Early spreading history of the Indian Ocean between India and Australia

A revised model of seafloor spreading between India and Australia from the inception of spreading 125 m.y. to the change to a new system at 90 m.y. stems from the wider recognition of the M-series of magnetic anomalies off the southwestern margin of Australia, from a revised pole of opening between Australia and Antarctica, and by the extension in the central Wharton Basin of the Late Cretaceous set of magnetic anomalies back to 34. The phase of spreading represented by the later anomalies has been extended back to 90 m.y. in order to give a resolved pole that describes the rotation of India from Australia consistent with the M-series anomalies, DSDP site ages, and fracture zone trends. An abandoned spreading ridge in the Cuvier Abyssal Plain indicates a ridge jump within the first ten million years of spreading. Elsewhere, two kinds of ridge jump (one to the continental margin of Australia or India, the other by propagation of the spreading ridge into adjacent compartments thereby causing them to fuse), are postulated to account for other observations.     

The single largest oceanic plateau: Ontong Java–Manihiki–Hikurangi

Oceanic plateaus are mafic igneous provinces commonly thought to derive from ascending mantle plumes. By far the largest, the Ontong Java Plateau (OJP) was emplaced ca. 120 Ma, with a much smaller magmatic pulse of ca. 90 Ma. Of similar age and composition, the Manihiki and Hikurangi Plateaus (MP and HP) are separated from the OJP by ocean basins formed during the Cretaceous long normal magnetic period. I present new seafloor fabric data that indicate the three plateaus formed as one (OJMHP). The data support previous interpretations that the Osbourn Trough is the relict of the spreading center that separated the MP and HP but they require a different interpretation than prevailing tectonic models for the Ellice Basin. Closely spaced, large offset, fracture zones in the Ellice Basin bound former right-stepping spreading segments that separated the OJP and MP. The MP was emplaced near the axis of the Pacific–Phoenix ridge and additional plateau fragments formerly bordered its eastern margins. Following OJMHP break-up, seafloor spreading removed these fragments to the east and SSE, together with the symmetric conjugates to the extant Phoenix magnetic lineations.     

The Argo Abyssal Plain

Eighteen geophysical transects were made in the Argo Abyssal Plain to study the magnetic anomalies, bathymetry and seismic structure. Magnetic anomalies were identified as being the Mesozoic anomalies M-10 to M-25, increasing in age from the Java Trench to the northwest continental shelf of Australia. A new bathymetric map shows that the Argo Abyssal Plain is bounded by the 5600-m contour and reaches a maximum depth of 5730 m against the inshore side of the Exmouth Plateau. Joey Rise was found to limit the Argo Abyssal Plain on the southwest. Continuous seismic profiles, sonobuoy data and seismic data from other cruises permit one to contour the depths to oceanic basement. Numerous diapir-like structures were observed, but their nature and origin is obscure.     

Age and structure of the southern Rockall Trough new evidence

The Rockall Trough separates the Rockall Plateau microcontinent from the shelf and slope west of the British Isles. The structure and age of the trough has been the source of considerable discussion. Although widely considered to be of oceanic origin, postulated ages for the spreading range from Permian to Cretaceous. New seismic profiles linked to the IPOD sites in the Bay of Biscay and to oceanic anomalies of known age are used to present a new assessment of the age and structure of the southern Rockall Trough. It is concluded that about 120 km of ocean crust is present in the trough and that spreading took place in the Albian-Maastrichtian interval.     

Summary of Arctic Geophysics

The Cenozoic history of the Eurasian Basin is well understood because it involves the Eurasian (EU) and North American (NA) plates and is therefore constrained by data from more southerly regions and contains a readilly decipherable magnetic pattern. Reconstruction of the older portion of the Arctic Ocean is more difficult; however, information collected on ice station CESAR in 1983, interpreted in conjunction with regional geologic and geophysical data, provides insight into its oceanic affinities and age. A dredged outcrop of the Alpha ridge, consisted of weathered fragmental alkaline volcanic rocks. Refraction data reveal a thick crust nearly 40 km and a high velocity lower crust on this ridge. The basal layer velocity is typical of all plateaus known to be oceanic crust. It is hypothesized that Alpha Ridge therefore represents a late Cretaceous oceanic plateau. A more recent analogue is the Iceland-Faeroe Ridge. The magnetic information are shown to be consistent with this interpretation of oceanic crust. The Amerasia Basin is closed by rotating the Arctic-Alaska plate against NA during the Cretaceous. This reconstruction, its timing and its position are consistent with the geology of the Canadian Arctic Islands and Alaska.     

Tectonic history of the Shikoku marginal basin

Studies of marine magnetic anomaly data from the Shikoku basin reveal magnetic lineations which strike northwest almost parallel to the trend of the Palau-Kyushu ridge. The lineation pattern is best developed in the western part of the basin and we can confidently identify a sequence of anomalies 7 through 5E between the base of the Palau-Kyushu ridge and the center of the basin. In the eastern part of the basin the basement morphology is rough and complex and magnetic anomalies can not be identified unequivocally. We infer that the Palau-Kyushu ridge and the Izu-Bonin island arc began separating about 27 m.y. B.P. An interval of rapid separation (4.2 cm/yr) occurred between 26 and 22.5 m.y. B.P. which approximately coincides with a period of intense volcanic activity in Japan. The observed magnetic lineation pattern and basement morphology can be best explained if the Shikoku basin formed at a two-limb spreading system during the Late Oligocene to Middle Miocene. Subsequently the eastern half of the basin was disrupted by fractures as the Iwo-Jima ridge collided with the Japanese islands. The accretionary process which formed the crust of the Shikoku marginal basin appears similar to that operating at mid-ocean ridges of the world.     

Evolution of the Tasman Sea reappraised

We reexamined available marine magnetics data from the Tasman Sea and reidentified sea-floor spreading anomalies in the southern portion of the basin. From the revised magnetic lineations and fracture zones we calculated new finite rotations which descrobe the evolution of the basin in terms of a simple two-plate spreading system active between about 82 and 60 m.y. ago. Allowing for the probable continental origin of the Dampier ridge, the predicted displacement of the western margin of the Lord Howe rise relative to the eastern Australian margin in the northern Tasman basin is consistent with their observed separation. Thus, the controversial episode of subduction of Tasman basin crust at the east Australian margin proposed in earlier studies is no longer necessary.     

Lithospheric model with thick oceanic crust at the continental boundary: A mechanism for shallow spreading ridges in young oceans

In a general lithospheric model of a simple divergent ocean and continental margin that satisfies the constraints of isostasy and gravity anomalies, the free-air gravity anomaly at the margin is modelled by an oceanic crust that thickens exponentially toward the margin from its common value of 6.4 km about 600 km from the margin to 17.7 km at the margin; this postulated thickening is supported empirically by seismic refraction measurements made near continental margins. The thickness of the oceanic crust matches that of the continental lithosphere at breakup, as observed today in Afar and East Africa, and is interpreted as the initial oceanic surface layer chilled against the continental lithosphere. With continued plate accretion, the chilled oceanic crust thins exponentially to a steadystate thickness, which is achieved about 40 m.y. after breakup. These findings contrast with the generally held view that the oceanic crust has a uniform thickness.During the first 40 m.y. of spreading, the thicker oceanic crust, of density 2.86 g/cm³, displaces the denser (3.32 g/cm³) subjacent material; by isostasy, the spreading ridge and the rest of the seafloor thus stand higher in younger( <40m.y.) oceans than they do in older(>40m.y.) oceans. This is postulated to be the cause of the empirical relationship between the crestal depth of spreading ridges and the age (or half-width) of ocean basins.     

Sea-floor spreading and the early opening of the North Atlantic

We have examined available magnetic and gravity data bearing on the initiation of sea-floor spreading in the North Atlantic between Ireland and Newfoundland. The change in character of the magnetic field on the continental margin on either side of the Atlantic from a landward magnetic quiet zone to a seaward “noisy”, magnetic signature is postulated to be related to a change from continental to oceanic crust. Sea-floor spreading between Ireland and Newfoundland was initiated during the long normal geomagnetic polarity interval in the Late Cretaceous. Rockall Trough may have opened at this time. At the end of the normal polarity interval (Late Santonian) the ridge axis jumped westward to bypass Rockall Trough and the related offset initiated the Charlie Gibbs fracture zone.A reconstruction is presented for the relative position between North America and Europe prior to the initiation of sea-floor spreading in the Late Cretaceous.     

Late Cretaceous genesis of the Kula plate

We propose that the Pacific-Kula ridge began spreading approximately 85 m.y. B.P., during Late Cretaceous time. Extrapolation of the Great Magnetic Bight backwards in time results in an implausible ridge configuration. This implies that plate velocity vectors for the Pacific, Kula, and Farallon plates were not constant during this interval. Evidence for splitting of the Kula plate from the Pacific plate along the Chinook trough is the relationship between the north-striking Amlia and Adak fracture zones, the Chinook and Emperor troughs, and the magnetic lineations south of the Aleutian trench. If this hypothesis is correct, it will require that Mesozoic reconstructions of the Pacific basin and their relation to Cenozoic reconstructions be re-examined. A previously unrecognized Mesozoic plate may be required. We propose calling this the Izanagi plate.     

The evolution of the Parece Vela Basin,eastern Philippine Sea

The Parece Vela Basin is a back-arc basin. It is approximately 5000 m deep and is divided into two topographic provinces by the north-trending Parece Vela Rift. The western province is thinly sedimented and topographically rough. The eastern province is blanketed by a thick apron of volcaniclastic sediments which were derived from the West Mariana Ridge. The Parece Vela Rift is composed of a series of discrete deeps and troughs with depths commonly of 6 km and locally exceeding 7 km.Petrologic and seismic refraction data indicate that the Parece Vela Basin is of oceanic character.Low-amplitude, nort-trending, lineated magnetic anomalies are present in the basin and appear symmetric about a line near the Parece Vela Rift. In the central latitudes of the basin seafloor spreading anomalies 10 (30 m.y. B.P.) to 5E or 5D (18 or 17 m.y. B.P.) can be identified. The uncertainty in identifying the youngest anomaly may be due to ridge jumps near the end of spreading. Spreading may have started slightly later in the northern end of the basin. Anomalies in the eastern province are disrupted and are difficult to correlate. DSDP results indicate post-spreading volcanism on the eastern side of the basin and this may have degraded the anomalies. The age obtained in the western province of the basin at DSDP Site 449 (～25m.y. B.P.) is in close agreement with that obtained from the magnetic data (～26m.y. B.P.).It is hypothesized that subduction was occurring at a west-dipping subduction zone east of the Palau-Kyushu Ridge in the Early Oligocene. This volcanic arc split about 31 or 32 m.y. ago and interarc spreading was initiated between the Palau-Kyushu Ridge (which then became a remnant arc) and the West Mariana Ridge. The Parece Vela Basin formed between the ridges by two-limb seafloor spreading. Spreading stopped about 17 or 18 m.y. ago.Like certain other marginal basins, the Parece Vela Basin is deeper than predicted from depth vs. age curves. The average heat flow for the Parece Vela Basin is in agreement with that predicted from heat flow vs. age curves.The origin of the Parece Vela Rift is unclear. It may represent the extinct spreading center or may be a postspreading feature.     

Emplacement processes of proto-arc basalt in the Izu–Bonin–Mariana arc system

Ascertaining the emplacement mechanism of oceanic basaltic lavas is important in understanding how ocean floor topography is produced and oceanic plates evolve, particularly during the early stages of crustal development of a supra-subduction zone. A detailed study of the volcanic stratigraphy at International Ocean Discovery Program (IODP) Site U1438 in the Amami Sankaku Basin, west of the Kyushu–Palau Ridge, has revealed the development of lava accretion and ridge topography on the Philippine Sea plate at about 49 Ma. Igneous basement rocks penetrated at Site U1438 are the uppermost 150 m of ~6 km-thick oceanic crust, and comprise, in a downhole direction, sheet flows (12.6 m), lobate sheet flows (61.3 m), pillow lavas (50.7 m), and thin sheet flows (25.3 m). The lowermost sheet flows are intercalated with layers of limestone and epiclastic tuff. Lithofacies analysis reveals that the lowermost sheet flows, limestone, and tuff formed on an axial rise, the pillow lavas were emplaced on a ridge slope, and the lobate sheet flows formed off ridge on an abyssal plain. The lithofacies of the basement basalt corresponds to the upper portions of fast-spreading oceanic crust, suggesting that subduction initiation was associated with intermediate to fast rates of seafloor spreading. The surface sheet flows are olivine–clinopyroxene-phyric basalt and differ from the lower basalt flows that contain phenocrysts of olivine and plagioclase, with or without clinopyroxene. The depleted chrome-spinel composition and olivine–clinopyroxene phenocryst assemblage in the surface sheet flows suggests a slight contribution of water for magma generation not present for the lower basalt flows. Considering the lithological difference between the backarc and forearc oceanic crust in the Izu–Bonin–Mariana arc, with sheet flow dominant in the former, seafloor spreading occurred faster in the later stage of subduction initiation.     

Progress in understanding the age and origin of the alpha ridge, arctic ocean

Multiparameter geophysical measurements and geological samples from CESAR suggest that close to Canada the Alpha Ridge is oceanic in nature and was built in part by volcanic activity. It is unclear whether this part of the ridge formed in an intraplate or a plate margin environment. Estimates from paleontological, heat flow and magnetic data place the construction of a volcanic ridge within the Cretaceous period between about 120 and 80 Ma ago, the interval in which the Canada Basin seafloor formed but clearly before the creation of the Lomonosov Ridge. The place of the Makarov Basin in this chronology remains unclear.     

Breakup of Australia and Antarctica estimated as mid-Cretaceous (95 ± 5 Ma) from magnetic and seismic data at the continental margin

A positive magnetic anomaly marks the seaward edge of the magnetic quiet zone along the southern margin of Australia eastward between 114° and 131°E and along the conjugate Antarctic margin between 105° and 132°E. This anomaly was originally interpreted as the oldest seafloor-spreading anomaly—A22, revised by Cande and Mutter to A34—in the Southeast Indian Ocean, but is better modelled as the edge effect at the continent-ocean boundary (COB) constrained by seismic data. Continental crust abuts the oceanic sequence of normal and reversed spreading blocks, truncated within the Cretaceous normal interval at an extrapolated age of 96 Ma, rounded to 95 ± 5 Ma to take into account the uncertainty of the initial spreading rate and of the location of the COB. The occurrence of the anomaly on both margins defines this as the age of breakup. Farther east between 131° and 139°E on the Australian margin, the COB anomaly is modelled as due to the same kind of effect but with successively younger ages of truncation to 49 Ma, interpreted as indicating the most recent ridge-crest jumping to the Australian COB. The magnetic data from the conjugate sector of Antarctica, albeit scanty, are consistent with this interpretation.The 95 ± 5 Ma age of breakup coincides with that of the breakup unconformity in southern Australia, expressed by a short mid-Cretaceous lacuna in the Otway Basin between faulted Early Cretaceous rift-valley sediments of the Otway Group and the overlying Late Cretaceous Sherbrook Group, and by an unconformity of similar age in the Great Australian Bight Basin.     

洋中脊扩张速率对洋壳速度结构的约束

洋中脊速度结构是揭示大洋岩石圈演化过程的重要约束.为探讨不同扩张速率下洋中脊的洋壳速度结构特征,挑选了全球152处快速(全扩张速率 90mm·a-1)、慢速(全扩张速率20～50mm·a-1)和超慢速(全扩张速率20mm·a-1)扩张洋中脊和非洋中脊的洋壳1-D地震波速度结构剖面,通过筛选统计、求取平均值等方法对分类的洋壳1-D速度结构进行对比研究,获得了不同扩张速率下洋中脊洋壳速度结构差异以及洋中脊与非洋中脊洋壳速度结构差异的新认识:(1)快速、慢速和超慢速扩张洋中脊的平均正常洋壳厚度分别为6.4km、7.2km和5.3km,其中洋壳层2的厚度基本相似,洋壳厚度差异主要源自洋壳层3;其洋壳厚度变化范围分别为4.9～8.1km、4.6～8.7km和4.2～10.2km,随着洋中脊扩张速率减小,洋壳厚度的变化范围逐渐增大;(2)快速扩张洋中脊的洋壳速度大于慢速和超慢速,可能与快速扩张脊洋壳生成过程中深部高密度岩浆上涌比较充足有关;(3)非洋中脊(10Ma)的洋壳比洋中脊(10Ma)的洋壳厚～0.3km,表明洋壳厚度与洋壳年龄有一定的正相关性.     

中国海-西太平洋地区典型剖面的重-磁-震联合反演研究

重-磁-震联合反演是获取地壳结构的重要方法.此次研究,我们主要基于全球最新的水深、重磁异常、沉积物厚度等数据,结合实测地震数据和前人研究成果,分析了中国海-西太平洋地区的莫霍面展布特征,并利用重磁震联合反演方法获得了跨越中国海-西太平洋典型剖面的地壳结构和异常体分布,揭示了陆壳到洋壳的典型变化规律.结果表明,从浙江地区到马里亚纳俯冲带,地壳结构大致呈现由厚到薄、由老到新、由复杂到简单的特征.浙江地区（扬子块体和华夏块体）地壳结构复杂,三层结构明显,地壳内断裂带发育,并伴有广泛的岩浆侵入;东海地区莫霍面起伏剧烈,地壳厚度变化较大,冲绳海槽地壳明显减薄,是其过渡壳性质的体现;西菲律宾海盆、九州-帕劳海脊、帕里西维拉海盆、马里亚纳俯冲带等构造单元地壳结构相对简单,二层结构明显.其中,西菲律宾海盆和帕里西维拉海盆地壳内部磁异常变化较为剧烈,海盆扩张过程中形成的磁异常体分布广泛,地壳厚度（5~8 km）明显小于陆壳;九州-帕劳海脊地壳厚度可达~20 km,缺失中地壳,表现为岛弧地壳结构;同源的西马里亚纳岛弧和东马里亚纳火山弧地壳结构相似,浅层磁异常体分布广泛,西马里亚纳岛弧地壳厚度（~17 km）略小于东马里亚纳火山弧（~20 km）,体现了裂离的不对称性;马里亚纳海槽具有正常的洋壳结构（~7 km）,但扩张中心未发生明显破裂.对比各构造单元地壳结构的异同点,我们进一步认识到,陆壳与洋壳之间不是孤立的,陆壳可能会演化出洋壳的结构或组分,板块的演化总是处于动态循环过程中.此研究加深了我们对中国海-西太平洋深部构造特征的整体理解,促进了我们对大陆边缘演化与板块相互作用的认识,深化了我国管辖海域及邻近地区的基础地质调查.     

岩浆与构造作用对洋壳厚度的影响——以西北印度洋为例

洋中脊及邻区洋盆的洋壳厚度能很好地反映区域岩浆补给特征,对于研究洋中脊内部及周缘岩浆活动和构造演化过程具有很好的指示意义.西北印度洋中脊作为典型的慢速扩张洋中脊,其扩张过程与周缘构造活动具有很强的时空关系.本文利用剩余地幔布格重力异常反演了西北印度洋洋壳厚度,由此分析区域内洋壳厚度分布和岩浆补给特征.研究发现,西北印度洋洋壳平均厚度为7.8 km,受区域构造背景影响厚度变化较大.根据洋壳厚度的统计学分布特征,将区域内洋壳分为三种类型:薄洋壳(小于4.5 km)、正常洋壳(4.5~6.5 km)和厚洋壳(大于6.5 km),根据西北印度洋中脊周缘(~40 Ma内)洋壳厚度变化特征可将洋中脊划分为5段,发现洋中脊洋壳厚度受区域构造活动和地幔温度所控制,其中薄洋壳主要受转换断层影响造成区域洋壳厚度减薄,而厚洋壳主要受地幔温度和地幔柱作用影响,并在S4洋中脊段显示出较强的热点与洋中脊相互作用,同时微陆块的裂解和漂移也可能是导致洋壳厚度差异的原因之一.     

Cretaceous episodic growth of the Japanese Islands

Abstract The Japanese Islands formed rapidly in situ along the eastern Asian continental margin in the Cretaceous due to both tectonic and magmatic processes. In the Early Cretaceous, huge oceanic plateaus created by the mid-Panthalassa super plume accreted with the continental margin. This tectonic interaction of oceanic plateau with continental crust is one of the significant tectonic processes responsible for continental growth in subduction zones. In the Japanese Islands, Late Cretaceous-Early Paleogene continental growth is much more episodic and drastic. At this time the continental margin uplifted regionally, and intra-continent collision tectonics took place in the northern part of the Asian continent. The uplifting event appears to have been caused by the subduction of very young oceanic crust (i.e. the Izanagi-Kula Plate) along the continental margin. Magmatism was also very active, and melting of the young oceanic slab appears to have resulted in ubiquitous plutons in the continental margin. Regional uplift of the continental margin and intra-continent collision tectonics promoted erosion of the uplifted area, and a large amount of terrigenous sediment was abruptly supplied to the trench. As a result of the rapid supply of terrigenous detritus, the accretionary complexes (the Hidaka Belt in Hokkaido and the Shimanto Belt in Southwest Japan) grew rapidly in the subduction zone. The rapid growth of the accretionary complexes and the subduction of very young, buoyant oceanic crust caused the extrusion of a high-P/T metamorphic wedge from the deep levels of the subduction zone. Episodic growth of the Late Cretaceous Japanese Islands suggests that subduction of very young oceanic crust and/or ridge subduction are very significant for the formation of new continental crust in subduction zones.     

New constraints on the tectonic evolution of the eastern Indian Ocean

From marine magnetic anomaly studies, a fossil spreading ridge is identified beneath the Nicobar Fan in the northwestern Wharton Basin. Several north-south-trending transform faults offset this ridge left-laterally east of the 86°E transform fault. Our findings show that this ridge, which was part of the plate boundary between the Indian and Australian plates, ceased its spreading shortly after formation of magnetic anomaly 20 (～ 45.6m.y. B.P.). Since the breakup of Australia and Antarctica probably occurred sometime between 110 and 90 m.y. B.P., we suggest that the Indian, Australian, and Antarctic plates were moving relative to one another from about 90 to 45 m.y. B.P. A triple junction would have existed in the southeastern Indian Ocean during that period of time. At anomaly 19 time (～ 45m.y. B.P.), the junction became inactive, and Australia and India became a single plate. The northwest-southeast-trending Southeast Indian Ridge was formed by connecting the India-Antarctica spreading center with the Australia-Antarctica spreading center. Its activity has continued to the present time.     

Accretion and tectonic erosion processes revealed by the mode of occurrence and geochemistry of greenstones in the Cretaceous accretionary complexes of the Idonnappu Zone, southern central Hokkaido, Japan

The Cretaceous accretionary complexes of the Idonnappu Zone in the Urakawa area are divided into five lithological units, four of which contain greenstone bodies. The Lower Cretaceous Naizawa Complex consists of two lithologic units. The Basaltic Unit (B‐Unit) is a large‐scale tectonic slab of greenstone, consisting of depleted tholeiite similar to that of the Lower Sorachi Ophiolite (basal forearc basin ophiolite) in the Sorachi‐Yezo Belt. The Mixed Unit of Naizawa Complex (MN‐Unit) contains oceanic island‐type alkaline greenstones which occur as slab‐like bodies and faulted blocks with tectonically dismembered trench‐fill sediments. Repeated alternations of the two units in the Naizawa Complex may have been formed by the collision of seamounts with forearc ophiolitic body (Lower Sorachi Ophiolite) in the trench. The Upper Cretaceous Horobetsugawa Complex structurally underlies the Naizawa Complex in its original configuration, and it also contains greenstone bodies. Greenstones in the MH‐Unit occur as blocks and sedimentary clasts in a clastic matrix, and exhibit depleted tholeiite and oceanic‐island alkaline basalt/tholeiite chemistry. This unit is interpreted as submarine slide and debris flow deposits. Greenstones in the PT‐Unit occur at the base of several chert‐clastic successions. Most of the greenstones are severely sheared and show normal‐type mid‐ocean ridge basalt composition. The PT‐Unit greenstones are considered to have been derived from abyssal basement peeled off during accretion. The different accretion mechanism of the greenstones in the Naizawa and Horobetsugawa complexes reflects temporal changes in subduction zone conditions. Seamount accretion and tectonic erosion were dominant in the Early Cretaceous, due to highly oblique subduction of the old oceanic crust and minimal sediment supply. Whereas, thick sediments with minor mid‐ocean ridge basalt and olistostrome accreted in the Late Cretaceous, due to near‐orthogonal subduction of young oceanic crust with voluminous sediment supply.