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GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Primary terms
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downwelling
The assembly of Pangaea: geodynamic conundrums revisited
Archean Cratons: Terms, Concepts, and Analytical Approaches
Dense melt residues drive mid-ocean-ridge “hotspots”
ABSTRACT The geodynamic origin of melting anomalies found at the surface, often referred to as “hotspots,” is classically attributed to a mantle plume process. The distribution of hotspots along mid-ocean-ridge spreading systems around the globe, however, questions the universal validity of this concept. Here, the preferential association of hotspots with slow- to intermediate-spreading centers and not fast-spreading centers, an observation contrary to the expected effect of ridge suction forces on upwelling mantle plumes, is explained by a new mechanism for producing melting anomalies at shallow (<2.3 GPa) depths. By combining the effects of both chemical and thermal density changes during partial melting of the mantle (using appropriate latent heat and depth-dependent thermal expansivity parameters), we find that mantle residues experience an overall instantaneous increase in density when melting occurs at <2.3 GPa. This controversial finding is due to thermal contraction of material during melting, which outweighs the chemical buoyancy due to melting at shallow pressures (where thermal expansivities are highest). These dense mantle residues are likely to locally sink beneath spreading centers if ridge suction forces are modest, thus driving an increase in the flow of fertile mantle through the melting window and increasing magmatic production. This leads us to question our understanding of sub–spreading center dynamics, where we now suggest a portion of locally inverted mantle flow results in hotspots. Such inverted flow presents an alternative mechanism to upwelling hot mantle plumes for the generation of excess melt at near-ridge hotspots, i.e., dense downwelling of mantle residue locally increasing the flow of fertile mantle through the melting window. Near-ridge hotspots, therefore, may not require the elevated temperatures commonly invoked to account for excess melting. The proposed mechanism also satisfies counterintuitive observations of ridge-bound hotspots at slow- to intermediate-spreading centers, yet not at fast-spreading centers, where large dynamic ridge suction forces likely overwhelm density-driven downwelling. The lack of observations of such downwelling in numerical modeling studies to date reflects the generally high chemical depletion buoyancy and/or low thermal expansivity parameter values employed in simulations, which we find to be unrepresentative for melting at <2.3 GPa. We therefore invite future studies to review the values used for parameters affecting density changes during melting (e.g., depletion buoyancy, latent heat of melting, specific heat capacity, thermal expansivity), which quite literally have the potential to turn our understanding of mantle dynamics upside down.
Geochemical Heterogeneity of Indian Ocean MOR Mantle
The role of megacontinents in the supercontinent cycle
Investigating the formation of the Cretaceous Western Interior Seaway using landscape evolution simulations
Mantle earthquakes, crustal structure, and gravitational instability beneath western North Island, New Zealand
P-wave tomography of potential convective downwellings and their source regions, Sierra Nevada, California
Craton formation: Internal structure inherited from closing of the early oceans
Mount Etna–Iblean volcanism caused by rollback-induced upper mantle upwelling around the Ionian slab edge: An alternative to the plume model
Influence of dynamic topography on sea level and its rate of change
In sand-shale sequences, subsidence due to compaction may be as important as eustatic and tectonic effects in controlling relative sea-level variations that determine sediment accommodation space and extent of marine transgressions. Furthermore, stratigraphic configuration in coastal settings is more sensitive to relative sea-level changes than either continental or deeper marine environments. Thus, the contribution to local sea-level variations due to compaction and its effect on coastal stratigraphy merit special attention. Compaction in turn is determined by the control of stratigraphic architecture on dewatering, by depositional timing, and by the physical properties of the compacting sediments. In modeling contemporary hydrogeology in these settings, compaction plays a double role: (1) it affects aquifer geometry by way of changes in sediment accommodation space and depositional environment, and (2) it determines petrophysical properties that control water extraction and the ensuing subsidence. We have explored this interaction by using a numerical sedimentary process model coupled with a single-phase flow simulator to study the interaction of sedimentation, dewatering, and compaction in coastal environments. The results explain several features of the sedimentary record and present-day morphology observed in data from the Gulf of Mexico. Although compaction may be hard to separate from other causes of relative sea-level change in ancient environments, careful interpretation of sedimentation timing and quantitative modeling can identify the effect of each. The identification of sedimentation-compaction regimes in coastal settings provides a useful conceptual tool for the geologic interpretation of ancient coastal deposits identified in seismic and well-log data. Hydrogeologic modeling performed on deposits that are simulated by sedimentary process models benefits from a richness of detail that can enhance hydrogeologic predictions. Additionally, the values of petrophysical properties required for hydrogeologic modeling (such as porosity, permeability, and compaction coefficient) can be better constrained by this dual modeling approach, because they result from the prior geologic sedimentation and compaction model, which limits their range of possible values.