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1.
A geophysical study was conducted over the Mid-Atlantic Ridge between 32–39°N and 30–40°W. A particularly deep fracture was observed which offset the ridge crest 110 km in the vicinity of 33°N. A pole of relative motion between the North American and African plates was deduced from this fracture zone as being at 63.1°N, 17°W.  相似文献   

2.
A detailed submersible investigation of a 20-km segment of the East Pacific Rise near 12°50′N between the Orozco and Clipperton fracture zones has resulted in the localization of 24 active hydrothermal vent fields and over 80 sites of sulfide accumulations. The active vents range from low-temperature vents characterized by exotic benthic communities to high-temperature “black smokers” and the deposition of polymetallic sulfides. The study is based upon a combination of fine scale topography obtained using the SEABEAM sonar system on N/O “Jean Charcot”, camera lowerings along the axis using the RAIE vehicle, and 32 dives by the submersible “Cyana” operating from N/O “Le Suroit”. The observations made between the Orozco and Clipperton fracture zones show topographic highs situated along the strike of the accreting plate segment separated by a small ridge offset at 11°49′N. This offset divides this portion of the ridge into two separate spreading segments each of which has a primary topographic high along strike. Secondary highs are associated with each segment of the ridge separated by either small offsets (or relay zones) or in some cases, zones where spreading centers overlap. Dives made on the tops of both primary highs (12°50′N and 11°30′N) confirm the presence inferred from previous surface work of high-temperature vent fields while one reconnaissance dive (14°20′N) near the Orozco fracture zone/ridge axis intersection reveals the absence of any hydrothermal activity in the present or recent past. The vast majority of vent fields investigated were found at the topographic high near 12°50′N, are associated with the most recent period of volcanism, and are confined to lava ponds situated within the axial graben.  相似文献   

3.
The Kane fracture zone has been traced as a distinct topographic trough from the Mid-Atlantic Ridge near 24°N to the 80-m.y. B.P. isochron (magnetic anomaly 34) on either side of the ridge axis for a total of approximately 2800 km. Major changes in trend of the fracture zone occur at approximately 72 m.y. B.P. (anomaly 31 time) and approximately 53–63 m.y. B.P. (anomaly 21–25 time) which are the result of major reorientations in spreading directions in the central Atlantic Ocean.  相似文献   

4.
西南印度洋岩浆补给特征研究:来自洋壳厚度的证据   总被引：1，自引：0，他引：1       下载免费PDF全文
西南印度洋中脊为典型的超慢速扩张洋中脊,其岩浆补给具有不均匀分布的特征.洋壳厚度是洋中脊和热点岩浆补给的综合反映,因此反演洋壳厚度是研究大尺度洋中脊和洋盆岩浆补给过程的一种有效方法.本文通过对全球公开的自由空气重力异常、水深、沉积物厚度和洋壳年龄数据处理得到剩余地幔布格重力异常,并反演西南印度洋地区洋壳厚度,定量地分析了西南印度洋的洋壳厚度分布及其岩浆补给特征.研究发现,西南印度洋洋壳平均厚度7.5 km,但变化较大,标准差可达3.5 km,洋壳厚度的频率分布具有双峰式的混合偏态分布特征.通过分离双峰统计的结果,将西南印度洋洋壳厚度分为0~4.8 km的薄洋壳、4.8~9.8 km的正常洋壳和9.8~24 km的厚洋壳三种类型,洋中脊地区按洋壳厚度变化特征可划分为7个洋脊段.西南印度洋地区薄洋壳受转换断层控制明显,转换断层位移量越大,引起的洋壳减薄厚度越大,减薄范围与转换断层位移量不存在明显相关性.厚洋壳主要受控于该区众多的热点活动,其中布维热点、马里昂热点和克洛泽热点的影响范围分别约340 km,550 km和900 km.Andrew Bain转换断层北部外角形成厚的洋壳,具有与快速扩张洋中脊相似的转换断层厚洋壳特征.  相似文献   

5.
Movement between the Africa and Antarctica plates is at present accomplished by sea-floor spreading on the Southwest Indian Ocean Ridge. This movement may be described in terms of an angular rotation vector. Bathymetric and magnetic observations from marine geophysical surveys near the Bouvet triple junction, at 52°S, 15°E and in the environs of the Prince Edward Islands are combined with spreading azimuths derived from earthquake fault plane solutions to define this vector. The rotation pole which describes the motion is located at 10.7°N, 40.9°W and the angular velocity is 1.44 × 10?7 deg/yr. This pole is significantly different from some other poles obtained by global closure or vector addition. The possibility that the differences may be due to Africa being split into two plates is investigated but there would have to be convergence across the African Rift system for this possibility to be true. Closure of the vector velocity triangle around the Central Indian triple junction is checked by using the pole derived in this study and published poles and rates for the Africa/India and Antarctica/India motions to determine this triangle. The triangle is found to close when errors in the Africa/India and Antarctica/India motions are taken into account. This suggests that it is errors in the data that cause the differences between the observed and predicted poles.  相似文献   

6.
Data gathered by recent “Islas Orcadas” cruises reveal the seafloor spreading pattern for a region south of the Agulhas/Falkland fracture zone system. The presence of a magnetic anomaly bight about the Agulhas Plateau indicates that the Agulhas Plateau may have developed at the site of a tectonic plate triple junction during the Late Cretaceous. A westward jump in the seafloor spreading center during the Late Maestrichtian (anomaly 34?31) reduced the offset across the Falkland/Agulhas fracture zone system and resulted in the formation of two conjugate aseismic ridges here described as the Meteor and Islas Orcadas Rises. The magnetic lineation pattern in the Agulhas Basin suggests that a tectonic plate (Malvinas Plate) existed during Campanian to Maestrichtian times. Relative rates of motion are calculated for Antarctica, South America, and Africa for the Late Cretaceous.  相似文献   

7.
A longitudinal seismic reflection profile along the east flank of Reykjanes Ridge, from Charlie fracture zone to the vicinity of Iceland, has important implications both for bottom water movement and for hypotheses of crustal generation at the axis of the mid-oceanic ridge. In this paper bottom water movement is considered. Between 52°N and 57°N Reykjanes Ridge is cut by about 12 fractures whose trend, inferred from other data, is approximately east-west. North of 57° there is little or no indication of east-west fracturing. Fracture valley bottoms are typically 1 km below the surrounding basement level; sediment fills are about 0.5 km; present bottoms are 2.1 to 2.8 km below sea level. Depositional asymmetry is apparent in 9 cases, 7 of which have the deepest and generally least reflective bottom at the northern edge. This suggests predominately west-flowing bottom currents, carrying Norwegian Sea overflow water through the fracture valleys, a result consistent with previously published data.  相似文献   

8.
An asymmetric pattern is observed in the orientation of minor fracture zones about the axis of the Mid-Atlantic Ridge at five sites where relatively detailed studies have been made between latitudes 22°N and 51°N. The minor fracture zones intersect the axis of the Mid-Atlantic Ridge in an asymmetric V-shaped configuration. The V's point south north of the Azores triple junction (38°N latitude) and point north south of that junction.The rates and directions of sea-floor spreading are related to the asymmetric pattern of minor fracture zones at the sites studied. Half-rates of sea-floor spreading averaged between about 0 and 10 m.y. are unequal measured perpendicular to the ridge axis. The unequal half-rates of spreading are faster to the west north of the Azores triple junction and faster to the east south of that junction. The half-rates of sea-floor spreading calculated in the directions of the asymmetric minor fracture zones are equal about the ridge axis within the uncertainty of the direction determinations.A discrepancy exists between minor fracture zones that form an asymmetric V about the axis of the Mid-Atlantic Ridge, and major fracture zones that follow small circles symmetric about the ridge axis. To reconcile this discrepancy it is proposed that minor fracture zones are preferentially reoriented under the influence of a stress field related to interplate and intraplate motions. Major fracture zones remain symmetric about the Mid-Atlantic Ridge under the same stress field due to differential stability between minor and major structures in oceanic lithosphere. This interpretation is supported by the systematic variation in the orientation of minor fracture zones and the equality of sea-floor spreading half-rates observed about lithospheric plate boundaries.  相似文献   

9.
The major tectonic elements of the Azores triple junction have been mapped using long-range side-scan sonar. The data enable the Mid-Atlantic Ridge axis to be located with a precision of a few kilometres. Major faults and other tectonic and volcanic elements of the ridge maintain their regional trend of 010° to 020° past the triple junction area. There is no oblique spreading, and only minor transform offsets of the Mid-Atlantic Ridge occur here. The main effect of the triple junction or Azores hot spot is to diminish the amplitude of the median valley to 200 m or less. There is no axial high: a topographic high seen on several profiles is located to the east of the Mid-Atlantic Ridge spreading axis and does not appear to have any fundamental significance.The third arm of the triple junction includes the Azores srreading centre which appears to have developed as a series of en echelon rifted basins (the Terceira Rift) extending from Formigas Trough at 36.8°N, 24.5°W to a point near 39.3°N, 28.8°W. There are indications that recent activity in the spreading centre may be concentrated in a series of ridges which flank the older rifted basins. Until recently the northwest end of the Terceira Rift was connected to the Mid-Atlantic Ridge axis either directly at an RRR junction, or via a transform fault. The triple junction has probably moved south during the last 6 Ma to a positin on the Mid-Atlantic Ridge near 38.7°N.Initiation of the Azores spreading centre may have occurred during the 36 Ma B.P. rearrangement of poles, with an RFF triple junction north from the East Azores fracture zone to the North Azores fracture zone and transferring a wedge of European plate to the African plate.The tectonic elements revealed by this study are in good agreement with inferred earthquake mechanisms and with the RM2 plate tectonic model of Minster and Jordan, but east-west motion between North America and Africa does not seem to be compatible with the other motions at the triple junction unless it is of very recent (2>3 Ma) origin.  相似文献   

10.
Between 67 and ～40 Ma ago a northwest-southeast-trending fracture system over 8000 km long split the Pacific plate and accumulated at least 1700 km of dextral offset between the east and west portions. This system, here named the Emperor fracture zone (EFZ) system, consisted of several segments, one along the present trace of the Emperor trough and another along the Line Islands, joined by short spreading ridges. The EFZ terminated at its northern end against the Kula-Pacific ridge, and at its southern end in a ridge-transform system, called the Emperor spreading system, which extended to the west, north of Australia.The finite angular velocity vector describing the relative motion between the East and West Pacific plates is ～0.6°/Ma about a pole at 36°N, 70°W. This vector, added to the known Early Tertiary motion of the Pacific plate with respect to the global hotspot reference frame, accounts in large part for the NNW trend of the Emperor seamount chain relative to the WNW Hawaiian trend, without violation of the integrity of the Antarctic plate. The Meiji-Emperor and Emperor-Hawaiian bends date, respectively, the initiation (～67 Ma ago) and cessation (～40 Ma ago) of seafloor spreading on the Emperor spreading system.The postulated Early Tertiary relative motion along the EFZ between the East and West Pacific plates explains (1) the present misalignment of the two sets of magnetic bights of the Pacific, (2) the abrupt truncation of eastern Pacific bathymetric lineaments against the Emperor trough and Line Islands, (3) the contrast in paleolatitude between the eastern and western Pacific as indicated by paleomagnetic and sedimentologic studies, and (4) the anomalous gravity signature of the central Hawaiian ridge that indicates that the ridge loaded thin hot lithosphere.  相似文献   

11.
The palaeomagnetism of Middle Triassic (224 ± 5 m.y.) igneous rocks from the Ischigualasto-Ischichuca Basin (67°40′W, 30°20′S) was investigated through 86 oriented hand samples from 11 sites. At least one reversal of the geomagnetic field has been found in these rocks. Nine sites yield a palaeomagnetic pole at 239°E, 79°S (α95 = 15°, k = 13).The K-Ar age determinations of five igneous units of the Puesto Viejo Formation give a mean age of 232 ± 4 m.y. (Early Triassic). The palaeomagnetism of six igneous units of the Puesto Viejo Formation (68°W, 35°S) was investigated through 60 oriented samples. These units, two reversed relative to the present magnetic field of the Earth and four normal, yield a pole at 236°E, 76°S (α95 = 18°, k = 14).Data from the Puesto Viejo Formation indicate, for the first time on the basis of palaeomagnetic and radiometric data, that the Illawarra Zone, which defines the end of the Kiaman Magnetic Interval, extends at least down to 232 ± 4 m.y. within the Early Triassic. The palaeomagnetic poles for the igneous rocks of the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation form an “age group” with the South American Triassic palaeomagnetic poles (mean pole position: 239°E, 77°S; α95 = 6.6°, k = 190). The Middle and Upper Permian, Triassic and Middle Jurassic palaeomagnetic poles for South America would define a “time group” reflecting a quasi-static interval (mean pole position: 232°E, 81°S; α95 = 4°, k = 131).  相似文献   

12.
A seafloor electrical conductivity profile resulting from a more thorough analysis of magnetotelluric data from station S.F. Revisited than previously presented is compared to an earlier profile at Farewell to Aggy, station III. Both stations are located over the same interfracture zone segment of the Pacific plate, the first roughly 700 km off the coast of California (position 31°18′N, 128°20′W, water depth 4.5 km, plate age 30 m.y. estimated from nearby magnetic reversal number 12), the second approximately 800 km to the NNE of the mainland of Hawaii (position 26°32′N, 151°20′W, depth 5.3 km, age 72 m.y. estimated from adjacent magnetic reversal 30–31).The seafloor impedances at S.F. Revisited are only mildly polarized and their interpretation in terms of an isotropic, horizontally layered structure suggests the occurrence at about 85 km depth of a highly conducting layer with a conductance exceeding by roughly 4 × 103 S, an otherwise monotonically increasing conductivity trend. The implied dependence of conductivity with depth is therefore similar to that found earlier for station III, however with the following differences: the high-conductivity layer at station III occurs at a greater depth (140 km), it appears to have a slightly reduced excess conductance over the background, 3.5 × 103 S although this evidence should be used with caution, and the lithospheric conductivity at station III, surprisingly seems to be somewhat higher, an effect possibly related to the proximity of the Hawaiian chain and to its generic processes.  相似文献   

13.
Rare earths (RE) in basalts erupted within the rift of the Mid-Atlantic Ridge show a progressive change from light-RE enriched to depleted patterns from the Azores Platform (40°N) down to 33°30′N. South, the pattern remains light-RE depleted as along other “normal ridge” segments. A progressive increase in chemical variability of the basalts towards the Azores is also noted.The latitudinal RE profile and corresponding ΣFeO/ΣFeO + MgO variations, together, indicate that the origin of these basalts cannot be accounted for simply by considering variable extents of partial melting of a single mantle source and subsequent fractional crystallization during the ascent of the magmas. These two processes produce only second-order effects on the RE patterns. The data requires the presence of a distinct, light-RE richer, mantle source beneath the Azores Platform relative to that of south of 33°30′N and an intermediate zone where both mantle types mix. The relative contribution of the Azores mantle source to the mix appears to decrease fairly regularly southward along the ridge and becomes negligible at 33°30′N. Increasing chemical variability of the basalts towards the Azores is probably caused by correspondingly larger extent of fractional crystallization at shallow depth, and/or greater variability in the extent of partial melting, apparently subsequent to, and superimposed on the mixing of the two mantle sources.The combined morphological, geophysical and RE evidence along the profile are consistent with a model suggesting upwelling of a major blob (plume) under the Azores Plateau; and reveal the present extent of the blob's overflow and mixing with the asthenosphere depleted in large ionic lithophile trace elements. The influence of the Azores blob is geochemically detectable up to 1000 km southwestward beneath the ridge axis.  相似文献   

14.
Double-couple point-source parameters for 11 of the largest intraplate earthquakes in the northern Indian Ocean during the last 20 y were determined from a formal inversion of long-period P and SH waveforms. Nine of the events have centroid depths at least 17 km below the seafloor, well into the upper mantle; two have centroid depths as great as 39 km. Using the source mechanisms of these earthquakes, we distinguish two major intraplate tectonic provinces in the northern Indian Ocean. To the west of the Ninetyeast Ridge, in the southern Bay of Bengal, intraplate earthquakes have thrust-faulting mechanisms with P axes oriented N-S. The centroid depths of these earthquakes range from 27 to 39 km below the seafloor. Lithospheric shortening in this region is thus accomplished by thrust faulting in the strong core of the oceanic upper mantle, while other geophysical evidence suggests that shallow sedimentary and crustal layers apparently deform predominantly by folding. In the immediate vicinity of the Ninetyeast Ridge, earthquakes display strike-slip mechanisms with left-lateral motion on planes parallel to the ridge. This type of faulting occurs from at least 10°S to the northern end of the Ninetyeast Ridge near 10°N, where the ridge meets the Sunda Arc. Seismic activity diminishes to the east of the Ninetyeast Ridge, but is also characterized by strike-slip faulting. Despite these variations in deformational style, the inferred orientation of greatest compressive stress in the northern Indian Ocean displays a consistent long-wavelength pattern over a large portion of the Indian plate, varying smoothly from nearly N-S in the Bay of Bengal to NW-SE in the northeastern Indian Ocean. This plate-wide stress pattern and the high level of intraplate seismicity in the northern Indian Ocean are likely the results of substantial resistance, along the Himalayan continental collision zone, to the continued northward motion of the western portion of the Indian plate. Oceanic intraplate earthquakes in other regions, where the level of deviatoric stress associated with the long-wavelength part of the stress field is likely to be smaller, need not be comparably reliable indicators of the plate-wide stress field.  相似文献   

15.
The ocean-continent boundary in the Newfoundland Basin is defined as the seaward limit of a continental margin magnetic smooth zone. East of the Grand Banks this boundary is marked by a prominent NNE-trending magnetic anomaly that is correlated with the J-Anomaly (115 m.y.). South of Flemish Cap the smooth zone boundary strikes approximately 060° and is approximately 15 m.y. younger. Magnetic anomaly trends suggest two directions of motion during separation of Iberia and North America. The first phase of motion, commencing at J-Anomaly time with a spreading center strike of 015°, produced a rifted margin along the Grand Banks south of the Newfoundland Seamounts. No spreading occurred north of the seamounts during this phase, implying a counter-clockwise rotation of Iberia and no Grand Banks-Galicia Bank separation. The second phase began at about 102 m.y. with a shift of the pole of rotation to a location near Paris, producing a ridge orientation of approximately 060°. This spreading center extended north and east into the northern Newfoundland Basin and Bay of Biscay, producing a rifted margin south of Flemish Cap and opening of Biscay. This ridge geometry produced a component of extension across the Newfoundland Fracture Zone and the southeastward migration of the resultant “leaky” transform fault between 102 m.y. and the next pole shift produced the volcanic edifice of the Southeast Newfoundland Ridge. Fracture zone trends during this phase also exerted strong control on volcanism within the Newfoundland Seamount province; this activity ceased at about 97 m.y. The date at which the second phase ended is not well defined by presently available data. A RRR triple-junction existed in the northeastern Newfoundland Basin-western Biscay region for a short time prior to anomaly33/34 (80 m.y.) which marks the inception of a continuous Mid-Atlantic Ridge spreading center between the Newfoundland and Charlie Gibbs Fracture Zones.  相似文献   

16.
Marine magnetic data collected along 37 east-west lines off the Washington and Oregon coast between 42°N and 48°N westward to 144°W have been interpreted. Our interpretation shows that decrease in the offsets of the anomalies across the Surveyor fracture zone and the presence of undisturbed north-south lineations east of it are the result of continuous asymmetric spreading along the Surveyor fracture zone. The survey has delineated the extension of the Blanco fracture zone north-westward to about 133°W. Movement along the Blanco fracture zone was initiated about 15 million years ago. The undisturbed north-south trend of the magnetic anomalies between latitudes 42°N and 48°N and longitudes 133°W and 136°W is interpreted as the interval (22 to 15 my) during which the Juan de Fuca and Gorda rises were one continuous structure. West of 137° the Surveyor, Sedna and three minor fracture zones are mapped.  相似文献   

17.
超慢速扩张西南印度洋中脊岩浆的集中供给在空间维度上表现为岩浆扩张段（NVR）与相邻的非转换断层不连续带（NTD）地壳结构的差异,而在时间维度上表现为离轴与沿轴地壳结构的差异.为了进一步揭示岩浆集中供给的时空分布特征,本文选取西南印度洋中脊热液区2010年海底地震仪深部探测中平行于洋中脊距轴部偏北约10 km的离轴测线d0d10,使用射线追踪正演和反演的方法,得到了NVR和NTD北侧离轴区域的地壳及上地幔P波速度结构,并与轴部速度结构进行了对比分析.研究结果表明：（1）NTD北侧离轴区域的地壳厚度约5.2 km,其厚度明显大于轴部NTD下方地壳厚度（~3.2 km）,由此推测洋脊轴部NTD区域形成的地壳在不断减薄;（2）NVR北侧离轴区域的地壳厚度约7.0 km,其厚度亦大于轴部NVR地壳厚度（~5.8 km）,表明在洋中脊演化过程中洋脊轴区域的岩浆供给在不断减少,其活动性在不断减弱.  相似文献   

18.
Study of focal mechanisms of earthquakes in the Near and Komandorsky Islands indicate that there are several distinct zones of tectonic activity. South of the Near Islands, normal faulting occurs in the trench east of 172°E and low-angle thrusting dominates the Aleutian ridge. Mechanisms indicate underthrusting as far west as Mednyy Island with strike-slip faulting restricted to the south and west of Beringa Island. A zone of northeast striking left-lateral faulting near 1645.°E is proposed to separate the Aleutian Ridge from Kamchatka Peninsula. This motion, as well as faulting north of the Komandorsky Islands, may be related to the existance of a buffer plate comprising the Aleutian Ridge in the Komandorsky Islands. Active subduction terminates near 173°E and the faulting north of the Komandorsky Islands may, in part, be due to the bouyancy of a remnant slab. Depth phase modelling indicates bulletin-reported depths are overestimated due to a misidentification of depth phases.  相似文献   

19.
The spatiotemporal distribution of Cretaceous–Paleogene granitic rocks in southwestern Japan is investigated to understand the origin of the granitic batholith belt and to reconstruct the tectonic setting of emplacement. New U–Pb zircon ages for 92 samples collected from a region measuring 50 km (E–W) by 200 km (N–S) reveals a stepwise northward younging of granitic rocks aged between 95 and 30 Ma with an age‐data gap between 60 and 48 Ma. Based on the spatiotemporal distribution of granite ages, we examine two plausible models to explain the pattern of magmatic activity: (i) subduction of a segmented spreading ridge and subsequent slab melting (ridge‐subduction model), and (ii) subduction with a temporally variable subduction angle and corresponding spatial distribution of normal arc magmatism (subduction angle model). We optimize the model parameters to fit the observed magmatism in time and space, and compare the best‐fit models. As to ridge subduction model, the best‐fit solution indicates that the spreading ridge started to subduct at approximately 100 Ma, and involved a 45‐km‐wide section of the ridge segment, a subduction obliquity of 30°, and a slow migration velocity (～1.6 cm/y) of the ridge. These values are within the ranges of velocities observed for present‐day ridge subduction at the Chile trench. On the other hand, the best‐fit solution of subduction angle model indicates that the subduction angle decreases stepwise from 37° at 95 Ma, 32° at 87 Ma, 22° at 72 Ma, to 20° at 65 Ma, shifting magmatic region towards the continental side. These results and comparison, together with constraints on the geometry of the tectonic setting provided by previous studies, suggest that the ridge subduction model better explains the limited duration of magmatism, although both models broadly fit the data and cannot be ruled out.  相似文献   

20.
Active oceanic ridges are part of the global system of diverging plate boundaries encircling the Earth. They represent weak zones of the lithosphere. They are isostatically equilibrated. The system as a whole is considered to be well adapted to the present field of plate driving forces. The search for regularities in the pattern of active oceanic ridges may, therefore, provide valuable information as to the large-scale characteristics of structures and processes in the Earth’s mantle. Two large belts of active oceanic ridges are envisaged: (1) The semi-circular belt bordering the Pacific plate which extends from South of Tasmania to Northwest of Vancouver Island over a length of 20,000 km. It appears to encircle a center P1 in the central Pacific region. (2) The circum-African belt bordering the African plate which extends from the Azores to the Gulf of Aden over a length of 24,000 km. It appears to encircle a center A1 in central Africa. The attempt is made to determine the position of these centers. Extent and position of the ridge systems are described by 34 fixed points. Points R01–R20 mark the circum-African ridge system, points R21-R34 the Pacific ridge system. A least-squares adjustment is used to determine the optimum position of the centers P1 and A1. Center P1 of the Pacific ridge system is located at 169.8°W/2.6°S. Center A1 of the circum-African ridge system is located at 11.6°E/2.4°N. The location error of the centers is less than 2.8°. In view of the great extent of the ridge systems, and considering the fact that the location of P1 and A1 is based on independent data sets, the nearly antipodal and equatorial position of the centers is remarkable. The newly defined centers P1 and A1 are located close to the Pacific pole P, at 170°W/0°N, and the African pole A, at 10°E/0°N. Within the limits of error the center P1 coincides with pole P, the center A1 with pole A. Originally, these poles were introduced in order to describe a fundamental hemispherical symmetry which is apparent in the evolution of the Earth’s lithosphere during the last 180 Ma. The new results confirm the unique position of poles P and A in the global tectonic framework.  相似文献   

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