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91.
The Bremen ocean bottom tiltmeter is a new 6000 m-depth deep sea instrument for autonomous observation of sea floor tilt with
signal periods longer than 7.5 s. The instrument also records vertical acceleration in the frequency range from DC to 1 Hz.
The tiltmeter has an Applied Geomechanics Inc. 756 wide angle biaxial bubble tilt sensor with a resolution of 1.0μ rad (0.2
arc second). A Kistler Corp. MEMS accelerometer of type Servo K-Beam 8330A2.5 with about 10−5m/s2 resolution is used for the acceleration measurements. An Oceanographic Embedded Systems AD24 24 bit Sigma-Delta converter,
which is controlled by a low-power Persistor Inc. embedded computer system of type CF 2, samples the data. The duration of
tiltmeter operation is more than one year, which is controlled by the battery life. In our design the tiltmeter does not need
active leveling devices, i.e., servo motors or other moving components to adjust sensors or frame. We designed the instrument
for deployments by means of a remote operated vehicle. Since May 2005 the Bremen ocean bottom tiltmeter has recorded sea floor
deformation and seismicity level in the Logatchev hydrothermal vent field, Mid-Atlantic Ridge. The tiltmeter is a part of
the monitoring system of project ‘Logatchev Long-Term Environmental Monitoring,’ called LOLEM, of the German research program
with the name ‘Schwerpunktprogramm 1144: Vom Mantel zum Ozean.’ 相似文献
92.
Norbert J. Schulz Robert S. Detrick Stephen P. Miller 《Marine Geophysical Researches》1988,10(1-2):41-57
Magnetic data collected in conjunction with a Sea Beam bathymetric survey of the Mid-Atlantic Ridge south of the Kane Fracture
Zone are used to constrain the spreading history of this area over the past 3 Ma. Two-dimensional forward modeling and inversion
techniques are carried out, as well as a full three-dimensional inversion of the anomaly field along a 90-km-long section
of the rift valley. Our results indicate that this portion of the Mid-Atlantic Ridge, known as the MARK area, consists of
two distinct spreading cells separated by a small, zero-offset transform or discordant zone near 23°10′ N, The youngest crust
in the median valley is characterized by a series of distinct magnetization highs which coalesce to form two NNE-trending
bands of high magnetization, one on the northern ridge segment which coincides with a large constructional volcanic ridge,
and one along the southern ridge segment that is associated with a string of small axial volcanos. These two magnetization
highs overlap between 23° N and 23°10° N forming a non-transform offset that may be a slow spreading ridge analogue of the
small ridge axis discontinuities found on the East Pacific Rise. The crustal magnetizations in this overlap zone are generally
low, although an anomalous, ESE-trending magnetization high of unknown origin is also present in this area. The present-day
segmentation of spreading in the MARK area was inherited from an earlier ridge-transform-ridge geometry through a series of
small (∼ 10 km) eastward ridge jumps. These small ridge jumps were caused by a relocation of the neovolcanic zone within the
median valley and have resulted in an overall pattern of asymmetric spreading with faster rates to the west (14 mm yr−1) than to the east (11 mm yr−1). Although the detailed magnetic survey described in this paper extends out to only 3 Ma old crust, a regional compilation
of magnetic data from this area by Schoutenet al. (1985) indicates that the relative positions and dimensions of the spreading cells, and the pattern of asymmetric spreading
seen in the MARK area during the past 3 Ma, have characterized this part of the Mid-Atlantic Ridge for at least the past 36
Ma. 相似文献
93.
印度洋无震海岭及海底高原的初步研究 总被引:1,自引:0,他引:1
根据1986年10月-1987年5月第三次南极考察和首次环球科学考察所获的印度洋实测重力资料,对印度洋一些典型构造进行了分析研究。初步主人为:无震海岭,海底高原和大洋中脊都有着复杂的壳-幔结构,其上都伴有一个布格异常的低值带,但引种布格局异常低值原因却不尽相同。虽然上述3者都是大洋中的隆起地带,但前两者的地壳增厚,莫氏丰下拱,软流圈变深,影响布格异常的主要因素是其下存在着一个较大的负山根。相反,在 相似文献
94.
95.
222Rn was measured in the near-bottom waters of the continental slope of the Mid-Atlantic Bight. Separate measurements of the 222Rn supported by dissolved 226Ra allowed the excess 222Rn that is derived from the underlying sediments to be distinguished. Measurements of production of 222Rn by the sediments were used to calculate fluxes of 222Rn from sediments that would be expected as a result of molecular diffusion. On the upper slope and on the lower slope excess 222Rn standing crops were, respectively, greater than and consistent with fluxes of radon from sediments by molecular diffusion as are typical of most ocean environments. On the middle slope, however, observed excess 222Rn concentrations and standing crops were significantly lower than what would be expected from the calculated fluxes from the underlying sediments. This unusual feature of low radon concentrations on the middle slope is referred to as the low-radon zone (LRZ). This LRZ was always present over several years and seasons, but was variable in intensity (excess-radon concentration and standing crop) and in location on the slope. Low concentrations of suspended particulate matter and low current velocities observed by others in the same region are consistent with low mixing as a possible cause of the LRZ. Radon profile shapes and recent work by others on near bottom mixing due to interactions between topography and internal waves, however, suggest that high mixing due to internal waves is a more likely cause of the LRZ. 相似文献
96.
Submersible observations and photogeology document dramatic variations in the distribution of young volcanic rocks, faulting,
fissuring, and hydrothermal activity along an 80 km-long segment of the Mid-Atlantic Ridge south of the Kane Transform (MARK
Area). These variations define two spreading cells separated by a cell boundary zone or a small-offset transform zone. The
northern spreading cell is characterized by a median ‘neovolcanic’ ridge which runs down the axis of the median valley floor
for 40 km. This edifice is as much as 4 km wide and 600 m high and is composed of very lightly sedimented basalts inferred
to be < 5000 years old. It is the largest single volcanic constructional feature discovered to date on the Mid-Atlantic Ridge.
The active Snake Pit hydrothermal vent field is on the crest of this ridge and implies the presence of a magma chamber in
the northern spreading cell. In contrast, the southern cell is characterized by small, individual volcanos similar in size
to the central volcanos in the FAMOUS area. Two of the volcanos that were sampled appear to be composed of dominantly glassy
basaltic rocks with very light sediment cover; whereas, other volcanos in this region appear to be older features. The boundary
zone between the two spreading cells is intensely faulted and lacks young volcanic rocks. This area may also contain a small-offset
( < 8 km) transform zone. Magmatism in the northern cell has been episodic and tens of thousands of years have lapsed since
the last major magmatic event there. In the southern cell, a more continuous style of volcanic accretion appears to be operative.
The style of spreading in the southern cell may be much more typical for the Mid-Atlantic Ridge than that of the northern
cell because the latter is adjacent to the 150 km-offset Kane Transform that may act as a thermal sink along the MAR. Such
large transforms are not common on the MAR, therefore, lithosphere produced in a spreading cell influenced by a large transform
may also be somewhat atypical. 相似文献
97.
Jean-Christophe Sempéré Jian Lin Holly S. Brown Hans Schouten G. M. Purdy 《Marine Geophysical Researches》1993,15(3):153-200
Analysis of Sea Beam bathymetry along the Mid-Atlantic Ridge between 24°00 N and 30°40 N reveals the nature and scale of the segmentation of this slow-spreading center. Except for the Atlantis Transform, there are no transform offsets along this 800-km-long portion of the plate boundary. Instead, the Mid-Atlantic Ridge is offset at intervals of 10–100 km by nontransform discontinuities, usually located at local depth maxima along the rift valley. At these discontinuities, the horizontal shear between offset ridge segments is not accommodated by a narrow, sustained transform-zone. Non-transform discontinuities along the MAR can be classified according to their morphology, which is partly controlled by the distance between the offset neovolcanic zones, and their spatial and temporal stability. Some of the non-transform discontinuities are associated with off-axis basins which integrate spatially to form discordant zones on the flanks of the spreading center. These basins may be the fossil equivalents of the terminal lows which flank the neovolcanic zone at the ends of each segment. The off-axis traces, which do not lie along small circles about the pole of opening of the two plates, reflect the migration of the discontinuities along the spreading center.The spectrum of rift valley morphologies ranges from a narrow, deep, hourglass-shaped valley to a wide valley bounded by low-relief rift mountains. A simple classification of segment morphology involves two types of segments. Long and narrow segments are found preferentially on top of the long-wavelength, along-axis bathymetric high between the Kane and Atlantis Transforms. These segments are associated with circular mantle Bouguer anomalies which are consistent with focused mantle upwelling beneath the segment mid-points. Wide, U-shaped segments in cross-section are preferentially found in the deep part of the long-wavelength, along-axis depth profile. These segments do not appear to be associated with circular mantle Bouguer anomalies, indicating perhaps a more complex pattern of mantle upwelling and/or crustal structure. Thus, the long-recognized bimodal distribution of segment morphology may be associated with different patterns of mantle upwelling and/or crustal structure. We propose that the range of observed, first-order variations in segment morphology reflects differences in the flow pattern, volume and temporal continuity of magmatic upwelling at the segment scale. However, despite large first-order differences, all segments display similar intra-segment, morphotectonic variations. We postulate that the intra-segment variability represents differences in the relative importance of volcanism and tectonism along strike away from a zone of enhanced magma upwelling within each segment. The contribution of volcanism to the morphology will be more important near the shallowest portion of the rift valley within each segment, beneath which we postulate that upwelling of magma is enhanced, than beneath the ends of the segment. Conversely, the contribution of tectonic extension to the morphology will become more important toward the spreading center discontinuities. Variations in magmatic budget along the strike of a segment will result in along-axis variations in crustal structure. Segment mid-points may coincide with regions of highest melt production and thick crust, and non-transform discontinuities with regions of lowest melt production and thin crust. This hypothesis is consistent with available seismic and gravity data.The rift valley of the Mid-Atlantic Ridge is in general an asymmetric feature. Near segment mid-points, the rift valley is usually symmetric but, away from the segment mid-points, one side of the rift valley often consists of a steep, faulted slope while the other side forms a more gradual ramp. These observations suggest that half-grabens, rather than full-grabens, are the fundamental building blocks of the rift valley. They also indicate that the pattern of faulting varies along strike at the segment scale, and may be a consequence of the three-dimensional, thermo-mechanical structure of segments associated with enhanced mantle upwelling beneath their mid-points. 相似文献
98.
Laura S. L. Kong Robert S. Detrick Paul J. Fox Larry A. Mayer W. B. F. Ryan 《Marine Geophysical Researches》1988,10(1-2):59-90
High-resolution Sea Beam bathymetry and Sea MARC I side scan sonar data have been obtained in the MARK area, a 100-km-long
portion of the Mid-Atlantic Ridge rift valley south of the Kane Fracture Zone. These data reveal a surprisingly complex rift
valley structure that is composed of two distinct spreading cells which overlap to create a small, zero-offset transform or
discordant zone. The northern spreading cell consists of a magmatically robust, active ridge segment 40–50 km in length that
extends from the eastern Kane ridge-transform intersection south to about 23°12′ N. The rift valley in this area is dominated
by a large constructional volcanic ridge that creates 200–500 m of relief and is associated with high-temperature hydrothermal
activity. The southern spreading cell is characterized by a NNE-trending band of small (50–200 m high), conical volcanos that
are built upon relatively old, fissured and sediment-covered lavas, and which in some cases are themselves fissured and faulted.
This cell appears to be in a predominantly extensional phase with only small, isolated eruptions. These two spreading cells
overlap in an anomalous zone between 23°05′ N and 23°17′ N that lacks a well-developed rift valley or neovolcanic zone, and
may represent a slow-spreading ridge analogue to the overlapping spreading centers found at the East Pacific Rise. Despite
the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10–17 km wide rift valley floor.
Block faulting along near-vertical, small-offset normal faults, accompanied by minor amounts of back-tilting (generally less
than 5°), begins within a few km of the ridge axis and is largely completed by the time the crust is transported up into the
rift valley walls. Features that appear to be constructional volcanic ridges formed in the median valley are preserved largely
intact in the rift mountains. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor
features, are the major geological processes occurring outside of the rift valley. The morphological and structural heterogeneity
within the MARK rift valley and in the flanking rift mountains documented in this study are largely the product of two spreading
cells that evolve independently to the interplay between extensional tectonism and episodic variations in magma production
rates. 相似文献
99.
Digital filters designed using wavelet theory are applied to high resolution deep-towed side-scan sonar data from the median valley walls, crestal mountains, and flanks of the Mid-Atlantic Ridge at 29°10 N. With proper tuning, the digital filters are able to identify the location, orientation, length, and width of highly reflective linear features in sonar images. These features are presumed to represent the acoustic backscatter from axis-facing normal faults. The fault locations obtained from the digital filters are well correlated with visual geologic interpretation of the images. The side-scan sonar images are also compared with swath bathymetry from the same area. The digitally filtered bathymetry images contain nine of the eleven faults identified by eye in the detailed geologic interpretation of the side-scan data. Faults with widths (measured perpendicular to their strike) of less than about 150 m are missed in the bathymetry analysis due to the coarser resolution of these data. This digital image processing technique demonstrates the potential of wavelet-based analysis to reduce subjectivity and labor involved in mapping and analyzing topographic features in side-scan sonar and bathymetric image data. 相似文献
100.
Based on a new quantitative analysis of sidescan sonar data combined with coring, we propose a revised model for the origin
for Mediterranean Ridge mud volcanism. Image analysis techniques are used to produce a synthetic and objective map of recent
mud flows covering a 640 × 700 km2 area, which represents more than half of the entire Mediterranean Ridge mud belt. We identify 215 mud flows, extruded during
the last 37,000–60,000 years. This time period corresponds to the limit of penetration of the sonar, that we evaluate through
geoacoustic modeling of the backscattered signal returned by the mud breccia-hemipelagites contact, and calibrate by coring.
We show that during this period, at least 96% of the mud volume has been extruded at the Mediterranean Ridge-Hellenic backstop
contact, the remaining being scattered over the prism. We suggest that the source is a Messinian (5–6 Ma) mud reservoir that
remained close to the backstop contact, at variance with the classical transport-through-the-wedge model. A revised mud budget
indicates that steady-state input is not needed. We propose that the source layer was deposited in deep and narrow pre-Messinian
basins, sealed by Messinian evaporites, and finally inverted in post-Messinian times. Onset of motion of the Anatolia-Aegea
microplate in the Pliocene resulted in change from slow to fast convergence, triggering shear partitioning at the edges of
the backstop and basin inversion. Mud volcanism initiation is probably coeval with the latest events of this kinematic re-organization,
i.e. opening of the Corinth Gulf and activation of the Kephalonia fault around 1–2 Ma. 相似文献