Some thoughts on the stability of cratonic lithosphere: Effects of buoyancy and viscosity (original) (raw)
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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.
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
Rejuvenation and erosion of the cratonic lithosphere
Nature geoscience, 2008
Th e crust of cratons is characterized by rocks of mostly Archaean age and a relatively fl at topography close to sea level. Th e cores of most cratons were formed during the fi rst half of the Earth's history and stabilized by events known collectively as 'cratonization' , mostly around 2,500 million years ago 1,2 but continuing into the Palaeoproterozoic era 3 . Th e conventional view is that a craton is forever: aft er formation by Archaean or early Proterozoic geodynamic processes they built the stable cores of continents, and since then have served only as inert objects onto which laterformed continental crust is accreted 3 . As such, a craton moves around the Earth's surface as part of a lithospheric plate, but is not aff ected by the opening and closing of ocean basins, or by the return of material into the mantle. Th e stability of cratons owes much to their thick, cold and chemically depleted mantle lithosphere. Because of its relatively low temperature, this mantle lithosphere has a high viscosity, which prevents deformation and thus preserves the craton root 4-6 : craton roots are 'unconditionally stable' for plausible viscosity ratios between the lithosphere and underlying asthenosphere 7 . However, a purely temperature-controlled boundary between lithosphere and asthenosphere would render the lithosphere gravitationally unstable because of its higher density 6 . Th erefore, bouyancy must be achieved by compositional diff erences 8 , which are oft en due to the relatively low iron content of the lithosphere. Diamonds, we are told, are also forever. Until recently, the occurrence of, and exploration for, diamonds was embedded in this cratonic paradigm: diamonds were thought to be exclusively Archaean in age like the lithosphere they are part of 9 , and their presumed restriction to cratonic kimberlites was a logical consequence of the thick lithosphere here, which reaches below the 160 km needed to stabilize diamonds 10 .
Large-scale crustal heterogeneities and lithospheric strength in cratons
Earth and Planetary Science Letters, 1998
The rheology and thermal structure of the continental lithosphere are intimately linked. In old cratons, the effective elastic thickness of the lithosphere has been estimated by various spectral (inverse) methods based on the correlation between topography and gravity anomalies. Estimates vary within a very large range from ³40 km to 120 km depending on the method used. In this paper, we use forward models to account for lateral variations in mechanical properties and their effect on the equivalent elastic thickness (EET) of the lithosphere. From these models, which allow brittleelastic-ductile rheologies and mechanical discontinuities (faults), we have calculated the strain=stress distributions and displacement fields. Vertical integration of the stress permits a local determination of the effective elastic thickness. The computed displacements were used to calculate related Bouguer and free-air gravity anomalies and compare them with the observations. The analysis is applied to the 2000-Ma Kapuskasing uplift (in the Superior Province of the Canadian Shield) where the presence of a high-density block in the upper crust is due to the upthrusting of midcrustal rocks along a major thrust fault. The study shows that the stability of this structure on geologic time scales requires a strong lower crustal rheology, a cold geotherm, and the fault to be healed. This study also shows that, because of stress dependence of the non-linear rheology, crustal heterogeneities may cause significant (³40%) local reductions of the lithospheric strength. Away from the Kapuskasing structure, the average strength of the lithosphere remains high (EET ³ 100 km). Conventional methods for estimating the elastic thickness would not resolve such local strength reductions in cratons, but would predict, depending on the method used, highly overestimated or instead, underestimated EET.
Earth and Planetary Science Letters, 2008
Cratons form the cores of continents and were formed within a narrow window of time (2.5-3.2 Gy ago), the majority having remained stable ever since. Petrologic evidence suggests that the thick mantle roots underlying cratons were built by underthrusting of oceanic and arc lithosphere, but paradoxically this requires that the building blocks of cratons are weak even though cratons must have been strong subsequent to formation. Here, we propose that one form of thickening could be facilitated by thrusting of oceanic lithospheres along weak shear zones, generated in the serpentinized upper part of the oceanic lithosphere (crust + mantle) due to hydrothermal interaction with seawater. Conductive heating of the shear zones eventually causes serpentine breakdown at~600°C, shutting down the shear zone and culminating in craton formation. However, if shear zones are too thin, serpentine breakdown and healing of the shear zone occurs too soon and underthrusting does not occur. If shear zones are too thick, serpentine breakdown takes too long so healing and lithospheric thickening is not favored. Shear zone thicknesses of~18 km are found to be favorable for craton formation. Because the maximal depth of seawater-induced serpentinization into the lithosphere is limited by the depth of the isotherm for serpentine breakdown, shear zone thicknesses should have increased with time as the Earth's heat flux and depth to the serpentine breakdown isotherm decreased and increased, respectively, with time. We thus suggest that the greater representation of cratons in the late Archean might not necessarily be explained by preferential recycling in the early Archean but may simply reflect preferential craton formation in the late Archean. That is, our model predicts that the early Archean was too hot, the Phanerozoic too cold, and the late Archean just right for making cratons.
On the relations between cratonic lithosphere thickness, plate motions, and basal drag
Tectonophysics, 2002
An overview of seismic, thermal, and petrological evidence on the structure of Precambrian lithosphere suggests that its local maximum thickness is highly variable (140 -350 km), with a bimodal distribution for Archean cratons (200 -220 km and 300 -350 km). We discuss the origin of such large differences in lithospheric thickness, and propose that the lithospheric base can have large depth variations over short distances. The topography of Bryce Canyon (western USA) is proposed as an inverted analog of the base of the lithosphere.
Cratonic root beneath North America shifted by basal drag from the convecting mantle
Nature Geoscience, 2015
Stable continental cratons are the oldest geologic features on the planet. They have survived 3.8 to 2.5 billion years of Earth's evolution 1,2 . The key to the preservation of cratons lies in their strong and thick lithospheric roots, which are neutrally or positively buoyant with respect to surrounding mantle 3,4 . Most of these Archaean-aged cratonic roots are thought to have remained stable since their formation and to be too viscous to be a ected by mantle convection 2,3,5 . Here we use a combination of gravity, topography, crustal structure and seismic tomography data to show that the deepest part of the craton root beneath the North American Superior Province has shifted about 850 km to the west-southwest relative to the centre of the craton. We use numerical model simulations to show that this shift could have been caused by basal drag induced by mantle flow, implying that mantle flow can alter craton structure. Our observations contradict the conventional view of cratons as static, non-evolving geologic features. We conclude that there could be significant interaction between deep continental roots and the convecting mantle.