摘要:

Fracture characteristics measured in the Sheeted Dike Complex of the Troodos ophiolite, Cyprus, indicate a decrease in fracture aperture and trace length with depth. Fractures, generally of a contraction‐on‐cooling origin, tend to be oriented either parallel or perpendicular to the dike margins. High‐temperature hydrothermal epidote and quartz fracture fillings are overprinted by late, low‐temperature calcite fracture fillings. Paleopermeabilities estimated on the basis of these data using three methods of permeability determination (parallel plate, matrix addition, and a stochastic approach) indicate higher on‐axis and slightly lower, late, possibly off‐axis, permeabilities than those measured in present‐day

ISSN:0148-0227    

DOI:10.1029/95JB01575

年代:1995

数据来源: WILEY

摘要:

Quantitative relationships are demonstrated between dike density and hydrothermal alteration, magnetization, location of volcanic massive sulfide (VMS) ore bodies and a number of other properties in a 40 by 15 km segment of the Extrusive Series of the Troodos Ophiolite. Low‐temperature alteration extends from the sediment‐extrusive interface to close to the 25% dike density surface, while the onset of greenstone type alteration is closely associated with the 50% dike density surface. These associations are explained in terms of the regional scale variations in the dike density contour surfaces following the form of the top of the gabbroic layer of the ophiolite. An upper magnetic zone, characterized by stable high remanence, terminates just below the 25% dike density surface. A deeper magnetic zone, characterized by high induced magnetization, straddles the top of the Sheeted Complex. Volcanic massive sulfide bodies are concentrated in a narrow depth range centered about 0.1 km above the 25% dike density surface. A model for the postridge crest evolution of ore bodies accounts for this optimum depth range for preservation. Physical property changes with dike density, and the depth distribution of VMS ore bodies and prospects for the ophiolite as a whole support this model. Similar relationships appear to be present in the Samail, Macquarie Island, Newfoundland and Chilean ophiolites and Icelandic crust. As a result of relationships found in the Troodos ophiolite, an alternative is proposed to the current lithologic profile for Ocean Drilling Program Hole 5

ISSN:0148-0227    

DOI:10.1029/95JB01481

年代:1995

数据来源: WILEY

摘要:

The diffusion model is potentially useful for quantifying the effect of downslope gravity transport on sedimentation rate variations, which are commonly found between Deep Sea Drilling Project and Ocean Drilling Program sites. If appropriate, the model and stratigraphy could be used, for example, to constrain the amount and timing of fault block rotation. Deep Tow profiler records from the French‐American Mid‐Ocean Undersea Study area of the Mid‐Atlantic Ridge are examined to determine whether they are consistent with a diffusion model. Sedimentary contacts with basement show that a variety of processes may be operating, some of which are inconsistent with the diffusion model. For example, there are moats around some contacts which are typical of scouring or nondeposition due to high current velocities. More than half of the contacts, however, show onlapping, which is qualitatively consistent with the diffusion model. Sediments generally fill low areas of the topography and have smooth surfaces, as expected from a diffusion model. Where the fluxes of sediment entering a basin are constant, the diffusion model predicts that the sediment surface should evolve to a parabola (the steady state solution). Some 20 curved surfaces in the profiler data were digitized and least squares parabolas fitted to them with rms errors of less than 1 m. The slopes of the model parabolas provide values for the ratio of sediment flux to diffusivity at the edges of the basins (steep surfaces are produced by low diffusivity or high fluxes). This ratio is combined with estimates of the fluxes to determine the apparent sediment diffusivity,Kapp, for eacri basin. Flux is estimated by assuming the abyssal hill topography acts as a simple sediment trap, so that the lateral flux equals the width of a basin's pelagic catchment area times the area's mean sedimentation rate S. Using this method, medianKappis 0.04–0.11 m2yr−1(assumingS= 10–30 m m.y.−1). Variations inKappand the assumptions behind the diffusion model are discussed, and alternative causes of sediment surface tilts are considered, in particular differential compaction, which is modelled using a simple porosity

ISSN:0148-0227    

DOI:10.1029/95JB01974

年代:1995

数据来源: WILEY

摘要:

Bathymetry and magnetic studies (part of the Trans Indian Ocean Geotraverse investigations) in the northeastern Indian Ocean revealed seafloor topographic features, magnetic lineations (19 through 32B) and abandoned spreading centers. The seafloor topography of the Ninetyeast Ridge is relatively wider and shallower south of 15°S. The magnetic anomalies indicate nine fracture zones. Two of them are newly identified. Some of the fracture zones are reflected in the bathymetry. Abandoned spreading centers between 86°E Fracture Zone (FZ) and 92°E FZ are interpreted as the western extensions of the Wharton Ridge. They ceased spreading along with other spreading centers in the Wharton Basin soon after the formation of magnetic anomaly 19 (around 42 Ma) and merged the Indian and Australian plates as single Indo‐Australian plate. The pattern of magnetic lineations between 86°E FZ and 90°E FZ indicate a series of southerly ridge jumps at anomalies 30, 26 (Royer et al., 1991 and other workers) and 19. These ridge jumps transferred portions of the Antarctic plate to the Indian plate. The captured portions and offset along 86°E FZ between India‐Antartica Ridge and Wharton Ridge resulted in an anomalous extra oceanic crust between 86°E FZ and Ninetyeast Ridge spanning 11°

ISSN:0148-0227    

DOI:10.1029/94JB02464

年代:1995

数据来源: WILEY

摘要:

Two sets of hydrogeologic tests conducted at Ocean Drilling Program (ODP) Hole 892 on the Oregon Accretionary Prism provided the opportunity to determine hydrogeologic properties of an active accretionary prism fault zone. The first set of tests consisted of shipboard packer tests conducted during ODP Leg 146 (fall 1992), while the second set of tests were constant‐drawdown and constant‐discharge tests conducted in fall 1993 using the submersibleAlvin. Pressure response during the first set of tests suggests that fractures remained open until excess fluid pressure (relative to hydrostatic) dropped below 0.315 to 0.325 MPa (λ* ∼ 0.53 to 0.54, where λ* = (pore pressure ‐ hydrostatic)/(lithostatic‐hydrostatic)). Analysis of the packer test data suggested an apparent background pressure of 0.25 MPa (λ* ∼ 0.42 to 0.50). Because the borehole had been open for 12 hours prior to the packer tests, formation pore pressures may have exceeded this value prior to drilling of the borehole. These overpressures dissipated by the time the second set of tests were conducted. One possible explanation for this decay is that the borehole may provide a vertical conduit between the overpressured zone and overlying or underlying sediments that had previously been hydraulically separated from the overpressured zone. The second set of tests were conducted at pressures (≤0.019 MPa or λ* ∼ 0.03) below that estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less than estimated for the packer tests (when fractures were open). Constraints on fluid flow rate along the fault are provided by observed displacement in a bottom‐simulating reflector (BSR) at its intersection with the fault zone. The closed‐fracture transmissivities are insufficient to produce flow rates capable of displacing the BSR; therefore open‐fracture transmissivities under conditions of elevated pore pressure are inferred to be necessary for the observed BSR displacement. In addition, calculated rates of specific discharge through the fault zone are 2 to 3 orders of magnitude lower than discharge measured at an associated seafloor vent site; fluid flow must become spatially or temporally focused as it moves up the fault

ISSN:0148-0227    

DOI:10.1029/95JB02152

年代:1995

数据来源: WILEY

摘要:

New heat flow data on the southeastern United States passive margin show that measured, uncorrected flux averages 49±11.8 mW m−2through old (∼175 Ma) oceanic crust. Nonuniform thermal gradients were measured at about half of the 114 penetrations that comprise the data set and over the full range of water depths (1900 m to 4250 m) at which data were collected. With the simplifying assumption that the nonuniform gradients were caused by a step function change in bottom water temperatures at some time before the heat flow cruises, concave down (decreasingdT/dzwith depth) and concave up (increasingdT/dz) sediment thermal gradients can be explained by respective average temperature decreases and increases of 0.1–0.2 K amplitude occurring 28–35 days before the measurements. Thermal gradients throughout the entire region are strongly influenced by oceanographic phenomena and locally by the presence of subsurface diapiric structures, while sediment thickness variations and lateral differences in sedimentation rate and sediment lithology appear to exercise relatively little control over thermal regimes. Mean reduced heat flow in the study area is estimated at ∼49 mW m−2by decompacting and back stripping the 5–8 km of sediment deposited on the margin since the formation of the underlying oceanic crust. This value agrees with previous measurements made on younger crust in the Blake Ridge area but is significantly higher than the reduced heat flow value in a similar passive margin setting at the Baltimore Canyon Trough. We use the calculated average thermal gradient value and an assumption of constant conductivity to estimate temperatures of 19.5°C to 24.5°C at the bottom‐simulating reflector (BSR) on the

ISSN:0148-0227    

DOI:10.1029/95JB01860

年代:1995

数据来源: WILEY

摘要:

The OCEAN experiment is a detailed geophysical study of a region of the Cape Verde basin. A dense network of new magnetic and gravity profiles has enabled us to constrain the spreading rate history of the region and the location of fracture zones. The main features on the gravity profiles are lineated perpendicular to the seafloor spreading magnetic anomaly lineations. Significant along‐axis variability in spreading history suggests that the Mid‐Atlantic Ridge behaved as a series of loosely coupled segments within which spreading was fundamentally asymmetric. Such variability is associated with a minor jump in the ridge axis which changes the offset and expression of one of the fracture zones. Deep seismic reflection and refraction lines were oriented parallel and perpendicular to the magnetic lineations; seismic reflections occur at all levels within the crust, decreasing in amplitude and coherence below the level of the Moho. Analysis of the subbasement reflectivity provides compelling evidence that at least two major sets of dipping structure are present and are imaged separately on the two perpendicular sets of seismic profiles. Dipping reflections on flow line (“dip”) profiles, which are interpreted as faults due to their association with offsets in the basement surface, appear to strike parallel to the paleoridge axis. The majority of reflections that may be identified as faults dip toward the west, and although basement topography suggests that east dipping faults are also present, no reflections may be interpreted unambiguously as such. East dipping reflections observed only in the middle to lower crust have a more obscure origin. Dipping reflections seen on isochron (“strike”) profiles show clear contrasts in strength, lateral coherence, depth, and dip population; a number of these strike parallel to flow lines. Comparing reflection and refraction data shows that both the layer 2/layer 3 boundary and the Moho are marked by a change in the character of reflections and suggests that they may represent important structural, as well as seismological, boundaries within the oceanic

ISSN:0148-0227    

DOI:10.1029/95JB01376

年代:1995

数据来源: WILEY

摘要:

The Ponga Unit lies in the core of the Cantabrian/Asturian arc, a thrust system which was emplaced and then noncoaxially refolded during the Variscan Orogeny. Alternative models for thrust system geometry prior to refolding of the Ponga Unit have been proposed. Although the Ponga Unit becomes populated with steeply plunging folds under each model, the alternative models imply differing kinematic histories for these folds. Paleomagnetic data were acquired from 22 sites in grey and red Carboniferous limestones around these folds in order to determine the rotational histories of the structures and hence to identify the appropriate geological model for initial thrust system geometry. Low anisotropy of magnetic susceptibility indicated that the samples were suitable for rigid‐body rotation studies. Four remanence components were identified on stepwise demagnetization: A very low temperature, randomly orientated componentV; a low‐temperature componentLpresent between 50°C and 200°C (64/349, α95= 4.4°), clustered around the Recent field in geographic coordinates; an intermediate‐temperature componentIpresent between 200°C and 400–600°C, of reverse polarity and shallow inclination and showing significant between‐site dispersion; and a high‐temperature componentH(30/342, α95= 21°), lying close to the Eocene reference direction in geographic coordinates. TheIcomponent fails a fold test based upon a first deformation phase (D1) hanging wall anticline and is therefore a remagnetization. TheIcomponent consistently gives positive fold tests of the steeply plunging fold structures when the structures are unfolded about their plunging hinges. This demonstrates that these folds result from north‐south shortening of essentially homoclinal westerly dipping D1 hanging wall ramps and that alternative models involving steeply dipping interference folds arising from complex hanging wall deformation during D1 are invalid. A partial restoration of the thrust system based on the former model with the paleomagnetic data pinned to a calculated Stephanian reference direction shows that the thrust front was approximately north‐south in trend in the Ponga Unit, as it was farther south in the Esla Unit, where anticlockwise rotations and dextral wrench faulting have subsequently occurred. The data also record a tilt to the north of up to 30°, which has occurred incrementally since the end

ISSN:0148-0227    

DOI:10.1029/95JB01482

年代:1995

数据来源: WILEY

摘要:

Paleomagnetic results from the Early Cretaceous Ponta Grossa dike swarm are reported. These dikes crosscut sediments and basement rocks associated with the Ponta Grossa Arch, a tectonic feature in the eastern border of the Paleozpic‐Mesozoic Parana Basin. These dikes are vertical or subvertical and trend mainly NW and NE. Samples from 127 dikes were submitted to both alternating field and thermal demagnetizations; 121 of them yielded characteristic directions interpreted as original thermal remanent magnetizations. Both normal and reversed polarities as well as intermediate directions (6 dikes) are recorded. The paleomagnetic pole is located at 30.3°E, 82.4°S (N= 115; α95= 2.0°;k= 43.8). This pole along with the available paleomagnetic poles for the Paraná Basin volcanics (Serra Geral Formation), recalculated in this paper in the light of reported40Ar/39Ar ages describe an apparent polar wander path segment corresponding to a clockwise rotation (∼6.9°) of the South American plate. This movement is compatible with the initial phase of the South Atlantic opening with rifting progressing from south tp north. However, the calculated plate velocity is higher than the predicted velocities reported so far based on oceanic floor magnetic anomalies. The new data reported in this paper allowed the calculation of a mean Early Cretaceous pole (60.3°E, 83.8°S;N= 7, α95= 2.9°) which can be discriminated from the Middle‐Late Jurassic (191.3°E, 86.2°S;N= 3, α95= 7.8°) and Late Cretaceous (346.5°E, 84.1°S;N= 5, α95= 4.9°) mean poles for South America, indicating a more complex movement of the continent than the simple east‐west

ISSN:0148-0227    

DOI:10.1029/95JB01681

年代:1995

数据来源: WILEY

摘要:

Empirical and analytical techniques for modeling ionospheric fields in Magsat data have been developed that facilitate ionospheric field removal from uncorrected anomalies to obtain better estimates of regional lithospheric anomalies. This task has been accomplished for equatorial ΔX, ΔZ, and ΔBcomponent and polar ΔZand ΔBcomponent measurements. The techniques for modeling ionospheric fields have been integrated into a processing sequence that incorporates some of the important data‐processing techniques developed during the last decade. Data‐processing techniques include retention of common signal in a correlation analysis of adjacent passes; analysis of field differences between dawn and dusk data at points where their orbits cross; and retention of common signal in a covariant spherical harmonic analysis procedure. Results suggest that implementation of the above processing scheme leads to the mapping of the most robust magnetic anomalies of the lithosphere (vector components as well as

ISSN:0148-0227    

DOI:10.1029/95JB01237

年代:1995

数据来源: WILEY