40Ar/ 39Ar and K–Ar geochronological age constraints for the inception and early evolution of the Izu–Bonin – Mariana arc system

K–Ar and ⁴⁰Ar/³⁹Ar dates are presented for locations in the Izu–Bonin – Mariana (IBM) forearc (Ocean Drilling Program (ODP) sites 786 & 782, Chichijima, Deep Sea Drilling Program (DSDP) sites 458 & 459, Saipan), and Palau on the remnant arc of the Kyushu–Palau Ridge. For a number of these locations, the ⁴⁰Ar/³⁹Ar plateau and ³⁶Ar/⁴⁰Ar versus ³⁹Ar/⁴⁰Ar isochrons give older ages than the K–Ar results. The most important results are: (i) at site 786, initial construction of the proto-IBM (now forearc) basement occurred at least by ca 47–45 Ma, consistent with the age of the immediately overlying sediments (middle Eocene nannofossil Zone CP13c); the younger pulse of construction dated at ca 35 Ma by K–Ar could not be confirmed by ⁴⁰Ar/³⁹Ar analysis; (ii) ⁴⁰Ar/³⁹Ar ages for the initial construction of the Mariana portion of the IBM system are as old as those of the Izu–Bonin portion, for example at site 458, initial construction commenced at least by ca 49 Ma and at ca 47 Ma at Saipan (Sankakayuma Formation); and (iii) a combination of K–Ar and ⁴⁰Ar/³⁹Ar ages indicate continued boninite magmatism in the Izu–Bonin forearc (and remnant arc at Palau) until ca 35 Ma. Subduction inception including boninite series rocks along most of the exposed length of the IBM system, clearly preceded by some 5 million years the Middle Eocene (ca 43.5 Ma) change in Pacific plate motion. Boninitic series magmatism persisted at locations now exposed in the forearc for ～ 15 million years after arc inception concurrently with low-K tholeiitic series eruptions from a subaerial arc system, established at ≥ 40 Ma, on the Kyushu–Palau Ridge. For the Mariana portion of the IBM system, reconstruction of the proto-arc places this activity adjacent to the concurrent but orthogonally spreading Central Basin Ridge of the West Philippine Basin. It is possible that a combination of subduction of a young North New Guinea Plate beneath newly created back-arc basin crust may account for some of the features of the Mariana system. It is clear, however, that the understanding of the processes of subduction initiation and early IBM arc development is incomplete.     

The migration history of the Nazca Ridge along the Peruvian active margin: a re-evaluation

The collision zone of the 200 km wide and 1.5 km high Nazca Ridge and the Peruvian segment of the convergent South American margin between 14°S and 17°S is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc. This is evident by a narrowing of the shelf, a westward shift of the coastline and the presence of marine terraces. As the Nazca Ridge is oblique with respect to both trench and convergence direction of the Nazca Plate, it migrates southward along the active plate boundary. For reconstructing the migration history of the Nazca Ridge, this study uses updated plate motion data, resulting from a revision of the geomagnetic time scale. The new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of ∼75 mm/a (before 10.8 Ma), ∼61 mm/a (10.8-4.9 Ma), and ∼43 mm/a (4.9 Ma to present). Intra-plate deformation associated with mountain building in the Peruvian Andes since the Miocene reduces the relative convergence rate between Nazca Plate and Peruvian forearc. Taking an intra-plate deformation at a rate of ∼10 mm/a, estimated from space-geodetic and geological data, into account, does not significantly reduce these lateral migration velocities. Constraining the length of the original Nazca Ridge by its conjugate feature on the Pacific Plate yields a length of 900 km for the subducted portion of the ridge. Using this constraint, ridge subduction began ∼11.2 Ma ago at 11°S. Therefore, the Nazca Ridge did not affect the northern sites of Ocean Drilling Program (ODP) Leg 112 located at 9°S. This is supported by benthic foraminiferal assemblages in ODP Leg 112 cores, indicating more than 1000 m of subsidence since at least Middle Miocene time, and by continuous shale deposition on the shelf from 18 to 7 Ma, recorded in the Ballena industrial well. At 11.5°S, the model predicts the passage of the ridge crest ∼9.5 Ma ago. This agrees with the sedimentary facies and benthic foraminiferal stratigraphy of ODP Leg 112 cores, which argue for deposition on the shelf in the Middle and Late Miocene with subsequent subsidence of a minimum of several hundred meters. Onshore at 12°S, the sedimentary record shows at least 500 m uplift prior to the end of the Miocene, also in agreement with the model.     

Drainage density on progressively tilted surfaces with different gradients,Wheeler Ridge,California

Wheeler Ridge in the Southern San Joaquin Valley, California, is an anticlinal fold which has been progressively uplifted during the last 250 ka. Drainage networks on the ridge become younger as the anticline's eastern tip is approached. Because of the fold's asymmetric shape, surfaces on opposite flanks of the ridge have similar ages but very different gradients. The ridge provides important insights into drainage development on progressively tilted surfaces, as existing studies are restricted to static topography. A surface gradient of between 4·8° and c.10° is needed to initiate channel networks. This gradient threshold is consistent with previous studies of the gradient and upslope area needed to incise a channel through overland flow. Comparison of coeval drainage networks on opposite flanks of the ridge allows the controversial relationship between drainage density and gradient to be investigated. A lower valley density is observed on the higher gradient flank of Wheeler Ridge. Field observations from the ridge indicate that this inverse relationship is associated with hillslope erosion by shallow mass-wasting, the rate of which increases rapidly as a threshold gradient is approached. Comparison of data from Wheeler Ridge with other field studies and numerical models, shows that the form of the relationship between gradient and drainage density is process-dependent. A positive correlation occurs when erosion is a result of overland flow, whilst a negative correlation occurs where erosion is dominated by shallow mass-wasting. Copyright © 1999 John Wiley & Sons, Ltd.     

台湾岛西南海域福尔摩沙海脊冷泉区地形地貌特征分析

利用水深数据和ROV近海底影像资料,对福尔摩沙海脊冷泉区的海底地形地貌和冷泉系统的海底表征进行了描述和分析,并讨论了二者之间的响应关系。结果表明,相对于船载多波束数据而言,近海底多波束测深系统所获得的数据能更高精度地反映冷泉区海底地形地貌特征,是研究冷泉系统不可或缺的基础资料。基于ROV近海底观测影像资料,福尔摩沙海脊冷泉系统整体表现为局部被化能自养生物群落覆盖并有流体喷口零星分布的巨大自生碳酸盐岩岩丘,海底表征主要包括形态各异的自生碳酸盐岩结壳或岩体、化能自养生物群落、流体喷口、还原性沉积物等几种形式。研究表明,福尔摩沙海脊冷泉区的地形地貌特征与冷泉系统海底表征具有良好的响应关系,并且该区的地形地貌特征主要受控于出露于海底的自生碳酸盐岩的形态特征及规模。首次揭示了福尔摩沙海脊冷泉区地形地貌特征与其海底表征之间的响应关系,以期为后续的冷泉研究提供必要的背景资料支持。     

赤道东经90°海岭活塞柱状心钙同位素曲线特征及其古环境意义

A δ~(44) Ca curve from shells of the planktonic foraminifera Globigerinoides sacculifer in calcareous biogenic oozes has been extracted from the Nintyeast Ridge in the Indian Ocean since 300 ka. By combining terrigenous inputs(e.g., grain size, magnetic susceptibility, and turbidite frequency) with the oceanic productivity(e.g., biogenic content and Neogloboquadrina dutertrei content), it is found that the curve's variations are closely related to the historical evolution of the oceanic calcium cycle. The δ~(44)Ca value is in lower tendency and has small oscillation during Marine Isotope Stage(MIS) 6, when the supply of terrigenous detrital is highest. In contrast, during MIS 3,5 and 7, the δ~(44) Ca values are in higher tendency, and their fluctuations are consistent with the variations of the productivity proxies. These results suggest that the calcium isotopes are mainly influenced by the input of the Himalayan erosion products to the northern Indian Ocean. In addition, the developmental stages of calcareous planktons may have a secondary impact on the fluctuations of the calcium isotope ratio of sea water.     

卡尔斯伯格脊 60°~61°E洋脊段多波束后向散射特征及其对构造与岩浆作用强度的指示

多波束声纳数据可以有效记录海底地形地貌和底质特征信息。本文利用船载多波束数据对慢速扩张的卡尔斯伯格脊60°~61°E洋脊段的典型构造地貌单元的后向散射强度特征进行了研究,在此基础上,分析了该洋脊段的构造和岩浆作用强度特征。结果表明,洋脊段I以构造拉张作用占主导,脊轴及附近后向散射强度为-29 dB左右,裂谷壁高差可达1 200 m以上,裂谷内断裂发育,裂谷侧翼高度与裂谷宽度的比值为78.7~126.2,裂谷两侧翼部线性构造较少,但轴向正断层面更宽,倾角更小;与洋脊段裂谷中段相比,末端火山活动频率较低但喷发规模较大,火山机构数量和体积也更大,且可发育深大断裂获取深部热源。洋脊段II以岩浆作用占主导,脊轴及附近后向散射强度达-35 dB,裂谷内轴向火山脊发育,裂谷壁高差小于500 m,裂谷侧翼高度与裂谷宽度的比值为77.6~116.8,裂谷两侧翼部线性构造数量众多、长宽比较大且呈近似对称,相邻线性构造之间沉积物广泛分布。通过提取挖掘与底质属性密切相关的多波束后向散射强度数据,结合海底地形地貌的分析,可以为洋中脊的构造和岩浆作用强度的定量研究提供有效的证据。     

西南印度洋脊“断桥”热液区深部地质构造研究

Polymetalic sulfide is the main product of sea-floor hydrothermal venting, and has become an important sea-floor mineral resources for its rich in many kinds of precious metal elements. Since 2007, a number of investigations have been carried out by the China Ocean Mineral Resources Research and Development Association(COMRA)cruises(CCCs) along the Southwest Indian Ridge(SWIR). In 2011, the COMRA signed an exploration contract of sea-floor polymetallic sulfides of 10 000 km2 on the SWIR with the International Seabed Authority. Based on the multibeam data and shipborne gravity data obtained in 2010 by the R/V Dayang Yihao during the leg 6 of CCCs21, together with the global satellite surveys, the characteristics of gravity anomalies are analyzed in the Duanqiao hydrothermal field(37°39′S, 50°24′E). The "subarea calibration" terrain-correcting method is employed to calculate the Bouguer gravity anomaly, and the ocean bottom seismometer(OBS) profile is used to constrain the two-dimensional gravity anomaly simulation. The absent Moho in a previous seismic model is also calculated.The results show that the crustal thickness varies between 3 and 10 km along the profile, and the maximum crustal thickness reaches up to 10 km in the Duanqiao hydrothermal field with an average of 7.5 km. It is by far the most thicker crust discovered along the SWIR. The calculated crust thickness at the Longqi hydrothermal field is approximately 3 km, 1 km less than that indicated by seismic models, possibly due to the outcome of an oceanic core complex(OCC).     

热液喷口探测化学传感器的研制及应用*

海底热液喷口周围的水体具有显著的浊度和化学组分异常, 是寻找热液喷口的重要标志。文章提出了一种海底热液喷口的探测技术方法, 设计了低功耗化学传感器。该化学传感器可搭载在相关平台, 实时探测水体的Eh、H₂S、pH及CO₃^2-等电位值, 结合浊度异常, 可以推断热液喷口的位置。在西南印度洋中脊海试结果表明, 该化学传感器可有效探测由热液活动产生的水体异常, 是一种探测海底热液喷口的有效技术。     

Sea-floor fissures, biological communities and sediment fatty acids of the Northern Okushiri Ridge, Japan Sea: Implications for possible methane seepage

A biological community was discovered in the Northern Okushiri Ridge, northeastern Japan Sea. The community was closely associated with sea-floor fissures, and presumed to be supported by methanotrophic and/or thiotrophic bacterial production. Sediments inside of and in the vicinity of the fissures were collected, and the short-chain (C_9–20) sediment fatty acids were analyzed for amounts and compositions. The fatty acid compositions were compared with those from a known methane seep and a submarine volcano in the Sagami Bay, central Japan, and from a whale skeleton at the Torishima Seamount, northwestern Pacific Ocean. As a result, a close relationship between the sediments from the Northern Okushiri Ridge, the known methane-seep, and the whale skeleton was found. This finding represents the first discovery of methane seepage and associated biological communities in the Japan Sea. This also supports the hypothesis that the eastern margin of the northern Japan Sea is at the early stage of new subduction.