Correction [to “Hotspot melting generates both hotspot volcanism and a hotspot swell?” by Jason Phipps Morgan, W. Jason Morgan, and Evelyn Price] (original) (raw)
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Hotspot melting generates both hotspot volcanism and a hotspot swell
Journal of Geophysical Research, 1995
Two prominent features mark the passage of oceanic lithosphere over a hotspot. The first is the initiation of oceanic volcanism leading to a chain of islands or seamounts. The second is the generation of a ~1-km-high, ~1000-km-wide bathymetric swell around the volcanic island chain. Here we show that recent estimates for the volume of hotspot volcanism and the size of the swell suggest a shared origin: swell relief is created by the density reduction created by melting beneath the hotspot. This results in a seafloor age dependence to swell size and volcanism along the Hawaiian chain: beneath younger, thinner lithosphere the hotspot undergoes more decompression melting, resulting in both a larger swell volume and greater island building. For rapidly moving plates the swell root residue from hotspot melting is dragged away from the hotspot by the overriding lithosphere; its buoyancy induces further spreading and thinning of swell root material, producing, for example, the characteristic bow-shaped form of the 0-5 Ma section of the Hawaiian swell. This post emplacement spreading and thinning of the swell root may be the reason for the ~5 m.y. duration of later stage melting and volcanism along the Hawaiian hotspot chain. The ~5 m.y. timescale for spreading of the swell root implies a characteristic viscosity of the depleted swell root of ~1-3×1020 Pa s, which is less fluid than underlying, less melted asthenosphere. Melt extraction at the hotspot is our preferred mechanism for the increase in viscosity of the swell root relative to underlying asthenosphere.
Variation of the apparent compensation depth of hotspot swells with age of plate
Earth and Planetary Science Letters, 1988
Most oceanic hotspot swells are associated with anomalously shallow seafloor and broad geoid anomalies. These depth and geoid anomalies are generally positively correlated and linearly related. The linear relationship can be used to infer an apparent depth of isostatic compensation of the swell. We have analysed geoid and depth anomalies over 23 oceanic hotspot swells (almost the majority of oceanic hotspots) and deduced the apparent depth of compensations of these features. The data indicate a linear increase of the compensation depth with the square root of seafloor age t, on the order of 7 km/(m.y.) 1/2. This trend is comparable to that predicted by conductive cooling models for the deepening of the isotherms inside the oceanic lithosphere.
Underplating of the Hawaiian Swell: evidence from teleseismic receiver functions
Geophysical Journal International, 2010
The Hawaiian Islands are the canonical example of an age-progressive island chain, formed by volcanism long thought to be fed from a hotspot source that is more or less fixed in the mantle. Geophysical data, however, have so far yielded contradictory evidence on subsurface structure. The substantial bathymetric swell is supportive of an anomalously hot upper mantle, yet seafloor heat flow in the region does not appear to be enhanced. The accumulation of magma beneath pre-existing crust (magmatic underplating) has been suggested to add chemical buoyancy to the swell, but to date the presence of underplating has been constrained only by local active-source experiments. In this study, teleseismic receiver functions derived from seismic events recorded during the PLUME project were analysed to obtain a regional map of crustal structure for the Hawaiian Swell. This method yields results that compare favourably with those from previous studies, but permits a much broader view than possible with activesource seismic experiments. Our results indicate that the crustal structure of the Hawaiian Islands is quite complicated and does not conform to the standard model of sills fed from a central source. We find that a shallow P-to-s conversion, previously hypothesized to result from the volcano-sediment interface, corresponds more closely to the boundary between subaerial and subaqueous extrusive material. Correlation between uplifted bathymetry at ocean-bottomseismometer locations and presence of underplating suggests that much of the Hawaiian Swell is underplated, whereas a lack of underplating beneath the moat surrounding the island of Hawaii suggests that underplated crust outward of the moat has been fed from below by dykes through the lithosphere rather than by sills spreading from the island centre. Local differences in underplating may reflect focusing of magma-filled dykes in response to stress from volcanic loading. Finally, widespread underplating adds chemical buoyancy to the swell, reducing the amplitude of a mantle thermal anomaly needed to match bathymetry and supporting observations of normal heat flow.
Geophysical Journal International, 2007
The surface topography, gravity and geoid associated with loads situated at the base of the thermal lithosphere are computed. The model lithosphere is composed of a viscous lower layer with depth-dependent viscosity overlain by an elastic lid. When the viscosity varies strongly with depth, the amplitude of the surface topography decreases rapidly as the wavenumber characterizing the mass anomaly increases. The response for this two-layer lithosphere differs substantially from the response computed for the loading from below of an elastic lid. If the 200km wavelength geoid anomalies observed in the oceans are due to convective instabilities at the base of the lithosphere, a positive geoid anomaly with an amplitude reaching 30 to 50cm is expected over a cold downwelling for ages greater than 10 Myr. No detectable topography associated with the lineations should be present for ages greater than 20 Myr. Our model predicts that some topographic anomalies might be visible for younger ages. If 1000 km broad swells are due t o light material at a depth of about 100 km, the associated geoid-over-topography ratios are quite reduced compared to those predicted by the long-wavelength linear relationship usually employed for estimating the apparent compensation depth.
Special Paper 430: Plates, Plumes and Planetary Processes, 2007
During the roughly year-long Seismic Wave Exploration in the Lower Lithosphere (SWELL) pilot experiment in 1997/1998, eight ocean bottom instruments deployed to the southwest of the Hawaiian Islands recorded teleseismic Rayleigh waves with periods between 15 and 70 s. Such data are capable of resolving structural variations in the oceanic lithosphere and upper asthenosphere and therefore help understand the mechanism that supports the Hawaiian Swell relief. The pilot experiment was a technical as well as a scientific feasibility study and consisted of a hexagonal array of Scripps Low-Cost Hardware for Earth Applications and Physical Oceanography (L-CHEAPO) instruments using differential pressure sensors. The analysis of eightyfour earthquakes provided numerous high-precision phase velocity curves over an unprecedentedly wide period range. We find a rather uniform (unaltered) lid at the top of the lithosphere that is underlain by a strongly heterogeneous lower lithosphere and upper asthenosphere. Strong slow anomalies appear within ~300 km of the island chain and indicate that the lithosphere has most likely been altered by the same process that causes the Hawaiian volcanism. The anomalies increase with depth and reach well into the asthenosphere, suggesting a sublithospheric dynamic source for the swell relief. The imaged velocity variations are consistent with thermal rejuvenation, but our array does not appear to have covered the melt-generating region of the Hawaiian hotspot.
Bathymetry and the geoid anomaly of the northern flank of the Hawaiian swell is broader and higher than the southern flank, and it is characterized by higher heat flow than the axis or southern flank. It is here proposed that the northern flank of the Hawaiian swell has been augmented by heat conducted from the hotspot conduit into the upper mantle then transported northward of the volcanic axis by flow in the upper mantle (¾325º) that is more northerly than Pacific plate motion (292º). By assuming that the deep upper mantle is decoupled from the Pacific plate and is flowing at 325º to the northwest, changes in direction and rate of volcanic propagation and in geochemistry along individual volcanic segments of the Hawaiian volcanic chain can be interpreted in terms of tank experiment results showing that a volcanic hotspot conduit breaks into diapirs when tilted by mantle flow. Hawaiian volcanoes are aligned in en-echelon segments, and the Hawaiian Islands are the two most recent segments. For an individual segment, older northwestern volcanoes are aligned nearly parallel to the 292º plate motion direction, and they propagated to the southeast at approximately the same rate as the 92 km=m.y. speed of northwestward plate motion. In contrast, the alignment of the younger southeastern volcanoes is close to 325º, and they show a conspicuous acceleration in propagation of volcanism marked by out-of-sequence eruptions. Within the model proposed here, diapirs rise from instability nodes that develop along the tilted conduit of a mantle hotspot plume as it is sheared in the direction of deep upper-mantle flow and each diapir gives rise to a single volcanic center. As tilting progresses, diapirs form at lower levels along the conduit in more upstream positions of the mantle flow zone, rise sequentially into the decoupled lithosphere, erupt sequentially, and are translated in the direction of plate motion (older, northwestern Hawaiian Islands). Eventually, flow in the highly tilted conduit is impeded to the degree that the remaining upstream conduit breaks into a number of diapirs that rise together into the lithosphere. These late diapirs, translated as a group aligned in the direction of horizontal mantle flow, erupt over a relatively short time span and show out-of-sequence volcanism (younger, southeastern Hawaiian Islands). At this stage, a new cycle of rising and tilting will initiate the next en-echelon segment.
Earth and Planetary Science Letters, 1989
The Bermuda Rise is a broad area of anomalously shallow seafloor presumed to be of thermal origin dating to the middle Eocene. This and other midplate swells have been modeled both by elevated temperature in a convecting layer at the base of the plate and as thermal expansion within the conducting portion of the lithosphere. Using recently developed techniques for the estimation of lithospheric flexure and temperature structure from geoid, bathymetry, and heat flow data, we place constraints on the depth of the thermal load and limit the possible thermal mechanisms causing this swell. Our data include depth anomalies calculated from DBDB5C bathymetry data, geoid anomalies using data from both Seasat and GEOS-3 altimeter missions, and published heat flow values. The first technique we employ to determine the thermo-mechanical properties of the swell is a forward filtering technique with which we determine flexural rigidity and compensation depth by successive fits to topography filtered to separate subsurface and surface loading. The second method is an admittance technique, with which we directly determine the geoid to topography ratios corresponding to the volcano and the swell. Both linear filtering and admittance approaches yield similar conclusions concerning the elastic plate thickness and compensation depth of the Bermuda Rise, indicating that the effective elastic thickness for the plate supporting the volcanoes is 30 + 5 km and the compensation depth for the swell is 55 + 10 kin. Finally, we use the linear programming technique to obtain extremal bounds on temperature as a function of depth and time in the lithosphere, We find that the depth and geoid anomalies, heat flow, and elastic thickness observations are inconsistent with a mechanism for formation of the Bermuda Rise which consists solely of thermal anomalies confined to depths greater than 50 km at the initiation of hotspot activity. If we use thermal rejuvenation alone to model the Bermuda Rise, the observations require that it be accompanied by substantial reheating to shallow depth in the lithosphere. On the basis of the data presented in this paper we cannot distinguish between a reheating model and a convection model for the origin of the Bermuda Rise though based on our geothermal modeling we preclude convective support models that do not change temperatures within the thermal plate.
Sedimentary evidence for moderate mantle temperature anomalies associated with hotspot volcanism
Plates, plumes, and paradigms, 2005
One of the characteristics of deep-rooted mantle plume models, whether driven by thermal or compositional buoyancy, and the hotspot volcanism with which they are associated is the presence of anomalously hot asthenosphere underlying the lithospheric plate. The presence of hot, upwelling, low-density asthenosphere causes shallowing of the seafloor over the plume center. It is commonly assumed that as the plume mantle disperses, greater subsidence occurs compared to normal oceanic crust. However, in this paper analysis of the sedimentary cover from a range of hotspot-related seamounts, plateaus, and ridges of various ages from all major ocean basins shows either no subsidence anomalies or only moderate ones that can be linked to hot asthenosphere during hotspot magmatism. Assuming that all the uplift is caused by excess mantle heat, temperature anomalies rarely exceed 100 °C for a plume head ~100 km thick and could be somewhat lower if dynamic flow or composition are important causes of uplift. On the Ontong-Java Plateau, Mid-Pacific Mountains, Emperor Seamounts, Hess Rise, and MIT Guyot, subsidence is slower than for normal oceanic lithosphere, suggesting either colder than normal mantle temperatures or, more likely, the formation of a buoyant lithospheric root under the hotspot province at the time of its formation.
Thermoelastic stress in oceanic lithosphere due to hotspot reheating
Journal of Geophysical Research, 1991
We investigate the effect of hotspot reheating on the intraplate stress field by modeling the threedimensional thermal stress field produced by nonuniform temperature changes in an elastic plate. Temperature perturbations are calculated assuming that the lithosphere is heated by a source in the lower part of the thermal lithosphere. A thermal stress model for the elastic lithosphere is calculated by superposing the stress fields resulting from temperature changes in small individual elements. The stress in an elastic plate resulting from a temperature change in each small element is expressed as an infinite series, wherein each term is a source or an image modified from a closed-form half-space solution. We apply the thermal stress solution to midplate swells in oceanic lithosphere with various thermal structures and plate velocities. Our results predict a stress field with a maximum deviatoric stress on the order of 100 MPa (1 kbar) covering a broad area around the hotspot plume. The predicted principal stress orientations show a complicated geographical pattern, with horizontal extension perpendicular to the hotspot track at shallow depths and compression along the track near the bottom of the elastic lithosphere. Although stress data near oceanic swells are limited, the source parameters of intraplate earthquakes near several hotspots are consistent with the thermal stress model. These results indicate that thermal stress due to reheating may be an important contributor to stress fields near hotspots in old oceanic lithosphere. . Bratt et al. [1985] were able to fit these observations using an elastic half-space model in which thermal stresses were relieved on time scales shorter than the age of the lithosphere. The effects of thermoelastic stress in oceanic lithosphere have also been identified at fracture zones. Parmentier and Haxby [1986] and Haxby and Parmentier [1988] found that geoid profiles across fracture zones showed clear evidence of bending induced by thermal stresses associated with lithospheric cooling. It has also been suggested that transform faults may result from thermal stresses [Turcotte, 1 9 7 4 a; Sandwell, 1986].
On the shallow origin of hotspots and the westward drift of the lithosphere
in Foulger, G.R., Natland, J.H., Presnall, D.C., and Anderson, D.L., eds., Plates, plumes, and paradigms: Geological Society of America Special Paper 388, p. 735–749, doi: 10.1130/2005.2388(42)., 2005
Intraplate migrating hotspots, which are unrelated to rifts or plate margins in general, regardless of their origin in the mantle column, indicate relative motion between the lithosphere and the underlying mantle in which the hotspot source is located. Pacific plate hotspots are sufficiently fixed relative to one another to represent an independent reference frame to compute plate motions. However, the interpretation of the middle asthenosphere rather than the deep lower mantle as the source for intraplate Pacific hotspots has several implications. First, decoupling between the lithosphere and subasthenospheric mantle is greater than recorded by hotspot volcanic tracks (>100 mm/yr) due to undetectable shear in the lower asthenosphere below the magmatic source. The shallower the source, the larger the décollement. Second, computation of the westward drift is linked to the Pacific plate and assumes that the deep lower mantle, below the decoupling zone, sources the hotspots above. The Pacific plate is the fastest plate in the hotspot reference frame and dominates the net rotation of the lithosphere. Therefore, if decoupling with the subasthenospheric mantle is larger, the global westward drift of the lithosphere must be faster than present estimates, and may possibly vary between 50 and 90 mm/yr. In this case, all plates, albeit moving at different velocities, move westward relative to the subasthenospheric mantle. Finally, faster decoupling can generate more shear heating in the asthenosphere (even >100 °C). This amount of heating, in an undepleted mantle, could trigger scattered intraplate Pacific volcanism itself if the viscosity of the asthenosphere is locally higher than normal. The Emperor-Hawaiian bend can be reproduced when bent viscosity anisotropy in the asthenosphere is included. Variations in depth and geometry in the asthenosphere of these regions of higher viscosity could account for the irregular migration and velocities of surface volcanic tracks. This type of volcanic chain has different kinematic and magmatic origins from the Atlantic hotspots or wetspots, which migrate with or close to the oceanic spreading center and are therefore plate margin related.