共查询到20条相似文献，搜索用时 31 毫秒
1.
Peter Lonsdale 《Marine Geophysical Researches》1986,8(3):203-242
A Seabeam reconnaissance of the 400 km-long fast-slipping (88 mm yr-1) Heezen transform fault zone and the 55 km-long spreading center that links it to Tharp transform defined and bathymetrically described several types of ridges built by tectonic uplift and volcanic construction. Most prominent is an asymmetric transverse ridge, at which abyssal hills adjacent to the fault zone have been raised 2–3 km above normal rise-flank depths. Topographic and petrologic evidence suggests that this uplift, which has produced a 5400 m scarp from the crest of the ridge to the floor of a 10 km-wide transform valley, is caused by rapid serpentinization of upper mantle which has been exposed to hydrothermal circulation by fault-zone fracturing of an unusually thin crust. Transverse ridges have been thought atypical of fast-slipping transforms. One class of volcanic ridge more common at these sites is the overshot ridge, formed by prolongation of spreading-center rift zones obliquely across the transform. Overshot ridges are well developed at Heezen transform, especially at the eastern end where an eruptive rift zone extending 60 km from the southern tip of the East Pacific Rise has built a transform-parallel ridge that fills the eastern transform valley. Obliteration of fault-zone structure by ridges overshooting from the spreading center intersections means that the topography of the aseismic fracture zones is not just inherited from that of the active transform fault zone. The latter has several en echelon and overlapping fault traces, linked by short oblique spreading axes that generally form pull-apart basins rather than volcanic ridges. Interpretation of the origin and pattern of the fault zone's tectonic and volcanic relief requires refinement of the plate geography and history of this part of the Pacific-Antarctic boundary, using new Seabeam and magnetic traverses to supplement and adjust the existing geophysical data base. 相似文献
2.
Coupled seasonal variability in the South China Sea 总被引：2，自引：0，他引：2
The present study documents the relationship between seasonal variations in sea surface temperature (SST) and precipitation in the South China Sea (SCS) region. There are strong interactions between the atmosphere and ocean in the seasonal variations of SST and precipitation. During the transition to warm and cold seasons, the SST tendency is primarily contributed by net heat flux dominated by shortwave radiation and latent heat flux with a complementary contribution from ocean advection and upwelling. The contribution of wind-driven oceanic processes depends on the region and is more important in the northern SCS than in the southern SCS. During warm and cold seasons, local SST forcing contributes to regional precipitation by modulating the atmospheric stability and lower-level moisture convergence. The SST difference between the SCS and the western North Pacific influences the convection over the SCS through its modulation of the circulation pattern. 相似文献
3.
Martin R. Fisk Robert A. Duncan Christopher G. Fox Jeffrey B. Witter 《Marine Geophysical Researches》1993,15(4):283-296
The Mendocino Fracture Zone, a 3,000-km-long transform fault, extends from the San Andreas Fault at Cape Mendocino, California due west into the central Pacific basin. The shallow crest of this fracture zone, known as the Mendocino Ridge, rises to within 1,100 m of the sea surface at 270 km west of the California Coast. Rounded basalt pebbles and cobbles, indicative of a beach environment, are the dominant lithology at two locations on the crest of Mendocino Ridge and a40Ar/39 Ar incremental heating age of 11.0 ± 1.0 million years was determined for one of the these cobbles. This basalt must have been erupted on the Gorda Ridge because the crust immediately to the south of the fracture zone is older than 27 Ma. This age also implies that the crest of Mendocino Ridge was at sea level and would have blocked Pacific Ocean eastern boundary currents and affected the climate of the North American continent at some time since the late Miocene. Basalts from the Mendocino Fracture Zone (MFZ) are FeTi basalts similar to those commonly found at intersections of mid-ocean ridges and fracture zones. These basalts are chemically distinct from the nearby Gorda Ridge but they could have been derived from the same mantle source as the Gorda Ridge basalts. The location of the 11 Ma basalt suggests that Mendocino Ridge was transferred from the Gorda Plate to the Pacific Plate and the southern end of Gorda Ridge was truncated by a northward jump in the transform fault of MFZ. 相似文献
4.
Much of the relief of the abyssal hills covering the ocean basins is believed to originate from faulting of oceanic crust at mid-ocean ridges. The timescale over which faults grow is controversial, however, with some authors arguing that faults continue to grow in places for 0.5 m.y. or more based on increasing relief of fault scarps with distance from ridge axes. We examine Deep Tow profiler records of the Galapagos Spreading Centre, in which basement reflections allow scarp relief to be measured beneath the sediment cover, and find that relief does not increase but decreases systematically to 40 km off-axis (1.5 Ma seafloor). Since reversal of fault offsets is unlikely in this tectonic setting, we interpret this result as indicating that variations in fault statistics could reflect temporal variations in the tectonic or volcanic state of the ridge crest, not necessarily progressive fault growth with age as previously assumed. Resolving the issue of fault longevity will therefore require independent data on the timing of fault growth and distribution of present growth activity. We suggest some possible alternative indicators of fault longevity and discuss more generally the implications of volcanic flows to studies of faulting at ridges. 相似文献
5.
This study presents results of surveys conducted along the slow to ultra-slow spreading axis of the Northern North Fiji Basin (NNFB), including the Hazel Holmes, Tripartite and South Pandora Ridges, and the newly discovered Futuna and North Cikobia spreading centers. Spreading segments along these axes display highly contrasted axial morphologies, ranging from a rift valley to a prominent axial high. In some places, abrupt inversions of topography are observed between neighboring segments. Detailed analyses of bathymetry and backscatter maps reveal that axial highs are spotted with numerous coalescent volcanoes forming features ranging from irregular terrains to well-organized ridges. The volcanic edifices are distributed over a wide neovolcanic zone, which corresponds to the axial relief, suggesting on important contribution of volcanism to the relief construction. Comparisons between various ridge-shaped segments reveal that axial volcano-tectonic patterns are directly related to the local magma production and delivery, in a context of tectonic extension related to plate divergence, and suggest that coalescent volcanoes are fed from multiples short-lived and unconnected magma lenses. In the competition between horizontal and vertical accretion of oceanic crust, the spreading centers of the NNFB represent a special case where lava production is locally high enough and spreading rate is low enough to allow prominent axial highs to develop. The along axis morphologic variability is related to intermittent volcanic activity that may result from rapid temporal and spatial variations in the distribution of upper mantle convection cells below accretion centers, superimposed on the regional thermal anomaly located under the whole basin. 相似文献
6.
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
7.
Multiphase rifts tend to produce fault populations that evolve by the formation of new faults and reactivation of earlier faults. The resulting fault patterns tend to be complex and difficult to decipher. In this work we use seismic reflection data to examine the evolution of a normal fault network in the Oseberg Fault Block in the northern North Sea Rift System – a rift system that experienced Permian – Early Triassic and Middle Jurassic – Early Cretaceous rifting and exhibits N-S, NW-SE and NE-SW oriented faults.Both N-S- and NW-SE-striking faults were established during the Permian – Early Triassic rifting, as indicated by Triassic growth packages in their hanging walls. In contrast, the NE-SW-striking faults are younger, as they show no evidence of Permian – Early Triassic growth, and offset several N-S- and NW-SE-striking faults. Structural analysis show that a new population of NW-SE-striking faults formed in the Lower – Middle Jurassic (inter-rift period) together with reactivation of N-S-striking Permian – Early Triassic faults, indicating a NE-SW inter-rift extension direction.During the Middle Jurassic – Early Cretaceous rifting, faults of all orientations (N-S, NW-SE and NE-SW) were active. However, faults initiated during the Middle Jurassic – Early Cretaceous rifting show mainly N-S orientation, indicating E-W extension during this phase. These observations suggest a reorientation of the stress field from E-W during the Permian – Early Triassic rift phase to NE-SW during inter-rift fault growth and back to E-W during the Middle Jurassic – Early Cretaceous rift phase in the Oseberg area. Hence, the current study demonstrates that rift activity between established rift phases can locally develop faults with new orientations that add to the geometric and kinematic complexity of the final fault population. 相似文献
8.
The right-lateral Blanco Transform Fault Zone (BTFZ) offsets the Gorda and the Juan de Fuca Ridges along a 350 km long complex zone of ridges and right-stepping depressions. The overall geometry of the BTFZ is similar to several other oceanic transform fault zones located along the East Pacific Rise (e.g., Siquieros) and to divergent wrench faults on continents; i.e., long strike-slip master faults offset by extensional basins. These depressions have formed over the past 5 Ma as the result of continual reorientation of the BTFZ in response to changes in plate motion. The central depression (Cascadia Depression) is flanked by symmetrically distributed, inward-facing back-tilted fault blocks. It is probably a short seafloor spreading center that has been operating since about 5 Ma, when a southward propagating rift failed to kill the last remnant of a ridge segment. The Gorda Depression on the eastern end of the BTFZ may have initially formed as the result of a similar occurrence involving a northward propagating rift on the Gorda ridge system. Several of the smaller basins (East Blanco, Surveyor and Gorda) morphologically appear to be oceanic analogues of continental pull-apart basins. This would imply diffuse extension rather than the discrete neovolcanic zone associated with a typical seafloor spreading center. The basins along the western half of the BTFZ have probably formed within the last few hundred thousands years, possibly as the result of a minor change in the Juan de Fuca/Pacific relative motion. 相似文献
9.
Arthur J. Miller Fei Chai Sanae Chiba John R. Moisan Douglas J. Neilson 《Journal of Oceanography》2004,60(1):163-188
Decadal-scale climate variations in the Pacific Ocean wield a strong influence on the oceanic ecosystem. Two dominant patterns
of large-scale SST variability and one dominant pattern of large-scale thermocline variability can be explained as a forced
oceanic response to large-scale changes in the Aleutian Low. The physical mechanisms that generate this decadal variability
are still unclear, but stochastic atmospheric forcing of the ocean combined with atmospheric teleconnections from the tropics
to the midlatitudes and some weak ocean-atmosphere feedbacks processes are the most plausible explanation. These observed
physical variations organize the oceanic ecosystem response through large-scale basin-wide forcings that exert distinct local
influences through many different processes. The regional ecosystem impacts of these local processes are discussed for the
Tropical Pacific, the Central North Pacific, the Kuroshio-Oyashio Extension, the Bering Sea, the Gulf of Alaska, and the California
Current System regions in the context of the observed decadal climate variability. The physical ocean-atmosphere system and
the oceanic ecosystem interact through many different processes. These include physical forcing of the ecosystem by changes
in solar fluxes, ocean temperature, horizontal current advection, vertical mixing and upwelling, freshwater fluxes, and sea
ice. These also include oceanic ecosystem forcing of the climate by attenuation of solar energy by phytoplankton absorption
and atmospheric aerosol production by phytoplankton DMS fluxes. A more complete understanding of the complicated feedback
processes controlling decadal variability, ocean ecosystems, and biogeochemical cycling requires a concerted and organized
long-term observational and modeling effort.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
Cenozoic structures in the Bohai Bay basin province can be subdivided into eleven extensional systems and three strike-slip systems. The extensional systems consist of normal faults and transfer faults. The normal faults predominantly trend NNE and NE, and their attitudes vary in different tectonic settings. Paleogene rifting sub-basins were developed in the hanging walls of the normal faults that were most likely growth faults. Neogene–Quaternary sequences were deposited in both the rifting sub-basins and horsts to form a unified basin province. The extensional systems were overprinted by three NNE-trending, right-lateral strike-slip systems (fault zones). Although the principal displacement zones (PDZ) of the strike-slip fault zones are developed only in the basement and lower basin sequences in some cross sections, the structural deformation characteristics of the upper basin sequences also indicate that they are basement-involved, right-lateral strike-slip fault zones. According to the relationships between faults and sedimentary sequences, the extensional systems were mainly developed from the middle Paleocene to the late Oligocene, whereas the strike-slip systems were mainly developed from the Oligocene to the Miocene. Strike-slip deformation was intensified as extensional deformation was weakened. Extensional deformation was derived from horizontal tension induced by upwelling of hot mantle material, whereas strike-slip deformation was probably related to a regional stress field induced by plate movement. 相似文献
11.
冬季北太平洋西部上层海洋的热量输送 总被引：1，自引：2，他引：1
用海气界面净热量收支和1950－1979年表层水温资料，计算了冬季北太平洋西部上层海洋热通量散度场，指出冬季北太平洋西部黑潮将大量低纬暖水输送到中高纬度海域，在30－35°N最大；亲潮将极地冷水沿千岛群岛向南输送，在45－50°N最大；两者在40°N附近相遇，混合减弱后沿纬向东传。同时用EOF分析方法对热通量散度距平场分型，前3个主要型分别为：黑潮亲潮偶合型、北太平洋海流型和冷平流优势型。最后还揭示了第一主要型与北太平洋副热带高压之间有意义的相关关系。 相似文献
12.
Fault linkage patterns in rift basins are very common. While the process of fault linkage may be very short, it changes sedimentation patterns before and after linkage and controls the development of half-grabens. The propagation and growth of boundary faults can be divided into a simple fault propagation pattern and a fault growth-linkage pattern. Based on structural style, sedimentation patterns and oil-source correlations in the Erlian Basin, three types of grabens and petroleum systems can be identified. The Abei graben is controlled by a single boundary fault and forms an independent petroleum system. The Saihantala graben is controlled by several linked faults, which also has an independent petroleum system. The Wuliyasitai Depression controlled by two boundary fault segments which results in two petroleum systems. As not all half-grabens develop one petroleum system; they may, therefore, have two or more systems. The relay ramps between fault segments before linkage are the locations of deposition of sands and conglomerates, and consequently, are the focus areas for stratigraphic–lithologic trap exploration. 相似文献
13.
《Deep Sea Research Part I: Oceanographic Research Papers》2000,47(2):159-175
The seasonal cycle of phosphate in the world ocean is described using all historical data (over 170,000 profiles) held at the U.S. National Oceanographic Data Center and World Data Center-A for Oceanography. Generally, phosphate concentrations are depressed in the season of highest primary production, in conformance with phosphate's role as a major nutrient. Mean phosphate concentration in the North Pacific are twice those in the North Atlantic. The largest seasonal differences occur in the sub-polar North Atlantic and Pacific, where changes in concentration are as large as a factor of two. Temperate and equatorial regions exhibit less seasonal variability. High latitudes, upwelling areas, and river mouths exhibit a notable seasonal signal in phosphate. Enrichment of phosphate from the Amazon and Orinoco rivers appears to dominate the seasonal signal in the tropical Atlantic. In fact, the extent of the rivers’ effects extend so far north into the North Atlantic gyre that it obscures the normal pattern of summer depletion occurring elsewhere in the basin. The seasonal signal in the tropical Pacific Ocean is a function of seasonal variability in the winds, which affect the strength of coastal upwelling. 相似文献
14.
侏罗纪洋壳为现存最古老的海洋地壳, 残留在地球表面上很少, 目前对于侏罗纪洋壳的断裂特征和构造变形了解很少。本文利用高分辨率的反射地震剖面精细解释了位于西太平洋的侏罗纪洋壳基底、沉积地层和断裂结构, 发现在研究区存在基底断层、沉积断层和垮塌断层三种类型的断裂构造, 并对其走向、倾角、断距等几何参数与变形特征进行了推测和定量研究。研究还发现, 基底断层是洋壳受到板块伸展拉张而产生的, 在后期海底沉积过程中持续发育并错断上覆沉积物, 在海底形成明显的断层陡坎。沉积断层是沉积地层自身重力作用的产物,受到沉积地层岩石性质的控制。垮塌断层是岩浆侵出或者侵入形成海山, 导致洋壳及其上覆沉积局部抬升并向两侧推移, 引起先存的基底断层和沉积断层重新错动产生的。研究区内切断洋壳基底和上覆沉积的活动断层的推测走向大体符合侏罗纪洋壳基底面起伏、重力异常骤变界面以及地磁异常条带等的走向, 表明这些断裂从侏罗纪洋中脊的海底扩张中演变而来, 并且持续活动至今。这些发育在古老洋壳上的断层能够长时间让水进入岩石圈并进入俯冲带及地球内部, 从而促进地球水循环。尽管目前尚未发现这些断裂产生大地震, 但这些断层可能随着板块俯冲而演变成俯冲带地震大断裂, 今后研究应该关注这类断层在靠近海沟之前的演化规律和潜在地震风险。 相似文献
15.
Swath bathymetric, sonar imagery and seismic reflection data collected during the SOPACMAPS cruise Leg 3 over segments of the Vitiaz Trench Lineament and adjacent areas provide new insights on the geometry and the stuctural evolution of this seismically inactive lineament. The Vitiaz Trench Lineament, although largely unknown, is one of the most important tectonic feature in the SW Pacific because it separates the Cretaceous crust of the Pacific Plate to the north from the Cenozoic lithosphere of the North Fiji and Lau Basins to the south. The lineament is considered to be the convergent plate boundary between the Pacific and Australian Plates during midde to late Tertiary time when the Vitiaz Arc was a continuous east-facing are from the Tonga to the Solomon Islands before the development of the North Fiji and Lau Basins. Progressive reversal and cessation of subduction from west to east in the Late Miocene-Lower Plioene have been also proposed. However, precise structures and age of initiation and cessation of deformation along the Vitiaz Trench Lineament are unknown.The lineament consists of the Vitiaz Trench and three discontinuous and elongated troughs (Alexa, Rotuma and Horne Troughs) which connect the Vitiaz Trench to the northern end of the Tonga Trench. Our survey of the Alexa and Rotuma Troughs reveals that the lineament is composed of a series of WNW-ESE and ENE-WSW trending segments in front of large volcanic massifs belonging to the Melanesian Border Plateau, a WNW trending volcanic belt of seamounts and ridges on Pacific crust. The Plateau and Pacific plate lying immediately north of the lineament have been affected by intense normal faulting, collapse, and volcanism as evidenced by a series of tilted blocks, grabens, horsts and ridges trending N 120° to N100° and N60°–70°. This tectonism includes several normal faulting episodes, the latest being very recent and possibly still active. The trend of the fault scarps and volcanic ridges parallels the different segments of the Vitiaz Trench Lineament, suggesting that tectonics and volcanism are related to crustal motion along the lineament.Although the superficial observed features are mainly extensional, they are interpreted as the result of shortening along the Vitiaz Trench Lineament. The fabric north of the lineament would result from subduction-induced normal faulting on the outer wall of the trench and the zig-zag geometry of the Vitiaz Trench Lineament might be due to collision of large volcanic edifices of the Melanesian Border Plateau with the trench, provoking trench segmentation along left-lateral ENE-WSW trending transform zones. The newly acquired bathymetric and seismic data suggest that crustal motion (tectonism associated with volcanism) continued up to recent times along the Vitiaz Trench Lineament and was active during the development of the North Fiji Basin. 相似文献
16.
Kastens Kim Bonatti Enrico Caress David Carrara Gabriela Dauteuil Olivier Frueh-Green Gretchen Ligi Marco Tartarotti Paola 《Marine Geophysical Researches》1998,20(6):533-556
Transverse ridges are elongate reliefs running parallel and adjacent to transform/fracture zones offsetting mid-ocean ridges. A major transverse ridge runs adjacent to the Vema transform (Central Atlantic), that offsets the Mid-Atlantic Ridge by 320 km. Multibeam morphobathymetric coverage of the entire Vema Transverse ridge shows it is an elongated (300 km), narrow (<30 km at the base) relief that constitutes a topographic anomaly rising up to 4 km above the predicted thermal contraction level. Morphology and lithology suggest that the Vema Transverse ridge is an uplifted sliver of oceanic lithosphere. Topographic and lithological asymmetry indicate that the transverse ridge was formed by flexure of a lithospheric sliver, uncoupled on its northern side by the transform fault. The transverse ridge can be subdivided in segments bound by topographic discontinuities that are probably fault-controlled, suggesting some differential uplift and/or tilting of the different segments. Two of the segments are capped by shallow water carbonate platforms, that formed about 3–4 m.y. ago, at which time the crust of the transverse ridge was close to sea level. Sampling by submersible and dredging indicates that a relatively undisturbed section of oceanic lithosphere is exposed on the northern slope of the transverse ridge. Preliminary studies of mantle-derived ultramafic rocks from this section suggest temporal variations in mantle composition. An inactive fracture zone scarp (Lema fracture zone) was mapped south of the Vema Transverse ridge. Based on morphology, a fossil RTI was identified about 80 km west of the presently active RTI, suggesting that a ridge jump might have occurred about 2.2 m.a. Most probable causes for the formation of the Vema Transverse ridge are vertical motions of lithospheric slivers due to small changes in the direction of spreading of the plates bordering the Vema Fracture Zone. 相似文献
17.
Ken C. Macdonald Paul J. Fox Steve Miller Suzanne Carbotte Margo H. Edwards Mark Eisen Daniel J. Fornari Laura Perram Rob Pockalny Dan Scheirer Stacey Tighe Charles Weiland Doug Wilson 《Marine Geophysical Researches》1992,14(4):299-344
SeaMARC II and Sea Beam bathymetric data are combined to create a chart of the East Pacific Rise (EPR) from 8°N to 18°N reaching at least 1 Ma onto the rise flanks in most places. Based on these data as well as SeaMARC II side scan sonar mosaics we offer the following observations and conclusions. The EPR is segmented by ridge axis discontinuities such that the average segment lengths in the area are 360 km for first-order segments, 140 km for second-order segments, 52 km for third-order segments, and 13 km for fourth-order segments. All three first-order discontinuities are transform faults. Where the rise axis is a bathymetric high, second-order discontinuities are overlapping spreading centers (OSCs), usually with a distinctive 3:1 overlap to offset ratio. The off-axis discordant zones created by the OSCs are V-shaped in plan view indicating along axis migration at rates of 40–100 mm yr–1. The discordant zones consist of discrete abandoned ridge tips and overlap basins within a broad wake of anomalously deep bathymetry and high crustal magnetization. The discordant zones indicate that OSCs have commenced at different times and have migrated in different directions. This rules out any linkage between OSCs and a hot spot reference frame. The spacing of abandoned ridges indicates a recurrence interval for ridge abandonment of 20,000–200,000 yrs for OSCs with an average interval of approximately 100,000 yrs. Where the rise axis is a bathymetric low, the only second-order discontinuity mapped is a right-stepping jog in the axial rift valley. The discordant zone consists of a V-shaped wake of elongated deeps and interlocking ridges, similar to the wakes of second-order discontinuities on slow-spreading ridges. At the second-order segment level, long segments tend to lengthen at the expense of neighboring shorter segments. This can be understood if segments can be approximated by cracks, because the propagation force at a crack tip is directly proportional to crack length.There has been a counter-clockwise change in the direction of spreading on the EPR between 8 and 18° N during the last 1 Ma. The cumulative change has been 3°–6°, producing opening across the Orozco and Siqueiros transform faults and closing across the Clipperton transform. The instantaneous present-day Cocos-Pacific pole is located at approximately 38.4° N, 109.5° W with an angular rotation rate of 2.10° m.y.–1 This change in spreading direction explains the predominance of right-stepping discontinuities of orders 2–4 along the Siqueiros-Clipperton and Orozco-Rivera segments, but does not explain other aspects of segmentation which are thought to be linked to patterns of melt supply to the ridge axis.There are 23 significant seamount chains in the mapped area and most are created very near the spreading axis. Nearly all of the seamount chains have trends which fall between the absolute and relative plate motion vectors. 相似文献
18.
Fracture and in-situ stress studies were conducted for unconventional prospect evaluation in the Silurian Qusaiba Shale, northern Saudi Arabia. Borehole image logs, oriented cores, seismic, and drilling observations were used in the studies. The fractures include natural fractures and induced fractures. The induced fractures were studied to assess the stress regime in terms of directions and magnitudes. The present day maximum horizontal in-situ stress trend varies from NNW-SSE to NNE-SSW, and shows a regional pattern dominated by Arabian plate tectonics. The relative magnitudes of the current day stresses are characteristic of an extensional to strike-slip regime. Natural fractures of microscopic (microfractures) to macroscopic (macrofractures) scales include extension fractures (joints/veins), and faults manifested as shear and hybrid (extensional-shear and compressional-shear) fractures. Joints clustering into zones are rare, unless when associated with fault zones. Over half of the faults (56%) show clustering into fault zones with their widths (thickness) varying by up to 5 orders of magnitude, and lengths and displacements varying by up to 4 orders of magnitudes respectively. The study identified five distinctive, regional, fracture sets: one gently dipping (bedding-parallel or at low angle to bedding) and up to four moderately to steeply dipping fracture sets: an easterly striking set is the oldest, followed by three younger major sets striking NNW-SSE, N-S, and NNE-SSW. The younger fractures are nearly parallel to the present day maximum horizontal in-situ stress. Crack-seal mechanism (natural hydrofracturing) dominates initial fracture growth, some with several phases of partial to complete mineralization or coating, dominated by calcite, quartz, and dolomite. Aqueous and hydrocarbon gaseous and fluid inclusions are common in the fractures' mineral filling. The regional nature of in-situ stresses and natural fractures means their occurrence, orientation, relative dominance, and relative age and relative apertures are easier to predict and manipulate for well planning and completion, including hydrofracturing. Forward modeling shows that natural fracture network are not critically stressed under reservoir conditions but when subjected to massive hydrofracture stimulation they and the bedding discontinuities form the seeds for the growth of a complex hydrofracture network that potentially grows out of presumed stress-barriers. Lack of stress rotation around faults in wells supports the modeling results. Microseismic monitoring gives time-lapse (incremental) microseismic events of two types; random and linear patterns parallel to maximum horizontal in-situ stress and the predominant natural fracture trend. Bulk microseismic cloud has no unique link to fault trends mapped from high resolution borehole images. This finding challenges the usability of uncalibrated microseimic monitoring of massive hydrofracturing to map faults. 相似文献
19.
Specific features of variations in the bottom topography of the mid-ocean rift zones with intermediate spreading rates are considered in this work. The rift zones with a transition morphology are analyzed, and the main features of the transition topography are distinguished. Several successive stages of topographic variations, each of which is characterized by a specific relative position of the topographic features of the rift zone cross section, have been distinguished based on an analysis of the character of rift zone topographic variations in going from axial rises to rift valleys. The specific features of variations in the structural segmentation of rift zones with intermediate spreading rates, depending on the morphological changes, have been established. The thermal models of the structure of the rift zone magmatic formations have been considered, and the geodynamic relations of the magmatic systems with the specific features of the morphology and the structural segmentation of the mid-ocean axial zones with the intermediate spreading rates have been discussed. 相似文献