Australian-Antarctic Discordance Research Articles

Large along-axis variations in magma supply and tectonism of the Southeast Indian Ridge near the Australian-Antarctic Discordance

We analyzed seafloor morphology and geophysical anomalies of the Southeast Indian Ridge (SEIR) to reveal the remarkable changes in magma supply along this intermediate fast-spreading ridge. We found systematic differences of the Australian-Antarctic Discordance (AAD) from adjacent ridge segments with the residual mantle Bouguer gravity anomaly (RMBA) being more positive, seafloor being deeper, morphology being more chaotic, M factors being smaller at the AAD. These systematic anomalies, as well as the observed Na8.0 being greater and Fe8.0 being smaller at AAD, suggest relatively starved magma supply and relatively thin crust within the AAD. Comparing to the adjacent ridges segments, the calculated average map-view M factors are relatively small for the AAD, where several Oceanic Core Complexes (OCCs) develop. Close to 30 OCCs were found to be distributed asymmetrically along the SEIR with 60% of OCCs at the northern flank. The OCCs are concentrated mainly in Segments B3 and B4 within the AAD at ≈124°–126°E, as well as at the eastern end of Zone C at ≈115°E. The relatively small map-view M factors within the AAD indicate stronger tectonism than the adjacent SEIR segments. The interaction between the westward migrating Pacific mantle and the relatively cold mantle beneath the AAD may have caused a reduction in magma supply, leading to the development of abundant OCCs.

The geotectonic features of the Southeast Indian Ridge and its current research progress

The Southeast Indian Ridge (SEIR) is the fastest spreading ridge in the Indian Ocean, with an average half spreading rate of 25−50 mm/a. The crust produced by the SEIR accounts for more than half of the total Indian oceanic crust. It represents a pivotal element in shaping the current tectonic settings of the Indian Ocean. Compared with the Southwest Indian Ridge (SWIR) and Northwest Indian Ridge (NWIR), the temporal and spatial evolution of the SEIR has been more complex, as reflected by the present-day tectonic and geomorphological diversity of its segmentation pattern. In this study, we integrate the topographic and geomorphological features, the gravimetric and geomagnetic signatures, and the MORB geochemical characteristics from the SEIR and the adjacent areas, in order to define the segmentation of the ridge, to trace its evolution through time, to characterize the heterogeneity degree of the mantle source beneath the ridge and to explore potential genetic links between regional intraplate volcanisms and the spreading ridge. The study shows that the axial bathymetry of SEIR undulates along the ridge, with the shallowest depth at the Amsterdam-St. Paul Plateau (ASP) and the deepest ones at the Australian-Antarctic Discordance (AAD). Using bathymetric, gravity and geomagnetic data, thirty-one first-order ridge segments can be distinguished. The boundaries of each segment coincide with transform faults. The eastern Indian Ocean expanded owing to three main successive phases of oceanic accretion each characterized by a distinct orientation: northwestward phase before 84 Ma, a north-south one between 84 and 38 Ma and a northeastward phase after 38 Ma. The mantle source of the SEIR is heterogeneous, especially in ASP and AAD regions. While the ASP results from a hotspot-ridge interaction, the AAD represents the isotopic boundary between Indian and the Pacific Ocean mantle domains. In the eastern Indian Ocean, there are many off-axis oceanic plateaux, such as the Kerguelen Plateau, Broken Ridge, Ninetyeast Ridge, Naturaliste Plateau and Wallaby-Cuvier Plateau. The buildings of these off-axis oceanic plateaux are either related to plume activity, and/or to presence of remnants of continental crust material embedded in the shallow mantle. The SEIR integrated with the NWIR at ~38 Ma, while the SWIR wedged into the NWIR-SEIR sometime after or before 38 Ma at the Rodrigues Triple Junction. The heavy isotopic compositions of SEIR are similar to NWIR and the northeast end of SWIR. Compared with Pacific-Antarctic Ridge, the Indian ridge systems show obvious Dupal anomaly, indicating the larger-scale mantle heterogeneities. This study can help better understand the evolution of the eastern Indian Ocean, the ridge systems and tectonic patterns of the entire Indian Ocean. It can also enhance our knowledge on Gondwana continent rifting and the evolution of Indian Ocean.

An isotopically distinct Zealandia–Antarctic mantle domain in the Southern Ocean

The mantle sources of mid-ocean ridge basalts beneath the Indian and Pacific oceans have distinct isotopic compositions with a long-accepted boundary at the Australian–Antarctic Discordance along the Southeast Indian Ridge. This boundary has been widely used to place constraints on large-scale patterns of mantle flow and composition in the Earth’s upper mantle. Sampling between the Indian and Pacific ridges, however, has been lacking, especially along the remote 2,000 km expanse of the Australian–Antarctic Ridge. Here we present Sr, Nd, Hf and Pb isotope data from this region that show the Australian–Antarctic Ridge has isotopic compositions distinct from both the Pacific and Indian mantle domains. These data define a separate Zealandia–Antarctic domain that appears to have formed in response to the deep mantle upwelling and ensuing volcanism that led to the break-up of Gondwana 90 million years ago, and currently persists at the margins of the Antarctic continent. The relatively shallow depths of the Australian–Antarctic Ridge may be the result of this deep mantle upwelling. Large offset transforms to the east may be the boundary with the Pacific domain. A separate mantle domain, distinct from both the Pacific and Indian domains, exists beneath the Southern Ocean, according to isotope compositions of samples from the Australian–Antarctic ridge.

Hot and cold zones of the Southeast Indian Ridge and their influence on the peculiarities of its structure and magmatism (Numerical and Physical Modelling)

The paper describes the specific features of the bottom topography and morphostructural segmentation along the strike of the Southeast Indian Ridge (SEIR) and in the zones of influence of the Amsterdam–St. Paul hot spot and the anomalous zone of the relatively cold mantle in the area of the Australian–Antarctic discordance. Numerical estimates of changes of thermal state and strength of the crust in axial and off-axial zones of the SEIR were performed. Сorrelation between the thermal–rheological settings in the axial zone of the ridge with the seabed topography and the morphostructural segmentation and magmatism has been established. The numerical modelling results make it possible to assume the presence of along-axis asthenospheric flows under the axial zone of the SEIR. One of them, which was initiated by the Amsterdam–St. Paul point and the Kerguelen plume, is oriented from west to east, and the second, located east of the Australian–Antarctic discordance, is oriented from east to west. Taking into account the numerical modelling results of the thermal regime and the change in thickness of the brittle layer of the axial lithosphere, we performed a physical modelling of the influence of temperature anomalies in the mantle on the peculiarities of crustal deformation in the axial zones of the ridge. The experimental modelling showed that the presence of a thermal anomaly in the sublithosphere mantle in the form of a local heat source (hot spot) will noticeably influence the geometry of the rift axis and its position in relation to the hot spot. An area of anomalous topography forms under the influence of the hot spot, traces of which are preserved in the off-axis spreading flank zones, as in the case of the Amsterdam–St. Paul hot spot. More contrasting and dissected topography forms in zones with a relatively low typical mantle temperature in the process of crustal accretion.

Global equivalent magnetization of the oceanic lithosphere

As a by-product of the construction of a new World Digital Magnetic Anomaly Map over oceanic areas, we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the World's ocean. This map reveals consistent patterns related to the age of the oceanic lithosphere, the spreading rate at which it was formed, and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere. As for the age, the equivalent magnetization decreases significantly during the first 10–15 Myr after its formation, probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration, then increases regularly between 20 and 70 Ma, reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization. As for the spreading rate, the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate, with a threshold at ∼40 km/Myr, in agreement with an independent global analysis of the amplitude of Anomaly 25. This result, combined with those from the study of the anomalous skewness of marine magnetic anomalies, allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate. Finally, specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures, such as the cold Australian–Antarctic Discordance, marked by a lower magnetization, and several hotspots, marked by a high magnetization.

Passive-margin prolonged volcanism, East Australian Plate: outbursts, progressions, plate controls and suggested causes

Prolonged intraplate volcanism along the 4000 km-long East Australian margin for ca 100 Ma raises many genetic questions. Studies of the age-progressive pulses embedded in general basaltic activity have spawned a host of models. Zircon U–Pb dating of inland Queensland central volcanoes gives a stronger database to consider the structure and origin of Australian age-progressive volcanic chains. This assists appraisal of this volcanism in relation to plate motion and plate margin tectonic models. Inland Queensland central volcanoes progressed south-southeast from 34 to 31 Ma (∼5.4 cm/yr) until a surge in activity led to irregular southerly progression 31 to 28 Ma. A new inland southeastern Queensland central volcano line (25 to 22 Ma), from Bunya Mountains to North Main Range, followed 3 Ma behind the adjacent coastal progression. The Australian and Tasman Sea age-progressive chains are compared against recent plate motion modelling (Indian Ocean hotspots). The chain lines differ from general vector traces owing to west-facing swells and cessations in activity. Tectonic processes on the eastern plate margin may regulate these irregularities. These include subduction, rapid roll-back and progressive detachment of the Loyalty slab (43 to 15 Ma). West-flowing Pacific-type asthenosphere, related to perturbed mantle convection, may explain the west-facing volcanic surges. Such westward Pacific flow for over 28 Ma is known at the Australian–Antarctic Discordance, southeast of the present Australian plume sites under Bass Strait–West Tasman Sea. Most basaltic activity along eastern Australia marks asthenospheric melt injections into Tasman rift zone mantle and not lithospheric plate speed. The young (post-10 Ma) fields (Queensland, Victoria–South Australia) reflect new plate couplings, which altered mantle convection and stress regimes. These areas receive asthenospheric inputs from deep thermal zones off northeast Queensland and under Bass Strait.

Seismic attenuation in the eastern Australian and Antarctic plates, from multiple ScS waves

The attenuation of seismic shear waves in the mantle beneath the eastern Australian and Antarctic plates is analyzed using a large data set of multiple ScSn waves, reflected n times at the core-mantle boundary and (n-1) times at the surface. The data are the transverse components of deep earthquakes from the subduction zones north and east of Australia, recorded at stations in Antarctica, Australia, Indonesia, New Caledonia and New Zealand. The data are filtered with narrow band-pass filters at 5 frequencies in the range 0.013-0.040 Hz. The ScSn+1/ScSn amplitude ratios of successive ScS phases are compared to the ratios computed for synthetic seismograms for the same paths and same focal mechanisms, in order to eliminate the effects of source radiation and geometric attenuation. The synthetic seismograms are computed from a summation of toroidal modes for the 1-D reference model PREM (Dziewonski & Anderson 1981). The observed to computed spectral ratios appear consistent for similar paths. They reveal that the attenuation is not frequency dependent, that the contribution of scattering to attenuation is low, and that the PREM model is a valuable reference model for the study region at the considered frequencies. An inversion of the data at 0.026 Hz is performed to retrieve the quality factor Q in the upper mantle, in regions defined using a priori constraints inferred from seismic shear velocities. Q-values close to those of PREM are found beneath the Australian and Antarctic cratons, lower values beneath the Eastern Australian Phanerozoic margin, and very low values beneath the oceanic region between Australia and Antarctica, where ridges and a triple junction are present. The Australian-Antarctic Discordance along the South-Indian ridge appears as an exception with a Q-value close to those of stable continents. The highest Q-values are found beneath the subduction zones, a feature which is not apparent in global attenuation models possibly because of its narrow lateral extension, and because it extends at depths larger than those sampled by surface waves. Despite limitations due to the uneven distribution of the ScSn bounce points at the surface and to the difficulty of collecting a large number of high quality data, our approach appears very promising. It is complementary to the more widely used determination of seismic attenuation using surface waves because it provides increased depth coverage, and a broader spectral coverage. It therefore has a considerable potential in future investigations of mantle structure and dynamics.

Goldschmidt Abstracts 2010 – H

Recent advances in analytical mass spectrometry techniques dramatically increases the number of isotopic data. Nowadays, the complete analysis of Sr, Nd, Pb, Hf and He isotopic compositions for a set of samples has become achievable. Working with such a multidimensional dataset implies a new approach in data interpretation, preferably based on statistical analysis techniques. Principal Component Analysis (PCA) is a powerful mathematical tool to study this type of multidimensional data set. The goal of PCA is to reduce the number of dimensions in a data set by keeping only those that contribute most to its variance. Samples collected during the PACANTARCTIC 2 cruise fill a sampling gap from 53° to 41° S of the Pacific Antarctic Ridge (PAR). Sr, Nd, Pb, Hf and He isotopic compositions of this new mid-ocean ridge basalt collection are shown together with published data from 66°S to 53°S and from the EPR. Using the PCA tool, it becomes possible to get a statistical picture of the geochemical variations along the entire PAR, from the Australian Antarctic Discordance (AAD) to the Juan Fernandez microplate. Based on the incomplete sampling of the ridge a previous study led to the identification of a large scale division of the south Pacific mantle with a limit at the latitude of Easter Island [1]. The complete dataset reveals a different geochemical profile. Along the Pacific ridge, a large scale variation reaches an extremum which corresponds to a less ‘depleted’ isotopic signature at the Juan Fernandez microplate latitude. Hot spot-ridge interactions are marked by anomalies superimposed on this curve. The PCA method allows to interpret this large scale variation as a progressive geochemical change of the depleted matrix of the mantle. This variation is unrelated to the effect of the hot spot-ridge interactions.

Australian Antarctic Discordance as a simple mantle boundary

Several complex models require unique mantle conditions to explain the Australian Antarctic Discordance (AAD), an unusually deep and rugged section of the Southeast Indian Ridge (SEIR) between ∼120°–128°E. Seismic evidence suggests the AAD is instead the manifestation of two contrasting mantle domains converging along its eastern edge. Variations in axial morphology and flanking topographic relief along the SEIR arise as ridge segments to the west (Indian mantle) grade into a cooler melting regime while those to the east (Pacific mantle) are more magmatically robust. Seismic refraction data show crustal thickness decreases from the west into the AAD at a rate of 0.1 km/100 km, then rapidly increases from 4.8 ± 0.4 km to 7.3 ± 0.2 km across the eastern border. The AAD thus appears to be the terminal end of a long‐wavelength reduction in melt supply at what may be the simplest global example of a mantle boundary.

Distribution of recycled crust within the upper mantle: Insights from the oxygen isotope composition of MORB from the Australian‐Antarctic Discordance

Geochemical heterogeneity within the mantle has long been recognized through the diversity of trace element and radiogenic isotopic compositions of mantle‐derived rocks, yet the specific origin, abundance, and distribution of enriched material within the mantle have been difficult to quantify. In particular, the origin of the distinctive geochemical characteristics of Indian mantle has been debated for decades. We present new laser fluorination oxygen isotope measurements of mid‐ocean ridge basalt from the Australian‐Antarctic Discordance (AAD), an area where a particularly abrupt transition occurs between Pacific‐type mid‐ocean ridge basalts (MORB) and Atlantic‐type MORB. These data show no distinction in average δ18O between Pacific‐ and Atlantic‐type MORB, indicating that the origin of Indian‐type mantle cannot be attributed to the presence of pelagic sediment. The combined radiogenic isotope, δ18O, and trace element characteristics of Indian‐type MORB at the AAD are consistent with contamination of the Indian upper mantle by lower crustal material. We also present a compilation of available laser fluorination δ18O data for MORB and use these data to evaluate the nature and percentage of enriched material within the upper mantle globally. Data for each ocean basin fit a normal distribution, with indistinguishable means and standard deviations, implying that the variation in δ18O of MORB reflects a stochastic process that operates similarly across all ocean basins. Monte Carlo simulations show that the mean and standard deviation of the MORB data are robust indicators of the mean and standard deviation of the parent distribution of data. Further, although some skewness in the data cannot be ruled out, Monte Carlo results are most consistent with a normal parent distribution. This similarity in characteristics of the δ18O data between ocean basins, together with correlations of δ18O with radiogenic isotope and trace element characteristics of subsets of the data, suggest that the upper mantle globally contains an average of ∼5–10% recycled crustal material and that the depleted mantle in the absence of this component would have δ18O of ∼5.25‰. The Monte Carlo simulations also suggest that additional oxygen isotope data may be used in the future to test the ability of geodynamical models to predict the physical distribution of enriched domains within the upper mantle.

Integrating deep Earth dynamics in paleogeographic reconstructions of Australia

It is well documented that the Cenozoic progressive flooding of Australia, contemporaneous with a eustatic sea level fall, requires a downward tilting of the Australian Plate towards the SE Asian subduction system. Previously, this large-scale, mantle-convection driven dynamic topography effect has been approximated by computing the time-dependent vertical shifts and tilts of a plane, but the observed subsidence and uplift anomalies indicate a more complex interplay between time-dependent mantle convection and plate motion. We combine plate kinematics with a global mantle backward-advection model based on shear-wave mantle tomography, paleogeographic data, eustatic sea level estimates and basin stratigraphy to reconstruct the Australian flooding history for the last 70 Myrs on a continental scale. We compute time-dependent dynamic surface topography and continental inundation of a digital elevation model adjusted for sediment accumulation. Our model reveals two evolving dynamic topography lows, over which the Australian plate has progressively moved. We interpret the southern low to be caused by sinking slab material with an origin along the eastern Gondwana subduction zone in the Cretaceous, whereas the northern low, which first straddles northern Australia in the Oligocene, is mainly attributable to material subducted north and northeast of Australia. Our model accounts for the Paleogene exposure of the Gulf of Carpentaria region at a time when sea level was much higher than today, and explains anomalous Late Tertiary subsidence on Australia's northern, western and southern margins. The resolution of our model, which excludes short-wavelength mantle density anomalies and is restricted to depths larger than 220 km, is not sufficient to model the two well recorded episodes of major transgressions in South Australia in the Eocene and Miocene. However, the overall, long-wavelength spatio-temporal pattern of Australia's inundation record is well captured by combining our modelled dynamic topography with a recent eustatic sea level curve. We suggest that the apparent Late Cenozoic northward tilting of Australia was a stepwise function of South Australia first moving away northwards from the Gondwana subduction-related dynamic topography low in the Oligocene, now found under the Australian–Antarctic Discordance, followed by a drawing down of northern Australia as it overrode a slab burial ground now underlying much of the northern half of Australia, starting in the Miocene. Our model suggests that today's geography of Australia is strongly dependent on mantle forces. Without mantle convection, which draws Australia down by up to 300 m, nearly all of Australia's continental shelves would be exposed. We conclude that dissecting the interplay between eustasy and mantle-driven dynamic topography is critical for understanding hinterland uplift, basin subsidence, the formation and destruction of shallow epeiric seas and their facies distribution, but also for the evolution of petroleum systems.

Constraints on asthenospheric flow from the depths of oceanic spreading centers: The East Pacific Rise and the Australian‐Antarctic Discordance

The fastest spreading center, the East Pacific Rise (EPR), is consistently deeper than most other spreading centers. Its average depth along the ∼5000 km length is ∼3100 m, while the mean depth along the adjacent, slower spreading Pacific‐Antarctic Rise (PAR) is ∼2700 m. The deepest spreading center is the ∼4000 m deep, ∼1000 km long Australian Antarctic Discordance (AAD). Analytic and numerical models show that dynamic thinning of asthenosphere can explain the magnitude and wavelength of the depth anomalies along the EPR and AAD. Previous models did not show such significant depth anomalies along spreading centers because the equations used to describe flow of thin viscous asthenosphere were linearized. At the EPR, fast plate divergence thins the asthenosphere by both sequestering it into diverging lithosphere and dragging it with the plates in contrast to the slower spreading but faster migrating PAR. The AAD asthenosphere is starved because of the restriction of asthenospheric flow due to nearby thick continental lithospheric roots combined with a moderately fast spreading rate. A narrow range of asthenospheric properties explains observed depth anomalies for both the AAD and EPR. If the asthenosphere is ∼100°C hotter, and so ∼10 kg/m3 less dense than the underlying asthenosphere, then the model requires an average asthenospheric thickness of ∼250 km and a viscosity of ∼1019 Pa s. Thinner asthenosphere works in the model if it has a lower density and lower viscosity. Although the source of the asthenosphere is not critical to our models, we assume that it is supplied by upwelling of depleted mantle plumes in contrast to enriched plumes that feed oceanic island basalts.

Constraints on the mantle temperature gradient along the Southeast Indian Ridge from crustal structure and isostasy: implications for the transition from an axial high to an axial valley

Axial and ridge flank depths on the Southeast Indian Ridge (SEIR) systematically increase for 2500 km from 90°E to 120°E approaching the Australian-Antarctic Discordance. The SEIR also experiences an abrupt change in ridge axis morphology near 103°30′E with axial highs found to the west and axial valleys to the east. Since the spreading rate is constant throughout this region, these variations have been ascribed to an along-axis gradient in mantle temperature. A seismic refraction experiment provides information on the crustal thickness and seismic velocity structure of two segments with differing axial morphology. Segment P2, centred near 102°E with an axial high has a mean crustal thickness of 5.9 ± 0.2 km, while the mean crustal thickness is 5.3 ± 0.3 km at Segment S1 with an axial valley and centred near 109°45′E. Isostatic compensation of the difference in crustal thickness and density structure between the two segments only accounts for 33 m of the 198 m difference in average ridge flank depth between the two segments. The remaining depth difference must result from a difference in mantle density. Melt production models imply a mantle temperature difference of 11–13.5 °C to produce the observed difference in crustal thickness. Isostatic compensation of the two segments requires that the resulting density difference must extend to about 300 km in the mantle. The transition in axial morphology along the SEIR is very abrupt occurring over a narrow zone within a single segment in which the transition is complete. If a linear mantle temperature gradient is assumed, the temperature difference across the transition segment is only 2.4 °C. The change in axial morphology is accompanied by abrupt changes in other parameters including abyssal hill height, magnetic anomaly amplitude, layer 2a thickness and the presence or absence of an axial magma lens. The abrupt, coincident change in a number of parameters with a very small change in mantle temperature strongly suggests a threshold change between two distinctly different modes of crustal accretion. The trigger for the transition appears to be whether a steady-state crustal magma lens can be maintained.

Long-wavelength tilting of the Australian continent since the Late Cretaceous

Global sea level and the pattern of marine inundation on the Australian continent are inconsistent. We quantify this inconsistency and show that it is partly due to a long wavelength, anomalous, downward tilting of the continent to the northeast by 300 m since the Eocene. This downward tilting occurred as Australia approached the subduction systems in South East Asia and is recorded by the progressive inundation of the northern margin of Australia. From the Oligocene to the Pliocene, the long wavelength trend of anomalous topography shows that the southern margin of Australia is characterized by relative subsidence. We quantify the anomalous topography of the Australian continent by computing the displacement needed to reconcile the interpreted pattern of marine incursion with a predicted topography in the presence of global sea level variations. On the southern margin, long wavelength subsidence was augmented by at least 250 m of shorter wavelength anomalous subsidence, consistent with the passage of the southern continental margin over a north–south elongated, 500 km wide, topographic anomaly approximately fixed with respect to the mantle. The present day reconstructed position of this depth anomaly is aligned with the Australian Antarctic Discordance and is consistent with the predicted passage of the Australian continent over a previously subducted slab. Both the long-wavelength continental tilting and smaller-scale paleo-topographic anomaly on the southern Australian margin may have been caused by subduction-generated dynamic topography. These new constraints on continental vertical motion are consistent with the hypothesis that mantle convection induced topography is of the same order of magnitude as global sea level change.

Глубинное строение Антарктической литосферной плиты по границе вдоль срединно-океанических хребтов по секущим вертикальным разрезам и латеральным сечениям на различных глубинах

3D vertical structure of the Antarctic Plate's boundary along longitudinal cross-sections and lateral slices at different depths is displayed through a distribution of density anomalies relative to Preliminary Reference Earth Model (PREM) using the harmonic coefficients of the EGM96 geoid model. Features of interaction between the Antarctic Plate and other plates are shown with our gravimetric tomography data over more than 40,000 km along the palte boundary. Two bodies (plumes) dominate in the mantle. Less dense masses ascend from depth of 2800 km and then split up at the depth of 200 km as three branches to the Australian Antarctic Discordance (AAD), the Ross Sea and the Nazca plate. Dense masses descend from a surface as subducted slabs and collect at depths of 60 km and 280 km. It was discovered in the AAD area and the Nazca Ridge that thinning hot masses penetrate into the colder crust and lithosphere.

Crustal thickness variations along the Southeast Indian Ridge (100°–116°E) from 2‐D body wave tomography

Axial morphology along the Southeast Indian Ridge (SEIR) systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100°–116°E, west of the Australian‐Antarctic Discordance (AAD). Basalt geochemistry has a consistent Indian–mid‐ocean ridge basalt (MORB) type isotopic signature, so changes in axial topography are attributed to variations in both mantle temperature and melt supply. Wide‐angle seismic refraction lines were shot to four ocean bottom hydrophones within SEIR segments P1, P2, S1, and T, where each segment is characterized by a different morphology. We constructed 2‐D crustal velocity models by jointly inverting hand‐picked P wave refraction (Pg) and Moho reflection (PmP) traveltime data using a top‐down, minimum‐structure methodology. The results show a 1.5 km eastward decrease in crustal thickness across the study area, with segment averages ranging from 6.1 km at P1 to 4.6 km at T. Melt generation models require a ∼30°C decrease in mantle temperature toward the AAD to account for the crustal thickness trend. Significant changes in axial morphology accompany small‐scale variations in crustal thickness, consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature, melt supply, and cooling from hydrothermal circulation. Layer 3 thins by 3.0 km as layer 2 thickens by 1.4 km between segments P1 and T, reflecting the eastward decrease in melt supply and increase in melt lens depth. The trade‐off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material.

Thermally induced brittle deformation in oceanic lithosphere and the spacing of fracture zones

Brittle deformation of oceanic lithosphere due to thermal stress is explored with a numerical model, with an emphasis on the spacing of fracture zones. Brittle deformation is represented by localized plastic strain within a material having an elasto-visco-plastic rheology with strain softening. We show that crustal thickness, creep strength, and the rule governing plastic flow control the formation of cracks. The spacing of primary crack decreases with crustal thickness as long as it is smaller than a threshold value. Creep strength shifts the threshold such that crust with strong creep strength develops primary cracks regardless of crustal thicknesses, while only a thin crust can have primary cracks if its creep strength is low. For a thin crust, the spacing of primary cracks is inversely proportional to the creep strength, suggesting that creep strength might independently contribute to the degree of brittle deformation. Through finite versus zero dilatation in plastic strain, associated and non-associated flow rule results in nearly vertical and V-shaped cracks, respectively. Changes in the tectonic environment of a ridge system can be reflected in variation in crustal thickness, and thus related to brittle deformation. The fracture zone-free Reykjanes ridge is known to have a uniformly thick crust. The Australian-Antarctic Discordance has multiple fracture zones and thin crust. These syntheses are consistent with enhanced brittle deformation of oceanic lithosphere when the crust is thin and vice versa.

Heat flow from the Southeast Indian Ridge flanks between 80°E and 140°E: Data review and analysis

We analyze available heat flow data from the flanks of the Southeast Indian Ridge adjacent to or within the Australian‐Antarctic Discordance (AAD), an area with patchy sediment cover and highly fractured seafloor as dissected by ridge‐ and fracture‐parallel faults. The data set includes 23 new data points collected along a 14‐Ma old isochron and 19 existing measurements from the 20‐ to 24‐Ma old crust. Most sites of measurements exhibit low heat flux (from 2 to 50 mW m−2) with near‐linear temperature‐depth profiles except at a few sites, where recent bottom water temperature change may have caused nonlinearity toward the sediment surface. Because the igneous basement is expected to outcrop a short distance away from any measurement site, we hypothesize that horizontally channelized water circulation within the uppermost crust is the primary process for the widespread low heat flow values. The process may be further influenced by vertical fluid flow along numerous fault zones that crisscross the AAD seafloor. Systematic measurements along and across the fault zones of interest as well as seismic profiling for sediment distribution are required to confirm this possible, suspected effect.

Thermal regime of the Southeast Indian Ridge between 88°E and 140°E: Remarks on the subsidence of the ridge flanks

The flanks of the Southeast Indian Ridge are characterized by anomalously low subsidence rates for the 0–25 Ma period: less than 300 m Ma−1/2 between 101°E and 120°E and less than 260 m Ma−1/2 within the Australian‐Antarctic Discordance (AAD), between 120°E and 128°E. The expected along‐axis variation in mantle temperature (∼50°C) is too small to explain this observation, even when the temperature dependence of the mantle physical properties is accounted for. We successively analyze the effect on subsidence of different factors, such as variations in crustal thickness; the dynamic contribution of an old, detached slab supposedly present within the mantle below the AAD; and depletion in ϕm, a parameter here defined as the “ubiquitously distributed melt fraction” within the asthenosphere. These effects may all contribute to the observed, anomalously low subsidence rate of the ridge flanks, with the most significant contribution being probably related to the depletion in ϕm. However, these effects have a deep‐seated origin that cannot explain the abruptness of the transition across the fracture zones that delineate the boundaries of the AAD, near 120°E and near 128°E, respectively.

The tilting continent: A new constraint on the dynamic topographic field from Australia

A pronounced latitudinal asymmetry in the present-day morphology of the Australian continental shelf is reflected in Neogene stratigraphic relationships. The northern Australian margin has a broad shelf, typically 200–500 km wide and a Neogene record of stratal onlap. Relative to the continent, sea levels are currently as high as at any stage during the Neogene. In contrast, the southern shelf is typically less than 100 km wide and shows a record of progressive offlap with Neogene palaeo-shorelines commonly many hundreds of kilometres inland, at elevations up to ∼ 250 m above present-day sea level. This continental-scale ‘reciprocal’ stratigraphy implies 250–300 m N-down, SSW-up apparent vertical motion with respect to sea level since the mid-Miocene. The apparent vertical motion can be attributed to variations in dynamic topography and the geoid along Australia's NNE plate circuit; specifically, the movement of the southern margin off the dynamic topography low, geoid low presently centred on the Australian–Antarctic discordance, and the northern margin towards a dynamic topography low, geoid high associated with the subduction realm to the north. Variations in the geoid appear to account for about 10% of the total apparent motion, depending on assumptions about how the geoid field has evolved during Australia's northward motion. This inferred Neogene, continental-scale dynamic N–S tilting rate of ∼15–20 m/myr provides a compelling new constraint on the nature of the Earth's dynamic topographic field.