Thermal Evolution Of Oceanic Lithosphere Research Articles

Comprehensive plate models for the thermal evolution of oceanic lithosphere

Seafloor spreading and the cooling of oceanic lithosphere is a fundamental feature of plate tectonics in the Earth, the details of which are unveiled by modeling with constraints from mineral physics and geophysical observations. To work toward a more complete model of the thermal evolution of oceanic lithosphere, we investigate the contributions of axial hydrothermal circulation, oceanic crust, and temperature‐pressure‐dependent thermal properties. We find that models with only temperature‐dependent properties disagree with geophysical observations unless properties are artificially modified. On the other hand, more comprehensive models are in better agreement with geophysical observations. Our preferred model requires a thermal expansivity reduction of 15% from a mineral physics estimate, and predicts a plate thickness of about 110–130 km. A principal result of our analysis is that the oceanic crust is a major contributor to the cooling of oceanic lithosphere. The oceanic crust acts as an insulating lid on the mantle, causing the rate of lithospheric cooling to increase from “crustal” values near the ridge to higher mantle values at old‐age. Major consequences of this insulation effect are: (a) low seafloor subsidence rate in proximity to ridge axes (<5 Ma), (b) the thermal structure of oceanic lithosphere is significantly warmer than previous models, (c) seafloor heat flow is significantly lower over young (<35 Ma) seafloor compared to simple models, (d) a low net seafloor heat flux (∼27 TW), and (e) temperature at the base of the seismogenic zone extends to 700–800°C mantle.

Localization of geoid anomalies and the evolution of oceanic lithosphere: A case study from the Mendocino Fracture Zone

The thermal evolution of oceanic lithosphere is investigated by focusing on geoid offsets occurring across the Mendocino Fracture Zone, where plates with different ages are juxtaposed. Various processing techniques have been devoted to separate the geoid signal of the age contrast from regional and shorter wavelength components unrelated to the thermal structure of lithosphere. Nevertheless, due to processing differences, estimates of geoid offsets vary, and no agreement on the thermal evolution of oceanic lithosphere has been found so far. In this study, we propose to use a continuous wavelet analysis to accurately characterize the components of the geoid at different spatial scales and to estimate a new geoid slope‐age relationship from localized signals. We also apply the same wavelet transform on a set of synthetic geoid calculated with different assumptions on plate cooling. The comparison of the observed geoid offsets with those predicted from cooling models indicates that our approach can successfully remove unwanted regional contributions and isolate the geoid signature due to lithospheric cooling. Our results suggest that, contrary to previous studies, geoid slopes measured at the Mendocino Fracture Zone are compatible with both the half‐space cooling model and the plate model.

Subsidence of normal oceanic lithosphere, apparent thermal expansivity, and seafloor flattening

Seafloor topography has been a key observational constraint on the thermal evolution of oceanic lithosphere, which is the top boundary layer of convection in Earth's mantle. At least for the first ~70 Myr, the age progression of seafloor depth is known to follow the prediction of half-space cooling, and the subsidence rate is commonly believed to be ~350 m Ma−1/2. Here we show that, based on a new statistical analysis of global bathymetry, the average subsidence rate of normal oceanic lithosphere is likely to be ~320 m Ma−1/2, i.e., ~10% lower than the conventional value. We define the ‘normal’ seafloor as regions uncorrelated with anomalous crust such as hotspots and oceanic plateaus, but the lower subsidence rate appears to be a stable estimate, not depending on how exactly we define the normal seafloor. This low subsidence rate can still be explained by half-space cooling with realistic mantle properties, if the effective thermal expansivity of a viscoelastic mantle is taken into account. In light of a revised model of half-space cooling, the normal seafloor unperturbed by the emplacement of anomalous crust exists for all ages, and the so-called seafloor flattening seems to be mostly caused by hotspots and oceanic plateaus.

Sublithospheric small‐scale convection and its implications for the residual topography at old ocean basins and the plate model

The seafloor topography and heat flux differ significantly from the predictions of the half‐space cooling (HSC) model at old ocean basins. Understanding the deviations has important implications for thermal evolution of oceanic lithosphere and large‐scale mantle dynamics. A widely used model that explains significant fraction of the deviations is the plate model, but the dynamical feasibility of the plate model has never been demonstrated. In this study, we investigated the effects of sublithospheric small‐scale convection (SSC) and of internal heating on seafloor heat flux and topography and mantle thermal structure, and we examined the dynamic feasibility of the plate model by formulating high‐ resolution two‐dimensional numerical models of mantle convection with strongly temperature‐ and depth‐dependent rheology. We found that mantle convection with tectonic plates often leads to formation of a broad thermal anomaly below old lithosphere where the mantle is not cooled by subducted slabs and heat transfer is less efficient because of thick lithosphere, especially when significant internal heating is present. This trapped heat may exist in the middle mantle when the SSC is absent, and it may also be redistributed by the SSC to shallow depths to reheat the lithosphere and to homogenize mantle temperature. When internal heating accounts for >∼60% of the total heat output, the trapped heat may provide sufficient heat supply to preferentially reheat old lithosphere via SSC while maintaining uniform mantle temperature. We suggest that the trapped heat and the SSC are responsible for the residual heat flux and topography at old ocean basins relative to the HSC model predictions. Our models also show that for the plate model to be dynamically viable, both the SSC and significant internal heating (>60%) are necessary. This is because only the SSC in a mantle with significant internal heating can erode and reheat the lithosphere while maintaining a nearly constant mantle temperature below lithosphere, which is the basic assumption of the plate model. With the viscosity structure and internal heating rate for the present‐day mantle, we think that the plate model is dynamically viable.

Extraction of a lithospheric cooling signal from oceanwide geoid data

The geoid, an equipotential surface of Earth’s gravity field, reflects the distribution of mass within the planet and hence a variety of geodynamic processes. Because these data are dominated by sublithospheric processes, notably mantle convection, they have not played a major role in the ongoing debate concerning models of the thermal evolution of oceanic lithosphere. Application of spatial filtering to the age derivative of the geoid, however, extracts an age-dependent signal which reflects lithospheric thermal evolution. The data are much better fit by a thin (about 100 km thick) thermal plate than by a cooling halfspace, and so provide a valuable constraint complementary to and consistent with the variations in oceanic depth and heat flow with age.

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

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.

Constraints on lithospheric thermal structure for the Indian Ocean from depth and heat flow data

Models for the thermal evolution of oceanic lithosphere are primarily constrained by variations in seafloor depth and heat flow with age. These models have been largely based on data from the Pacific and Atlantic Ocean basins. We construct seafloor age relations for the Indian Ocean which we combine with bathymetric, sediment isopach and heat flow data to derive curves for depth and heat flow versus age. Comparison of these curves with predictions from three thermal models shows that they are better fit by the shallower depths and higher heat flow for the GDH1 model, which is characterized by a thinner and hotter lithosphere than previous models.

North American Plate Dynamics

Deformation within the North American plate in response to various tectonic processes is modeled using an elastic finite element analysis. A coarse grid contains 328 elements and 190 nodes, while a fine grid contains 718 elements and 396 nodes for a spatial resolution of 2°–3°. Abundant information about the present‐day state of intraplate stress constrains the modeling. The dominant pattern that must be fit by all acceptable models is an ENE trend for the maximum compressive stress for most of the plate east of the Rocky Mountains. The tectonic processes considered in the modeling include ridge forces associated with the normal thermal evolution of oceanic lithosphere, shear and normal stresses transmitted across transforms, normal stresses transmitted across convergent boundaries, stresses due to horizontal density contrasts within the continent, and shear tractions applied along the base of the plate. Model stresses are calculated with respect to a lithostatic reference stress state. Distributed ridge forces of magnitude 2×1012 N/m predict deviatoric stresses of the order of 20–40 MPa that are capable of accounting for the dominant observed ENE stress trend. Driving drag models also fit the trend but are not preferred because of a predicted tenfold increase in compressive stress magnitudes from east to west across the plate. Assuming that ridge forces account for the dominant ENE stress trend, bounds may be placed on other possible forces. For example, shear stresses transmitted across transform boundaries along the San Andreas and Caribbean are small, of the order of 5–10 MPa. Also, compressive stresses of the order of 5–10 MPa transmitted across the major transforms improve the fit to the data. Compressive stresses across convergent margins along the Aleutians and the Middle America trench are important.