Joanne M. Whittaker

Constraining the Jurassic extent of Greater India: Tectonic evolution of the West Australian margin

Alternative reconstructions of the Jurassic northern extent of Greater India differ by up to several thousand kilometers. We present a new model that is constrained by revised seafloor spreading anomalies, fracture zones and crustal ages based on drillsites/dredges from all the abyssal plains along the West Australian margin and the Wharton Basin, where an unexpected sliver of Jurassic seafloor (153 Ma) has been found embedded in Cretaceous (95 My old) seafloor. Based on fracture zone trajectories, this NeoTethyan sliver must have originally formed along a western extension of the spreading center that formed the Argo Abyssal Plain, separating a western extension of West Argoland/West Burma from Greater India as a ribbon terrane. The NeoTethyan sliver, Zenith and Wallaby plateaus moved as part of Greater India until westward ridge jumps isolated them. Following another spreading reorganization, the Jurassic crust resumed migrating with Greater India until it was re-attached to the Australian plate ∼95 Ma. The new Wharton Basin data and kinematic model place strong constraints on the disputed northern Jurassic extent of Greater India. Late Jurassic seafloor spreading must have reached south to the Cuvier Abyssal Plain on the West Australian margin, connected to a spreading ridge wrapping around northern Greater India, but this Jurassic crust is no longer preserved there, having been entirely transferred to the conjugate plate by ridge propagations. This discovery constrains the major portion of Greater India to have been located south of the large-offset Wallaby-Zenith Fracture Zone, excluding much larger previously proposed shapes of Greater India.

An open-source software environment for visualizing and refining plate tectonic reconstructions using high-resolution geological and geophysical data sets

We describe a powerful method to explore spatio-temporal relationships within geological and geophysical data sets by analyzing the data within the context of tectonic reconstructions. GPlates is part of a new generation of plate reconstruction software that incorporates functionality familiar from GIS software with the added dimension of geological time. Here we use GPlates to reconstruct geological terranes, geophysical grids, and paleomagnetic data within alternative tectonic models of the assembly of Western Australia and the configuration of Rodinia. With the ability to rapidly visualize a diverse range of geological and geophysical constraints within different reconstructions, users can easily investigate the implications of different tectonic models for reconciling a variety of observations and make more informed choices between different models and data.

Ridge subduction sparked reorganization of the Pacific plate-mantle system 60-50 million years ago

A reorganization centered on the Pacific plate occurred ~53–47 million years ago. A “top-down” plate tectonic mechanism, complete subduction of the Izanagi plate, as opposed to a “bottom-up” mantle flow mechanism, has been proposed as the main driver. Verification based on marine geophysical observations is impossible as most ocean crust recording this event has been subducted. Using a forward modeling approach, which assimilates surface plate velocities and shallow thermal structure of slabs into mantle flow models, we show that complete Izanagi plate subduction and margin-wide slab detachment induced a major change in sub-Pacific mantle flow, from dominantly southward before 60 Ma to north-northeastward after 50 Ma. Our results agree with onshore geology, mantle tomography, and the inferred motion of the Hawaiian hot spot and are consistent with a plate tectonic process driving the rapid plate-mantle reorganization in the Pacific hemisphere between 60 and 50 Ma. This reorganization is reflected in tectonic changes in the Pacific and surrounding ocean basins.

Onset of Antarctic Circumpolar Current 30 million years ago as Tasmanian Gateway aligned with westerlies

Earth’s mightiest ocean current, the Antarctic Circumpolar Current (ACC), regulates the exchange of heat and carbon between the ocean and the atmosphere, and influences vertical ocean structure, deep-water production and the global distribution of nutrients and chemical tracers. The eastward-flowing ACC occupies a unique circumglobal pathway in the Southern Ocean that was enabled by the tectonic opening of key oceanic gateways during the break-up of Gondwana (for example, by the opening of the Tasmanian Gateway, which connects the Indian and Pacific oceans). Although the ACC is a key component of Earth’s present and past climate system, the timing of the appearance of diagnostic features of the ACC (for example, low zonal gradients in water-mass tracer fields) is poorly known and represents a fundamental gap in our understanding of Earth history. Here we show, using geophysically determined positions of continent–ocean boundaries, that the deep Tasmanian Gateway opened 33.5 ± 1.5 million years ago (the errors indicate uncertainty in the boundary positions). Following this opening, sediments from Indian and Pacific cores recorded Pacific-type neodymium isotope ratios, revealing deep westward flow equivalent to the present-day Antarctic Slope Current. We observe onset of the ACC at around 30 million years ago, when Southern Ocean neodymium isotopes record a permanent shift to modern Indian–Atlantic ratios. Our reconstructions of ocean circulation show that massive reorganization and homogenization of Southern Ocean water masses coincided with migration of the northern margin of the Tasmanian Gateway into the mid-latitude westerly wind band, which we reconstruct at 64° S, near to the northern margin. Onset of the ACC about 30 million years ago coincided with major changes in global ocean circulation and probably contributed to the lower atmospheric carbon dioxide levels that appear after this time.

Insights on the kinematics of the India‐Eurasia collision from global geodynamic models

The Eocene India-Eurasia collision is a first order tectonic event whose nature and chronology remains controversial. We test two end-member collision scenarios using coupled global plate motion-subduction models. The first, conventional model, invokes a continental collision soon after ∼60 Ma between a maximum extent Greater India and an Andean-style Eurasian margin. The alternative scenario involves a collision between a minimum extent Greater India and a NeoTethyan back-arc at ∼60 Ma that is subsequently subducted along southern Lhasa at an Andean-style margin, culminating with continent-continent contact at ∼40 Ma. Our numerical models suggest the conventional scenario does not adequately reproduce mantle structure related to Tethyan convergence. The alternative scenario better reproduces the discrete slab volumes and their lateral and vertical distribution in the mantle, and is also supported by the distribution of ophiolites indicative of Tethyan intraoceanic subduction, magmatic gaps along southern Lhasa and a two-stage slowdown of India. Our models show a strong component of southward mantle return flow for the Tethyan region, suggesting that the common assumption of near-vertical slab sinking is an oversimplification with significant consequences for interpretations of seismic tomography in the context of subduction reference frames.

Displacement along the Red River Fault constrained by extension estimates and plate reconstructions

Significant E-W extension and/or compression must have been generated by displacements along the Red River Fault (RRF) since its curvature does not match a small circle centered at the Euler pole for the Indochina–south China plate pair. The amount of extension perpendicular to the RRF offshore Vietnam depends on the magnitude of left-lateral displacement along the RRF. In general, the larger the left-lateral displacement along the fault, the smaller the amount of E-W extension. All purely strike-slip models of the opening of the South China Sea that assume large displacements (>250 km) along the RRF encounter major problems because they imply little extension, or even considerable shortening, offshore east Vietnam. This is inconsistent with the presence of large elongated basins offshore Vietnam. Using a plate tectonic model, we compare continental extension values implied by different magnitudes of displacement along the RRF with crustal stretching estimates derived from 2-D profiles modeled from gravity data. We utilize 2-D gravity forward models to restore the extended continental margin crust to its original position prior to extension. We find that substantial amounts of extension for offshore Vietnam can only be modeled assuming moderate displacements along the RRF compatible with the presence of a southward subducting proto–South China Sea. The total amount of ENE-WSW extension offshore northern Vietnam constrained by our 2-D gravity profiles and gravity inversion increases southward from 36 to 89 km along the Yinggehai Basin. These values of ENE-WSW extension are consistent with 250 km of left-lateral displacement along the RRF.

Community infrastructure and repository for marine magnetic identifications

Magnetic anomaly identifications underpin plate tectonic reconstructions and form the primary data set from which the age of the oceanic lithosphere and seafloor spreading regimes in the ocean basins can be determined. Although these identifications are an invaluable resource, their usefulness to the wider scientific community has been limited due to the lack of a central community infrastructure to organize, host, and update these interpretations. We have developed an open-source, community-driven online infrastructure as a repository for quality-checked magnetic anomaly identifications from all ocean basins. We provide a global sample data set that comprises 96,733 individually picked magnetic anomaly identifications organized by ocean basin and publication reference, and provide accompanying Hellingerformat files, where available. Our infrastructure is designed to facilitate research in plate tectonic reconstructions or research that relies on an assessment of plate reconstructions, for both experts and nonexperts alike. To further enhance the existing repository and strengthen its value, we encourage others in the community to contribute to this effort.

Semiautomatic fracture zone tracking

Oceanic fracture zone traces are widely used in studies of seafloor morphology and plate kinematics. Satellite altimetry missions have resulted in high-resolution gravity maps in which all major fracture zones and other tectonic fabric can be identified, and numerous scientists have digitized such lineaments. We have initiated a community effort to maintain low-cost infrastructure that allows seafloor fabric lineaments to be stored, accessed, and updated. A key improvement over past efforts is our processing software (released as a GMT5 supplement) that allows for semiautomatic corrections to previously digitized fracture zone traces given improved gridded data sets. Here we report on our seafloor fabric processing tools, which complement our database of seafloor fabric lineations, magnetic anomaly identifications, and plate kinematic models.

Development of the Australian-Antarctic depth anomaly

The oceanic Australian-Antarctic Discordance (AAD) contains two unusual features: (1) N–S trending anomalously deep bathymetries and (2) rough basement morphologies in young ( 45° spreading obliquities.

How supercontinents and superoceans affect seafloor roughness

Seafloor roughness varies considerably across the world’s ocean basins and is fundamental to controlling the circulation and mixing of heat in the ocean and dissipating eddy kinetic energy. Models derived from analyses of active mid-ocean ridges suggest that ocean floor roughness depends on seafloor spreading rates, with rougher basement forming below a half-spreading rate threshold of 30–35 mm yr-1 (refs 4, 5), as well as on the local interaction of mid-ocean ridges with mantle plumes or cold-spots. Here we present a global analysis of marine gravity-derived roughness, sediment thickness, seafloor isochrons and palaeo-spreading rates of Cretaceous to Cenozoic ridge flanks. Our analysis reveals that, after eliminating effects related to spreading rate and sediment thickness, residual roughness anomalies of 5–20 mGal remain over large swaths of ocean floor. We found that the roughness as a function of palaeo-spreading directions and isochron orientations indicates that most of the observed excess roughness is not related to spreading obliquity, as this effect is restricted to relatively rare occurrences of very high obliquity angles (>45°). Cretaceous Atlantic ocean floor, formed over mantle previously overlain by the Pangaea supercontinent, displays anomalously low roughness away from mantle plumes and is independent of spreading rates. We attribute this observation to a sub-Pangaean supercontinental mantle temperature anomaly leading to slightly thicker than normal Late Jurassic and Cretaceous Atlantic crust, reduced brittle fracturing and smoother basement relief. In contrast, ocean crust formed above Pacific superswells, probably reflecting metasomatized lithosphere underlain by mantle at only slightly elevated temperatures, is not associated with basement roughness anomalies. These results highlight a fundamental difference in the nature of large-scale mantle upwellings below supercontinents and superoceans, and their impact on oceanic crustal accretion.