeny and older events (e.g., Proterozoic basement and Paleozoic arc rocks). The Ouachita Orogeny was a collision event involving North America and a continental plate to the south, and the results of this study indicate that La Olivina is located above the southern plate. The paragneiss xenoliths overlap in isotopic composition with Carboniferous flysch units exposed in the Marathon uplift of west Texas. These sedimentary rocks and the sedimentary protoliths of the paragneiss xenoliths were derived from the southern plate and deposited before the orogeny in the ocean basin that separated North America and the southern plate. The paragneisses were not metamorphosed to granulite facies until mid‐Tertiary time. Pre‐Cenozoic and mid‐Tertiary crustal evolution followed very different paths in northern Mexico. For example, Nd isotopic evidence for crustal recycling is much more evident in rocks associated with the Paleozoic convergence than in rocks produced during mid‐Tertiary magmatism. Furthermore, mafic rocks are very rare in the pre‐Cenozoic xenolith suite, but they dominate the mid‐Tertiary one. These contrasting characteristics of the pre‐Cenozoic and mid‐Tertiary xenolith suites are interpreted to reflect differences in tectonic environment and crustal thickness. Preceding the collision event, the southern plate had an Andean‐type margin, and the abundant evidence for crustal recycling during this time implies that the crust was thick. The rarity of pre‐Cenozoic mafic xenoliths suggests that Proterozoic and Paleozoic lower crust may have delaminated in response to crustal overthickening assoc

ISSN:0148-0227    

DOI:10.1029/92JB01493

年代:1992

数据来源: WILEY

摘要:

Finite element calculations of magma flow in dikelike channels with length‐to‐width ratios of 1000:1 or more have been used to investigate the coupling between thermal and dynamical regimes due to temperature‐dependent viscosity and dissipation. Steady state solutions with realistic thermal and dynamical parameter values have been obtained. The models show that the onset of solidification on the boundaries of a basaltic or andesitic dike, as predicted by idealized laminar flow models, can be prevented or significantly delayed by a small amount of transverse flow induced by rising bubbles, boundary roughness, or turbulence. This effect will reduce the critical initial widths of dikes estimated by Bruce and Huppert (1989). In the absence of transverse flow, the bulk temperature of the magma may actually increase slightly with distance along the dike as a result of viscous dissipation even while solidification is occurring on the walls of the dike. With converging or necking dike walls it is found that boundary temperatures fall to a minimum and then increase with distance along a dike even if viscous heating is neglected. For viscous heating to offset significant rates of heat loss (2 kW/m2) in a plane‐parallel 1‐m‐wide basaltic dike, an average flow velocity of 2.7 m/s driven by a pressure gradient of 1.7 MPa/km is required. A 7% void fraction caused by exsolution of volatiles or chamber gas in the magma will produce this pressure gradient. The ease of producing such a gradient by reducing the density of the magmatic column with addition of a gas phase makes it likely that flows of basaltic magma could be maintained in dikes tens of kilometers long. Furthermore, a gas phase may be important for the propagation of the fracture prior to the initial injection of magma during dike emplacement. Rapid transport of more silicic magmas through dikes is inhibited by the requirement of large driving pressure gradients exceeding several hundred megapascals per kilometer. However, the pressure gradient can be substantially reduced if the magma is a heterogenous mixture of a predominantly silicic component with a more mafic component. Pipe flow experiments involving molten polymers that exhibit dynamical similarity to magmas strongly suggest that unmixing occurs when a two‐component magma mixture, in which the components have different viscosities, rises within a dike. Within a few dike widths of the inlet the less viscous mafic component encapsulates the more viscous silicic part, effectively lubricating the passage of the more viscous component. Compositional variations in the Obsidian Dome volcano support the occurrence of this pressure reduction or self‐lubrication mechanism. Such a self‐lubrication process often may be necessary to permit very viscous magmas to reach Earth's surface. If so, chemical and lithologic zoning would be anticipated as a common feature of near‐surface intrusions or lava flows that are characterized by a high silica and/or high crystal content such as at Deadman Dome in Long

ISSN:0148-0227    

DOI:10.1029/92JB01244

年代:1992

数据来源: WILEY