摘要:

A total of 152 paleomagnetic samples was collected from 23 sites distributed over six Paleozoic formations, with ages ranging between Middle Ordovician and Early Permian, in the Tangshan type section in the Nanjing area of South China. Stepwise thermal demagnetization has successfully resolved two magnetization components from these samples. Component A with northerly shallow tilt‐corrected directions resides in approximately 95% of the total number of samples analyzed; this component passes fold tests at the 99% significance level in three formations. The overall mean direction of component A is 350/−3 (tilt‐corrected, a95=7.2). Component B, isolated from 19 samples in two formations (Cl and D3), yields negative fold tests at the 95% significance level and is, therefore, younger than the Mesozoic folding. The overall site mean direction of component B is 354/−4 (in situ, a95=8.4). The tilt‐corrected direction of component A clearly resembles the in situ direction of component B, indicating that both components are likely to be of approximately the same age. We infer that they are both remagnetizations, associated with the deformation. A review of the Paleozoic paleomagnetic data reported by other authors from adjacent areas shows that similar remagnetization situations may prevail. It is concluded that the northeast part of south China has experienced pervasive remagnetizations at the time of folding. The direction of this folding‐induced remagnetization yields a new well‐defined paleopole position (56°N, 314°E) for south China. Although this paleopole is significantly different from Mesozoic poles reported to date, we infer that a Triassic to Jurassic age is most likely for this

ISSN:0148-0227    

DOI:10.1029/92JB02405

年代:1993

数据来源: WILEY

摘要:

Geological application of paleomagnetic data generally relies on equating radiometric or biostratigraphic age with that of the characteristic remanence. Yet the standard of proof of paleomagnetic age is often unsatisfactory and is extremely difficult to achieve where contact tests or conglomerate tests are unavailable. In this study of the Ntonya igneous ring complex the thermoremanent (TRM) origin of one component of natural remanent magnetization (NRM) is confirmed by paleointensity determination and its carrier is identified mineralogically, and the age of this TRM is estimated by extrapolation from40Ar/39Ar mineral ages. The characteristic remanence in the Ntonya Ring Structure, which was determined by Briden (1968) as D = 311° I = +43° α95= 1.9°, pole 28°N 15°W, is shown to be a TRM in single‐domain/pseudo‐single‐domain magnetite, acquired in a field of approximately half the present geomagnetic field strength. The40Ar/39Ar mineral ages (biotites 471.5 ± 7 Ma; hornblendes 510 ± 7 Ma) and the high precision of the paleomagnetic directions combine in a model of steady cooling (∼ 4.6° m.y.−1) that leads us to the conclusion that the thermoremanence was acquired over approximately 13 m.y., 522 ± 13 m.y. ago. In passing it is shown that the isothermal (IRM) component that dominates the remanence is carried by multidomain magnetite. The study shows how remanence age may be determined with precision in the absence of geological constraints. It makes the Ntonya pole a key point in the African (and Gondwanan) apparent polar wander path, correlating with Middle Cambrian glacial deposits in the Sahara at about

ISSN:0148-0227    

DOI:10.1029/92JB01254

年代:1993

数据来源: WILEY

摘要:

Precessing elliptical motion (PEM) is an important possible trajectory of the virtual geomagnetic pole (VGP), an expected result of the superposition of dipole precession on nondipole elliptical motion as proposed and reported on in the literature. The complex plane spectral signature of a simple PEM combination is shown here to be bichromatic in general, consisting of two peaks at frequencies ωaand ωb. Associated with these two peaks is a frequency pair (‐ ωo, ωo) from which each of the peaks at ωaand ωbis shifted by an amount ωp. This shift corresponds to the frequency of precession of the axis of the elliptical motion modified by the precession. Shifts towards positive (negative) frequencies correspond to counterclockwise (clockwise) precessions. The complex plane spectrum associated with a PEM can be strongly asymmetrical and does not provide the investigator with a straightforward means of extracting the frequencies of the component precessional and elliptical motions which may belie processes fundamental to the geomagnetic field. To overcome this problem, an enhanced complex plane spectral analysis is developed which directly identifies both the frequencies of the component precessional and elliptical motions. Examples of the application of this method are given using discretely sampled, synthetic VGP tim

ISSN:0148-0227    

DOI:10.1029/92JB02575

年代:1993

数据来源: WILEY

摘要:

The Born and Rytov approximations, widely used for solving scattering problems, are of limited utility for low‐frequency electromagnetic scattering in geophysical applications where conductivity can vary over many orders of magnitude. We present four new, relatively simple nonlinear estimators that can be used for rapid electromagnetic modeling. The first, termed the static localized nonlinear approximation, is designed specifically to correct the magnitude of the electric field internal to the scatterer. The second, termed the localized nonlinear approximation, improves the estimate of the phase of the scattered field and includes some of the cross‐polarization effects due to full wave scattering. Two further new estimators, based on the Rytov transformation (the localized nonlinear Rytov and the static localized nonlinear Rytov approximations) are designed to further improve the estimation of the phase of the scattered field, especially at high frequency and for larger size scatterers. Although these approximations are nonlinear functions in conductivity, they are generally much faster to compute than the full forward problem, and are almost as efficient as the Born or Rytov approximations. Moreover, the enhanced accuracy of the new estimators has made us optimistic about their application to low‐frequency three‐dimensional inverse problems in electromagnetics. The approximations developed in this paper will also be applicable to fields such as quantum mechanics, optics, ultrasonics, and sei

ISSN:0148-0227    

DOI:10.1029/92JB02324

年代:1993

数据来源: WILEY

摘要:

Seismic reflection data from Mesozoic oceanic crust in the western North Atlantic, in the vicinity of the Blake Spur Fracture Zone, have imaged a number of short, subhorizontal events in the upper 1 km of the crystalline crust. Wide‐angle expanding spread profile (ESP) data from the same region indicate the presence of first‐order velocity discontinuities in this depth interval. Using ESP data reprocessed to enhance precritical reflection events, we investigate the relationship between the precritical and postcritical reflections with three different approaches: amplitude analysis, forward modeling, and waveform inversion. The precritical reflections have high apparent reflection coefficients (∼ 0.1) and may be divided into two types: those which can be traced out to critical ranges (5–6 km), representing velocity steps, and those which fade out at ranges of 3–4 km, representing shorter‐wavelength velocity variations. Synthetic seismogram modeling indicates that the velocity steps occur within a 50 m depth interval, while the shorter‐wavelength features may correspond to high‐ or low‐velocity zones with a thickness of 20 m (10% of the dominant seismic wavelength) or less. We investigate the velocity structure in detail by full waveform inversion of a selected portion of ESP data, transformed into the intercept time‐slowness domain. The inversion method minimizes the sample‐by‐sample misfit between the data and reflectivity synthetics in a least squares sense. A conjugate gradient algorithm was used, starting from a velocity structure derived from forward modeling of wide‐angle ESP data. The inversion tightly constrains short‐wavelength components of the velocity structure, and indicates that intracrustal reflections result both from major velocity steps and from zones of alternating high and low velocity. Synthetic seismograms generated from sonic log data from Deep Sea Drilling Project hole 418A show reflections of similar amplitude and character to those seen in the ESP data. These reflections probably originate from rather subtle features of the velocity‐depth profile, related primarily to changes in porosity in the pillow lava sequence. A deeper reflector, 800 ms below oceanic basement in the Blake Spur Fracture Zone, may correspond to the top of a zone

ISSN:0148-0227    

DOI:10.1029/92JB02136

年代:1993

数据来源: WILEY

摘要:

The tectonic evolution of the Indian Ocean ridge‐ridge‐ridge triple junction, the junction of the Central Indian Ridge (CIR), Southeast Indian Ridge (SEIR) and Southwest Indian Ridge (SWIR), is inferred by mapping structures along its traces on the three plates with a long‐range side scan sonar, Geological Long‐Range Inclined Asdic (GLORIA). The sonar images to the west of the triple junction show two different styles of tectonic evolution. Some sections of the trace on the African plate are marked by a SWIR‐facing scarp containing fine lineaments that are probably normal faults. These correspond with sections of the trace on the Antarctic plate of the same age that have a sharp intersection of SWIR and SEIR abyssal hills that is to be expected from a simple ridge‐ridge‐ridge triple junction evolution (a “herringbone” pattern of isochrons). In contrast there are other sections of the trace on the African plate that have a blocky structure with no definite termination of CIR abyssal hills against those of the SWIR. The corresponding regions of the Antarctic plate show large 10‐ to 30‐km‐long overlapping fault scarps, tapering to the northeast and ending in asymmetric valleys to the southwest. These major normal faults are each offset by 1–6 km, forming an en echelon series. The two different styles of tectonic structures indicate two different modes of evolution, although why the triple junction has evolved with two different modes is unclear. Periods of asymmetric spreading in the CIR magnetic anomaly sequence collected on the African plate may possibly indicate that the CIR and SEIR have been offset by a fracture zone at the triple junction during one of these two modes. Echo sounder traverses on the Antarctic and African plates commonly show a steady shallowing of the CIR and SEIR seafloor toward the traces, a steep scarp facing the SWIR and a broad marginal deep at the base of this scarp. This rift flank shape resembles that of the SWIR valley's rift shoulders close to the present triple junction (Mitchell, 1991a), suggesting that the mechanical evolution has been similar to the present day and the relief due to the rift flank uplift has been preserved by the cooling and strengthening of the lithosphere with age away from the ridge axes. If the shape of the rift shoulder uplift along the trace on the Antarctic plate is interpreted with a simple flexural model involving an end‐loaded elastic plate, the inferred rigidity (1019‐1021N m or 2–6 km effective elastic thickness) is consistent with the suggestion that the isostatic uplift occurred in young, weak lithosphere, close to the triple junction. Interestingly, the observed ∼1‐km relief of the trace escarpment and the estimated rigidity suggest that the uplift was produced by a load (2–5 × 1011N/m) that is equivalent to half the present cross‐sectional area of the SW

ISSN:0148-0227    

DOI:10.1029/92JB00582

年代:1993

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/92JB02048

年代:1993

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/92JB02647

年代:1993

数据来源: WILEY

摘要:

Large‐scale magmatic activity, ranging from late Oligocene to Quaternary, is associated with the Red Sea‐Gulf of Aden rifting throughout the Arabian passive margin. The Southern Yemen area represents the southernmost extremity of this magmatic range, facing the Afar area, and provides a means of studying the magmatic records of early stages of rifting (30–16 Ma) in a plume‐related context. We investigate major and trace elements, and Nd, Sr and Pb isotopes of a bimodal series of transitional affinity consisting of (1) thick olivine‐basalt traps overlain by ignimbritic rhyolites, (2) basaltic, rhyolitic, trachytic and peralkaline dykes with a prevailing N120–140° E orientation, and (3) gabbroic, syenitic and granitic plutons. Major and some trace element variations from basalts to felsic rocks are consistent with low‐pressure fractional crystallization. Mass balance calculations using major elements suggest the fractionation of clinopyroxene (6–9%), olivine (∼6%), plagioclase (42–43%), magnetite (∼12%), apatite (1–2%) ± alkali feldspar (16%). However, LILE (large ion lithophile element) enrichment and high initial87Sr/86Sr ratios (up to 0.7074 and 0.710 in rhyolites and pantellerites, respectively) require the felsic rocks to be generated through significant crustal assimilation. Nd and Sr isotopic ratios of the rhyolitic traps can be reproduced by bulk mixing between magmas similar to the underlying basaltic unit and the Arabian Proterozoic basement. On the other hand, an assimilation‐fractional crystallization process is required to account for the isotopic diversity of the rhyolitic and peralkaline dykes. Rhyolites can be derived from a basaltic liquid by a moderate fractionation rate (F= 0.47) and a high crustal assimilation rate (R= 0.45), whereas the pantellerites require more significant fractionation rate (F= 0.07) and a very low assimilation rate (R= 0.05). Elemental and isotopic signatures of the basalts do not support a significant crustal contribution in their formation and their isotopic diversity (87Sr/86Sr from 0.7034 to 0.7051,143Nd/144Nd from 0.512676 to 0.513045 and206Pb/204Pb from 17.96 to 18.66) mainly reflects mantle source heterogeneities. Sr‐Nd‐Pb isotopic data are consistent with a binary mixing between depleted and enriched source regions. The depleted end‐member corresponds to an asthenospheric reservoir approaching that producing mid‐ocean ridge basalts at Gulf of Aden/Red Sea spreading centers. The enriched reservoir, intermediate between enriched mantle I and II end‐members, is supposed to be located within old subcontinental lithosphere related to the Pan‐African orogenic events. Unlike the modern volcanics from Afar, no HIMU (high U/Pb ratio) signature has been recognized in our sampling. This rules out any significant chemical influence of the Afar plume upon early rift‐related volcanism in Southern Yemen and suggests a continental rift initiation of passive type. However, one can suspect that the Afar plume may have supplied the excess heat required to produce so voluminous traps and to trigger melting in the lithospheric mantle, making the distinction between

ISSN:0148-0227    

DOI:10.1029/92JB02314

年代:1993

数据来源: WILEY

摘要:

The Henry Mountains represent one of three major igneous centers of the Colorado Plateau interior during mid‐Tertiary (25–30 Ma) time. These intrusions occur ∼1000 km from the paleotrench and are unrelated to subduction in a classical sense, despite many compositional similarities to orogenic magmatic rocks. Most of the intrusive volume (95%) consists of plagioclase‐hornblende porphyry of intermediate composition, with minor syenite making up the remainder, and both suites appear to have cooled rapidly as evidenced by the fine‐grained texture of their groundmass. Although many elemental variations can be explained by fractional crystallization, Sr, Nd, Pb, and O isotope systematics require open system interaction of mantle derived magmas and Proterozoic amphibolite crust. The plagioclase‐hornblende porphyry evolved via assimilation fractional crystallization (AFC) in deep‐crustal magma chambers. Syenite porphyry evolved in two distinct stages. First, AFC produced a shallow, zoned, subintrusive magma chamber containing Ne‐normative syenite magma. Second, as magma was withdrawn, batches of Ne‐normative magma mixed with tonalitic melts produced by fusion of amphibolite country rock to produce Q‐normative syenite porphyry. Trace element abundance patterns suggest that both plagioclase‐homblende and syenite porphyry were derived from the same mantle source. A higher degree of partial melting may explain the silica‐oversaturated character and lower trace element abundances in the plagioclase‐hornblende porphyry relative to the syenite porphyry. Trace‐element systematics also indicate that high large ion lithophile (LILE) to high field strength element (HFSE) ratios, typical of subduction‐derived magmas, are characteristic of the source and have not been imposed solely by crustal contamination. Therefore, the porphyries of the Henry Mountains appear to be related to contemporaneous voluminous regional magmatism of the western United States as part of a large‐scale igneous system with arc‐like affinities. The relatively minor volume of igneous rocks implies that the Colorado Plateau acted as a structural barrier to the ascen

ISSN:0148-0227    

DOI:10.1029/92JB02689

年代:1993

数据来源: WILEY