Donald W. Forsyth

Gravity anomalies of the ridge-transform system in the South Atlantic between 31 and 34.5° S: Upwelling centers and variations in crustal thickness

To decipher the distribution of mass anomalies near the earths surface and their relation to the major tectonic elements of a spreading plate boundary, we have analyzed shipboard gravity data in the vicinity of the southern Mid-Atlantic Ridge at 31–34.5° S. The area of study covers six ridge segments, two major transforms, the Cox and Meteor, and three small offsets or discordant zones. One of these small offsets is an elongate, deep basin at 33.5° S that strikes at about 45° to the adjoining ridge axes.By subtracting from the free-air anomaly the three-dimensional (3-D) effects of the seafloor topography and Moho relief, assuming constant densities of the crust and mantle and constant crustal thickness, we generate the mantle Bouguer anomaly. The mantle Bouguer anomaly is caused by variations in crustal thickness and the temperature and density structure of the mantle. By subtracting from the mantle Bouguer anomaly the effects of the density variations due to the 3-D thermal structure predicted by a simple model of passive flow in the mantle, we calculate the residual gravity anomalies. We interpret residual gravity anomalies in terms of anomalous crustal thickness variations and/or mantle thermal structures that are not considered in the forward model. As inferred from the residual map, the deep, major fracture zone valleys and the median, rift valleys are not isostatically compensated by thin crust. Thin crust may be associated with the broad, inactive segment of the Meteor fracture zone but is not clearly detected in the narrow, active transform zone. On the other hand, the presence of high residual anomalies along the relict trace of the oblique offset at 33.5° S suggests that thin crust may have been generated at an oblique spreading center which has experienced a restricted magma supply. The two smaller offsets at 31.3° S and 32.5° S also show residual anomalies suggesting thin crust but the anomalies are less pronounced than that at the 33.5° S oblique offset. There is a distinct, circular-shaped mantle Bouguer low centered on the shallowest portion of the ridge segment at about 33° S, which may represent upwelling in the form of a mantle plume beneath this ridge, or the progressive, along-axis crustal thinning caused by a centered, localized magma supply zone. Both mantle Bouguer and residual anomalies show a distinct, local low to the west of the ridge south of the 33.5° S oblique offset and relatively high values at and to the east of this ridge segment. We interpret this pattern as an indication that the upwelling center in the mantle for this ridge is off-axis to the west of the ridge.

Geochemical constraints on initial and final depths of melting beneath mid-ocean ridges

The comparison of the composition and volume of mid-ocean ridge basalts to predicted values inferred from experimental and theoretical studies is one of the primary tools for mapping thermal structure and the geometry of melting beneath mid-ocean ridges. Several indicators of either the pressure or degree of melting are commonly employed: Na2O concentration in melts is roughly inversely proportional to the extent of melting; (Srn/Yb)N is affected by the proportion of melt that is generated in the garnet stability field (deeper than 60 km); and FeO content increases with increasing pressure of melting. Crustal thickness also depends on the geometry and extent of melting. K2O/TiO2 is used as an indicator of the enrichment of the mantle source. For normal ridges, we find that regional averages of Fe(8) (FeO normalized to 8 wt % MgO to correct for fractionation) are strongly dependent on enrichment. Thus if Fe(8) is used to infer to the pressure of melting, correction for the effects of compositional heterogeneity of the source is needed. Even with the correction for heterogeneity, however, it is difficult to correct for all fractionation effects accurately enough to use FeO as a depth indicator except in the most extreme cases. Mantle heterogeneity also affects Na(8) and (Sm/Yb)N, but the correction for enrichment by linear regression of the Na(8) and (Sm/Yb)N to a common reference value of K2O/TiO2 does not change the relationship between (Sm/Yb)N and Na(8) that is used to estimate depth and degree of melting. In simple melting models, the composition and crustal thickness depend on the rate of pressure release melting per kilometer of uplift; the initial depth of onset of melting; the final depth of termination of melting; the nature of melt equilibration, i.e., batch or fractional melting; and the geometry of upwelling, i.e., passive or dynamic flow. We find that melting beneath normal ridges commences in a narrow depth range in the spinel-garnet transition zone (about 60–70 km in depth), suggesting a much smaller variation in potential temperature beneath normal ridges (<60°K) than previously suggested (250°K, Klein and Langmuir, 1987; Langmuir et al., 1992). The difference is due primarily to the component of Fe(8) variation caused by mantle heterogeneity in the global data set and to difficulties in correcting FeO for fractionation. The initial depth of melting beneath hotspots is greater than beneath normal ridges, indicating significantly higher temperatures. Instead of melting continuing to the base of the crust beneath normal ridges, it ceases at a variety of depths beneath the crust. Variations in final depth of melting may be due to cooling to the surface, effects of transform faults, and the local, relative importance of dynamic upwelling.

Geophysical evidence from the MELT area for compositional controls on oceanic plates

Magnetotelluric and seismic data, collected during the MELT experiment at the southern East Pacific Rise, constrain the distribution of melt beneath this mid-ocean-ridge spreading centre and also the evolution of the oceanic lithosphere during its early cooling history. Here we focus on structures imaged at distances ∼100 to 350 km east of the ridge crest, corresponding to seafloor ages of ∼1.3 to 4.5 million years (Myr), where the seismic and electrical conductivity structure is nearly constant and independent of age. Beginning at a depth of about 60 km, we image a large increase in electrical conductivity and a change from isotropic to transversely anisotropic electrical structure, with higher conductivity in the direction of fast propagation for seismic waves. Conductive cooling models predict structure that increases in depth with age, extending to about 30 km at 4.5 Myr ago. We infer, however, that the structure of young oceanic plates is instead controlled by a decrease in water content above a depth of 60 km induced by the melting process beneath the spreading centre.

Isostatic compensation of tectonic features of the Mid‐Atlantic Ridge: 25–27°30′S

We investigate the isostatic compensation of tectonic features along the Mid-Atlantic Ridge 25°–27°30′S through a detailed examination of the relationship between bathymetry and gravity anomalies The study area includes three ridge segments, spreading at an average full rate of 35 mm/yr, their flanks out to about 6 m.y. old crust, and the intervening Rio Grande and Moore fracture zones. In a three-dimensional analysis of gravity and Sea Beam bathymetry data, we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models. Positive residual gravity anomalies over the northern wall of the Rio Grande fracture zone indicate that in spots the crust is 2–3 km thinner than average there, in contrast to the Moore fracture zone where little thinning is observed. The greater than average depths of the fracture zones as a whole are not locally compensated by thin crust, but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle. A narrow linear ridge that crosses one of the inactive branches of the Moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner, not thicker crust. There is no indication of thicker crust beneath topographic highs at the inside corners of ridge-transform intersections. Presumably, these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies. There is a residual gravity low associated with an unusually shallow 15- to 20-km section of the ridge segment between the Rio Grande and Moore offsets where the median valley virtually disappears. Our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust, much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation. Two-dimensional Fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented. This implies that the tectonic processes that control the gross ridge-transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure. Statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average, effective, elastic plate thickness in the study area is about 6 km.

The paradox of the axial profile: Isostatic compensation along the axis of the Mid‐Atlantic Ridge?

Mantle Bouguer gravity anomalies (MBA) and bathymetry on three profiles covering more than 1000 km along the axis of the Mid-Atlantic Ridge (MAR) are highly correlated, suggesting that along-axis topographic relief is locally compensated by variations in crustal thickness and/or mantle density structure. The quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin, narrow elastic strip, representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak, bounding normal faults. The paradox is that across-axis profiles show that the median valley is an uncompensated feature, apparently created by a dynamic mechanism. New, extensive off-axis coverage of the MAR from 31° to 36°S shows that the high correlation does not persist outside the axial zone. We suggest that the on-axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along-axis variations in crustal thickness, both of which contribute to the MBA. If the mechanism is extension of a brittle-ductile lithosphere, the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis. A three-dimensional (3-D) thermal model incorporating passive mantle flow, hydrothermal circulation, the plate boundary geometry, and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography. We model the expected topography using cross-axis sections of the 3-D thermal model and a 2-D finite element model of an extending lithosphere that incorporates temperature- and strain-dependent rheology, as well as the flexural response of a thickening plate. The predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry. At the 18 mm yr−1 spreading half-rate in our South Atlantic survey, the 2-D models also reproduce the observed rapid transition along the axis between rift valley/no rift valley morphology. We conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the Southern MAR and the along-axis correlation between MBA and bathymetry.

Array Analysis of Two‐Dimensional Variations in Surface Wave Phase Velocity and Azimuthal Anisotropy in the Presence of Multipathing Interference

Multipath propagation of surface waves introduces distortions in waveforms that can bias array measurements of phase velocities. We present a method for array analysis of laterally and azimuthally varying phase velocities that represents the incoming wavefield from each earthquake as the sum of two interfering plane waves. This simple approximation successfully represents the amplitude and phase variations for most earthquakes recorded in the MELT Experiment on the East Pacific Rise in the period range from 16 to 67 s. The inversion for velocities automatically reduces the importance of data from earthquakes or periods that are not described well by this approximation. Each iteration in the inversion involves two stages: a simulated annealing inversion for the best wave parameter description of each event, and a linearized inversion for velocities and changes in the wave parameters. At 29 s period, the two-plane-wave solutions indicate that nearly every signal is significantly affected by multipathing. The larger of the two plane waves typically has an apparent azimuth of propagation that is within a few degrees of the great circle path. The smaller wave is more scattered, differing in apparent azimuth from the larger wave by an average of about 13° at 29 s. Both lateral and azimuthal variations in Rayleigh wave phase velocity in the study area are significant, although it is possible to trade off azimuthal anisotropy with rapid and probably unrealistic lateral variations in velocity. Apparent azimuthal anisotropy reaches 5 to 6%, with the fast direction approximately perpendicular to the ridge.

Anomalous upper mantle beneath the Australian-Antarctic discordance

Abstract Within the Australian-Antarctic discordant zone, residual depth anomalies approach 1000 m. In sea floor younger than 10 Ma that is more than 500 m deeper than expected, Rayleigh wave phase velocities are significantly faster than in sea floor of comparable age in the Pacific. In this area, the shear wave velocity in the 20–40 km depth range is unusually fast, indicating that the lithosphere develops more rapidly than usual from an asthenosphere that is perhaps cooler than average. In sea floor that is older than 10 Ma, phase velocities are anomalously fast and independent of the residual depth. Beneath this older sea floor, the low-velocity zone in the oceanic mantle is much less pronounced than beneath sea floor of comparable age in the Pacific.

Asymmetric mantle dynamics in the MELT region of the East Pacific Rise

Abstract The mantle electromagnetic and tomography (MELT) experiment found a surprising degree of asymmetry in the mantle beneath the fast-spreading, southern East Pacific Rise (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Scheirer et al., Science 280 (1998) 1221–1224; Evans et al., Science 286 (1999) 752–756). Pressure-release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick (Canales et al., Science 280 (1998) 1218–1221). Seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric (Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Evans et al., Science 286 (1999) 752–756) at depths of several tens of kilometers, melt is more abundant beneath the Pacific plate to the west of the axis than beneath the Nazca plate to the east. MELT investigators attributed the asymmetry in melt and geophysical properties to several possible factors: asymmetric flow passively driven by coupling to the faster moving Pacific plate; interactions between the spreading center and hotspots of the south Pacific; an off-axis center of dynamic upwelling; and/or anomalous melting of an embedded compositional heterogeneity (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Evans et al., Science 286 (1999) 752–756). Here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies. Asthenospheric flow from hotspots in the Pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies.

Determinations of focal depths of earthquakes associated with the bending of oceanic plates at trenches

Abstract The earthquakes examined in this paper are all within the oceanic lithosphere and are associated with the bending of plates before subduction. Accurate determinations of the depth of these earthquakes are needed to study the stress pattern within a bending plate. Routinely-determined depths of shallow sub-oceanic earthquakes published in bulletins are unreliable. The depths can be accurately determined to within a few kilometers if the original seismograms from these events are studied. In some cases, the reflected phases pP and pwP can be clearly identified. There exists the possibility that the wave reflected at the water-air interface, pwP, may be misidentified as pP, leading to erroneous estimates of depth. Additional methods of analysis, such as surface wave radiation patterns or the apparent frequency-dependence of reflection at the crust-water interface, can remove this possible source of confusion. One of the most powerful techniques for depth analysis is the modelling of long-period waveforms. The pattern of stresses within the bending oceanic lithosphere revealed by the depths and focal mechanisms of these intraplate earthquakes is one of horizontal, deviatoric tension down to a depth of about 25 km, with horizontal compression at greater depths.

Convective upwelling in the mantle beneath the Gulf of California

In the past six million years, Baja California has rifted obliquely apart from North America, opening up the Gulf of California. Between transform faults, seafloor spreading and rifting is well established in several basins. Other than hotspot-dominated Iceland, the Gulf of California is the only part of the world’s seafloor-spreading system that has been surrounded by enough seismometers to provide horizontal resolution of upper-mantle structure at a scale of 100 kilometres over a distance great enough to include several spreading segments. Such resolution is needed to address the long-standing debate about the relative importance of dynamic and passive upwelling in the shallow mantle beneath spreading centres. Here we use Rayleigh-wave tomography to image the shear velocity in the upper 200 kilometres or so of the mantle. Low shear velocities similar to those beneath the East Pacific Rise oceanic spreading centre underlie the entire length of the Gulf, but there are three concentrated locations of anomalously low velocities spaced about 250 kilometres apart. These anomalies are 40 to 90 kilometres beneath the surface, at which depths petrological studies indicate that extensive melting of passively upwelling mantle should begin. We interpret these seismic velocity anomalies as indicating that partial melting triggers dynamic upwelling driven by either the buoyancy of retained melt or by the reduced density of depleted mantle.