The mid-Proterozoic or "boring billion" exhibited extremely stable environmental conditions, with little change in atmospheric oxygen levels, and mildly oxygenated shallow oceans. A limited number of passive margins with extremely long lifespans are observed from this time, suggesting that subdued tectonic activity-a plate slowdown-was the underlying reason for the environmental stability. However, the Proterozoic also has a unique magmatic and metamorphic record; massif-type anorthosites and anorogenic Rapakivi granites are largely confined to this period and the temperature/pressure (thermobaric ratio) of granulite facies metamorphism peaked at over 1500 °C/GPa during the Mesoproterozoic. Here, we develop a method of calculating plate velocities from the passive margin record, benchmarked against Phanerozoic tectonic velocities. We then extend this approach to geological observations from the Proterozoic, and provide the first quantitative constraints on Proterozoic plate velocities that substantiate the postulated slowdown. Using mantle evolution models, we calculate the consequences of this slowdown for mantle temperatures, magmatic regimes and metamorphic conditions in the crust. We show that higher mantle temperatures in the Proterozoic would have resulted in a larger proportion of intrusive magmatism, with mantle-derived melts emplaced at the Moho or into the lower crust, enabling the production of anorthosites and Rapakivi granites, and giving rise to extreme thermobaric ratios of crustal metamorphism when plate velocities were slowest.
Remote sensing for archaeological investigations using surface response is reasonably well established, however, remote subsurface exploration is limited by depth and penetration and ground resolution. Furthermore, the conservation of archaeological sites requires constant monitoring capability, which is often not feasible between annual field seasons, but may be provided by modern satellite coverage. Here we develop an approach using Sentinel-1 C-band radar backscatter, and Sentinel-2 multispectral data, to map and characterise the site of Qubbet el-Hawa, Egypt. The multispectral bands analysed show similar sensitivity to satellite imagery. However, the radar backscatter is sensitive to exposed known structures, as well as disturbances to soil textural/composition profile due to excavation/erosion. Sub-resolution features such as causeways manifest as a 'radar-break' in the backscatter - a discontinuity in otherwise continuous radar units. Furthermore, the finite subsurface response in the backscatter under the arid conditions of the site means we are able to delineate some shallow subsurface structures and map their orientation beneath the surface in areas not yet excavated. The sensitivity of Sentinel-1 backscatter to soil disturbance and human activity at Qubbet el-Hawa, and the short (~12 day) recurrence time of the satellites, makes it an important tool in heritage conservation.
The development of plate tectonics from a pre-plate tectonics regime requires both the initiation of subduction and the development of nascent subduction zones into long-lived contiguous features. Subduction itself has been shown to be sensitive to system parameters such as thermal state and the specific rheology. While generally it has been shown that cold-interior high-Rayleigh-number convection (such as on the Earth today) favours plates and subduction, due to the ability of the interior stresses to couple with the lid, a given system may or may not have plate tectonics depending on its initial conditions. This has led to the idea that there is a strong history dependence to tectonic evolution—and the details of tectonic transitions, including whether they even occur, may depend on the early history of a planet. However, intrinsic convective stresses are not the only dynamic drivers of early planetary evolution. Early planetary geological evolution is dominated by volcanic processes and impacting. These have rarely been considered in thermal evolution models. Recent models exploring the details of plate tectonic initiation have explored the effect of strong thermal plumes or large impacts on surface tectonism, and found that these ‘primary drivers’ can initiate subduction, and, in some cases, over-ride the initial state of the planet. The corollary of this, of course, is that, in the absence of such ongoing drivers, existing or incipient subduction systems under early Earth conditions might fail. The only detailed planetary record we have of this development comes from Earth, and is restricted by the limited geological record of its earliest history. Many recent estimates have suggested an origin of plate tectonics at approximately 3.0 Ga, inferring a monotonically increasing transition from pre-plates, through subduction initiation, to continuous subduction and a modern plate tectonic regime around that time. However, both numerical modelling and the geological record itself suggest a strong nonlinearity in the dynamics of the transition, and it has been noted that the early history of Archaean greenstone belts and trondhjemite–tonalite–granodiorite record many instances of failed subduction. Here, we explore the history of subduction failure on the early Earth, and couple these with insights from numerical models of the geodynamic regime at the time. This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics'.
Abstract Interactions among tectonics, volcanism, and surface weathering are critical to the long‐term climatic state of a terrestrial planet. Volcanism cycles greenhouse gasses into the atmosphere. Tectonics creates weatherable topography, and weathering reactions draw greenhouse gasses out of the atmosphere. Weathering depends on physical processes governed partly by surface temperature, which allows for the potential that climate‐tectonic coupling can buffer the surface conditions of a planet in a manner that allows liquid water to exist over extended timescales (a condition that allows a planet to be habitable by life as we know it). We discuss modeling efforts to explore the level to which climate‐tectonic coupling can or cannot regulate the surface temperature of a planet over geologic time. Thematically, we focus on how coupled climate‐tectonic systems respond to the following: (1) changes in the mean pace of tectonics and associated variations in mantle melting and volcanism, (2) large‐amplitude fluctuations about mean properties such as mantle temperature and surface plate velocities, and (3) changes in tectonic mode. We consider models that map the conditions under which plate tectonics can or cannot provide climate buffering as well as models that explore the potential that alternate tectonic modes can provide a level of climate buffering that allows liquid water to be present at a planet's surface over geological timescales. We also discuss the possibility that changes in the long‐term climate state of a planet can feedback into the coupled system and initiate changes in tectonic mode.
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