<p> The foot of the continental slope is an important topographical feature of the continental margin. Its the basis for coastal states to extend its continental shelf rights and to delimit the outer limit of the continental shelf beyond 200 nautical miles. Its also an important technical parameter that the Commission on the Limits of the Continental Shelf pays special attention to when considering the submissions of coastal states. The formulation of the continental shelf regime in Article 76 of the United Nations Convention on the Law of the Sea originates from the typical passive continental margin. However, due to the diversity and complexity of the global continental margin, especially the transformation and influence of late tectonic activities and sedimentation on the continental margin, the seabed topography is extremely complex and changeable, which makes it very difficult to identify the foot of the continental slope. In addition, in order to obtain the largest extent of the outer continental shelf, each coastal state has interpreted the relevant provisions of the foot of the continental slope in their own favor, making the foot of the continental slope a hot and controversial issue in the delimitation of the outer continental shelf. Based on the provisions of the United Nations Convention on the Law of the Sea and the "Scientific and Technical Guidelines of the Commission on the Limits of the Continental Shelf" on the foot of the continental slope, combined with the geological characteristics of different types of continental margins and the delimitation practice of various coastal states, the determination of the base of the continental slope, the selection of the point of greatest change and the application of the evidence to the contrary are discussed.</p><p> <br></p>

塞舌尔微陆块及其北部区域的形成过程与冈瓦纳大陆解体和马达加斯加-塞舌尔-印度复杂的形成演化过程密切相关。通过对2016年中国-塞舌尔大陆边缘海洋地球科学联合调查航次所取得的高精度多波束数据和重力数据处理分析,首次得到了塞舌尔海台北部的高精度地形图和重力异常图。应用构造地貌学分析方法,结合该区域地形及地质、地球物理等资料,进一步探讨了该区域的构造演化过程。研究发现塞舌尔海台北部发育3条向海方向延伸的狭长条带状海脊和平坦的深海平原,与塞舌尔陆块被阿米兰特海沟所切割,是一独立的构造单元。岩石地球化学证据和构造演化历史表明,塞舌尔海台北部可能在印度洋早期扩张阶段形成,向海方向延伸的海脊是由岩浆沿转换断层薄弱带喷发形成的不连续海脊。

Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs.

The mean composition of mid‐ocean ridge basalts (MORB) is determined using a global data set of major elements, trace elements, and isotopes compiled from new and previously published data. A global catalog of 771 ridge segments, including their mean depth, length, and spreading rate enables calculation of average compositions for each segment. Segment averages allow weighting by segment length and spreading rate and reduce the bias introduced by uneven sampling. A bootstrapping statistical technique provides rigorous error estimates. Based on the characteristics of the data, we suggest a revised nomenclature for MORB. “ALL MORB” is the total composition of the crust apart from back‐arc basins, N‐MORB the most likely basalt composition encountered along the ridge >500 km from hot spots, and D‐MORB the depleted end‐member. ALL MORB and N‐MORB are substantially more enriched than early estimates of normal ridge basalts. The mean composition of back‐arc spreading centers requires higher extents of melting and greater concentrations of fluid‐mobile elements, reflecting the influence of water on back‐arc petrogenesis. The average data permit a re‐evaluation of several problems of global geochemistry. The K/U ratio reported here (12,340 ± 840) is in accord with previous estimates, much lower than the estimate of Arevalo et al. (2009). The low Sm/Nd and 143Nd/144Nd ratio of all morb and N‐MORB provide constraints on the hypothesis that Earth has a non‐chondritic primitive mantle. Either Earth is chondritic in Sm/Nd and the hypothesis is incorrect or MORB preferentially sample an enriched reservoir, requiring a large depleted reservoir in the deep mantle.

Many oceanic island basalts show sublinear subparallel arrays in Sr-Nd-Pb isotopic space. The depleted upper mantle is rarely a mixing end-member of these arrays, as would be expected if mantle plumes originated at a 670-kilometer boundary layer and entrained upper mantle during ascent. Instead, the arrays are fan-shaped and appear to converge on a volume in isotopic space characterized by low (87)Sr/(86)Sr and high (143)Nd/(144)Nd, (206)Pb/(204)Pb, and (3)He/(4)He ratios. This new isotopic component may be the lower mantle, entrained into plumes originating from the core-mantle boundary layer.

Geophysical, petrological, and geochemical data provide important clues about the composition of the deep continental crust. On the basis of seismic refraction data, we divide the crust into type sections associated with different tectonic provinces. Each shows a three‐layer crust consisting of upper, middle, and lower crust, in which P wave velocities increase progressively with depth. There is large variation in average P wave velocity of the lower crust between different type sections, but in general, lower crustal velocities are high (>6.9 km s−1) and average middle crustal velocities range between 6.3 and 6.7 km s−1. Heat‐producing elements decrease with depth in the crust owing to their depletion in felsic rocks caused by granulite facies metamorphism and an increase in the proportion of mafic rocks with depth. Studies of crustal cross sections show that in Archean regions, 50–85% of the heat flowing from the surface of the Earth is generated within the crust. Granulite terrains that experienced isobaric cooling are representative of middle or lower crust and have higher proportions of mafic rocks than do granulite terrains that experienced isothermal decompression. The latter are probably not representative of the deep crust but are merely upper crustal rocks that have been through an orogenic cycle. Granulite xenoliths provide some of the deepest samples of the continental crust and are composed largely of mafic rock types. Ultrasonic velocity measurements for a wide variety of deep crustal rocks provide a link between crustal velocity and lithology. Meta‐igneous felsic, intermediate and mafic granulite, and amphibolite facies rocks are distinguishable on the basis of P and S wave velocities, but metamorphosed shales (metapelites) have velocities that overlap the complete velocity range displayed by the meta‐igneous lithologies. The high heat production of metapelites, coupled with their generally limited volumetric extent in granulite terrains and xenoliths, suggests they constitute only a small proportion of the lower crust. Using average P wave velocities derived from the crustal type sections, the estimated areal extent of each type of crust, and the average compositions of different types of granulites, we estimate the average lower and middle crust composition. The lower crust is composed of rocks in the granulite facies and is lithologically heterogeneous. Its average composition is mafic, approaching that of a primitive mantle‐derived basalt, but it may range to intermediate bulk compositions in some regions. The middle crust is composed of rocks in the amphibolite facies and is intermediate in bulk composition, containing significant K, Th, and U contents. Average continental crust is intermediate in composition and contains a significant proportion of the bulk silicate Earth's incompatible trace element budget (35–55% of Rb, Ba, K, Pb, Th, and U).

Plume–ridge interaction in the Reykjanes Ridge and Iceland region is graphically demonstrated by several V-shaped ridges surrounding the spreading axis, indicating mantle flow away from Iceland. It also has significant geochemical effects. Regionally, incompatible element concentrations increase northwards coinciding with decreasing depth and increasing crustal thickness, depth of melting and proximity to Iceland. Major and trace element data show that isolated magma bodies feed individual volcanic systems along the ridge. Fractionation within these systems increases towards 60–61°N, where it coincides with the intersection of a V-shaped ridge, thicker crust and more abundant seamounts. Trace element, Nd and Sr isotopic data reveal dynamic melting and mixing within a southward-thinning, heterogeneous mantle wedge beneath the Reykjanes Ridge. Melting is variable and locally enhanced at 58°N, 59°N, 60°N and 61°N. A total of six mantle components are identified. Some are specific to Iceland whereas others are found only beneath the ridge axis. The geographical distribution of these components reflects their origin within the deep upper and lower mantle and subsequent translation by plume outflow along the entire length of the ridge.