Stewart Fishwick

Scales of transient convective support beneath Africa

Beneath sub-equatorial Africa, there is convincing evidence for a gigantic, slow, velocity anomaly within the lower mantle that is inferred to be a thermochemical superplume. Here, we define the spatial and temporal pattern of uplift across the northern edge of this putative superplume by analyzing stratal geometries and stacking velocities of Neogene sedimentary rocks. These marine deposits are imaged on a dense grid of seismic reflection profiles that straddle the southwest African coastal shelf. Inverse modeling of stacking velocity profiles demonstrates that there have been at least two significant phases of uplift. A post-Pliocene uplift event increases from 0 to ~500 m elevation over a distance of ~1000 km, in close agreement with geophysical estimates of the present-day variation of dynamic topography. An earlier phase of uplift occurred in middle Oligocene time (ca. 30–35 Ma ago). We propose a two-stage development of epeirogeny in which a Middle Oligocene phase of uplift may be related to initiation and growth of the African superplume. Post-Pliocene denudation records rapid (100–250 m/Ma) vertical motions generated by upper mantle convection, which vary on length scales of 500–1000 km. Our results support the existence of a convective circulation system that has operated on different temporal and spatial scales beneath the African plate.

3-D structure of the Australian lithosphere from evolving seismic datasets

During the last 20 years, seismic tomography has frequently been used to provide information on the structure of the lithosphere beneath the Australian continent. New tomographic models are presented using two complementary seismological techniques in order to illustrate the current state-of-knowledge. Surface wave tomography is the ideal method to obtain information of velocity variations across the whole continent. The latest models use data from over 13 000 source–receiver paths, allowing a higher resolution than in previous studies using the same technique. In Western Australia the results at 100 km depth clearly reveal the contrast in structure between the Pilbara and Yilgarn Cratons and the Capricorn Orogen. At greater depths, the Kimberley Block has a distinct fast velocity anomaly in comparison with the surrounding mobile belts. In the east of the continent, strong horizontal gradients in velocity indicate transitions in lithospheric structure, although the new high resolution models reveal a complexity in the transitions through central Victoria and New South Wales. Complementing the surface wave tomography, we also present the results from the inversion of over 25 000 relative arrival times from body wave phases recorded in southeast Australia and Tasmania. The body wave tomography uses the surface wave model to provide information on long-wavelength structure and absolute velocities that would otherwise be lost. The new results indicate a distinct boundary between the Delamerian and Lachlan orogens within the upper mantle, the location of which is consistent with an east-dipping Moyston Fault, as observed by deep seismic reflection profiling. The new models also confirm a distinct region of fast velocities beneath the central sub province of the Lachlan Orogen. A significant new observation is that the inferred eastern edge of this central sub-province has a strong correlation with the location of copper/gold deposits; a similar relationship is observed at a larger scale in Western Australia where mineral deposits appear to flank the regions of fastest velocity within the West Australian Craton.

Contrasts in mantle structure beneath Australia: Relation to Tasman Lines?

Surface‐wave tomography for the Australian region, using data mostly from portable seismic recorders, reveals a very strong contrast in seismic shear‐wave speed beneath central and western Australia and the east of the continent. Shear‐wave speeds faster than the continental average extend to at least 200 km depth in the cratonic zone to the west of 140°E. Along an approximately north‐south line there is then an eastward step to thinner lithosphere (∼150 km thick) but still with fast shear‐wave speeds. A further more irregular transition to the east marks the transition to lowered shear‐wave speeds. The eastern transition at 75 km depth is in close agreement with the original Tasman Line, whereas the two more westerly transitions do not bear a simple relation to the more recent group of Tasman Lines defined from crustal information (outcrop and inferred geophysical trends). The westward transition to the thickest coherent lithosphere (near 140°E) may well mark the edge of the ancient core of the continent, but the current mantle structures must bear the scars of the breakups and reassembly that have created the current Australian continent.

Towards a better understanding of African topography: a review of passive-source seismic studies of the African crust and upper mantle

Abstract Explaining the cause and support of Africas varied topography remains a fundamental question for our understanding of the long-term evolution of the continent. As geodynamical modelling becomes more frequently used to investigate this problem, it is important to understand the seismological results that can be incorporated into these models. Crustal thickness estimates are crucial for calculating components of topography that are isostatically compensated. Variations in seismic velocity help constrain variations in subsurface temperature and density and thus buoyancy; measurements of anisotropy can also be used to determine the contribution of the mantle flow field to dynamic topography. In this light, we review the results of passive seismic studies across Africa. At the continental scale there are significant differences in crustal models, meaning large uncertainties in corrections for isostatic topography. In east Africa, multiple seismic experiments have provided firm constraints on crustal and mantle structure. Tomographic images illuminate a broad (c. 500 km wide) low-velocity region in the upper mantle, with possible connection to the African Superplume in the lower mantle. These observations, alongside the variations in radial anisotropy, strongly suggest that the mantle flow field contributes significantly to the uplift of the region. Beneath southern Africa, low velocities are observed near the base of the continental lithosphere; the depth to transition zone discontinuities however suggests that they are not linked to the superplume beneath. It is thus less clear what role the sublithospheric mantle plays in supporting the regions high topography. Many of Africas secondary topographic features (e.g. Atlas, Hoggar, Bie Dome) are underlain by slow velocities at depths of 100–150 km and are adjacent to rapid changes in lithospheric thickness. Whether these variations in lithospheric structure promote small-scale convection or simply guide the larger-scale mantle flow field remains ambiguous.

Spatial and temporal patterns of Australian dynamic topography from River Profile Modeling

Despite its importance, the temporal and spatial evolution of continental dynamic topography is poorly known. Australias isolation from active plate boundaries and its rapid northward motion within a hot spot reference frame make it a useful place to investigate the interplay between mantle convection, topography, and drainage. Offshore, dynamic topography is relatively well constrained and can be accounted for by Australias translation over the mantles convective circulation. To build a database of onshore constraints, we have analyzed an inventory of longitudinal river profiles, which is sensitive to uplift rate history. Using independently constrained erosional parameters, we determine uplift rates by minimizing the misfit between observed and calculated river profiles. Resultant fits are excellent and calculated uplift histories match independent geologic constraints. We infer that western and central Australia underwent regional uplift during the last 50 Myr and that the Eastern Highlands have been uplifted in two stages. The first stage from 120 to 80 Ma, coincided with rifting along the eastern margin and its existence is supported by thermochronological measurements. A second stage occurred at 80–10 Ma, formed the Great Escarpment, and coincided with Cenozoic volcanism. The relationship between topography, gravity anomalies, and shear wave tomographic models suggest that regional elevation is supported by temperature anomalies within the lithospheres thermal boundary layer. Morphology and stratigraphy of the Eastern Highlands imply that these anomalies have been coupled to the base of the plate during Australias northward motion over the last 70 Myr.

Multiple mantle upwellings in the transition zone beneath the northern East-African Rift system from relative P-wave travel-time tomography

Mantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low-shear-velocity province beneath Southern Africa, the so-called African Superplume, to multiple lower-mantle sources along the rift. We present a new P-wave travel-time tomography model below the northern East-African, Red Sea, and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated data set allows us to image structures of 100 km length-scale down to depths of 700– 800 km beneath the study region. Our images provide evidence of two clusters of low-velocity structures consisting of features with diameter of 100–200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off-rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100 6 50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low-velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep-plume material below the transition zone may be spawning these upper mantle upwellings.

The Upper Mantle Seismic Velocity Structure of South-Central Africa and the Seismic Architecture of Precambrian Lithosphere Beneath the Congo Basin

The seismic architecture of the lithosphere beneath the Congo Basin is investigated using a new shear wave velocity model of the upper mantle for central and southern Africa derived from an inversion of Rayleigh wave group velocity measurements. The model is similar to other tomographic models derived from Rayleigh wave phase velocities, revealing a region of fast upper mantle velocities in the 50–100 km depth interval beneath the northwestern, central and southern portions of the basin, and slower upper mantle velocities beneath the northeastern part of the basin, as well as beneath Proterozoic mobile belts to the east and south of the basin. The upper mantle velocity pattern indicates that Proterozoic lithosphere may lie beneath the northeastern side of the basin, but it does not support the presence of Proterozoic lithosphere beneath the entire northern portion of the basin. This finding suggests that lithospheric structure beneath the basin is not uniform, as is commonly assumed in geodynamic models explaining how the basin formed. A second geodynamic implication concerns the Neoproterozoic rifting event that may have initiated basin subsidence. The proposed locations of the rifts are in the region of the velocity model where the velocities begin to change from faster to slower going from the center of the basin toward the northeast. Thus, the rifts may have formed along the border between two separate, smaller Archean blocks, as opposed to within the middle of a single, larger Archean block, alleviating the need to explain how a Neoproterozoic rift might form within the interior of a large Archean craton.

Small-scale thermal upwellings under the Northern East African Rift from S travel-time tomography

There is a long-standing debate over how many and what types of plumes underlie the East African Rift and whether they do or do not drive its extension and consequent magmatism and seismicity. Here we present a new tomographic study of relative teleseismic S and SKS residuals that expands the resolution from previous regional studies below the northern East African Rift to image structure from the surface to the base of the transition zone. The images reveal two low-velocity clusters, below Afar and west of the Main Ethiopian Rift, that extend throughout the upper mantle and comprise several smaller-scale (about 100 km diameter), low-velocity features. These structures support those of our recent P tomographic study below the region. The relative magnitude of S to P residuals is around 3.5, which is consistent with a predominantly thermal nature of the anomalies. The S and P velocity anomalies in the low-velocity clusters can be explained by similar excess temperatures in the range of 100–200°C, consistent with temperatures inferred from other seismic, geochemical, and petrological studies. Somewhat stronger VS anomalies below Afar than west of the Main Ethiopian Rift may include an expression of volatiles and/or melt in this region. These results, together with a comparison with previous larger-scale tomographic models, indicate that these structures are likely small-scale upwellings with mild excess temperatures, rising from a regional thermal boundary layer at the base of the upper mantle.

New approach to directional filtering of near-surface magnetic data

Topographical anomalies or compressed areas of soil caused by ploughing or more significant relic features such as ridge and furrow produce a response in near-surface magnetic surveys that are usually identified by their repetitive, linear pattern. While they are accurate recordings of the subsurface magnetic properties and micro-topographical features of the site, it is often the anomalies due to deeper features that are the primary focus of the survey. These target anomalies can be masked by the nearsurface pattern and it is therefore often preferable to remove these from the final presentation of the data. Two routines used for removing these features are common in commercial processing software. These are the directional pass/reject and cosine-taper filters. While these filtering techniques can dramatically improve the clarity of the data image, it is shown here that they make significant changes to the data that remain. As interpretation of near-surface magnetic data moves beyond image analysis towards more quantitative methods, it is important to ensure that the final processed data set represents as close as possible the response to the subsurface features of interest. Here an alternative filtering routine dependant on both the azimuth and power-content of the anomalies is proposed that overcomes the problems encountered by the traditional techniques. It is shown that patterns of agricultural linear anomalies can be removed from the data without significantly changing the properties of the desired responses and therefore quantitative interpretation can be subsequently carried out without the data being significantly compromised by the choice of previous processing techniques.