Garrett Ito

Mantle flow, melting, and dehydration of the Iceland mantle plume

Abstract Recent studies have shown that the extraction of water from the mantle due to partial melting beneath mid-ocean ridges may increase the viscosity of the residuum by 2–3 orders of magnitude. We examine this rheological effect on mantle flow and melting of a ridge-centered mantle plume using three-dimensional numerical models. Results indicate that the viscosity increase associated with dehydration prevents buoyancy forces from contributing significantly to plume upwelling above the dry solidus. Consequently, upwelling in the primary melting zone is driven passively by plate spreading and melt production rates are substantially lower than predicted by models that do not include the rheological effect of dehydration. Predictions of along-axis crustal thickness, bathymetric, and gravity variations are shown to be consistent with observations at Iceland and along the Mid-Atlantic Ridge. Furthermore, these predictions result from a model of a plume with relatively high excess temperature (180°C) and narrow radius (100 km) — properties that are consistent with estimates previously inferred from geochemical and seismological observations. Calculations of incompatible trace-element concentrations suggest that observed along-axis geochemical anomalies primarily reflect incompatible element heterogeneity of the plume source.

Reykjanes 'V'-shaped ridges originating from a pulsing and dehydrating mantle plume

Prominent crustal lineations straddle the Reykjanes ridge, south of Iceland (Fig. 1). These giant V-shaped features are thought to record temporal variations in magma production at the Reykjanes ridge axis, associated with along-axis flow of Icelandic plume material. It has been proposed that this flow is channelled preferentially along the ridge axis, and that temporal variability is induced by fluctuations of the Iceland plume itself or, alternatively, by relocations of the ridge axis on Iceland. Here I present a geodynamic model that predicts the formation of crustal V-shaped ridges from a pulsing and radially flowing mantle plume. In this model, plume pulses produce mantle temperature perturbations that expand away from the plume in all directions beneath the zone of partial melting. The melting zone has a high viscosity owing to mantle dehydration at the onset of partial melting. This high-viscosity region allows for reasonable variations in crustal thickness, produces crustal Vs that extend hundreds of kilometres along the axis, and prevents the plume material from being preferentially channelled along the ridge axis. The angle of the crustal V-shaped features relative to the ridge axis reflects the rate of lateral plume flow, which remains several times greater than the ridge half-spreading rate over the length of a crustal V. Consequently, this radially expanding plume produces lineations in crustal thickness and free-air gravity anomalies that appear to be nearly straight.

Crustal thickness along the western Galápagos Spreading Center and the compensation of the Galápagos hotspot swell

Wide-angle refraction and multichannel reflection seismic data show that oceanic crust along the Galapagos Spreading Center (GSC) between 97‡W and 91‡25PW thickens by 2.3 km as the Galapagos plume is approached from the west. This crustal thickening can account for V52% of the 700 m amplitude of the Galapagos swell. After correcting for changes in crustal thickness, the residual mantle Bouguer gravity anomaly associated with the Galapagos swell shows a minimum of 325 mGal near 92‡15PW, the area where the GSC is intersected by the Wolf^ Darwin volcanic lineament (WDL). The remaining depth and gravity anomalies indicate an eastward reduction of mantle density, estimated to be most prominent above a compensation depth of 50^100 km. Melting calculations assuming adiabatic, passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of V30‡C. The associated thermal expansion and increase in melt depletion reduce mantle densities, but to a degree that is insufficient to explain the geophysical observations. The largest density anomalies appear at the intersection of the GSC and the WDL. Our results therefore require the existence of compositionally buoyant mantle beneath the GSC near the Galapagos plume. Possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the Galapagos Islands that is later transported to the GSC. Our estimate for the buoyancy flux of the Galapagos plume (700 kg s 31 ) is lower than previous estimates, while the total crustal production rate of the Galapagos plume (5.5 m 3 s 31 ) is comparable to that of the Icelandic and Hawaiian plumes.

Interaction of mantle plumes and migrating mid‐ocean ridges: Implications for the Galápagos plume‐ridge system

We investigate the three-dimensional interaction of mantle plumes and migrating mid-ocean ridges with variable viscosity numerical models. Numerical models predict that along-axis plume width W and maximum distance of plume-ridge interaction xmax scale with (Q/U)½, where Q is plume source volume flux and U is ridge full spreading rate. Both W and xmax increase with buoyancy number ⊓b which reflects the strength of gravitational-versus plate-driven spreading. Scaling laws derived for stationary ridges in steady-state with near-ridge plumes are consistent with those obtained from independent studies of Ribe [1996]. In the case of a migrating ridge, the distance of plume-ridge interaction is reduced when a ridge migrates toward the plume because of the excess drag of the faster moving leading plate and enhanced when a ridge migrates away from the plume because of the reduced drag of the slower moving trailing plate. Given the mildly buoyant and relatively viscous plumes investigated here, the slope of the lithospheric boundary and thermal erosion of the lithosphere have little effect on plume flow. From observed plume widths of the Galapagos plume-migrating ridge system, our scaling laws yield estimates of Galapagos plume volume flux of 5–16×106km3 m.y.−1 and a buoyancy flux of ∼2×103 kg s−1. Model results suggest that the observed increase in bathymetric and mantle-Bouguer gravity anomalies along Cocos Plate isochrons with increasing isochron age is due to higher crustal production when the Galapagos ridge axis was closer to the plume several million years ago. The anomaly amplitudes can be explained by a plume source with a relatively mild temperature anomaly (50°–100°C) and moderate radius (100–200 km). Predictions of the along-axis geochemical signature of the plume suggest that mixing between the plume and ambient mantle sources may not occur in the asthenosphere but, instead, may occur deeper in the mantle possibly by entrainment of depleted mantle as the plume ascends from its source.

Geodynamics, Second Edition

Turcotte and Schuberts first edition of Geodynamics has become the premier textbook on applications of continuum physics to geologic problems. After 20 years of widespread use by students, educators, and researchers, the second edition of Geodynamics is a welcome revision. All individuals working in the solid Earth sciences should own this textbook. Geodynamics provides practical insight into the governing physics of many processes in geology and geophysics. The general approach is to emphasize geological phenomena, derive the governing equations, and then expand to a variety of applications. Most of the applications are presented in one-spatial dimension, and this provides an effective starting point for conveying the basic theory Derivations in multi-dimensions are limited, however, largely because tensor and index notations are not used.

Mantle temperature anomalies along the past and paleoaxes of the Galápagos spreading center as inferred from gravity analyses

To better understand the effects of hot spots on mid-ocean ridge thermal structure, we investigate the subsurface density structure of the Galapagos spreading center and nearby lithosphere. Using shipboard gravity and bathymetry data, we obtain maps of mantle Bouguer anomalies (MBA) by removing from the free-air gravity the attractions of seafloor topography and a 6-km-thick model crust. Comparison of observed and theoretical MBA profiles along isochrons for ages 0.0–7.7 Ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle. This result is consistent with the notion that seafloor topography along the Galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density, both of which are controlled by temperatures in the upper mantle where decompression melting occurs. Along the ridge axis, the MBA decreases from the east and west toward the Galapagos hot spot by ∼90 mGal, reaching a minimum nearest the hot spot at 91°W. Seafloor topography mirrors the MBA along axis, increasing by ∼1.1 km toward the hot spot. These variations in MBA and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50±25°C toward the hot spot. The predicted crustal thickening of 2–4 km nearest the hot spot accounts for 70–75% of the along-axis MBA and bathymetry anomalies; mantle density variations account for the rest of the anomalies. From the crustal isochron of age 7.7 Ma to the present-day axis, the along-isochron amplitudes of MBA decrease from ∼150 to ∼90 mGal. The corresponding along-isochron bathymetry anomalies decrease from ∼1.7 to ∼1.1 km. These observations along the paleoaxes of the Galapagos spreading center indicate that the axial temperature anomaly was 70% hotter in the past (86±25°C) and has steadily decreased to 50±25°C as the ridge axis migrated away from the Galapagos hot spot. These along-isochron temperature anomalies, however, have remained well below that estimated for the hot spot itself (200°C), indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10–20 times greater than that along the ridge axis over the past 7.7 m.y.

Effects of volcano loading on dike propagation in an elastic half-space

We use laboratory experiments and numerical models to examine the effects of volcano loading on the propagation of buoyant dikes in a two-dimensional elastic half-space. In laboratory experiments we simulate the propagation of buoyant dikes in an isotropic regional stress field by injecting air into tanks of solidified gelatin. A weight resting on the surface of the gelatin represents a volcanic load. A numerical model is used to simulate these experiments. Both experiments and numerical simulations show that as a dike ascends, it begins to curve toward the load in response to the local stress field imposed by the load. The lateral distance over which dikes curve to the load increases with the ratio of average pressure at the base of the load to the dike driving pressure. For realistic volcano and dike dimensions this pressure ratio is going to be large, suggesting that dikes can converge to a volcano over lateral distances several times the load width. Numerical calculations involving an anisotropic regional stress field, however, predict that the lateral extent of dike attraction shrinks as the regional horizontal compressive stress decreases relative to the vertical compressive stress. Dike focusing will be substantial if the regional differential stresses are less than the average pressure at the base of the load. If this is the case, then our models predict a positive feedback between the size of volcanoes and the area of dike attraction. This feedback may promote the development of large discrete volcanoes and also predicts a positive correlation between the spacing and sizes of adjacent volcanoes. To test this prediction, we examine nearest-neighbor pairs of the 21 largest volcanoes in the Cascade Range. The 14 pairs examined show a large range in volcano spacing (6–115 km) and a statistically significant correlation between spacing and average volcano height. This result is consistent with our model results and suggests that the local compressive stress induced by these volcanoes may be an important factor in controlling magma transport in the lithosphere.

Subsidence and growth of Pacific Cretaceous plateaus

Abstract The Ontong Java, Manihiki, and Shatsky oceanic plateaus are among the Earths largest igneous provinces and are commonly believed to have erupted rapidly during the surfacing of giant heads of initiating mantle plumes. We investigate this hypothesis by using sediment descriptions of Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drill cores to constrain plateau subsidence histories which reflect mantle thermal and crustal accretionary processes. We find that total plateau subsidence is comparable to that expected of normal seafloor but less than predictions of thermal models of hotspot-affected lithosphere. If crustal emplacement was rapid, then uncertainties in paleo-water depths allow for the anomalous subsidence predicted for plumes with only moderate temperature anomalies and volumes, comparable to the sources of modern-day hotspots such as Hawaii and Iceland. Rapid emplacement over a plume head of high temperature and volume, however, is difficult to reconcile with the subsidence reconstructions. An alternative possibility that reconciles low subsidence over a high-temperature, high-volume plume source is a scenario in which plateau subsidence is the superposition of (1) subsidence due to the cooling of the plume source, and (2) uplift due to prolonged crustal growth in the form of magmatic underplating. This prolonged crustal growth and uplift scenario may explain the low and thus submarine relief during plume initiation, the late stage eruptions found on Ontong Java (90 Ma) and Manihiki (∼70 Ma), a large portion of the high-seismic-velocity lower crust, and the widespread normal faults observed throughout and along the margins of the three plateaus. Such late stage underplating may have occurred continuously or in discrete stages over ∼30 m.y. and implies lower magmatic fluxes than previously estimated.

Morphology and segmentation of the western Galápagos Spreading Center, 90.5°–98°W: Plume‐ridge interaction at an intermediate spreading ridge

Author Posting.

Magmatic and tectonic extension at mid‐ocean ridges: 1. Controls on fault characteristics

We use 2-D numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges. Magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction, M, of the total spreading rate (i.e., M = 1 for fully magmatic spreading). Heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust. We examine a range of intrusion rates and axial thermal structures by varying M, spreading rate, and the efficiency of crustal cooling by conduction and hydrothermal circulation. Fault development proceeds in a sequential manner, with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis. Fault spacing and heave are primarily sensitive to M and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis. Contrary to what is often cited in the literature, but consistent with prior results of mechanical modeling, we find that thicker axial lithosphere tends to reduce fault spacing and heave. In addition, fault spacing and heave are predicted to increase with decreasing rates of off-axis lithospheric thickening. The combination of low M, particularly when M approaches 0.5, as well as a reduced rate of off-axis lithospheric thickening produces long-lived, large-offset faults, similar to oceanic core complexes. Such long-lived faults produce a highly asymmetric axial thermal structure, with thinner lithosphere on the side with the active fault. This across-axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes.