摘要:

Winona basin is a deep water sedimentary basin forming part of Explorer plate and located off the northern end of Vancouver Island, British Columbia. During 1975 a deep seismic sounding survey, using explosive charges and multiple hydrophones as receivers, was carried out over the basin. Three reversed refraction profiles were shot, one along and two across the basin. A preliminary look at data along the basin indicated reasonable lateral homogeneity so interpretation was based on both travel time and amplitude characteristics through the use of synthetic seismograms. However, the two profiles across the basin indicated significant lateral variations in crustal structure, so a ray tracing approach was used. The sediments of Winona basin have a maximum thickness of approximately 5 km, consisting of ∼2 km of low‐velocity (average ∼2.5 km/s) layers and ∼3 km of higher‐velocity (∼4.3 km/s) material. Beneath the sediments there is a rapid increase in velocity to ∼5.4 km/s, the top of the oceanic crust. Below this and along the basin, the interpretation indicates three zones with thicknesses of 1 to 3 km and having low‐velocity gradients; these being separated by transition zones with depth intervals less than 1 km and having high‐velocity gradients. The three zones with low‐velocity gradients are represented approximately by 1 km at 5.4 km/s, 2.7 km at 6.2 km/s, and 2.5 km at 6.7 km/s. With the inclusion of the zones of high‐velocity gradient, an average subsedimentary crustal velocity of ∼6.3 km/s and thickness of ∼8 km are obtained. The upper mantle velocity is ∼7.8 km/s. Across the basin, the crustal model shows layers dipping eastward from the westward side and dipping westward from the eastward side, forming a bowllike structure. Two‐dimensional gravity calculations based on the seismic model are compatible with a gravity profile derived from a free air anomaly map. The thick crust and structural variations are consistent with, but do not provide direct evidence for, the complex plate interactions embodied in the tectonic reconstructions

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00225

年代:1981

数据来源: WILEY

摘要:

A short survey with Gloria side‐scan sonar and other geophysical instruments has revealed new information about Charlie‐Gibbs fracture zone between 29° and 36°W. The traces of two transform faults have been clearly delineated. They fit small circles about the pole of rotation with an rms error of only about 1 km, but they do not always follow the deepest parts of the transform valleys. The transforms are joined by a short spreading center at 31°45′W. The median transverse ridge appears to have been produced by normal seafloor spreading at this center and bears identifiable Vine‐Matthews magnetic anomalies. A transverse ridge along the eastern inactive part of the northern transform may be an intrusive feature. Considerable thicknesses of sediment appear to have been deposited in the northern transform valley from Norwegian Sea overflow water passing through the fracture zone, but transverse ridges have prevented the sediment reaching the south

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00243

年代:1981

数据来源: WILEY

摘要:

New marine geophysical data are used to describe the structure and history of the Gulf of Aden. Magnetic anomaly data shows seafloor spreading magnetic anomalies on Sheba Ridge from the axial anomaly to anomaly 5 (10 m.y. B.P.) between the Owen fracture zone and 45°E and to anomaly 2′ (3 m.y. B.P.) or anomaly 3 (4 m.y. B.P.) west of 45°E. The data does not support the two episodes of seafloor spreading recently proposed. Landward of the seafloor spreading magnetic anomalies is a magnetic quiet zone of uncorrelatable anomalies. The magnetic quiet zone boundary is also a structural boundary effectively marking the edge of Sheba Ridge, with deeper basement lacking a significant topographic gradient found on the landward side. A magnetic quiet zone is found not only where Sheba Ridge splits continental lithosphere but also on East Sheba Ridge where the ridge splits the old oceanic lithosphere of the Owen and Somali basins. There the position occupied by the continental margin within the gulf is marked by nonmagnetic ridge complexes that stretch from the continents to the Owen fracture zone. The magnetic quiet zone boundary is not an isochron in either the Gulf of Aden or the Red Sea, suggesting that significant horizontal motions can occur prior to the initiation of seafloor spreading. The offset on the Dead Sea Rift is used to estimate that from 80 to 160 km of opening, amounting to between 65% and 200% extension of the initial rift valley, occurred in the Gulf of Aden and Red Sea prior to the establishment of a mid‐ocean ridge. It is suggested that the development of a new ocean basin occurs in two stages. The first involves diffuse extension over an area perhaps 100 km wide in a rift valley environment without an organized spreading center. This is followed by concentration of the extension at a single axis and the beginning of true seafloor spre

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00263

年代:1981

数据来源: WILEY

摘要:

A total of 560 samples from three areas in northeastern Baluchistan (the southern Sulaiman Range, the central Loralai Range, and the northern Kirthar Range) were analyzed using thermal demagnetization techniques. Thirteen formations of late Palaeozoic to early Tertiary age were studied palaeomagnetically. Inclinations of the obtained results show a general affinity with the Indian apparent polar wander path. Deviating declinations from the Loralai Range indicate a clockwise rotation over 50° with respect to the Indian shield. Secondary magnetization components probably of late Palaeocene to early Eocene age were consistently present in the samples from the Kirthar Range and the Sulaiman Range but were not observed in samples from the Loralai Range. Acquisition of these components is attributed to crustal upwarping during the Palaeocene, which is tentatively related to initial collision of continental Indo‐Pakistan with an island arc off south central Asia. The Baluchistan data support recent palaeomagnetic results from the Indus‐Tsangpo suture zone in Ladakh (NW Himalaya), which indicate that such an initial collision took place at very low northern palaeolatit

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00289

年代:1981

数据来源: WILEY

摘要:

The ophiolites, considered remnants of oceanic seafloor, provide a fruitful source of information on the physical and chemical behavior of the deeper uncored lithosphere. A study of the gabbros in a rather well‐preserved ophiolite from the western Alps (Montgenèvre, France) shows strong evidence for intraoceanic plate deformation. To characterize the different steps of deformation observed in the outcrops, a magnetic fabric study was conducted using the anisotropy of initial magnetic susceptibility (Aims) method. The main conclusions are (1) the Aims is controlled by the paramagnetic ferromagnesian minerals, such as hornblende, resulting from the ocean floor metamorphism close to the spreading ridge (partial amphibolitization) and (2) the magnetic lineation and foliation of the gabbros show a deformation path with increasing strain. A good correspondence between petrofabric and magnetic anisotropies is observed; both suggest that the deformation mechanism was simple she

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00307

年代:1981

数据来源: WILEY

摘要:

Petrographic studies show that hematite is present in the Moenkopi Formation in at least five and possibly six forms: (1) microcrystalline hematite, (2) crystals of specular hematite, (3) polycrystalline and monocrystalline grains, (4) grains of partly hematitized ilmenite, (5) grains composed of primary ilmenite‐hematite intergrowths, and (6) ultrafine pigment. The microcrystalline hematite and crystals of specular hematite are unequivocally authigenic. They form cement in interstitial and secondary voids, and they have replaced detrital iron‐bearing silicate minerals. Furthermore, microcrystalline hematite is superimposed on other authigenic cementing minerals such as potassium feldspar, calcite, dolomite, and quartz, and in some cases it has replaced authigenic pyrite. In addition, both microcrystalline and specularite crystals are common daughter products of intrastratally altered biotite grains. Thermodynamic considerations coupled with studies of hematite‐magnetite relationships in modern sediments indicate that most of the hematite in the polycrystalline grains, and probably the monocrystalline grains as well, was formed authigenically by post‐depositional replacement of detrital grains of magnetite. The ilmenite probably has similarly altered in situ to hematite. The only hematite of unquestionable detrital origin in the red beds is the hematite in the ilmenite‐hematite intergrowths (‘tiger striped’ grains) and that in monocrystalline detrital grains containing rutile exsolution platelets, both of which are products of high‐temperature processes. With the exception of the ultrafine pigment, each of the above forms is coarser grained than the superparamagnetic threshold for hematite, and therefore each contributes components of remanent magnetism to the rocks. Inasmuch as most of the hematite varieties represent authigenic products of intrastratal alterations that require considerable geologic time, we conclude that the red bed remanence is largely chemical remanent magnetization (CRM) acquired over long time intervals. The pigment in the Moenkopi red beds consists partly of authigenic ultrafine red iron oxide and partly of translucent microcrystalline hematite. The ultrafine red iron oxide may or may not be hematite, but even if it is, the grain size probably lies below the paramagnetic threshold for hematite. Much of the pigment, therefore, may not contribute greatly to the remanent magneti

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00317

年代:1981

数据来源: WILEY

摘要:

The spatial and temporal relationship of volcanism to the occurrence of large earthquakes and convergent plate motion is examined. The number of volcanic eruptions per year in a convergent zone is found to be linearly related to the aseismic slip component of plate motion. If the aseismic slip rate is low (coupling between converging plates is strong), then the primary manifestation of tectonic activity is the occurrence of large earthquakes with only infrequent volcanic activity. If, however, the aseismic slip rate is high (coupling is weak), then there are few large earthquakes, and volcanism is the principal manifestation of tectonic activity. This model is consistent with the spatial distribution of large earthquakes and active volcanoes in the circum‐Pacific area. It is tested by examining the extent of volcanic activity in the rupture zones of the 1952–1973 sequence of earthquakes in the Japan‐Kurile Islands area. The number of volcanic eruptions along these zones during the interval between large earthquakes is used to compute the aseismic slip rates for these segments, based on the relationship developed in this study. The aseismic slip rates so computed agree with those determined from the earthquake history of the area and rates of plate motion. The agreement suggests that in the interval between large earthquakes, the aseismic plate motion is manifested in a specific number of volcanic eruptions. Therefore in areas with adequate historical data it should be possible to use the model developed in this study to monitor volcanic eruptions for long‐term prediction of large eart

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00335

年代:1981

数据来源: WILEY

摘要:

The 214 m high Toktogul Dam will impound a 19.5‐km3reservoir on the Naryn River in central Kirgizia. The reservoir lies along the Talas‐Fergana fault, a major tectonic feature of central Asia. The nature of Cenozoic movements along the fault is in debate; however, there is considerable evidence of present‐day right‐lateral movement. The region is one of moderate to high seismicity. A magnitude 7.6 earthquake occurred in 1946, approximately 65 km from the dam, near the intersection of the Talas‐Fergana fault and Chatkal ranges. Construction of the dam was completed in 1975 and by late 1979 the water level in the reservoir reached three fourths its final planned depth. Low‐magnitude, induced earthquakes began as the water level first exceeded 100 m in late 1977. A microearthquake survey using portable instruments was carried out in July‐August 1978 and a seven‐station telemetered network was installed in October 1978. The largest of the induced earthquakes has been of magnitude 2.5; the activity is in the immediate vicinity of the dam at very shallow depths (2–5 km) and is related to water level. Preliminary analysis of data gathered during 1978–1979 shows a zone of activity, extending to 5 km depth, supporting geological evidence for an active reverse fault beneath the dam and parallel to the canyon of the Naryn River. Where the reservoir crosses the Talas‐Fergana fault, 15 km upstream from the dam, the water level has not yet reached 100 m and seismicity along the Talas‐Fergana has not shown any significant change related to f

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00345

年代:1981

数据来源: WILEY

摘要:

Slant stacking is used to transform an observed record section Φ(x,t) into its plane wave decomposition Ψ(p,τ). The invertible transformation untangles travel time triplications and removes focusing and defocusing effects, yielding signals with the plane wave amplitudes and phases. The principal arrival, composed of refraction and wide angle reflections, appears in these coordinates as a display of the intercept time function τ(p), which may be inverted by any of several currently popular methods. The transformation into the (p,τ) domain; is an objective procedure which removes many of the interpreter's difficulties by casting the picking process into much simpler variables. Moreover, the decomposition into plane waves directly yields the transient reflectivity matrix of the (one dimensional) medium. Several lines of argument lead us to conclude that the comparison of data and synthetic data should be done in the (p,τ) domain. The use of these techniques requires that the wave field be sampled with sufficient spatial density of receivers to avoid wave number aliasing. We show examples of plane wave decomposition for shallow data from the COCORP deep crustal reflection pr

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00359

年代:1981

数据来源: WILEY

摘要:

Previous estimates of the seismic energy released by lunar events have not properly accounted for instrument bandwidth, variations in corner frequency, and the effects of intense scattering. In this paper, equations are developed that include all of these effects and give realistic estimates for source parameters. These equations are applied to seismograms and displacement spectra from near‐surface and deep moonquakes to obtainM0(seismic moment),Ēs(seismic energy release),Eyr(seismic energy released annually by each lunar event class), Δσ (stress drop), andmb(body wave magnitude). The calculations yieldM0∼ 3 × 1021dyn cm,Ēs∼ 2 × 1017ergs, Δσ ∼ 400 bars, andmb∼ 5.0 for the largest shallow moonquakes; andM0∼ 5 × 1020dyn cm,Ēs∼ 1 × 1013ergs, Δσ ∼ 0.1 bars, andmb∼ 3.0 for large deep events. The average energy released annually isEyr= 2 × 1017erg/yr andEyr= 8 × 1013erg/yr by shallow and deep events, respectively; overall, lunar energy release is dominated by the shallow events. The energy released by the deep events may be accounted for by tidal dissipation, and deep event stress drops are comparable to and smaller than the calculated tidal stresses. A comparison of the above values with those observed terrestrially (Eyr∼ 1025ergs) and with the energy available from heat flow and tidal dissipation emphasizes the importance of tectonic style (e.g., plate tectonics) in determining the chara

ISSN:0148-0227    

DOI:10.1029/JB086iB01p00378

年代:1981

数据来源: WILEY