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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 ...

New thermal data from 18–24 Ma lithosphere on the Cocos Plate delineate contrasting subsurface thermal conditions in adjacent sections of crust. Heat flow through seafloor created at the East Pacific Rise is generally suppressed by ∼70% relative to conductive lithospheric cooling models, whereas heat flow through adjacent, similarly‐aged lithosphere generated at the Cocos‐Nazca Spreading Center is consistent with these models. The transition between thermal regimes is remarkably abrupt, only 2–5 km wide, indicating a shallow hydrothermal origin. The transition is more closely associated with differences in the distribution of basement outcrops than with tectonic boundaries, demonstrating the importance of the former in extracting heat from the lithosphere on a regional basis.

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Australia and India are conventionally thought to be contained in a single plate divided from an Arabian plate by the Owen Fracture Zone. We propose instead that motion along the nearly aseismic Owen Fracture Zone is negligible and that Arabia and India are contained within a single Indo‐Arabian plate, divided from the Australian plate by a diffuse boundary. This boundary, which trends E‐W from the Central Indian Ridge near Chagos Bank to the Ninetyeast Ridge, and north along the Ninetyeast Ridge to the Sumatra Trench, is a zone of concentrated seismicity and deformation heretofore characterized as “intraplate”. Plate motion inversions and an F‐ratio test show that relative motion data along the Carlsberg Ridge are fit significantly better by the new model. The misclosure of the Indian Ocean triple junction is reduced by 40%. The rotation vector of Australia relative to Indo‐Arabia is consistent with the seismologically observed ∼2 cm/yr of left‐lateral strike‐slip along the Ninetyeast Ridge, N‐S compression in the Central Indian Ocean, and the N‐S extension near Chagos. This boundary, possibly initiated in late Miocene time, may be related to the opening of the Gulf of Aden and the uplift of the Himalayas. The convergent segment of this boundary may represent an early stage of convergence preceding the initiation of subduction.