Long-lasting viscous drainage of eclogites from the cratonic lithospheric mantle after Archean subduction stacking (original) (raw)
Related papers
Geology, 2004
Subduction is the principal mechanism by which the hydrosphere and interior of Earth interact. Today, subduction involves the dehydration of ocean crust at depths of 60-120 km depending on the age of the slab. Release of the water leads to generation of arc magmas (future continental crust), and the slab is then transformed into denser eclogite that helps to pull more of the slab into the trench. However, it is unlikely that the first continental crust formed this way. Growing geochemical evidence indicates that large volumes of continental crust were produced over a short period of time in the Archean, when the planet was probably too hot for modern plate tectonics to operate. A significant increase in the kinetics of eclogite-forming reactions may have been the key to the transition from Archean to modern tectonics. Under the higher geothermal gradients of the Archean, tectonically buried ocean crust would have been severely dehydrated before reaching eclogite facies pressures. Because rapid eclogitization is dependent on water as a medium for advective ion transport, the very shallow dehydration in the Archean may have inhibited the formation of eclogite facies minerals. The importance of water in eclogite metamorphism is illustrated by a complex of partly eclogitized mafic granulites in Holsnøy, western Norway, in which reaction progress was limited by the availability of water. When water is scarce or absent, metastable granulite facies mineral assemblages can persist at eclogite facies depths owing to the extremely slow reaction kinetics when diffusion is the only chemical transport mechanism. Such dehydrated but uneclogitized mafic crust would have been very strong and too buoyant to sink into the mantle, and it may have formed the substrate for the first continental lithosphere.
Nature Communications , 2021
(Authors: Buntin S., Artemieva I.M., Malehmir A., Thybo H., et al.) The nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6-8 km thick body with extremely high velocity (Vp~8.5-8.6 km/s) and high density (>3.4 g/cm 3) immediately beneath equally thinned high-velocity (Vp~7.3-7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50-70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stability of cratonic crust.
Long-lived Paleoproterozoic eclogitic lower crust
Nature Communications
The nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6–8 km thick body with extremely high velocity (Vp ~ 8.5–8.6 km/s) and high density (>3.4 g/cm3) immediately beneath equally thinned high-velocity (Vp ~ 7.3–7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50–70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stabi...
Creation and preservation of cratonic lithosphere: seismic constraints and geodynamic models
2006
Cratons are areas of continental crust and lithosphere that exhibit long-term stability against deformation. The formation hypotheses of cratonic lithosphere invoke either plume interaction or a subduction related process. Seismic evidence suggests that cratonic lithosphere may have formed via thrust stacking of proto-cratonic lithosphere. We conducted numerical simulations and scaling analysis to test this hypothesis, as well as to elucidate mechanisms for stabilization. We found that formation of cratonic lithosphere via thrust-stacking is most viable for buoyant and viscous proto-cratonic lithosphere that is thin and/or possesses a low effective friction coefficient. These conditions lead to low integrated yield strength within proto-cratonic lithosphere. This allows the material to fail in response to convection-generated stresses and to thrust-stack above a mantle downflow. Thrust-stacking, in turn, generates a thickened cratonic root. The increased thickness of cratonic lithosphere provides a higher integrated yield stress within cratons, which is more conducive to stability subsequent to formation. Increased friction coefficient values, due to dehydration, can also provide higher integrated yield stresses within cratons. High yield stress, due to cratonic lithosphere exceeding a critical thickness and/or being dehydrated, can offset convective stresses and stabilize cratons. To provide long-term stability from the Archean onward, the integrated yield stresses must be great enough to offset mantle convection generated stresses, which can increase with time as the mantle viscosity increases due to cooling. Thin or rehydrated cratonic lithosphere may not provide stability against the increasing convective stresses, thus providing an explanation as to why some cratons are not long-lived.
Ephemeral isopycnicity of cratonic mantle keels
Nature Geoscience, 2013
Cratons, the ancient nuclei of continents that have been stable for billions of years, are underlain by keels of lithosphere with strongly melt-depleted compositions 1,2. These cratonic keels may have formed either from partial melting in a mantleplume environment 3,4 , or alternatively by melting at shallow depths in a subduction zone, during the successive accretion of slabs of oceanic lithosphere 5. The stability of cratonic keels has been attributed to a pervasive state of near-neutral buoyancy-isopycnicity-created by offsetting thermal and compositional effects on density 6. However, it is unclear how an isopycnic state can be sustained over geological time 2. Here we simulate the evolution of a simplified southern African cratonic keel, initiated in either a hot-plume or a cold-slab environment, over 3 billion years, using a numerical model that incorporates secular cooling of the mantle, coupled with gradual loss of radiogenic heating in the lithosphere. We find that the simulation that starts from a cold-slab environment best explains the subsidence history of the southern African craton 7. However, irrespective of how the cratonic keel formed, we find that the isopycnic state is inherently ephemeral: a cratonic keel that is approximately isopycnic under present conditions was more, or less, buoyant in the geologic past. The lithosphere, Earth's relatively rigid outer shell, moves coherently with plate motions above the underlying, more easily deformed asthenosphere. The lithosphere-asthenosphere boundary (LAB) thus forms a mechanical detachment that accommodates relative motion between the plate and underlying mantle; as such, it represents the most extensive plate boundary on Earth 1. Yet, owing to the scarcity of representative geological samples 8 , even fundamental physical parameters remain uncertain, particularly beneath Archaean cratons 9. The mantle underlying these regions may be compositionally stratified 10 , and possesses a distinctive, dehydrated and strongly melt-depleted composition that differs fundamentally from modern analogues 3. This mantle region, sometimes denoted as tectosphere 6 , is stabilized against convection under present mantle conditions owing to its intrinsic buoyancy and relatively high viscosity 2. The buoyancy and rheology of cratons under thermal conditions in the geologic past are unknown, however, particularly during the early stages of lithospheric evolution. The formation of cratonic lithospheric mantle (CLM) is contentious and various models have been proposed, of which two may be considered as thermal endmembers. One model invokes formation of CLM within a hot Archaean plume environment, where CLM peridotites represent residues and/or cumulates from high-degree partial melting at significant depth 3,4. A second model postulates formation of CLM by melting at shallower depths, associated with underthrusting and imbrication of subducted oceanic lithosphere 5. Both of these models attribute longevity of CLM to intrinsic buoyancy arising from a strongly meltdepleted composition, coupled with inherent strength (high effective viscosity) arising from dehydration 11. Although previous
Some thoughts on the stability of cratonic lithosphere: Effects of buoyancy and viscosity
Journal of Geophysical Research: Solid Earth, 1999
Continental cratons have not experienced major tectonic disruptions over a timescale of 109 years. The thickness of cratonic lithosphere also appears to have changed little over this timescale. These observations are often attributed to the presence of chemically buoyant and/or highly viscous subcratonic roots. Simple physical scaling relationships are developed to explore the buoyancy and/or viscosity conditions required to stabilize such roots against large-scale deformation and rapid remixing into the mantle. The scalings are tested using idealized numerical simulations with good general agreement. Applied to Earth, the scalings suggest that (1) buoyancy alone is unlikely to stabilize cratonic roots and (2) if root viscosity is to provide stability into the Archean, then roots must be 103 times as viscous as the mantle. Based on' available experimental data, root dehydration cannot account for the required viscosity increase. Temperature-dependent viscosity can stabilize roots, but it does so at the expense of stagnating the entire mantle lithosphere, i.e., at the expense of sacrificing plate tectonics. This suggests that the plastic yielding properties of rocks at low temperatures will need to be more directly accounted for in future experiments exploring root stability. cratonic lithosphere. Lithospheric history is more difficult to infer, but thermobarometric studies of diamond inclusions within kimberlites do suggest that cratonic lithosphere in the Archean was not appreciably thinner than at present [Boyd et al., 1985]. Understanding how cratons can remain stable while other continental regions undergo intense tectonic episodes and how cratonic lithosphere can remain thick in the face of vigorous mantle convection are long-standing related questions. One can imagine two end-member means by which sections of continents can remain stable geologically and in terms of deep lithospheric structure. Either
Archean cratons and mantle dynamics
Earth and Planetary Science Letters, 2005
The apparent stability of Archean cratons and cratonic keels for billions of years is a difficult observation for geodynamic modeling to explain. While it may be straight-forward to assert that chemical buoyancy and high viscosity are needed to stabilize cratons, there are many questions regarding craton formation, variability (both in space and time), and evolution that remain unanswered. In numerical studies, strong and buoyant cratonic keels survive relatively undeformed for several mantle overturn times (the equivalent of several hundred million years); extending this to several billion years remains a challenge. The strength required to stabilize keels in some of these numerical experiments exceeds reasonable estimates of the laboratory measurements of strength of mantle materials (including both the effects of temperature and melt-depletion). In addition, the most common explanation of keel formation, vertical stacking of subducted plate, requires the keel material to be deformable at the time of formation and soon afterward the keel material becomes strong enough to resist shearing. The extent to which cratonic keels interact with and influence the pattern of mantle convection, by nucleating small-scale edge-driven convection or by coupling plate motions to deeper mantle flow, remains an open question. D Archean cratons are relatively flat, stable regions of the crust that have remained undeformed since the Precambrian, forming the ancient cores of the continents . Most primary diamond deposits occur in Archean cratons and there are no recognized primary diamond deposits intruding rocks younger than 1.6 Gyr [1]. The graphite-diamond phase transition occurs at 120-150 km depths and low (b1000 8C) temperatures . Thus, the occurrence of diamonds provides an important constraint on the thermal state of the lower lithosphere. As illustrates, only relatively-low surface-heatflow geotherms pass through the diamond stability field in the 120-150 km depth range. Seismic velocities under Archean cratons are significantly faster than normal subcontinental mantle (e.g., ) and this suggests 0012-821X/$ -see front matter D
Compositional vs. thermal buoyancy and the evolution of subducted lithosphere
Geophysical Research Letters, 1994
We formulate 2-D Cartesian finite element models that explore the fate of compositionally defined lithosphere as it encounters a viscosity increase at the boundary between the upper and lower mantle. Subducted lithosphere is represented as a cold, stiff, layered composite of denser eclogite underlain by more buoyant harzburgite. Slabs impinging on a lower mantle 30 and 100 times more viscous than the upper mantle thicken and fold strongly as they penetrate the lower mantle. Approximately a factor of two thickening occurs via pure shear just above the discontinuity, with additional enhancement due to folding by over a factor of two. No separation of the individual slab components occurs at the discontinuity, and direct comparison with models in which compositional buoyancy is explicitly ignored indicates that slab evolution is largely controlled by the thermal buoyancy. These results are at odds with hypotheses about slab evolution in which the compositional buoyancy contributions lead to component separation and the formation of slab megaliths or a compositionally layered upper mantle.