Ridge Anomalies Research Articles

Duplex equilibria of Ural circulation anomalies

Atmospheric circulation anomalies over the Ural Mountains are crucial indicators of the anomalous downstream weather and climate over East Asia. Here, we provide a new perspective on the mechanism of Ural circulation anomalies. We use a simple theoretical model to determine that the relationship between the solar forcing and three Ural circulation patterns, namely, neutral type, trough anomaly and ridge anomaly, is a nonlinear relationship following the supercritical pitchfork bifurcation theory. The theory predicts that when the total solar irradiance (TSI) is below a critical value, trough and ridge anomalies represent duplex equilibria and are equally likely to occur at the same TSI. Based on 180 winter months record, we have estimated the bidimensional probability density of TSI and the monthly mean geopotential height at 500 hPa or zonal wind at 850 hPa over the Ural Mountains. Results show that Sc = 1360.9 W m−2 is a critical value of TSI, the neutral type pattern is the single circulation regime when TSI > Sc, whereas trough and ridge anomaly patterns are duplex circulation regimes when TSI < Sc. Besides, when TSI < Sc, during the same TSI range, trough and ridge anomaly events occur at nearly the same frequencies. These results generally agree with the theoretical model. We demonstrate that trough and ridge anomalies, as duplex equilibria, result from the large-scale zonal flow interacting with the Ural Mountains. Low TSI tends to strengthen the large-scale zonal flow over the Ural Mountains, hence inducing either a trough anomaly or ridge anomaly.

The aeromagnetic expression of New Zealand’s Alpine Fault: regional displacement and entrainment of igneous rock

ABSTRACTThe Alpine Fault is very well delineated by recent high-resolution aeromagnetic data sets over much of its length in New Zealand’s South Island. Aeromagnetic data acquired over parts of the West Coast, Tasman and Marlborough regions for the New Zealand Government reveal different types of total magnetic intensity anomalies associated with the fault that can be characterised as truncated, step, ridge, trough and broad anomalies. Fault-terminated Permian ultramafic rocks, Late Cretaceous basalt and some Early Cretaceous intrusions are attributed to the more prominent truncated anomalies. The pre-eminent step anomaly is associated with relatively magnetic Alpine Schist juxtaposed against variable but generally less magnetic Western Province rocks. Strongly positive ridge anomalies occurring in the Alpine Fault zone are associated with Mesozoic mafic metavolcanic and ultramafic rocks. A trough anomaly associated with the Alpine Fault occurs in the southernmost survey and may reflect some hydrothermal demagnetisation. Broad, low-amplitude anomalies underlying the hanging wall are attributed in one instance to the extension of the magnetic Hohonu Range granitoids and dikes extending southeast in the footwall and in other places as magnetic schist rocks in the hanging wall. The apparent great-circle displacement of the Alpine Fault based on offset of magnetic anomalies associated with the Dun Mountain Ultramafics Group rocks is c. 461 km, in close agreement with the more precise 459 km inferred from geological map offset.

Meandarra Gravity Ridge: symmetry elements of the gravity anomaly and its relationship to the Bowen–Gunnedah–Sydney basin system

The Meandarra Gravity Ridge is one of the major gravity features in eastern Australia. The source(s) for this gravity anomaly is problematic because the ridge, a gravity high, coincides for much of its length with the thickest parts of the Bowen and Gunnedah Basins, areas that would normally be expected to be regional gravity lows. Furthermore, the Meandarra Gravity Ridge is continuous across numerous structural elements and regional terranes, and there is no obvious expression of its source in the surface geology. Construction of a series of gravity profiles across the Meandarra Gravity Ridge in this study has provided detailed analyses of changes in the size and shape of the Bouguer anomaly along the ridge axis. These data suggest that the ridge is composed of a series of offset colinear anomalies, and demonstrate that the magnitude and symmetry of the Bouguer anomaly are different for each segment of the ridge. 3D-mesh diagrams of the total-field Bouguer gravity illustrate that the ridge may be subdivided into at least five segments, each of which is characterised by particular gravity values and shapes for the anomaly profile. 2D gravity modelling of selected Bouguer anomaly profiles suggests that the source for the Meandarra Gravity Ridge anomaly is a dense mafic body in the upper crust. This source has a preferred density contrast of +0.25 tm−3, and probably consists of a pile of mafic volcanics up to 9 km thick. The size and geometry of the modelled anomalous mass supports a rift-type origin for the source of the Meandarra Gravity Ridge. Our modelling predicts that the magnitude and symmetry of the Meandarra Gravity Ridge gravity anomaly are controlled by the geometry of an underlying mafic source and not by interference effects due to density contrasts between sedimentary and metamorphic rocks of adjacent orogens.

Global upper‐mantle structure from finite‐frequency surface‐wave tomography

We report global shear‐wave velocity structure and radial anisotropy in the upper mantle obtained using finite‐frequency surface‐wave tomography, based upon complete three‐dimensional Born sensitivity kernels. Because wavefront healing effects are properly taken into account, finite‐frequency surface‐wave tomography improves the resolution of small‐scale mantle heterogeneities, especially for deep anomalies that are constrained by the longest‐period surface waves. In our finite‐frequency model FFSW1, the globally averaged radial anisotropy shows a transition from positive (SH &gt; SV) to negative anisotropy (SV &gt; SH) at about 220 km, consistent with a change in the dominant mantle circulation pattern from predominantly horizontal flow at shallow depths to vertical flow at greater depths. The radial anisotropy beneath cratons and the old Pacific plate agrees well with previous studies. However, our model exhibits a strong negative radial anisotropy at depths greater than 120 km beneath mid‐ocean ridges, a feature that is not present in previous upper‐mantle models. More interestingly, the depth extent of the ridge anomalies is distinctly different beneath fast‐ and slow‐spreading centers; anomalies beneath fast‐spreading centers are stronger, but the strength decreases rapidly below 250 km. In contrast, beneath slow‐spreading centers such as the northern Mid‐Atlantic Ridge and the Red Sea, anomalies extend down at least to the top of the transition zone. The different depth extent of the ridge anomalies suggests that the primary driving force of slow‐spreading seafloor may be different from that of fast‐spreading seafloor and that active upwelling beneath slow‐spreading ridges may play a major role in the opening of the seafloor.

Rare gas systematics on the southernmost Mid‐Atlantic Ridge: Constraints on the lower mantle and the Dupal source

Concentrations and isotopic compositions of He, Ne, Ar, Kr, and Xe have been measured for mid‐ocean ridge basalt glasses from the Mid‐Atlantic Ridge Discovery section, centered at 47°30′S, thus extending the database for the 50°–53°S Shona section [Moreira et al, 1995]. The 44°–53°S part of the Mid‐Atlantic Ridge includes the Discovery and Shona bathymetrie and geochemical ridge anomalies [Douglass et al, 1999], which also appear clearly in the rare gas isotopic record. In addition to air, present at the surface or possibly mantle recycled, three source components are identified, upper mantle, primitive plume, and a Dupal‐related component. He and Ne isotopes indicate a very primitive source for both the Discovery and Shona plumes, which must originate in deep, poorly degassed mantle. Ne and Ar, corrected from air based on Ne systematics, reveal very consistent along‐strike He, Ne, and Ar isotopic patterns, also consistent with Xe data. These systematics provide evidence that plume argon has low 40Ar/36Ar and plume Xe low isotopic ratios relative to degassed mantle. A segment of the Discovery ridge anomaly shows a Dupal‐type, low 206Pb/204Pb component named LOMU (low μ, where μ = 238U/204Pb) by Douglass et al. [1999], and has radiogenic 4He/3He and 21Ne/22Ne, relatively elevated 20Ne/22Ne, mildly radiogenic 40Ar/36Ar, and low Xe isotopic ratios, possibly representing the Dupal rare gas signature. Interpretations of this component as either recycled oceanic crust, or delaminated subcontinental lithosphere are consistent with the rare gas systematics. In the former case, a maximum subduction age of 500 Ma can be calculated. In the latter case, the sublithospheric mantle should have a 40K/36Ar ratio 2–5 times lower than the convective mantle and a 238U/3He ratio 2–3 times higher.

Plume‐ridge interactions of the Discovery and Shona mantle plumes with the southern Mid‐Atlantic Ridge (40°‐55°S)

We report on 66 Pb, Sr, and Nd isotope analyses of basalts dredged along the Mid‐Atlantic Ridge (MAR) from 40° to 55°S. The results strongly indicate interaction and mixing between the off‐ridge Discovery and ridge‐centered Shona mantle plumes and the ambient asthenosphere beneath the MAR. In addition, the Bouvet mantle plume appears to be feeding the southernmost portion of the MAR as suggested earlier by le Roex et al [1987]. The Discovery and Shona plumes have enriched mantle and high‐μ(μ = 238U/204Pb) affinities, respectively. Their proximity to one another suggests a genetic relationship, probably associated with subducted altered oceanic crust recycled through the mantle with some sediment (Discovery) or without sediment (Shona). The Discovery Ridge Anomaly exhibits Pb, Sr, and Nd isotopic discontinuities resulting from southward preferential plume flow across the Agulhas transform beginning ∼13 Ma. The presence of a component with unusually low 206Pb/204Pb accompanied by high 87Sr/86Sr and low 208Pb/204Pb and 143Nd/144Nd in the Discovery Ridge Anomaly and to a lesser extent in the Shona Ridge Anomaly indicates three‐component mixing between the ambient asthenosphere, the Discovery and Shona plumes, and this low‐μ (LOMU) component which possibly represents subcontinental lithospheric mantle material. We also note that in Pb, Sr, and Nd isotopic space, ocean island basalts from the Tristan, Gough, and Discovery family of plumes could be interpreted as resulting from binary mixing between a generic plume component similar to Bouvet or the “C” component [Hanan and Graham, 1994] and the LOMU component, which progressively increases southward. The LOMU component seems to be a characteristic feature of the South Atlantic and Indian Ocean mantles and is thought to reside passively in the shallow mantle because of delamination of subcontinental lithospheric mantle following the breakup of Gondwana.

The Agulhas Magsat anomaly: Implications for continental break-up of Gondwana

A new processing procedure is implemented on Magsat data over the southern African region and surrounding oceans. The oceanic region of the ensuing map is interpreted in this study. The anomaly map is image processed and a composite image of Magsat with Seasat data is also presented. The Agulhas high-amplitude satellite-magnetic anomaly, situated off the southeastern coast of Africa in the southwestern Indian Ocean (31°E, 35°S), is clearly defined. In addition, a north-south oceanic anomaly bordering the west coast of southern Africa, including in part the Walvis Ridge anomaly, is now recognised. The Walvis Ridge and Agulhas oceanic anomalies are interpreted to represent thickened, remanently magnetised (during Cretaceous quiet times) ocean crust. Similarities between the Walvis Ridge and the Agulhas Magsat anomalies suggest a common genesis. The Agulhas anomaly is interpreted to represent the remnant scar of the process that led to fragmentation of Gondwana. It is hypothesised that fragmentation of Gondwana was initiated by an upwelling of hotter than normal asthenosphere centred directly below the Agulhas anomaly.

Evidence for a remanent contribution in Magsat data from the Cretaceous quiet zone in the south Atlantic

Indirect evidence for the presence of a TRM component in the MAGSAT elevation data in the South Atlantic is found through forward modeling of the prominent positive anomaly overlying the Walvis Ridge. The anomaly contrast between the Ridge and adjacent Angola and Cape Basins is too large to be easily explained by induced magnetization of even alkali basaltic crust. However, most of the Walvis Ridge and portions of the adjacent basins lie within the Cretaceous Quiet Zone where prolonged normal polarity may have produced enough volume of material to cause a measureable contribution at satellite elevation. An additional 3 A/m (spread over the entire layer 2 thickness) TRM contribution is required to account for the Walvis Ridge anomaly. This value is within the range found for the Cretaceous Quiet Zone anomalies in the MAGSAT data in the North Atlantic.

Viscous remanent magnetization model for the Broken Ridge Satellite Magnetic Anomaly

A range of crustal models for the interpretation of the Broken Ridge satellite magnetic anomaly have been constructed from bathymetric data. The source magnetizations were assumed to be constant in direction throughout the source region and parallel to the present earth's field at the center of the source region. The model calculations were made at the observation points of a selected set of Magsat Investigator B passes, with a variable magnetic field direction calculated from the MGST (4/81) field model. The horizontal perimeter of all the models was defined by the departure of the Broken Ridge bathymetry from an average oceanic depth of 4.5 km. A topographic model, with an upper surface defined by the bathymetry of the ridge and a lower surface defined by the average oceanic depth, requires a magnetization of 42 A/m to account for the observed anomaly amplitude. An Airy‐type isostatic model, with an upper surface defined by the bathymetry of the ridge and a lower surface predicted from the bathymetry, requires a magnetization of 6 A/m to account for the observed anomaly amplitude. A midcrust model, with an upper surface at the average oceanic depth and a lower surface at the average oceanic Moho, requires a magnetization of 13 A/m to account for the observed anomaly amplitude. A comparison of the locations of the model and observed anomaly peaks shows that the Broken Ridge anomaly can be modeled assuming present field directions. There is not sufficient sensitivity in the data to distinguish between the models on the basis of anomaly location. The model magnetizations vary from 6 A/m, where the whole crust is uniformly magnetized, to 42 A/m, where the topographic expression of the ridge is the source of the anomaly. The strength of the required magnetization contrasts cannot be accounted for by induced magnetization if normal oceanic crustal materials are assumed to be the source for the observed anomaly. Measured and modeled remanent magnetizations of seafloor rocks are typically within the lower part of the range of required values, but the higher magnetizations are associated with very young rocks and dredge samples. It is concluded that the source magnetizations have been enhanced by the development of a viscous remanence effect since the last reversal of the earth's field.