Three-Dimensional Velocity Field and Cross-Frontal Water Exchange in the Kuroshio Extension

A spread of warm water from the first crest of the Kuroshio Extension is periodically enhanced by northward warm water intrusions from the main current. The water type in the spread area was previously found to be the same as that in the Kuroshio front at depth. In looking for the possible mechanism responsible for the northward warm water intrusions, a dynamic analysis in the Kuroshio front was carried out by using CTD, ADCP, AVHRR and ARGOS buoy data, obtained in 1996 by the R.V. Hakuho Maru. Downstream, cross-stream and vertical velocities in the Kuroshio Extension were found by using a "stream coordinate system". The velocity field in the Kuroshio front at the first crest showed a double structure with two surface velocity maxima. In the inner part of the front, relatively high cross-stream (northward) and vertical (upward) velocities were found. Thus, this study suggests that while water particles flow downstream along the first stationary meander of the Kuroshio Extension, they also experience lateral and vertical movements which allow the deeper water from an upstream location to rise to the surface layer, and in certain locations to deflect northward. By assuming isopycnal movement and conservation of potential vorticity, it was found that in those locations where anticyclonic curvature of the meander increases, warm water is more likely to deflect northward. High ageostrophic components observed in the first 300 m of the water column are probably related to the relatively high cross-stream and vertical velocities in the inner part of the front.     

敦密断裂带白垩纪两期重要的变形事件

本文报道了敦密断裂带糜棱岩中黑云母~(40)Ar/~(39)Ar定年结果和大规模走滑-逆冲断裂的几何学、运动学特征及其形成时代,以便揭示断裂带两期变形事件的构造属性。黑龙江省密山市花岗质糜棱岩中黑云母~(40)Ar/~(39)Ar加权平均年龄为132.2±1.2Ma,它是敦密断裂带经历伸展事件的冷却年龄,也是东北亚大陆边缘在早白垩世Hauterivian期-Albian期发生强烈区域伸展作用的产物。密山市至辽宁省清原县系列大型走滑-逆冲断层和断层相关褶皱揭示出在晚白垩世晚期-末期发生右旋走滑-逆冲事件,该事件规模大,影响范围广,导致整个断裂带遭受到强烈改造,形成对冲式断裂系统。将研究区走滑-逆冲断裂与山东省郯庐断裂带中段挤压构造对比,认为郯庐断裂带北段和中段在晚白垩世末期都发生了强烈的走滑-逆冲事件,它们具有相同的构造特征和构造属性。     

Petrology,geochemistry and geodynamic setting of Eocene-Oligocene alkaline intrusions from the Alborz-Azerbaijan magmatic belt,NW Iran

The Kaleybar, Razgah and Bozqush (KRB) intrusions were studied to better understand subduction-related Eocene-Oligocene alkaline magmatism in NW Iran. The bulk of intrusions mainly consist of Si-undersaturated rocks including foid-bearing monzonite and syenite (nepheline syenite, pseudoleucite syenite) with some foid-bearing diorite and gabbro. In addition, they are spatially associated with Si-saturated rocks ranging in composition from monzo-diorite to syeno-granite. The main mafic rock-forming minerals of the studied rocks are olivine (Fo₄₄Fa₅₆), clinopyroxene (diopside to augite), biotite (Mg-biotite through Fe-biotite), amphibole (ferro-pargasite and magnesio-hastingsite with Mg#<0.55), and garnet (Ti-andradites). Based on whole rock geochemistry, the foid-syenites and associated rocks show mildly alkaline (shoshonitic) affinity. The content of SiO₂, K₂O?+?Na₂O, and K₂O/Na₂O ratio ranges from 47.8 to 60.7?wt.%, 5.31 to 16.33?wt.%, and 0.6 to 3.2, respectively. The intrusions are commonly metaluminous, with an aluminum-saturation index (ASI) ranging from 0.66 to 1.01. Almost all the rocks display similar arc-related geochemical features characterized by the enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) together with the depletion in high field strength elements (HFSE). The chondrite-normalized REE patterns show no to marked negative Eu anomaly (Eu/Eu*?=?0.55 to 1.12), (La/Yb)_N?=?8.16 to 31, (La/Sm)_N?=?2.80 to 10.59, and (Tb/Yb)_N?=?0.84 to 2.40. The evaluation of the REE patterns for the KRB magmas and the comparison of the trace element ratios with experimental studies indicate a chemically enriched lithospheric mantle source composed of garnet-spinel-lherzolite that have underwent a low degree of partial melting <5% to generate the KRB intrusions. Based on the present data, we infer that the mantle source was contaminated by a subduction component and the melting of the mantle lithosphere occurred by local extension in an overall convergent regime in NW Iran. The extension regime during the Eocene is proposed to be the result of the Neo-Tethys slab roll-back and the Sevan-Akera-Qaradagh (SAQ) slab break-off.     

3D seismic analysis of the structure and evolution of a salt-influenced normal fault zone: A test of competing fault growth models

In this paper we determine the structure and evolution of a normal fault system by applying qualitative and quantitative fault analysis techniques to a 3D seismic reflection dataset from the Suez Rift, Egypt. Our analysis indicates that the October Fault Zone is composed of two fault systems that are locally decoupled across a salt-bearing interval of Late Miocene (Messinian) age. The sub-salt system offsets pre-rift crystalline basement, and was active during the Late Oligocene-early Middle Miocene. It is composed of four, planar, NW–SE-striking segments that are hard- linked by N–S-striking segments, and up to 2 km of displacement occurs at top basement, suggesting that this fault system nucleated at or, more likely, below this structural level. The supra-salt system was active during the Pliocene-Holocene, and is composed of four, NW–SE-striking, listric fault segments, which are soft-linked by unbreached relay zones. Segments in the supra-salt fault system nucleated within Pliocene strata and have maximum throws of up to 482 m. Locally, the segments of the supra-salt fault system breach the Messinian salt to hard-link downwards with the underlying, sub-salt fault system, thus forming the upper part of a fault zone composed of: (i) a single, amalgamated fault system below the salt and (ii) a fault system composed of multiple soft-linked segments above the salt. Analysis of throw-distance (T-x) and throw-depth (T-z) plots for the supra-salt fault system, isopach maps of the associated growth strata and backstripping of intervening relay zones indicates that these faults rapidly established their lengths during the early stages of their slip history. The fault tips were then effectively ‘pinned’ and the faults accumulated displacement via predominantly downward propagation. We interpret that the October Fault Zone had the following evolutionary trend; (i) growth of the sub-salt fault system during the Oligocene-to-early Middle Miocene; (ii) cessation of activity on the sub-salt fault system during the Middle Miocene-to-?Early Pliocene; (iii) stretching of the sub- and supra-salt intervals during Pliocene regional extension, which resulted in mild reactivation of the sub-salt fault system and nucleation of the segmented supra-salt fault system, which at this time was geometrically decoupled from the sub-salt fault system; and (iv) Pliocene-to-Holocene growth of the supra-salt fault system by downwards vertical tip line propagation, which resulted in downward breaching of the salt and dip-linkage with the sub-salt fault system. The structure of the October Fault Zone and the rapid establishment of supra-salt fault lengths are compatible with the predictions of the coherent fault model, although we note that individual segments in the supra-salt array grew in accordance with the isolated fault model. Our study thereby indicates that both coherent and isolated fault models may be applicable to the growth of kilometre-scale, basin-bounding faults. Furthermore, we highlight the role that fault reactivation and dip-linkage in mechanically layered sequences can play in controlling the three-dimensional geometry of normal faults.     

Constraints on the timing and regional conditions at the start of the present phase of crustal extension in western Turkey,from observations in and around the Denizli region

The chronology of extension of the continental crust in western Turkey has been the subject of major controversies. We suggest that these difficulties have arisen in part because of past misuse of dating evidence; and in part because the assumption often made, that deposition of major terrestrial sedimentary sequences implies crustal extension to create the necessary accommodation space, is incorrect. We report evidence that the present phase of extension began in the Denizli region at ~ 7 Ma, around the start of the Messinian stage of the Late Miocene. This timing matches the estimated start of right-lateral slip on the North Anatolian Fault Zone, and corresponds to a substantial increase in the dimensions of the Aegean extensional province to roughly its present size: beforehand, between ~ 12 Ma and ~ 7 Ma, extension seems to have only occurred in the central part of this modern province. In some localities, terrestrial sedimentation that began before this start of extension continued into this extensional phase, both within and outside normal fault zones. However, in other localities within the hanging-walls of normal faults, the start of extension marked the end of sedimentation. Relationships between sedimentation and crustal extension in this region are thus not straightforward, and a simple correlation should therefore not be assumed in structural interpretations. During the time-scale of this phase of extension, the Denizli region has also experienced major vertical crustal motions that are unrelated to this extension. The northern part of this region, in the relatively arid interior of western Turkey, has uplifted by ~ 400 m since the Middle Pliocene, whereas its southern part, closer to the Mediterranean Sea and with a much wetter climate, has uplifted by ~ 1,200 m since the Early Miocene, by up to ~ 900 m since the Middle Pliocene, and by an estimated ~ 300 m since the Early Pleistocene. This regional uplift, superimposed on the local effects of active normal faulting, is interpreted as a consequence of lateral variations in rates of erosion. A reliable chronology for this phase of extension in western Turkey, in relation to changes in the geometry of motions of adjoining plates and Late Cenozoic environmental change, is now in place.     

Origin of fault domains and fault-domain boundaries (transfer zones and accommodation zones) in extensional provinces: Result of random nucleation and self-organized fault growth

In many extensional provinces, large normal faults dip in the same direction forming fault domains. Features variously named transfer faults, transfer zones, and accommodation zones (hereafter non-genetically referred to as fault-domain boundaries) separate adjacent fault domains. Experimental modeling of distributed extension provides insights on the origin, geometry, and evolution of these fault domains and fault-domain boundaries. In our scaled models, a homogeneous layer of wet clay or dry sand overlies a latex sheet that is stretched orthogonally or obliquely between two rigid sheets. Fault domains and fault-domain boundaries develop in all models in both map view and cross-section. The number, size, and arrangement of fault domains as well as the number and orientation of fault-domain boundaries are variable, even for models with identical boundary conditions. The fault-domain boundaries in our models differ profoundly from those in many published conceptual models of transfer/accommodation zones. In our models, fault-domain boundaries are broad zones of deformation (not discrete strike-slip or oblique-slip faults), their orientations are not systematically related to the extension direction, and they can form spontaneously without any prescribed pre-existing zones of weakness. We propose that fault domains develop because early-formed faults perturb the stress field, causing new nearby faults to dip in the same direction (self-organized growth). As extension continues, faults from adjacent fault domains propagate toward each another. Because opposite-dipping faults interfere with one another in the zone of overlap, the faults stop propagating. In this case, the geometry of the domain boundaries depends on the spatial arrangement of the earliest formed faults, a result of the random distribution of the largest flaws at which the faults nucleate.     

Timing of deformation in the Norseman-Wiluna Belt, Yilgarn Craton, Western Australia

Establishing relative and absolute time frameworks for the sedimentary, magmatic, tectonic and gold mineralisation events in the Norseman-Wiluna Belt of the Archean Yilgarn Craton of Western Australia, has long been the main aim of research efforts. Recently published constraints on the timing of sedimentation and absolute granite ages have emphasized the shortcomings of the established rationale used for interpreting the timing of deformation events. In this paper the assumptions underlying this rationale are scrutinized, and it is shown that they are the source of significant misinterpretations. A revised time chart for the deformation events of the belt is established. The first shortening phase to affect the belt, D₁, was preceded by an extensional event D_1e and accompanied by a change from volcanic-dominated to plutonic-dominated magmatism at approximately 2685–2675 Ma. Later extension (D_2e) controlled deposition of the ca 2655 Ma Kurrawang Sequence and was followed by D₂, a major shortening event, which folded this sequence. D₂ must therefore have started after 2655 Ma—at least 20 Ma later than previously thought and after the voluminous 2670–2655 Ma high-Ca granite intrusion. Younger transcurrent deformation, D₃–D₄, waned at around 2630 Ma, suggesting that the crustal shortening deformation cycle D₂–D₄ lasted approximately 20–30 Ma, contemporaneous with low-volume 2650–2630 Ma low-Ca granites and alkaline intrusions. Time constraints on gold deposits suggest a late mineralisation event between 2640–2630 Ma. Thus, D₂–D₄ deformation cycle and late felsic magmatism define a 20–30 Ma long tectonothermal event, which culminated with gold mineralisation. The finding that D₂ folding took place after voluminous high-Ca granite intrusion led to research into the role of competent bodies during folding by means of numerical models. Results suggest that buoyancy-driven doming of pre-tectonic competent bodies trigger growth of antiforms, whereas non-buoyant, competent granite bodies trigger growth of synforms. The conspicuous presence of pre-folding granites in the cores of anticlines may be a result from active buoyancy doming during folding.     

黑潮延伸体上游中尺度涡场的年代际振荡及其相关机制

黑潮延伸体上游区域的中尺度涡场的涡动能和涡特征尺度存在显著地年代际振荡,和黑潮延伸体路径的年代际变化有很好的相关性。当黑潮延伸体路径比较稳定时,其上游区域涡动能比较高,涡特征尺度比较大,反之相反。通过对黑潮延伸体上游区域的中尺度涡场进行集合分析发现:当黑潮延伸体处于稳定状态时,上游涡场几乎是各向均匀地,有轻微的径向伸长;而当黑潮延伸体处于不稳定状态时,上游的中尺度涡场有显著地纬向伸长。对与中尺度涡场的产生相关的线性斜压不稳定和正压不稳定进行了计算分析,结果显示,线性斜压不稳定不是控制中尺度涡场年代际变化的机制,而正压不稳定对中尺度涡场的年代际变化有积极的贡献。不稳定产生的中尺度涡之间存在非线性涡-涡相互作用。     

Baroclinic Buoyancy-Inertia Joint Stability Parameter

At present, the barotropic buoyant stability parameter has been derived from a vertical virtual displacement of a water parcel. The barotropic inertial stability parameter in the eccentrically cyclogeostrophic, basic current field was derived in 2003 from a horizontal cross-stream virtual displacement of a parcel. By expressing acceleration of a parcel due to a virtual displacement, which is arbitrarily sloping within a vertical section across the basic current, in terms of natural coordinates, we derived the vertical component of baroclinic buoyant stability parameter B ₂ ², the horizontal component of baroclinic inertial stability parameter I ₂ ², the baroclinic joint stability parameter J ², its buoyant component B ² and its inertial component I ². B ² is far greater than I ₂ ², and when neglecting relative vorticity except for vertical shear, a downward convex curve of J ² plotted against the slope of a virtual displacement follows a trend of B ² curve. If a parcel displaces along a horizontal surface or an isopycnal surface, however, B ² vanishes, and J ² becomes equal to I ². Actual parcel is apt to displace not only along the bottom slope, but also along the sea surface and an isopycnal interfacial surface, which is approximately equivalent to an isentropic surface, preferred by lateral mixing and exchange of momentum. Such actual displacement makes B ² vanishing, and grants I ² an important role. The present analysis of I ² examining effects due to curvature and horizontal and vertical shear vorticities are useful in deepening our understanding of baroclinic instability in actual oceanic streams.     

Kuroshio extension variability explored through assimilation of TOPEX/POSEIDON altimeter data into a quasi-geostrophic model

Mesoscale features in the eastward extension of the Kuroshio were investigated using assimilation of TOPEX/POSEIDON (T/P) data into a three-layer quasi-geostrophic model. The T/P data exhibited an elongated state of the southern recirculation gyre in 1993–95 and 1997, between whose two periods the gyre had a contracted state in 1995–96. A few stationary eddies were located in the southern gyre during the contracted state. The baroclinic instability, which was indicated by the phase shift from the uppermost-to the lowest-layer anomalies toward the downstream side, was evident near the Kuroshio Extension (KE) path. Since the instability never appeared in the artificial model without bottom topography, the topographic barrier for the eastward flow in the lowest layer was a necessary condition for the instability. The instability synchronized with the transition in the western region of the KE axis from the elongated to the contracted states. This evolution was interpreted as if the baroclinic instability played some part in the KE states and was a trigger for the transition from the elongated to the contracted states.