Secular cooling of the solid Earth, emergence of the continents, and evolution of Earth's external envelopes

The secular cooling of the mantle and of the continental lithosphere trigger an increase in the area of emerged land. The corollary increase in weathering and erosion processes has major consequences for the evolution of Earth's external envelopes. We developed a physical model to evaluate the area of emerged land as a function of mantle temperature, continental area, and of the distribution of continental elevations. Our numerical results show that less than 15% of Earth's surface consisted of emerged land by the end of the Archaean. This is consistent with many geological and geochemical observations. To estimate the secular cooling of the continental lithosphere, we combined thermo-mechanical models with fi eld observations. Our results, constrained by geological data, suggest that the Moho temperature has decreased by ~ 200ºC over 2.7 Ga in the Pilbara Craton. To evaluate the eff ect of continental growth on the evolution of the area of emerged land, we developed a model based on published thermal evolution models. Our results suggest that the area of emerged land was less than 5% of Earth's surface in the Archaean, and that it does not depend on crustal growth. This allows to reconcile the evolution of oceanic 87Sr/86Sr with early crustal growth models. Continents are enriched in phosphorus, which is essential to the biosphere. The emergence of the continents would thus have triggered an increase in the production of oxygen by photosynthetic micro-organisms, possibly contributing to the oxidation of the atmosphere 2.4 Ga ago

Insights on the kinematics of the India-Eurasia collision from global geodynamic models

The Eocene India-Eurasia collision is a first order tectonic event whose nature and chronology remains controversial. We test two end-member collision scenarios using coupled global plate motion-subduction models. The first, conventional model, invokes a continental collision soon after ∼60 Ma between a maximum extent Greater India and an Andean-style Eurasian margin. The alternative scenario involves a collision between a minimum extent Greater India and a NeoTethyan back-arc at ∼60 Ma that is subsequently subducted along southern Lhasa at an Andean-style margin, culminating with continent-continent contact at ∼40 Ma. Our numerical models suggest the conventional scenario does not adequately reproduce mantle structure related to Tethyan convergence. The alternative scenario better reproduces the discrete slab volumes and their lateral and vertical distribution in the mantle, and is also supported by the distribution of ophiolites indicative of Tethyan intraoceanic subduction, magmatic gaps along southern Lhasa and a two-stage slowdown of India. Our models show a strong component of southward mantle return flow for the Tethyan region, suggesting that the common assumption of near-vertical slab sinking is an oversimplification with significant consequences for interpretations of seismic tomography in the context of subduction reference frames

A deep subaqueous fan depositional model for the Palaeoarchaean (3.46 Ga) Marble Bar Cherts, Warrawoona Group, Western Australia

International audienceThe 3.46 Ga Marble Bar Chert Member of the East Pilbara Craton, Western Australia, is one of the earliest and best-preserved sedimentary successions on Earth. Here, we interpret the finely laminated thin-bedded cherts, mixed conglomeratic beds, chert breccia beds and chert folded beds of the Marble Bar Chert Member as the product of low-density turbidity currents, high-density turbidity currents, mass transport complexes and slumps, respectively. Integrated into a channel-levee depositional model, the Marble Bar Chert Member constitutes the oldest documented deep-sea fan on Earth, with thin-bedded cherts, breccia beds and slumps composing the outer levee facies tracts, and scours and conglomeratic beds representing the channel systems

Provenance of plumes in global convection models

In global convection models constrained by plume motions and subduction history over the last 230 Myr, plumes emerge preferentially from the edges of thermochemical structures that resemble present-day large low shear velocity provinces (LLSVPs) beneath Africa and the Pacific Ocean. It has been argued that large igneous provinces (LIPs) erupting since 200 Ma may originate from plumes that emerged from the edges of the LLSVPs and numerical models have been devised to validate this hypothesis. Although qualitative assessments that are broadly in agreement with this hypothesis have been derived from numerical models, a quantitative assessment has been lacking. We present a novel plume detection scheme and derive Monte Carlo-based statistical correlations of model plume eruption sites and reconstructed LIP eruption sites. We show that models with a chemically anomalous lower mantle are highly correlated to reconstructed LIP eruption sites, whereas the confidence level obtained for a model with purely thermal plumes falls just short of 95%. A network of embayments separated by steep ridges form in the deep lower mantle in models with a chemically anomalous lower mantle. Plumes become anchored to the peaks of the chemical ridges and the network of ridges acts as a floating anchor, adjusting to slab push forces through time. The network of ridges imposes a characteristic separation between conduits that can extend into the interior of the thermochemical structures. This may explain the observed clustering of reconstructed LIP eruption sites that mostly but not exclusively occur around the present-day LLSVPs

Cenozoic Uplift of south Western Australia as constrained by river profiles

The relative tectonic quiescence of the Australian continent during the Cenozoic makes it an excellent natural laboratory to study recent large-scale variations in surface topography, and processes that influence changes in its elevation. Embedded within this topography is a fluvial network that is sensitive to variations in horizontal and vertical motions. The notion that a river acts as a 'tape recorder' for vertical perturbations suggests that changes in spatial and temporal characteristics of surface uplift can be deduced through the analysis of longitudinal river profiles. We analyse 20 longitudinal river profiles around the Australian continent. Concave upward profiles in northeast Australia indicate an absence of recent surface uplift. In contrast, the major knickzones within longitudinal profiles of rivers in southwest Australia suggest recent surface uplift. Given the lack of recent large-scale tectonic activity in that region, this uplift requires an explanation. Applying an inverse algorithm to river profiles of south Western Australia reveals that this surface uplift started in the Eocene and culminated in the mid-late Neogene. The surface uplift rates deduced from this river profile analysis generally agree with independent geological observations including preserved shallow-marine sediment outcrops across the Eucla Basin and south Western Australia. We show that the interplay between global sea level and long-wavelength dynamic topography associated with south Western Australia's plate motion path over the remnants of an ancient Pacific slab is a plausible mechanism driving this surface uplift.Comment: 33 pages including 7 figures. Published in Tectonophysics, please see final manuscript ther

Dynamic topography of passive continental margins and their hinterlands since the Cretaceous

Even though it is well accepted that the Earth\u27s surface topography has been affected by mantle-convection induced dynamic topography, its magnitude and time-dependence remain controversial. The dynamic influence to topographic change along continental margins is particularly difficult to unravel, because their stratigraphic record is dominated by tectonic subsidence caused by rifting. We follow a three-fold approach to estimate dynamic topographic change along passive margins based on a set of seven global mantle convection models. We first demonstrate that a geodynamic forward model that includes adiabatic and viscous heating in addition to internal heating from radiogenic sources, and a mantle viscosity profile with a gradual increase in viscosity below the mantle transition zone, provides a greatly improved match to the spectral range of residual topography end-members as compared with previous models at very long wavelengths (spherical degrees 2-3). We then combine global sea level estimates with predicted surface dynamic topography to evaluate the match between predicted continental flooding patterns and published paleo-coastlines by comparing predicted versus geologically reconstructed land fractions and spatial overlaps of flooded regions for individual continents since 140 Ma. Modelled versus geologically reconstructed land fractions match within 10% for most models, and the spatial overlaps of inundated regions are mostly between 85% and 100% for the Cenozoic, dropping to about 75-100% in the Cretaceous. Regions that have been strongly affected by mantle plumes are generally not captured well in our models, as plumes are suppressed in most of them, and our models with dynamically evolving plumes do not replicate the location and timing of observed plume products. We categorise the evolution of modelled dynamic topography in both continental interiors and along passive margins using cluster analysis to investigate how clusters of similar dynamic topography time series are distributed spatially. A subdivision of four clusters is found to best reveal end-members of dynamic topography evolution along passive margins and their hinterlands, differentiating topographic stability, long-term pronounced subsidence, initial stability over a dynamic high followed by moderate subsidence and regions that are relatively proximal to subduction zones with varied dynamic topography histories. Along passive continental margins the most commonly observed process is a gradual motion from dynamic highs towards lows during the fragmentation of Pangea, reflecting the location of many passive margins now over slabs sinking in the lower mantle. Our best-fit model results in up to 500 (± 150) m of total dynamic subsidence of continental interiors while along passive margins the maximum predicted dynamic topographic change over 140 million years is about 350 (± 150) m of subsidence. Models with plumes exhibit clusters of transient passive margin uplift of about 200 ± 200 m, but are mainly characterised by long-term subsidence of up to 400 m. The good overall match between predicted dynamic topography to geologically mapped paleo-coastlines makes a convincing case that mantle-driven topographic change is a critical component of relative sea level change, and indeed the main driving force for generating the observed geometries and timings of large-scale continental inundation through time

Influence of mantle flow on the drainage of eastern Australia since the Jurassic Period

Recent studies of the past eastern Australian landscape from present-day longitudinal river profiles and from mantle flow models suggest that the interaction of plate motion with mantle convection accounts for the two phases of large-scale uplift of the region since 120 Ma. We coupled the dynamic topography predicted from one of these mantle flow models to a surface process model to study the evolution of the eastern Australian landscape since the Jurassic Period. We varied the rainfall regime, erodibility, sea level variations, dynamic topography magnitude, and elastic thickness across a series of experiments. The approach accounts for erosion and sedimentation and simulates catchment dynamics. Despite the relative simplicity of our model, the results provide insights on the fundamental links between dynamic topography and continental-scale drainage evolution. Based on temporal and spatial changes in longitudinal river profiles as well as erosion and deposition maps, we show that the motion of the Australian plate over the convecting mantle has resulted in significant reorganization of the eastern Australian drainage. The model predicts that the Murray river drained eastward between 150 and ∼120 Ma, and switched to westward draining due to the tilting of the Australian plate from ∼120 Ma. First order comparisons of eight modeled river profiles and of the catchment shape of modeled Murray-Darling Basin are in agreement with present-day observations. The predicted denudation of the eastern highlands is compatible with thermochronology data and sedimentation rates along the southern Australian margin are consistent with cumulative sediment thickness. © 2017. American Geophysical Union. All Rights Reserved.We thank Robert Moucha, Nicky White and an anonymous reviewer for their constructive comments and suggestions which have greatly improve this paper. The authors were supported by ARC grants IH130200012 and DE160101020

Special Libraries, May 1916

Volume 7, Issue 5https://scholarworks.sjsu.edu/sla_sl_1916/1004/thumbnail.jp

Topographic asymmetry of the South Atlantic from global models of mantle flow and lithospheric stretching

The relief of the South Atlantic is characterized by elevated passive continental margins along southern Africa and eastern Brazil, and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank. We investigate the origin of these topographic features in the present and over time since the Jurassic with a model of global mantle flow and lithospheric deformation. The model progressively assimilates plate kinematics, plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates, and predicts the evolution of mantle temperature, continental crustal thickness, long-wavelength dynamic topography, and isostatic topography. Mantle viscosity and the kinematics of the opening of the South Atlantic are adjustable parameters in thirteen model cases. Model predictions are compared to observables both for the present-day and in the past. Present-day predictions are compared to topography, mantle tomography, and an estimate of residual topography. Predictions for the past are compared to tectonic subsidence from backstripped borehole data along the South American passive margin, and to dynamic uplift as constrained by thermochronology in southern Africa. Comparison between model predictions and observations suggests that the first-order features of the topography of the South Atlantic are due to long-wavelength dynamic topography, rather than to asthenospheric processes. The uplift of southern Africa is best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle

The role of deep Earth dynamics in driving the flooding and emergence of New Guinea since the Jurassic

The paleogeography of New Guinea indicates fluctuating periods of flooding and emergence since the Jurassic, which are inconsistent with estimates of global sea level change since the Eocene. The role of deep Earth dynamics in explaining these discrepancies has not been explored, despite the strongly time-dependent geodynamic setting within which New Guinea has evolved. We aim to investigate the role of subduction-driven mantle flow in controlling long-wavelength dynamic topography and its manifestation in the regional sedimentary record, within a tectonically complex region leading to orogeny. We couple regionally refined global plate reconstructions with forward geodynamic models to compare trends of dynamic topography with estimates of eustasy and regional paleogeography. Qualitative corroboration of modelled mantle structure with equivalent tomographic profiles allows us to ground-truth the models. We show that predicted dynamic topography correlates with the paleogeographic record of New Guinea from the Jurassic to the present. We find that subduction at the East Gondwana margin locally enhanced the high eustatic sea levels from the Early Cretaceous (∼145 Ma) to generate long-term regional flooding. During the Miocene, however, dynamic subsidence associated with subduction of the Maramuni Arc played a fundamental role in causing long-term inundation of New Guinea during a period of global sea level fall. © 2017 Elsevier B.VThis research was undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government . SZ and RDM were supported by Australian Research Council grant IH130200012 and DP130101946 . NF was supported by Australian Research Council grant DE16010102