Matthew J. Fouch

Tectospheric structure beneath southern Africa

P-wave and S-wave delay times from the broadband data of the southern Africa seismic experiment have been inverted to obtain three-dimensional images of velocity perturbations in the mantle beneath southern Africa. High velocity mantle roots appear to extend to depths of at least 250 km, and locally to depths of 300 km beneath the Kaapvaal and Zimbabwe cratons. Thick roots are confined to the Archean cratons, with no evidence for similar structures beneath the adjacent Proterozoic mobile belts. The Kaapvaal craton was modified ca. 2.05 Ga by the Bushveld magmatic event, which affected a broad swath of cratonic mantle beneath and to the west of the exposed Bushveld Complex. The mantle beneath the extended Bushveld province is characterized by seismic velocities lower than those observed in regions of undisturbed cratonic mantle. The mantle beneath the Limpopo Belt, an Archean collisional zone sandwiched between the Kaapvaal and Zimbabwe cratons, exhibits a cratonic signature.

Shear wave splitting, continental keels, and patterns of mantle flow

In this study we investigated the origin of seismic anisotropy in the mantle beneath North America. In particular, we evaluated whether shear wave splitting patterns in eastern North America are better explained by anisotropy caused by lithospheric deformation, anisotropy due to mantle flow beneath the lithosphere, or a combination of both. We examined new measurements of shear wave splitting from the Missouri to Massachusetts broadband seismometer array (MOMA), the North American Mantle Anisotropy and Discontinuity experiment (NOMAD), as well as splitting parameters from several previous studies. We developed a simple finite difference model that approximates mantle flow around a complex, three-dimensional continental lithospheric keel. To evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern North America, we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model. Our results indicate that a significant portion of observed shear wave splitting in eastern North America can be explained by mantle flow around the continental keel. However, shear wave splitting patterns in a few regions of eastern North America indicate that a component of lithospheric anisotropy must exist, particularly in regions containing the largest keel thicknesses. For eastern North America, as well as for splitting observations in Australia, Europe, and South America, we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow.

An Overview of the Izu‐Bonin‐Mariana Subduction Factory

The Izu-Bonin-Mariana (IBM) arc system extends 2800km from near Tokyo, Japan to Guam and is an outstanding example of an intraoceanic convergent margin (IOCM). Inputs from sub-arc crust are minimized at IOCMs and output fluxes from the Subduction Factory can be more confidently assessed than for arcs built on continental crust. The history of the IBM IOCM since subduction began about 43 Ma may be better understood than for any other convergent margin. IBM subducts the oldest seafloor on the planet and is under strong extension. The stratigraphy of the western Pacific plate being subducted beneath IBM varies simply parallel to the arc, with abundant off-ridge volcanics and volcaniclastics in the south which diminish northward, and this seafloor is completely subducted. The Wadati-Benioff Zone varies simply along strike, from dipping gently and failing to penetrate the 660 km discontinuity in the north to plunging vertically into the deep mantle in the south. The northern IBM arc is about 22km thick, with a felsic middle crust; this middle crust is exposed in the collision zone at the northern end of the IBM IOCM. There are four Subduction Factory outputs across the IBM IOCM: (1) serpentinite mud volcanoes in the forearc, and as lavas erupted from along (2) the volcanic front of the arc and (3) back-arc basin and (4) from arc cross-chains. This contribution summarizes our present understanding of matter fed into and produced by the IBM Subduction Factory, with the intention of motivating scientific efforts to understand this outstanding example of one of earths most dynamic, mysterious, and important geosystems.

Mantle anisotropy beneath northwest Pacific subduction zones

To assess the location, strength, and orientation of seismic anisotropy in the southern Kuril, Japan, and Izu-Bonin subduction zones, we analyzed shear wave splitting in local S phases and teleseismic SKS phases. Fast directions from these phases are roughly parallel to the direction of absolute Pacific plate motion (∼WNW) in Izu-Bonin, roughly parallel to the strike of the trench (∼NNE) in central Honshu, and roughly N in the southern Kuril back arc in the vicinity of Sakhalin Island. Assuming that the orientation and strength of anisotropy does not vary with depth, modeling of splitting times from local S and teleseismic phases recorded at Sakhalin Island requires that the maximum depth of anisotropy lies between 480 km and 950 km. In contrast, splitting times for local S phases from events in Izu-Bonin rule out anisotropy in the transition zone. All the data are consistent with a model in which the lower transition zone (520–660 km) and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone (410–520 km), possibly due to preferred orientation of β spinel. Assuming that splitting in the upper mantle is produced by preferred orientation of olivine, the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones. The relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow, but that flow correlated with upper plate deformation must also exist.

Mantle seismic structure beneath the Kaapvaal and Zimbabwe Cratons

We present results of seismic tomography for a broad region of southern Africa using data from the seismic component of the Kaapvaal Project, a multinational, multidisciplinary experiment conducted in the late 1990s. Seismic images provide clear evidence of mantle structures that mimic the surface geology across the region and provide important constraints on subcrustal structure associated with Archean cratons. Specifically, a thick (~250 to 300km) mantle keel exists beneath the Kaapvaal craton; a slightly thinner (~225 to 250km) keel exists beneath the Zimbabwe craton and parts of the Archean Limpopo mobile belt. Mantle velocities lower than surrounding regions are evident across a broad swath beneath the surface expression of the Bushveld Complex, a ~2.05 Ga layered mafic intrusion. These reduced velocities may be due to mantle refertilization during intrusion of Bushveld magmas, or they may be caused by a thermal perturbation of more recent origin, perhaps related to the ~183 Ma Karoo magmatic event.

Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis

[1] We examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the South American subduction zone. Data is from the CHARGE network, which traversed Chile and western Argentina across two transects between 30� S and 36� S. Beneath the southern and northwestern parts of the network, fast polarization direction (j) is consistently trench-parallel, while in the northeast j is trench-normal; the transition between these two zones is gradual. We infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab. We explain our observations with a model in which eastward, Nazca-entrained asthenospheric flow is deflected by retrograde motion of the subducting Nazca plate. Resulting southward flow through this area produces N-S j observed in the south and northwest; E-W j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab. INDEX TERMS: 7203 Seismology: Body wave propagation; 7218 Seismology: Lithosphere and upper mantle; 8123 Tectonophysics: Dynamics, seismotectonics; 8150 Tectonophysics: Plate boundary—general (3040); 9360 Information Related to Geographic Region: South America. Citation: Anderson, M. L., G. Zandt, E. Triep, M. Fouch, and S. Beck (2004), Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis, Geophys. Res. Lett., 31, L23608, doi:10.1029/ 2004GL020906.

Seismic anisotropy, mantle fabric, and the magmatic evolution of Precambrian southern Africa

The observed seismic anisotropy of the southern African mantle from both shear-wave splitting and surface wave observations provides important constraints on modes of mantle deformation beneath this ancient continent. We find that the mantle anisotropy beneath southern Africa is dominated by deformational events in Archean times occurring within the lithosphere, rather than present-day processes in the sublithospheric mantle. Consequently, the distribution and magnitude of anisotropy provide valuable data to constrain the mantle’s role in the tectonic evolution of this region. The pattern of mantle anisotropy reveals several noteworthy characteristics. First, mantle anisotropy is closely associated with the Great Dyke of the Zimbabwe Craton, with values of the splitting fast polarization direction, ϕ, parallel to the Dyke. This correspondence with the Great Dyke is likely not due to the present-day Dyke structure but instead is most probably due to the emplacement of the Dyke parallel to pre-existing mantle fabric within the Zimbabwe craton. This deformation thus predates dike emplacement and is no younger than Neo-Archean in age. Second, there is a spatially continuous arc of mantle anisotropy extending from the western Kaapvaal Craton to the northeastern Kaapvaal and Limpopo Belt. All along the arc, ϕ is subparallel to the trend of the arc. Given the crust/mantle chronology associated with these regions, the anisotropy likely represents deformation that occurred at ~2.9 to ~2.6 Ga during collisional accretion of both the western Kimberley and northern Pietersburg blocks onto the seismically isotropic eastern shield of the Kaapvaal, with accretion on the northern ramparts of the Kaapvaal ultimately culminating in the Neo-Archean Limpopo orogen. The anisotropy-inferred arc of deformation reveals diverse zones of both strong and weak coupling between the crust and mantle, as measured by the coherence between mantle deformation and geologically-inferred surface deformation. In particular, there is high coherence between surface and mantle deformation at the southwestern and northeastern ends of the arc, which implies strong crust-mantle coupling in these regions. Conversely, apparent decoupling exists in the northwestern portion of the arc, where northeast to southwest trending anisotropy cuts across north to south trending structures, such as the surface outcrop and aeromagnetic expressions of the Kraaipan Greenstone Belts. Independent seismic evidence from seismic reflection profiling supports the conclusions that these north-south-trending crustal features are superficial and confined to the upper crust. We present evidence that the mantle fabric producing seismic anisotropy constitutes fossil structure in the mantle that is subsequently reactivated, much like the more commonly acknowledged reactivation of crustal structures. In particular, we argue that Neo-Archean collisional orogenesis imparted a mechanical anisotropy to the mantle that controlled the subsequent magmatic history of cratonic southern Africa. We furthermore suggest that four major Precambrian magmatic events: the Great Dyke, the Ventersdorp, Bushveld, and the Soutpansberg, all represent extensional failure along planes oriented parallel to the local splitting fast polarization direction. Each of these events is interpreted to be a collisional rift, similar to the Baikal rift of northern Eurasia, where the stress field associated with collision produces extension and rifting for orientations at a small angle to the direction of the collision. Precise crustal geochronology associates both Ventersdorp and Great Dyke magmatism with the earliest and latest phases of the Limpopo collision, respectively. Similarly, the Bushveld magmatic event is temporally linked to the ~ 2.0 Ga reactivation of Neo-Archean structures in the Limpopo and surrounding areas by the Magondi Orogen, and the Soutpansberg is related to the ~1.9 Ga Kheis Orogen. Since the timing of these basaltic intrusions is controlled by temporal variations in lithospheric stress associated with orogenesis, it implies either that the melting process is genetically related to the evolution of the far-field collision, or that there was a semi-permanent reservoir of basaltic magma residing in the sublithospheric mantle during the ~1 billion-year time period spanned by these magmatic events. The existence of an extensive magma reservoir would argue for elevated temperatures just beneath the lithosphere during this time. Splitting delay times, δ t , a measure of the magnitude of anisotropy, reveal geologically controlled variations in the strength of anisotropy. In particular, the Meso-Archean Kaapvaal shield, the area that was not exposed to ~2.9 Ga and later deformational events, is effectively isotropic. We observe two areas where the anisotropic/isotropic transition is relatively sharp. The north-south boundary appears to coincide with the east-west trending Thabazimbi-Murchison Lineament. In the west, the boundary has been observed in the vicinity of Kimberley, South Africa, near the Colesberg Magnetic Lineament. The Eastern Shield has been relatively devoid of the kind of rifting and magmatic events seen elsewhere in cratonic southern Africa since the Meso-Archean, suggesting that the Eastern Shield lithosphere is mechanically stronger than surrounding areas. This relative strength difference may in part be due to the absence of the mechanical anisotropy inferred for the surrounding areas.

Lowermost mantle anisotropy beneath the Pacific: Imaging the source of the Hawaiian plume

We utilized recordings of seismic shear phases provided by several North American broadband seismometer arrays to provide unique constraints on shear wave anisotropy beneath the northern and central Pacific Ocean. Using a new analysis method that reduces measurement errors and enables the analysis of a larger number of available waveforms, we examined relative travel times of teleseismic S and Sdiff that sample a large area of lowermost mantle structure. The results of this study provide evidence for small-scale lateral and depth variations in shear wave anisotropy for a broad region of the lowermost mantle beneath the Pacific Ocean. In particular, we image a localized zone of anomalously strong anisotropy whose strength increases toward the top of DQ beneath Hawaii. Our results, combined with a previous study of VP/VSH ratios, indicate that ancient subducted slab material may be responsible for observations beneath the northern Pacific, while lenses or layers of core^mantle boundary reaction products or partial melt, oriented by horizontal inflow of mantle material to the Hawaiian plume source, can explain observations beneath the central Pacific. fl 2001 Elsevier Science B.V. All rights reserved.

Shear wave anisotropy in the Mariana Subduction Zone

To determine the location, strength, and orientation of seismic anisotropy in the Mariana subduction zone beneath Guam, we evaluated shear wave splitting in local S, regional S and ScS, and teleseismic core phases such as SKS recorded at station GUMO. Fast directions from the local S phases have an average azimuth of −45°, and splitting times range from 0.1 s to 0.4 s. For local S phases from events within the southeastern half of the subducting slab, splitting parameters manifest minimal frequency dependence in both fast direction and splitting time. However, for the remaining local S phases in the data set, fast directions vary with frequency content. No well-constrained splitting parameters were obtained for the regional and teleseismic phases, but the particle motions of these unsplit phases are consistent with an average anisotropic fast direction of ∼−45°. Anisotropy due solely to olivine oriented by slab-entrained flow in the mantle wedge would produce local S fast directions at ∼−66°, and anisotropy due solely to fossil seafloor spreading in the subducting slab would yield fast directions at −20° to −30°. Neither of these predictions is consistent with the observed fast directions. However, the observed splitting, including the frequency-dependent fast directions, can be explained by models containing anisotropy in both the slab and wedge, and possibly (although not necessarily) anisotropy due to recent extension in the overriding Philippine sea plate.

Formation and evolution of Archaean cratons: insights from southern Africa

Abstract Archaean cratons are the stable remnants of Earth’s early continental lithosphere, and their structure, composition and survival over geological time make them unique features of the Earth’s surface. The Kaapvaal Project of southern Africa was organized around a broadly diverse scientific collaboration to investigate fundamental questions of craton formation and mantle differentiation in the early Earth. The principal aim of the project was to characterize the physical and chemical nature of the crust and mantle of the cratons of southern Africa in geological detail, and to use the 3D seismic and geochemical images of crustal and mantle heterogeneity to reconstruct the assembly history of the cratons. Seismic results confirm that the structure of crust and tectospheric mantle of the cratons differs significantly from that of post-Archaean terranes. Three-dimensional body-wave tomographic images reveal that high-velocity mantle roots extend to depths of at least 200 km, and locally to depths of 250–300 km beneath cratonic terranes. No low-velocity channel has been identified beneath the cratonic root. The Kaapvaal Craton was modified approximately 2.05 Ga by the Bushveld magmatic event, and the mantle beneath the Bushveld Province is characterized by relatively low seismic velocities. The crust beneath undisturbed Archaean craton is relatively thin (c. 35–40 km), unlayered and characterized by a strong velocity contrast across a sharp Moho, whereas post-Archaean terranes and Archaean regions disrupted by large-scale Proterozoic magmatic or tectonic events are characterized by thicker crust, complex Moho structure and higher seismic velocities in the lower crust. A review of Re-Os depletion model age determinations confirms that the mantle root beneath the cratons is Archaean in age. The data show also that there is no apparent age progression with depth in the mantle keel, indicating that its thickness has not increased over geological time. Both laboratory experiments and geochemical results from eclogite xenoliths suggest that subduction processes played a central role in the formation of Archaean crust, the melt depletion of Archaean mantle and the assembly of early continental lithosphere. Co-ordinated geochronological studies of crustal and mantle xenoliths have revealed that both crust and mantle have experienced a multi-stage history. The lower crust in particular retains a comprehensive record of the tectonothermal evolution of the lithosphere. Analysis of lower-crustal xenoliths has shown that much of the deep craton experienced a dynamic and proteracted history of tectonothermal activity that is temporally associated with events seen in the surface record. Cratonization thus occurred not as a discrete event, but in stages, with final stabilization postdating crustal formation.