Mantle Convective Circulation Research Articles

Cenozoic epeirogeny of the Arabian Peninsula from drainage modeling

It is generally accepted that the Arabian Peninsula has been uplifted by subcrustal processes. Positive residual depth anomalies from oceanic crust in the Red Sea and in the Gulf of Aden suggest that a region surrounding this peninsula is dynamically supported. Admittance calculations, surface wave tomography studies, and receiver function analyses all imply that regional topography is generated and maintained by some combination of mantle convective circulation and lithospheric thickness changes. Despite these significant advances, the spatial and temporal uplift rate history of the Arabian Peninsula is not well known. Here we show that a regional uplift rate history can be obtained by jointly inverting 225 longitudinal river profiles that drain this peninsula. Our strategy assumes that shapes of individual river profiles are controlled by uplift rate history and moderated by erosional processes. We used local measurements of incision rate to calibrate the relevant erosional parameters. In our inverse algorithm, uplift rate is permitted to vary smoothly as a function of space and time but upstream drainage area remains invariant. We also assume that knickzone migration is not lithologically controlled. Implications of these important assumptions have been investigated. Our results suggest that the Arabian Peninsula underwent two phases of asymmetric uplift during the last 20–30 Ma at rates of 0.05–0.1 mm a−1. The southwestern flank of the peninsula has been uplifted by 1.5–2.5 km. Regional stratigraphic constraints, the age and composition of volcanism, paleosol formation, incised peneplains, emergent marine terraces, and thermochronometric measurements corroborate our calculated patterns of uplift. Progressive development of three domal swells along the western margin of the peninsula is consistent with localized upwelling of hot asthenospheric mantle.

Spatial and temporal patterns of Australian dynamic topography from River Profile Modeling

Despite its importance, the temporal and spatial evolution of continental dynamic topography is poorly known. Australia's isolation from active plate boundaries and its rapid northward motion within a hot spot reference frame make it a useful place to investigate the interplay between mantle convection, topography, and drainage. Offshore, dynamic topography is relatively well constrained and can be accounted for by Australia's translation over the mantle's convective circulation. To build a database of onshore constraints, we have analyzed an inventory of longitudinal river profiles, which is sensitive to uplift rate history. Using independently constrained erosional parameters, we determine uplift rates by minimizing the misfit between observed and calculated river profiles. Resultant fits are excellent and calculated uplift histories match independent geologic constraints. We infer that western and central Australia underwent regional uplift during the last 50 Myr and that the Eastern Highlands have been uplifted in two stages. The first stage from 120 to 80 Ma, coincided with rifting along the eastern margin and its existence is supported by thermochronological measurements. A second stage occurred at 80–10 Ma, formed the Great Escarpment, and coincided with Cenozoic volcanism. The relationship between topography, gravity anomalies, and shear wave tomographic models suggest that regional elevation is supported by temperature anomalies within the lithosphere's thermal boundary layer. Morphology and stratigraphy of the Eastern Highlands imply that these anomalies have been coupled to the base of the plate during Australia's northward motion over the last 70 Myr.

A joint geochemical–geophysical record of time-dependent mantle convection south of Iceland

The North Atlantic V-Shaped Ridges (VSRs) provide a spatially extensive and clear record of unsteady mantle convective circulation over >40 My. VSRs are diachronous ridges of thick crust formed with a periodicity of ∼5 My along the Mid Atlantic Ridge, south of Iceland. We present data from a set of dredged basalt samples that shows chemical variation associated with two complete VSR crustal thickness cycles where they intersect the Mid Atlantic Ridge. The new dataset also records chemical variation associated with a VSR crustal thickness cycle along a plate spreading flow-line. Inverse correlations between crustal thickness and both incompatible trace element concentrations and incompatible element ratios such as Nb/Y and La/Sm are observed. Geochemical and crustal thickness observations can be matched using a time-dependent mid-ocean ridge melting model with a basal boundary condition of sinusoidally varying potential temperature. Our observations and models suggest that VSRs are generated when hot patches are carried up the plume stem beneath SE Iceland and spread radially outward within the asthenosphere. These patches are then drawn upward into the melting region when passing beneath the Mid Atlantic Ridge. The geometry of the VSRs and the size of the dynamically supported swell suggest that the Iceland Plume is the strongest plume in the Earth at present, with a volume flux of 49±14 km3yr−1.

Spatial and temporal patterns of Cenozoic dynamic topography around Australia

Despite its importance, the spatial and temporal pattern of dynamic topography generated by mantle convective circulation is poorly known. We present accurate estimates of dynamic topography from oceanic basins and continental margins surrounding Australia. Our starting point is measurement of residual depth anomalies on the oldest oceanic floor adjacent to the continental shelf. These anomalies were determined from a combined dataset of ~200 seismic reflection and wide‐angle images of well‐sedimented oceanic crust. They have amplitudes of between −1 km and +0.5 km, and their spatial variation is broadly consistent with long‐wavelength free‐air gravity and shallow seismic tomographic anomalies. Along the Northwest Shelf, a regional depth anomaly of −300 to −700 m intersects the adjacent continental shelf. The temporal evolution of this anomaly was determined by analyzing the stratigraphic architecture of an extensive carbonate platform, which fringes the shelf and records a dramatic switch from progradation to aggradation during Neogene times. Three‐dimensional seismic mapping calibrated by boreholes was used to calculate water‐loaded subsidence histories at rollover points of clinoforms along the shelf. At 9 ± 3 Ma, the rate of subsidence increases from 5 to up 75 m Myr−1, generating a subsidence anomaly of −300 to −700 m. The amplitude of this anomaly varies along the shelf and cannot be generated by glacio‐eustatic sea‐level variation. Instead, we propose that a regional subsidence episode, which affects both the proximal shelf and the distal oceanic basin, was generated by convective drawdown. By combining our results with other published estimates of uplift and subsidence, a map of Australia, which shows the spatial and temporal pattern of dynamic topography is presented. Most, but not all, of Australia's epeirogeny can be attributed to rapid northward motion of the Australian plate over a pre‐existing pattern of convective circulation.

Origin of volcanism on the flanks of the Pacific‐Antarctic ridge between 41°30′S and 52°S

Non–hot spot, intraplate volcanism is a common feature near the East Pacific Rise or Pacific‐Antarctic ridge. Volcanic ridges and seamount chains, tens to hundreds of kilometers long, are asymmetrically distributed about the ridge axis, with most volcanic features occurring on the Pacific plate. Their origins remain controversial. We have analyzed off‐axis volcanic ridges near the Pacific‐Antarctic ridge from bathymetry, backscatter, gravity, and geochemistry data of the Pacantarctic 2 cruise. K/Ar dating of samples dredged on these structures reveals a contrast of up to 3 Ma between the volcanoes and the underlying crust. The volcanic activity, as suggested by the strong backscatter in sonar images, appears to be limited to areas of seafloor younger than about 3 Ma. All surveyed ridges north of the Menard transform fault (TF) show recent activity close to the ridge axis and are not affected by faults. The off‐axis volcanic ridges south of the Menard TF show recent volcanic flows in their center and are affected by N‐S extension. Two different types of volcanoes can be characterized: conical, flat‐topped ones and rough, elongated ones associated with narrow, E‐W trending volcanic ridges, some of them showing strong backscattering on the EM12 imagery. From the morphology of the seamount chains and the ages of the lava samples, we infer that the magma source for the volcanic ridges is related to the feeding of the ridge axis through three‐dimensional mantle convective circulation. We suggest that a change in the relative plate motion since about 5 Ma might have induced an offset of the mantle upwelling circulation under the ridge axis so that anomalously hot mantle rises under the Pacific ridge flank. The kinematic change is also likely responsible for the tectonic deformation in the young lithosphere south of Menard TF.

Quantifying transient mantle convective uplift: An example from the Faroe‐Shetland basin

The Icelandic mantle plume generated maximum uplift in early Paleogene times. The Faroe‐Shetland basin, which fringes the North Atlantic margin of Europe, was close to the center of early Paleogene Icelandic plume activity. Three‐dimensional seismic reflection data from the Faroe‐Shetland basin reveal sedimentary geometries that allow a phase of transient uplift to be accurately reconstructed and quantified. Close to the Paleocene/Eocene boundary (circa 56 Ma), rapid uplift resulted in fluvial incision into marine sediments. This unconformity was buried by nonmarine sediments, recording the decay of transient uplift. Relief on the unconformity was ∼550 m, constraining the minimum amount of surface uplift. Some 60 m of this uplift can be attributed to the isostatic response to erosional unloading. Tectonic uplift of over 490 m peaked and decayed within 3 Ma. Rates of water‐loaded tectonic subsidence following peak uplift are several times greater than maximum expected postrift subsidence rates. The amplitude and duration of this transient effect is best explained by a mantle convective phenomenon. We suggest that a region of hot plume material flowed laterally beneath the lithosphere, producing transient uplift which decayed when plume material was advected farther away. Our analysis suggests that under certain circumstances, stratigraphic records can yield valuable quantitative information about aspects of mantle convective circulation.

Global surface heat flux anomalies from seismic tomography‐based models of mantle flow: Implications for mantle convection

We investigate the very long‐wavelength, global pattern of surface heat flux anomalies within the context of whole‐mantle and layered‐mantle anelastically compressible internal loading theories. Since the internal loading framework does not yield a direct estimate of the geotherm, we argue that accurate predictions for the surface heat flux may nevertheless be obtained by assuming that it is linearly related to the radial component of flow velocity at shallow depth in the mantle. The mantle convective circulation is assumed to be driven by density heterogeneity inferred from global seismic tomography models. Best results for the pattern of surface heat flux anomalies are obtained for models that significantly impede the circulation at a depth of 670 km. Total variance reductions of 60–65% (degree 1–5) are obtained when the viscosity profile includes a low‐viscosity asthenosphere. Within the context of our modeling assumptions, however, whole‐mantle circulation models provide best descriptions of the long‐wavelength nonhydrostatic gravity data. In order to resolve the gravity‐heat flux impasse that is revealed herein, we consider the possibility of modifying the a priori global seismic models employed in the calculations. We show that the rigidly layered‐mantle internal loading theory is equivalent to a theory in which no explicit flow‐blocking boundary condition is imposed at 670 km but in which the buoyancy field inferred from the a priori tomographic model is supplemented by flow‐blocking heterogeneity in the form of an appropriately constrained sheet mass load. We develop a general mathematical formalism describing how the introduction of appropriately constrained sheet mass loads allows the exact reconciliation of a number of a priori constraints or hypotheses concerning the structure of the circulation. Using this formalism, we explore the extreme nonuniqueness that not only characterizes internal loading theory inferences of the depth profile of mantle viscosity but also inferences of the radial style of the circulation. On this basis, we suggest that great caution is warranted with respect to tomography‐based inferences of mantle properties. Based on a viscosity profile whose depth dependence is close to that independently inferred within the context of postglacial rebound studies, we present plausible resolutions of the gravity‐heat flux impasse effected either within the framework of whole‐mantle or layered‐mantle circulation models.

The LAGEOS constraint on deep mantle viscosity: Results from a new normal mode method for the inversion of viscoelastic relaxation spectra

The recently observed acceleration in the node of the orbit of the LAGEOS satellite has previously been shown to provide a good constraint on the viscosity of the earth's mantle beneath the 670‐km seismic discontinuity. This paper explores the extent to which the previously inferred lower mantle viscosity may be traded off against variations of other properties of the radial viscoelastic structure. We show explicitly that the nontidal acceleration of rotation inferred from the LAGEOS data, and predicted as a consequence of the earth's response to the last deglaciation event of the current ice age, is relatively insensitive both to variations of lithospheric thickness and to the presence or absence of plausible chemical heterogeneity across the 670‐km interface. It therefore provides a high‐quality constraint on the viscosity of the deep mantle. With the upper mantle viscosity fixed at the nominal value of 1021 Pa s, the bounds on lower mantle viscosity νLM are such that 2×1021 Pa s ≤ νLM ≤ ∼6×1021 Pa s even when allowance is made for the presence of a viscous boundary layer at the base of the upper mantle such as would exist if the mantle convective circulation were layered.

Whole mantle convection and the thermal evolution of the earth

Three recent results in the fields of geophysics and geochemistry have combined to rejuvenate the study of the Earth's thermal history. The first, and perhaps most important, of these is the new understanding of the planetary accretion process which has been realized as a consequence of work by V.S. Safronov and others. This work shows that the terrestrial planets accrete on a time scale of ∼10 8 y by the accumulation of planetesimal sized objects and that they probably form sufficiently hot to induce ‘immediate’ differentiation of iron from silicates. The second important recent contribution has been from geochemistry and concerns the net complement of radioactivity of the Earth as a whole. Although the net complement of radioactive elements has usually been inferred on the basis of the assumption of equilibrium with the surface heat flow, a priori analysis based upon elemental condensation temperatures in the primitive solar nebula yields a model in which the net radioactivity is at least a factor of two less than would be required to explain the mean surface heat flow. There are good reasons to believe that this model is valid for the Earth. The third result has been the successful development of a technique for incorporating the influence of convective heat transfer in a thermal evolution model which does not require explicit solution of the dynamical equations. This paper begins with an analysis of the mantle convective circulation which is required to understand plate tectonics and continental drift, an analysis which presents the arguments for and against the crucial notion that the main circulation has a depth scale equal to the mantle thickness. Using the idea of ‘parameterized’ convection, the radial heat transfer by the mantle convective circulation is included in a thermal history model of the Earth. This model turns out to contain within it an interesting explanation of the fact that no crustal rocks with ages in excess of 3.8 Ga have ever been found, an observation that has heretofore proven enigmatic.

Mantle plumes and the thermal stability of the D″ layer

The interpretation of the seismic D″ layer at the base of the mantle as the lower thermal boundary layer of a whole mantle convective circulation has several important geophysical consequences. Because of the strong temperature dependence of mantle viscosity, which sharply reduces the viscosity in the boundary layer itself, this region could be strongly unstable against secondary convective instability. Linear stability analyses presented here show that this destabilization is in fact dramatic, the fastest growing disturbances being characterized by growth times on the order of 106 years and spatial scales on the order of 10² km for a wide variety of plausible lower mantle conditions. The highly nonlinear evolution of the resulting “thermal turbulence” may be such that fastest growing disturbances mature into thermal plumes which rise rapidly to the base of the lithosphere where they may induce intense partial melting.