摘要:

Rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks, provide a basis for interpretation of aeromagnetic data over volcanic terranes, and yield insights into the depositional and cooling histories of ash flow sheets. A rhyolitic ash flow sheet, the Miocene‐aged Tiva Canyon Member of the Paintbrush Tuff, contains both titanomagnetite phenocrysts, present in the magma prior to eruption, and cubic Fe‐oxide microcrystals that grew after emplacement. Systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the Nevada Test Site. Natural remanent magnetization varies from less than 1 × 10−4to more than 8 × 10−4A m3kg−1, and in‐phase magnetic susceptibility varies from less than 1 × 10−6to more than 10 × 10−6m3kg−1. The microcrystals, which include both magnetite and maghemite, have Curie points and maximum unblocking temperatures between 580°C and 640°C. Rock magnetic data, including in‐phase and quadrature magnetic susceptibilities as well as hysteresis parameters, demonstrate that these microcrystals are of superparamagnetic and single‐domain sizes. Titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section, whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15‐m‐thick zones at the top and bottom. Within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet. The decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break; possibly represented by a concentration of pumice. Laboratory heating produces changes in magnetic properties which vary complexly from sample to sample. Such changes include (1) decrease in saturation magnetization, (2) both increase and decrease in in‐phase and quadrature magnetic susceptibilities, and (3) shifts in coercivity spectra. The complex sample‐to‐sample variation in response to heating is attributed to difference

ISSN:0148-0227    

DOI:10.1029/93JB00355

年代:1993

数据来源: WILEY

摘要:

A stable prefolding magnetization has been discovered in pillow basalts of the Eocene lower Crescent Formation of the northern Olympic Mountains. The curved outcrop pattern of the Crescent Formation has been the target of several unsuccessful studies to test for oroclinal bending. The success of this study is due, in part, to the development of a small‐diameter electric core drill for sampling the fractured rims of basalt pillows. Thermal demagnetization produced stable endpoints by 580°C in 12 of the 34 sites sampled (large within‐site scatter was common in the remaining sites). Among the accepted sites, within‐site scatter was small and correction for bedding tilt significantly reduced the scatter between sites. The mean paleomagnetic pole for newly sampled pillow basalts (86.4° north latitude, 170.0° east longitude, A95=16.5°) is indistinguishable from the early to middle Eocene pole expected for North America. Including previous results from sites in subaerial basalt of the upper Crescent Formation in and near the easterm Olympic Mountains results in a more regional paleomagnetic pole (80.7° north latitude, 192.0° east longitude, A95=8.0°, N=46) that shows no significant rotation (0.8° ± 14.4°) or poleward displacement (−3.6 ± 8.5). Analysis of the magnetic mineralogy suggests that the remanence is early, probably primary. The pole, therefore, should be valid for tectonic interpretation of the region. A circular distribution of virtual geomagnetic poles after correction for bedding tilt supports the hypothesis that the northern Crescent Formation experienced deformation due to the rise, in a domelike fashion, of the sediments of the Olympic Core terrane. Erosion of a partial dome open to the west could have produced the curvature seen in the outcrop pattern of the Crescent Formation. The lack of significant rotation of the northernmost Coast Ranges contrasts with the net clockwise rotation seen to the south. The difference could be that irrotational northward translation (paleomagnetically insignificant yet geologically important), driven by oblique convergence, was accommodated by the north‐south trending strike‐slip faults to the east of

ISSN:0148-0227    

DOI:10.1029/93JB00709

年代:1993

数据来源: WILEY

摘要:

Paleopoles determined from seamount magnetic anomalies constitute the major data source for the Pacific apparent polar wander path, but relatively little is known about the processes of remanence acquisition in seamounts. Since magnetic anomalies reflect both natural remanence (NRM) and the induced field, it is important first to assess whether the NRM is likely to represent an original field direction and second to constrain the magnitude of the induced component. To this end, we present paleomagnetic data from an uplifted, subaerially exposed section through a seamount on La Palma, Canary Islands. The Pliocene Seamount Series of La Palma comprises a>6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts. The structural tilt of the Seamount Series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting (e.g., viscous remanence). The NRM provides a poor indication of the original magnetization direction, although the characteristic magnetization of many La Palma samples is compatible with the original pretilt direction. Hydrothermal alteration has resulted in the production of Ti‐poor magnetite and an increasing contribution of hematite with increasing degree of alteration. More importantly, well‐defined magnetization directions which deviate from any reasonable geomagnetic direction at La Palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization. Based on a comparison of the magnitude of low‐stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization, we conclude that viscous and induced magnetization probably account for 15–25% of the total magnetization of seamounts. The resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4° to 16° for Cretaceous seamounts in th

ISSN:0148-0227    

DOI:10.1029/93JB00932

年代:1993

数据来源: WILEY

摘要:

Multibeam echosounder surveys and seismic reflection profiling were conducted in the southern Lau back arc basin (SW Pacific). The bathymetric survey covered a 130‐km‐long section of the active spreading ridge (3 cm/yr half spreading rate), the N‐S trending Valu Fa Ridge (VFR), while seismic profiles concentrated on a 35‐km‐long ridge section. Analysis of these data shows a morphologically segmented ridge which is underlain by a continuous magma chamber. The ridge shows similarities, but also differences, to the segmentation model developed for mid‐ocean ridges. Analogous to this model the section of the VFR studied displays third‐ and fourth‐order segmentation. The amplitudes of the along‐axis undulations, however, are a factor of 5 to 10 greater than at mid‐ocean ridges. Additionally, the VFR has numerous small volcanic cones along the axis, which have previously only been observed at much slower spreading ridges. In cross section the VFR has a very steep, narrow triangular shape which is thought to result from the high viscosity of its andesitic lavas. The axial magma chamber appears to vary in width (1–4 km) and location relative to the axis beneath different morphological segments. Perhaps the most significant deduction from our combined data is that the magma chamber is widest beneath the most prominent axial discontinuity, a small overlapping spreading center (OSC). This is counter to observations from mid‐ocean ridges. Off‐axis traces of ridge segments allow us to reconstruct the evolution of this OSC for the past 220 kyr. We conclude that this back arc spreading axis shows a segmentation pattern which is similar to mid‐ocean ridges, but it is influenced and modified by the geochemistry of its lavas and its

ISSN:0148-0227    

DOI:10.1029/93JB00708

年代:1993

数据来源: WILEY

摘要:

Bathymetric Sea Beam mapping of a southward propagating rift system in the Lau Basin (SW Pacific) shows a morphotectonic segmentation of the two competing axes: Over a distance of 70 km the propagating axis develops from a narrow rift graben at the propagator tip, via a leaky graben, into a juvenile volcanic ridge, and finally into a fully developed spreading ridge. The competing dying spreading axis 70 km farther east shows steady deepening towards the north, combined with a change from a small volcanic ridge to a narrow depression before it is buried by sediments. The relay zone between the two axes contains two deep N‐S striking grabens; the eastern one, 3200 m deep, may act as a short‐lived spreading center. A NE‐trending string of slightly offset deep sediment‐filled basins is interpreted as fossil analoges to these grabens, originally generated in the southward migrated relay zone. The string of basins suggests an episodic rather than steady state southward migration. Propagation and opening of grabens led to a rotation of the inherited ridge‐parallel crustal fabric. However, the propagating tip and the grabens cut obliquely through the fabric thus generating small‐scale structural and morphological inhomogeneities of the crust. At the propagating tip, old and new tectonic lineaments closely

ISSN:0148-0227    

DOI:10.1029/93JB00707

年代:1993

数据来源: WILEY

摘要:

The Troodos complex of Cyprus has been interpreted as a slice of oceanic crust formed from ultramafic magma in a back‐arc basin. Recent investigations, however, have suggested that only the upper gabbros, the sheeted dike complex, and the lower lava sequence formed as a direct consequence of seafloor spreading and that the primary magma was of basaltic composition. According to this model, the upper lava suite and related ultramafic cumulates are not represented in the sheeted dike complex. A multivariate statistical analysis of major element compositions of 480 dikes and 130 volcanic glasses supports this suggestion. Statistically (98%), the dikes are similar to the glasses of the lower lava suite. This supports the model that the seafloor spreading related sequence of the Troodos ophiolite is relatively thin and of basaltic composition. The magma chambers represented by the ultramafic cumulates and associated gabbros and the upper lavas appear to have been emplaced after, or during, the termination of the major spreading event

ISSN:0148-0227    

DOI:10.1029/93JB00695

年代:1993

数据来源: WILEY

摘要:

The Paleogene Basement Series on Attu Island in the Western Aleutians is dominated by tholeiitic basalts chemically similar to mid‐ocean ridge basalts (MORB; La/Yb ∼ 2, ϵNd>+10.0,206Pb/204Pb<18.4). These basalts evolved in “open” magma systems. Contemporaneous rhyolites and albite granites are chemically analogous to silicic volcanic rocks of modern ocean ridges (e.g., Iceland, Galapagos Spreading Center). The occurrence of chemically arc‐type tholeiitic basalts, with high Th/La and La/Ta relative to MORB, suggests that a depleted MORB‐like mantle source was variably modified by a subduction component. The Attu Basement Series is not allochthonous. Geological and tectonic constraints imply formation in arc‐adjacent, transtensional rifts that developed in a strike‐slip regime that was established ∼43 m.y. ago. Rifting of this kind may have produced much of the Western Aleutian crust between ∼43 and 15 Ma. Subsequently, small volumes of crystal‐rich andesite and dacite were erupted on Attu Island as well as throughout the Western Aleutians. These strongly calcalkaline rocks (FeO*/MgO∼1.1, CaO/Al2O3∼0.35 at 65% SiO2) are chemically akin to magnesian andesites of Piip Volcano, a hydrothermally active seamount that overlies small dilational structures within the broadly transpressional regime of the modern western arc. This transpressional setting is inferred to have been established throughout the Western Aleutians approximately 15 m.y. ago. The switch from tholeiitic magmatism in a transtensional regime to strongly calcalkaline magmatism in a transpressional regime may have resulted from clogging of the Aleutian‐Kamchatka junction with buoyant, subduction‐related terranes. These terranes probably originated to the east and were transported by strike‐s

ISSN:0148-0227    

DOI:10.1029/93JB00714

年代:1993

数据来源: WILEY

摘要:

New Rb‐Sr and K‐Ar geochronological data are presented for the majority of known pre‐Cenozoic outcrops in Thurston Island, the Jones Mountains, and the western Eights Coast, which collectively represent the basement geology of the Thurston Island crustal block of West Antarctica. Almost all are of calc‐alkaline igneous or metaigneous rocks, and indicate long‐standing proximity to a magmatic arc. The observable history began with Late Carboniferous (309±5 Ma) emplacement of mantle‐derived orthogneiss precursors in eastern Thurston Island. Nd model ages from these and later igneous rocks suggest that the underlying crust is no older than about 1200–1400 Ma throughout the area. A variety of cumulate gabbros was emplaced soon after gneiss formation, followed by crust‐contaminated diorites that have Triassic mineral cooling dates of 240–220 Ma. In the nearby Jones Mountains, the oldest exposed rock is a muscovite‐bearing granite with an Early Jurassic age of 198±2 Ma; its initial87Sr/86Sr ratio of 0.710 and ϵNdtvalues of −5 to −7 indicate either anatexis or, at least, a high degree of crustal input during magma genesis. This belongs to a suite of such granites known throughout the Antarctic Peninsula and related to earliest rifting of the Gondwana supercontinent. The subsequent evolution of the Thurston Island area was dominated by I‐type magmatism, apparently in two major episodes at 152–142 Ma (Late Jurassic granites) and 125–110 Ma (Early Cretaceous bimodal suite). Most of these magmas had initial87Sr/86Sr ratios of 0.705–0.706 and ϵNdtvalues of +2 to −4 and were derived from slightly enriched mantle or from juvenile lower crust. They are thought to signify subduction of Pacific Ocean floor as in the adjacent parts of West Antarctica, although the Late Jurassic episode was of greater intensity in Thurston Island than elsewhere. The Cretaceous magmatism was intense and of Andean‐type. Between 100 and 90 Ma, volcanism in the Jones Mountains became predominantly silicic, with increasing incorporation of crustal components (initial87Sr/86Sr ratios of 0.706–0.709 and ϵNdtvalues of −3 to −6), as subduction‐rel

ISSN:0148-0227    

DOI:10.1029/93JB01157

年代:1993

数据来源: WILEY

摘要:

An analysis is presented for the variation in chemistry of 162 normal mid‐ocean ridge basalt glass groups, collected with an average spacing of about 10 km along a −1100 km section of the East Pacific Rise between 13° and 23°S. Long‐wavelength periodicities have been determined for oxides Al2O3, CaO, Cr2O3, FeO*, K2O, MgO, MnO, Na2O, NiO, P2O5, SiO2, TiO2; for ratios FeO*/MgO, K/P, and K/Ti; for FeO*, Na2O, and SiO2corrected for fractionation to 8.0 wt % MgO; and for bathymetry. The data were modeled as the sum of a long‐wavelength periodic component and a residual random component. The power spectrum of the periodic component was determined using the maximum entropy method. The spectral phase of significant spectral peaks was determined by cross correlation with a cosine wave with the wavelength of the respective peaks. In most cases the characteristics of the random component were found to be consistent with a normal distribution. Spatial aliasing considerations and the variance of the random component place confidence limits upon the spectral features. The results of the spectral analyses indicate substantial correlation in the power spectra of the oxides. The oxide data have spectral peaks with wavelengths in the vicinity of 575 and 225 km and a local minimum in spectral power at wavelengths between 300 and 500 km. In general, oxide components compatible in the crystallizing minerals olivine and plagioclase are approximately 180° out of phase with the incompatible oxides, indicating that fractionation is an important process in controlling compositional variations along this portion of the East Pacific Rise. Unlike the fractionation‐sensitive oxide data, periodic variations are not pronounced for K/Ti, indicating that the length scales of mantle composition being melted are uncorrelated with those of magmatic temperature variation. The analysis of fractionation‐corrected values of Na8.0and Fe8.0indicates that degree and depth of partial melting show a strong spectral peak near a wavelength about 430 km. There is also significant power in the spectrum of Na8.0near 260 km and of Fe8.0near 200 km, bounding the average spectral peak for the oxides at 225 km. There appears to be strong coupling between the degree and depth of melting, and magmatic temperature at two length scales, around 225 km and 400–600 km, corresponding to wavelengths of geoid undulations observed in the vicinity of the East Pacific Rise, and suggesting a relationship

ISSN:0148-0227    

DOI:10.1029/93JB00352

年代:1993

数据来源: WILEY

摘要:

The possibility that free SiO2may exist at the core mantle boundary motivates a first‐principles investigation of the equation of state and possible phase transitions of stishovite using periodic Hartree‐Fock theory. Using a moderately extended basis set, the calculated equation of state is shown to be in close agreement with experimental results taken under hydrostatic conditions. When fit to a third‐order Birch‐Murnaghan equation, the Hartree‐Fock total energies giveV0= 46.1 A3,K0= 328 GPa, and K′ = 4.0. Moreover, the calculated equation of state is very close to that recently obtained using the linearized augmented plane wave method. The bonding in stishovite is unchanged by compression to 80% of its zero‐pressure volume (corresponding to a pressure near 110 GPa). The possibility that stishovite might transform to a denser phase at high pressure is investigated by calculating the free energy of SiO2in the modified‐fluorite and α‐PbO2structures. The enthalpy of SiO2with the fluorite and modified‐fluorite structure is too high to allow those structures to be adopted. The 0 K enthalpy of silica in the α‐PbO2structure is within 10 kJ/mol of that of stishovite, and a transition to such a structure cannot be ruled out in the temperature regime of the lower mantle. However, the density of α‐PbO2structure SiO2is only 2% great

ISSN:0148-0227    

DOI:10.1029/93JB00783

年代:1993

数据来源: WILEY