摘要:

A model of the geomagnetic 60‐year variation is proposed. The 60‐year variation is regarded as a magnetohydrodynamic oscillation originating in a thin layer at the top of the core. Two types of magnetic field and two types of fluid motion are mainly considered in the layer, which are poloidal magnetic mode (1, 0), toroidal magnetic mode (2, 0), toroidal velocity mode (1, 0), and poloidal velocity mode (2, 0). When the magnetic force and the Coriolis force are dynamically dominant, these four fields make a closed system and cause magnetohydrodynamic oscillations in the layer. Steady magnetic fields of higher modes generated in the deeper part of the core induce fluctuating fields when they pass through the layer. The induced magnetic fields have a common period and two different types of phase. Observed amplitudes of these fields are mostly explained when the maximum velocity in the layer is 10

ISSN:0148-0227    

DOI:10.1029/93JB00754

年代:1993

数据来源: WILEY

摘要:

Paleomagnetic study of the shale to slate transition in the Ordovician Martinsburg Formation at Lehigh Gap, Pennsylvania reveals that the remanence of the relatively undeformed shales predates cleavage formation, and that the slates were remagnetized during cleavage development. The shales display two components of remanence. Component 1 (C1) was the only magnetization direction in the majority of the shales, and the first‐removed component in 10 shale specimens. Component 2 (C2) is observed as a single‐component remanence with a low (300–400°C) unblocking temperature range in samples from one shale station and as a second‐removed component in 10 shale specimens. The mean directions are (declination/inclination) 347°/−04° (346°/−47°) in situ (tilt‐corrected) for C1 and 80°/−63° (135°/−40°) for C2. The pencil slates and well‐cleaved slates both display single component remanences. The mean directions are 198°/−12° (201°/26°) for the pencil slates and 199°/24° (225°/54°) for the well‐cleaved slates. Using the relationship between cleavage development and remagnetization in the slates, the timing of cleavage development and folding, and the regional geology, we can constrain the relative ages of the remanences as follows: (1) the shale C1 direction is most likely prefolding and represents a primary upper Ordovician direction, (2) the slate directions are younger in age than the shale directions and are probably of Permo‐Carboniferous (Alleghenian) age. These results show that remagnetization occurred during development of slaty cleavage in these rocks. We interpret the mechanism for this remagnetization to have been strain‐induced dissolution and neocrystallization of magnetite. The additional difference in the characteristic directions of the pencil slates and well‐cleaved slates may be attributed to strain reorientation of the pencil slate remanence after new growth of magnetite. The unknown temporal relationship between dissolution and neocrystallization and strain precludes any quantitative attempt to correct remanence for strain in these samples. Our study shows that paleomagnetic studies of deformed sediments must consider the possibility of dissolution and neocrystallization of remanence ca

ISSN:0148-0227    

DOI:10.1029/93JB01088

年代:1993

数据来源: WILEY

摘要:

New paleomagnetic data presented here further constrain the relative motion of the Antarctic Peninsula relative to East Antarctica during the Mesozoic development of the southern ocean basins. The Antarctic Peninsula (AP) is one of four crustal blocks that define West Antarctica, the others being the Ellsworth‐Whitmore Mountains (EWM), Thurston Island‐Eights Coast (TI), and Marie Byrd Land (MBL). A Jurassic pole (∼155Ma) (124°E, 64°S,A95=7.1°,N=10 VGPs) was obtained from the AP block which suggests that the AP block rotated clockwise between ∼175 and ∼155 Ma due to significant early opening in the Weddell Sea basin. A new Early Cretaceous paleomagnetic pole (182°E, 74S,A95=5.9°,N=6 site mean VGPs) indicates that the AP block was in or near to its present‐day position with respect to East Antarctica by ∼130 Ma. Between ∼155 and 130 Ma, counterclockwise rotation of the AP‐TI blocks, together with the southward motion of East Gondwanaland, probably resulted in subduction of Weddell Sea ocean floor beneath the southern AP block and initiated the Palmer Land deformational event. A ∼130 Ma pole from the TI block requires clockwise rotation of the TI and possibly the EWM blocks between 130 and ∼110 Ma producing sinsistral strike‐slip motion between the EWM block and East Antarctica and dextral transpressional motion between the TI‐EWM blocks and the AP block. New AP block ∼110 Ma and ∼85 Ma poles from this study (199°E, 74°S,A95=6.9°,N=13 VGPs; 152°E, 86°S,A95 = 7.5°,N=6 VGPs, respectively) are similar to equivalent age poles from East Antarctica and suggest little or no relative motion between the Antarctic Peninsula and East Antarctica. Northern and southern Antarctic Peninsula mid‐Cretaceous poles are very much alike suggesting that the “S” shape of the Antarctic Peninsula

ISSN:0148-0227    

DOI:10.1029/93JB01089

年代:1993

数据来源: WILEY

摘要:

We present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20°40′S on the East Pacific Rise. A majority of the data used were collected during a cruise aboard the R/VMoana Waveduring which continuous SeaMARC II coverage was obtained. These data were combined with additional data to produce an anomaly map which extends to anomaly‐2‐aged crust. A three‐dimensional inversion in the presence of bathymetry was carried out for the area. The resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity. The results indicate consistent asymmetric spreading faster to the east, discontinuous high magnetizations in the discordant zone associated with the discontinuity, and southward migration of the feature at a rate of 90–100 mm/yr between Jaramillo and Brunhes time (0.95 to 0.73 Ma) with slowing during the Brunhes to less than 10 mm/yr. There is also an overlapping Jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the Nazca to the Pacific plate. In addition, areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector. Results from this second model support the presence of highly rotated pre‐Brunhes Nazca crust within Brunhes Pacific crust which has been deformed by bookshelf faulting. This indicates at least two episodes of crustal transfer from the Nazca plate to the Pacific plate. The discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the Nazca plate between the discontinuity and the Easter microplate to the south. The detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of

ISSN:0148-0227    

DOI:10.1029/92JB02913

年代:1993

数据来源: WILEY

摘要:

The recent R/VMaurice EwingEW9105 Hydrosweep survey of the East Pacific Rise (EPR) and adjacent flanks between 7°S and 9°S provides an excellent opportunity to explore the causal relationship between the ridge and the abyssal hills which form on its flanks. These data cover 100% of the flanking abyssal hills to 115 km on either side of the axis. We apply the methodology of Goff and Jordan (1988) for estimating statistical characteristics of abyssal hill morphology (rms height, characteristic lengths and widths, plan view aspect ratio, azimuthal orientation, and fractal dimension). Principal observations include the following: (1) the rms height of abyssal hill morphology is negatively correlated with the width of the 5‐ to 20‐km‐wide crestal high, consistent with the observations of Goff (1991) for northern EPR abyssal hill morphology; (2) the characteristic abyssal hill width displays no systematic variation with position relative to ridge segmentation within the EW9105 survey area, in contrast with observations of Goff (1991) for northern EPR abyssal hill morphology in which characteristic widths tend to be smallest at segment ends and largest toward the middle of segments; (3) abyssal hill rms heights and characteristic widths are very large just north of a counterclockwise rotating “nannoplate”, suggesting that the overlap region is being pushed northward in response to microplate‐style tectonics; and (4) within the 7°12′S–8°38′S segment, abyssal hill lineaments are generally parallel to the ridge axis, while south of this area, abyssal hill lineaments rotate with a larger “radius of curvature” than does the EPR axis approaching the EPR‐Wilkes r

ISSN:0148-0227    

DOI:10.1029/93JB01095

年代:1993

数据来源: WILEY

摘要:

Reinterpretation of newly published geophysical data (Kamesh‐Raju and Ramprasad, 1989) and older profiles of the Central Indian Basin, associated with similar studies of the Madagascar and Crozet basins, shows that the Indian Ocean Triple Junction trace on the Indian plate corresponds, at anomalies 23 and 22, to a N38°E offset of the magnetic lineations, oblique to both the Southeast Indian Ridge (SEIR) and Central Indian Ridge (CIR) spreading directions. The conjugate Triple Junction trace on the African plate identified in the Madagascar Basin is associated with a roughly north‐south offset, parallel to the Southwest Indian Ridge (SWIR) fracture zones. In order to account for these observations and the velocity triangle of the Indian, African, and Antarctic plates close to the Triple Junction, a ridge‐fault‐fault mode is proposed, with a propagatorlike SEIR‐CIR offset. The Triple Junction jumped between anomalies 24 and 23 and between anomalies 22 and 21, restoring a ridge‐ridge‐ridge configuration which immediately turned to a pseudo‐ridge‐ridge‐fault and later to a true ridge‐fault‐fault configuration. After the Triple Junction jump at anomaly 21, the former SEIR‐CIR offset was accommodated by a new CIR fracture zone. The lack of such a fracture zone prior to anomaly 21 suggests that either a pseudo‐ridge‐ridge‐fault or an unstable ridge‐ridge‐ridge configuration prevailed before anomaly 24, in agreement with the velocity triangles which predict more unstable Triple Junction modes. Both modes support the creation of numerous SWIR fracture zones, presently observed between 52°30′E and 59°30′E, as a consequence of the Triple Junction evolution between anomalies 29 and 24. This result suggests that the physiography of the SWIR

ISSN:0148-0227    

DOI:10.1029/93JB00438

年代:1993

数据来源: WILEY

摘要:

We examine the seismic structure of Cretaceous‐aged ocean crust north of the Blake Spur fracture zone in the western North Atlantic by using a combination of multichannel seismic and wide‐angle reflection/refraction data. Although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure, it displays large variations in crustal reflectivity on both ridge‐normal and ridge‐parallel profiles. The upper crust is highly reflective and contains both subhorizontal (or shallow dipping) events and more steeply dipping reflectors. Subhorizontal reflectors are typically present between 1.5 and 2 km depth and are, in some cases, laterally continuous for distances of 15 to 20 km. Generally, these events do not correlate with the depth of the seismic layer 2/layer 3 boundary determined from refraction data, and their origin is still poorly understood. The steeply dipping events are generally confined to the upper 2.5 km of the crust. We interpret these reflectors as the subsurface expression of ridge‐parallel extensional faults commonly mapped at mid‐ocean ridges. The lower crust in this area exhibits alternating regions of high and low reflectivity. The highly reflective zones are made up of packages of linear or concave‐upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust. Some of these dipping reflectors appear to cut through the whole crustal section, although most fade out in the acoustically transparent midcrust. On ridge‐normal profiles the majority of these events dip to the east, toward the paleo‐spreading center, whereas on ridge‐parallel profiles the events typically dip south, toward the Blake Spur fracture zone. The base of the crust is generally not associated with a strong, discrete Moho reflection but is a comparatively indistinct boundary usually associated with a 1‐ to 2‐km‐thick band of diffuse high reflectivity. We interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8–10 km. With the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge‐normal and ridge‐parallel profiles. We may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults. Alternatively, these events may represent two different classes of fault systems that form during different stages of the emplacement a

ISSN:0148-0227    

DOI:10.1029/93JB00557

年代:1993

数据来源: WILEY

摘要:

The Porcupine Seabight Basin is a north‐south trending, extensional sedimentary basin situated on the continental shelf west of Ireland. It is filled with sedimentary rocks which range in age from Devonian to present‐day. A large amount of geological and geophysical data is now available for this basin, mainly as a result of oil industry exploration. In this paper, information from 23 well logs and from regional seismic reflection surveys is used to determine the lithospheric stretching history of the basin. Subsidence analyses indicate that there was one main phase of stretching which began in late Liassic times (∼ 180 Ma), ending in earliest Cretaceous (∼ 145 Ma). Stretching factors at the northern end of the basin are relatively small (β = 1.1–1.7) but increase rapidly southwards along the axis of the Main Porcupine Basin. In the southern part of the region (the Seabight Basin), β is greater than 6. The simplest way to accommodate this rapid variation in stretching is by clockwise rotation of the Porcupine Ridge through approximately 20°, away from the Irish Shelf. Subsidence‐derived stretching values are in reasonable agreement with those determined previously from crustal thicknesses based on gravity models and deep seismic data (normal incidence and wide‐angle). The existence, location, and inferred age of the Porcupine Median Volcanic Ridge are also consistent with the subsidence‐derived stretching factors. After intermittent Paleocene igneous activity, an anomalous increase in the rate of subsidence occurred in the Eocene, between 55 and 42 Ma. There is little evidence that this rapid increase is caused by lithospheric stretching, and we conclude that it must be associated in some unknown way with melt generated by development of t

ISSN:0148-0227    

DOI:10.1029/93JB00890

年代:1993

数据来源: WILEY

摘要:

Reprocessed multichannel seismic profiles from the 9°N segment of the East Pacific Rise reveal prominent shallow subbasement events. These events are identified as wide‐angle reflections from the base of seismic layer 2A, based upon modeling of expanding spread profile data and velocity functions. The layer 2A reflections typically increase from 0.15 s after the seafloor reflection at the rise axis to 0.3–0.45 s within 1–2 km of the axis, corresponding to an increase in layer thickness of 200–600 m. No further systematic increase in layer thickness is observed, although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis. However, the intermittent character of the imaged layer 2A reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer. The base of layer 2A is interpreted as corresponding to the transition between the extrusive section, pillow basalts and sheet flows, and a sheeted dike complex. The rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone. Lateral variability of layer 2A can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2A are approximately half that of layer 2B. For an along‐axis profile, apparent along‐axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2A caused by wandering of the profile relative to axis. Within the resolution of the data, the time delay of the magma chamber reflection relative to the base of layer 2A

ISSN:0148-0227    

DOI:10.1029/93JB00886

年代:1993

数据来源: WILEY

摘要:

We have reprocessed seven cross‐axis common depth point (CDP) profiles between the Clipperton transform and the 9°17′N Deval (deviation in axial linearity) on the East Pacific Rise (EPR) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Forward modeling of cross‐axis CDP profiles suggests a segmented AMC in which significant variations in width occur across minor rise axis discontinuities (e.g., Devals). The modeled rise segment widths bounded by the 9°53′N‐9°35′N Devals, the 9°35′N‐9°17′N Devals, and south of the 9°17′N Deval were<0.7 km, 1.0–1.2 km, and 4.15 km, respectively. Transition in AMC width across these discontinuities is unclear due to the sparseness of cross‐axis line spacing; however, a simple association of Devals with decreased magma supply is doubtful: the minimum (250 m) and maximum (4150 m) AMC widths are found near the 9°35′N and 9°17′N Devals, respectively. The reprocessing of CDP profiles included repicking stacking velocities to ensure a proper stack of the AMC reflector and its associated diffractions, imaging postcritical reflections from the base of layer 2A, finite difference time migration, ray theoretical depth migration, and travel time modeling of AMC diffraction patterns. Constraints on AMC width were derived from forward modeling techniques based on analytic raytracing. Velocity models were constructed from SeaBeam bathymetry and modified expanding spread profile (ESP) velocity‐depth functions. ESP velocity models were altered to compensate for off‐axis thickening of layer 2A as imaged in the CDP reflection data. Continuous two‐dimensional velocity models constructed from modified ESP velocity‐depth functions and SeaBeam bathymetry should account for ray bending at the seafloor/basalt interface and any lateral velocity gradients induced by a thickening layer 2A. Stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration. Reflector positions were input into our forward modeling scheme to produce a zero‐offset travel time response of our migrated solution. The travel time response was then overlain on the stacked section to ensure an adequate match, especially to diffractions generated at the AMC edge. Forward modeling of AMC diffraction patterns reveals that original AMC width estimates were too large. The under‐migration of the AMC reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual CMP gathers, thus resulting in improperly collapsed diffraction hyperbolae. Ship wandering relative to the AMC edge can account for variations in AMC reflector amplitude and dropout on the along‐axis CDP line. The continuity of the AMC appears unbroken across several ridge axis discontinuities between the Clipperton transform and the 9°17′N Deval which suggests an AMC whose along‐axis dimension exceeds 75 km. Reflectivity modeling of CMP gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush. Such a thin layer of melt might inhibit along‐axis mixing of magmas and thereby account for variations in magma composition along the rise crest. This melt lens model for the AMC would also produ

ISSN:0148-0227    

DOI:10.1029/93JB00705

年代:1993

数据来源: WILEY