Carol A. Stein

Constraints on hydrothermal heat flux through the oceanic lithosphere from global heat flow

A significant discrepancy exists between the heat flow measured at the seafloor and the higher values predicted by thermal models of the cooling lithosphere. This discrepancy is generally interpreted as indicating that the upper oceanic crust is cooled significantly by hydrothermal circulation. The magnitude of this heat flow discrepancy is the primary datum used to estimate the volume of hydrothermal flow, and the variation in the discrepancy with lithospheric age is the primary constraint on how the hydrothermal flux is divided between near-ridge and off-ridge environments. The resulting estimates are important for investigation of both the thermal structure of the lithosphere and the chemistry of the oceans. We reevaluate the magnitude and age variation of the discrepancy using a global heat flow data set substantially larger than in earlier studies, and the GDH1 (Global Depth and Heat flow) model that better predicts the heat flow. We estimate that of the predicted global oceanic heat flux of 32×1012 W, 34% (11×1012 W) occurs by hydrothermal flow. Approximately 30% of the hydrothermal heat flux occurs in crust younger than 1 Ma, so the majority of this flux is off-ridge. These hydrothermal heat flux estimates are upper bounds, because heat flow measurements require sediment at the site and so are made preferentially at topographic lows, where heat flow may be depressed. Because the water temperature for the near-ridge flow exceeds that for the off-ridge flow, the near-ridge water flow will be even a smaller fraction of the total water flow. As a result, in estimating fluxes from geochemical data, use of the high water temperatures appropriate for the ridge axis may significantly overestimate the heat flux for an assumed water flux or underestimate the water flux for an assumed heat flux. Our data also permit improved estimates of the “sealing” age, defined as the age where the observed heat flow approximately equals that predicted, suggesting that hydrothermal heat transfer has largely ceased. Although earlier studies suggested major differences in sealing ages for different ocean basins, we find that the sealing ages for the Atlantic, Pacific, and Indian oceans are similar and consistent with the sealing age for the entire data set, 65±10 Ma. The previous inference of a young (∼20 Ma) sealing age for the Pacific appears to have biased downward several previous estimates of the global hydrothermal flux. The heat flow data also provide indirect evidence for the mechanism by which the hydrothermal heat flux becomes small, which has often been ascribed to isolation of the igneous crust from seawater due to the hydraulic conductivity of the intervening sediment. We find, however, that even the least sedimented sites show the systematic increase of the ratio of observed to predicted heat flow with age, although the more sedimented sites have a younger sealing age. Moreover, the heat flow discrepancy persists at heavily sedimented sites until ∼50 Ma. It thus appears that ∼100–200 m of sediment is neither necessary nor sufficient to stop hydrothermal heat transfer. We therefore conclude that the age of the crust is the primary control on the fraction of heat transported by hydrothermal flow and that sediment thickness has a lesser effect. This inference is consistent with models in which hydrothermal flow decreases with age due to reduced crustal porosity and hence permeability.

Plate tectonic models for Indian Ocean intraplatë deformation

Abstract The equatorial region of the conventionally defined Indo-Australian plate has long been recognized as containing a type example of intense “intraplate” deformation. We trace the development of tectonic models for the area to illustrate techniques for the analysis of such deformation. The identification of anomalous seismicity near the Ninetyeast and Chagos-Laccadive Ridges demonstrated the existence of the deformation. Focal mechanisms from recent and historic earthquakes showed strike-slip motion occurring along the Ninetyeast Ridge; seismic moment data allowed the rate to be estimated. Similar studies showed north-south tension in the Chagos Bank region and north-south compression in the region between the Ninetyeast and Chagos ridges. Global plate motion studies indicated non-closure of the Indian Ocean triple junction, suggesting the conventional plate geometry was inadequate for a rigid plate description of the area. Gravity and marine geophysical data indicated intense north-south compressional deformation south of the Bay of Bengal. These observations are reconciled by a plate motion model in which Australia and India lie on distinct plates divided by a boundary that intersects the Central Indian Ridge near the equator. In this model Arabia, usually considered a separate plate, has negligible motion relative to India. The resulting Euler vector for Australia relative to Indo-Arabia lies just east of the Central Indian Ridge, and predicts approximately 0.5–1.5 cm/yr compression in the Central Indian Basin and 1.5–2 cm/yr strike-slip motion along the northern Ninetyeast Ridge, consistent with the seismological and geophysical data. In contrast to conventional oceanic plate boundaries, the boundary deformation is distributed over a wide zone. This diffuse nature may reflect either the boundarys recent inception or slow rate of motion. Analysis of seismicity and deformation in the boundary zone should offer insights into the mechanics of its development and its implications for the evolution of plate boundaries.

Heat flow constraints on the South Pacific Superswell

The South Pacific superswell has been defined as a large region of anomalously shallow bathymetry, low Love wave velocities, and low effective elastic thicknesses relative to those predicted for its age. These phenomena have been interpreted as reflecting a combination of lithospheric reheating and thinning, and dynamic uplift due to mantle flow. We use heat flow data to better constrain the thermal structure of this region and examine the predictions of various possible models. The average heat flow for the superswell region does not differ significantly from that for lithosphere of similar ages elsewhere on the Pacific plate. Given their uncertainties, the heat flow data imply that thermal lithospheric thickness exceeds 60 km, but cannot discriminate between greater thicknesses. The lack of observed high heat flow appears not to be explained by biases due to water circulation in the thin sediment cover, since the superswell heat flow is not higher than for sites elsewhere with similar sedimentary environments. The Darwin Rise has been proposed as a fossil superswell in the Cretaceous, on the basis of the many similar characteristics to the South Pacific superswell. We find that the Darwin Rise heat flow values do not exceed those for similar ages elsewhere in the Pacific and Atlantic. This observation suggests that any thermal effects associated with the formation of the Darwin Rise are no longer present, and it is consistent with the idea of a fossil superswell. The surface heat flow data thus provide no evidence that the temperatures in the uppermost portion of the lithosphere; are significantly higher in the entire superswell region than in other areas of comparable age. This observation is intriguing given the suggestion that the thin effective elastic thicknesses inferred from seamount loading may reflect reheating of the lithosphere. Models in which the plate thicknesses and/or the basal temperatures are increased to yield temperatures high enough to explain the low effective elastic thicknesses predict surface heat flow much higher than observed. Reheating the lithosphere, as is proposed for hot spots, yields temperatures adequate to explain the effective elastic thicknesses only if reheating occurs at very shallow depths, and again implies a surface heat flow much greater than observed. Hence, unless shallow reheating is somehow localized beneath the seamounts, the thinner elastic thicknesses may reflect mechanical, rather than thermal, weakening of the lithosphere.

Preconceptions abound among students in an introductory earth science course

Although the Third International Mathematics and Science Study found that most 8th-grade students like science and feel that they are doing well in it [Geary, 1997], fewer than one-quarter of U.S. adults can define the term DNA and only one in 11 knows what a molecule is [Augustine, 1998]. Hence the motivated, bright young people described by the study somehow become scientific illiterates despite the best efforts of elementary, secondary, and college-level instructors. This phenomenon has prompted various investigations into reasons why students have difficulty learning science. One possibility is illustrated by the famed video [Shapiro etal, 1988] showing that most of the graduating Harvard seniors surveyed confidently attributed the cause of the seasons to changes in the distance between the Earth and Sun rather than to the Earths tilt. They had a clear conception of the answer, but it was wrong.

Estimation of oceanic hydrothermal heat flux from heat flow and depths of midocean ridge seismicity and magma chambers

The difference between the heat flow predicted by thermal models of the lithosphere and that measured at the seafloor can be used to estimate the cooling by hydrothermal circulation. However, this approach may yield an overestimate because measurements in thinly sedimented young crust are typically made at topographic lows where hydrothermal circulation may systematically lower heat flow. To circumvent this bias we estimate the cooling using the depths of midocean ridge earthquakes and magma chambers in addition to heat flow data. The results indicate deeper hydrothermal circulation at slow spreading ridges, and higher near-axial hydrothermal heat loss at fast spreading ridges. The predicted global hydrothermal heat flux is ∼80% of that for models constrained only by heat flow.

Constraints on the Central Indian Basin thermal structure from heat flow, seismicity and bathymetry

Abstract The lithosphere beneath the Central Indian Basin is characterized by high heat flow, widespread deformation of sediment and acoustic basement, and unusually high seismicity. The high heat flow suggests that temperatures in the lithosphere may be higher than expected for its age. We explore the constraints on the temperature distribution with depth by combining heat flow, bathymetry, and seismicity data. The high heat flow (~30 mW m−2 greater than expected) constraints the near-surface temperature distribution. The bathymetric depths (on average those expected for these lithospheric ages after corrections for sediment loads) constrain the integral of the temperature with depth. Since the depths of oceanic intraplate earthquakes appear to be limited by an isotherm (~ 750°C), the maximum earthquake depth (40 km) constrains the minimum depth of deformation and the maximum temperature there. The present thermal structure is investigated by examining different models matching these three observational constraints. First, the possibility is examined that the high heat flow results from anomalous lithosphere with a basal temperature or thickness different from typical oceanic lithosphere. In this case the temperature constraints from the earthquake depths restrict the additional heat flux to less than 20 mW m−2. However, since the heat flux anomaly requires plate thicknesses substantially thinner than usually assumed, it appears that at most 10 mW m−2 can be added by this mechanism. Second, the possibility is examined that the extra flux results from reheating the bottom portion of the lithosphere to asthenospheric temperatures. Substantial reheating 30–40 km below the surface is required to match the high heat flow. Such reheating should produce

Stress magnitude estimates from earthquakes in oceanic plate interiors

1 km of uplift, whereas the average basement depth is no shallower than expected for its age. Thus, significant deep lithospheric reheating cannot be widespread over the deformed region. Third, the effects of a temperature perturbation within the lithospheric column and the resulting change in surface heat flux with time are examined. A temperature perturbation at shallow depths could produce the present heat flow anomaly, but no evidence for shallow intrusion or other mechanism for such an effect exists. It is concluded that, despite the heat flow anomaly, the lack of an average bathymetric anomaly and the observation of seismicity to a depth of 40 km indicate that lithospheric temperatures in the Central Indian Basin are not significantly different from those expected for its age.

Was the Midcontinent Rift part of a successful seafloor-spreading episode?

We propose a method to estimate stress magnitudes in oceanic plate interiors from focal depths and focal mechanisms. Using a depth-dependent rheology, we show it is possible to estimate the differential stress (σ1–σ3), averaged over some reference lithospheric thickness. The resolving power of the method is investigated by evaluating the effect of uncertainties in parameters that are involved in the analysis. We apply the method to the Central Indian Ocean, where intraplate seismicity is high. From well-studied earthquakes we estimate differential stresses of the order of hundreds of rnegapascals. This result is consistent with the high level of stress that was found from numerical model calculations by Cloetingh and Wortel (1985, 1986). From the few intraplate events in the Pacific plate, we also estimate differential stresses in this area.

Comparison of plate and asthenospheric flow models for the thermal evolution of oceanic lithosphere

The ~1.1 Ga Midcontinent Rift (MCR), the 3000 km long largely buried feature causing the largest gravity and magnetic anomaly within the North American craton, is traditionally considered a failed rift formed by isolated midplate volcanism and extension. We propose instead that the MCR formed as part of the rifting of Amazonia (Precambrian northeast South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established. A cusp in Laurentias apparent polar wander path near the onset of MCR volcanism, recorded by the MCRs volcanic rocks, likely reflects the rifting. This scenario is suggested by analogy with younger rifts elsewhere and consistent with the MCRs extension to northwest Alabama along the East Continent Gravity High, southern Appalachian rocks having Amazonian affinities, and recent identification of contemporaneous large igneous provinces in Amazonia.

Extraction of a lithospheric cooling signal from oceanwide geoid data

Although seafloor depth and heat flow for young oceanic lithosphere can be described by modeling the lithosphere as the boundary layer of a cooling halfspace, a long standing question has been why data at older ages deviate from those expected for a halfspace. Two classes of models have been proposed for these deviations. In one, heat added from below “flattens” depth and heat flow. In the other, asthenospheric flow beneath the lithosphere perturbs the depths. We compare recent versions of the model classes: the GDH1 thin-lithosphere plate model [Stein and Stein, 1992] and an asthenospheric flow model [Phipps Morgan and Smith, 1992]. The plate model fits heat flow data better than the flow model for all cases considered, and topographic data in all but one case. The flow model significantly overpredicts depths for the North Atlantic, because the assumed asthenospheric flow in the plate motion direction would yield deepening for old ages rather than the observed flattening. Overall, the GDH1 global average model does better than this flow model, whose parameters were fit to specific plates. Moreover, plate models fit to specific plates do better than the flow model. Plate models thus appear more useful than this flow model, suggesting that deviations from a cooling halfspace are largely thermal in origin.