Section 4.8
In this chapter, we have paid renewed attention to a relatively wide range of formerly unrelated facts about the Earth – giving particular attention to its early Precambrian history; in the context of a relatively undiffereniated planet undergoing continuous degassing, a range of alternative interpretations has been presented. It appears that this attempt to bring together the many bits and pieces of uncoordinated geological history has achieved some success in that previously unrelated and puzzling facts now seem to fall in place within a simple physical framework. In other words, a new theory is beginning to take shape that can explain the tangled web of processes and resulting geological phenomena. The basis for reconsidering the many facets of Earth history is that the traditional idea of a hot, convecting globe – currently needed as the driving force for the purely hypothetical plate tectonic processes – has functioned like a Pandora’s box. Once reopened decades ago, it has continued to generate an endless series of both seemingly unmanageable problems and unrealistic solutions, turning Earth history into a chaotic mix of unrelated observations and ad hoc fixes. In an attempt to escape from this deadlock, we have gone back to ask fundamental questions about the planet’s very beginning, taking the view that the proto-Earth was born from a fast-spinning cold mix of mineral components and gas (primarily hydrogen).
In this quest, we have found reasons to ignore the classical planetesimal scenario and, instead, have built on the assumption that the Earth (along with the other planets) started out as an individual and relatively concentrated cloud of primitive particulate matter and gases that, prior to its final consolidation underwent a certain degree of internal mass segregation. Thus, a combination of magnetic and gravitational processes led to the formation of an iron-rich core – along with admixures of hydrogen and a range of other light components – and a surface layer of felsic (anorthositic) composition, including a concentration of the heavier radioactive elements. As the consolidation/condensation scenario is considered to have involved smaller particles, it is likely that the Earth solidified in a relatively cold state out of chemical equilibrium. At the high pressures of the deep interior, the store of hydrogen could then have ended up as solid state metalliferous hydrogen or in a variety of mineral combinations including a range of hydrides. That unoxidized hydrocarbons occur as inclusions in diamonds of suggested lower mantle provenance, and that methane and other hydrocarbons are being emitted continuously through the crystalline basement provide strong prima facie evidence that the overall internal temperature is relatively low. Hence, throughout its history, the Earth must have been out of thermochemical equilibrium. So, in the natural process of reaching internal stability, mass reorganization – aided by buoyant volatiles – has been at play, as might be expected, since the dawn of the planet, giving rise to a progressive evolutionary course of intermittent geological activity.
With the new starting point, a novel theory of the Earth’s history emerges, in which geological phenomena, varying in age from Archaean to Recent, form a natural coherent network. Degassing and the associated internal reorganization of planetary mass would naturally cause changes of spin rate, including long-term overall slowing, and episodic changes of spatial orientation of the entire body of the Earth (polar wander) thereby repositioning the equatorial bulge. The governing idea of the new theory is that these changes of planetary dynamics provide the principal driving forces behind geological processes. In other words, the tectonomagmatic history is intimately linked to changes in Earth’s rotation (see chapter 5). Furthermore, with the degassing-driven melt production in the topmost mantle having gradually built up an irregular low-velocity asthenosphere, the overlying more brittle ‘lithosphere’ would have been prone to detach from the deeper parts and, hence, events of latitude-dependant wrench deformation, operating in concert with changes in the Earth’s moment(s) of inertia, will have ensued. The principal tectonic system then becomes one of torsional lithospheric deformation, directly linked to shifts of planetary rotation (changes of spin rate as well as of relative axis-orientation), hence the term Global Wrench Tectonics.
It can be assumed that, even as early as the early Archaean, heating from combined radiogenic, tidal, and chemical processes gave rise to the effective degassing of the outer few hundred kilometres of the Earth, accompanied by the installation of partial melt pockets at near-surface levels, while the remaining part of the planetary body tended to maintain its original low temperature. Aided by buoyant volatiles, the outer layers of the geosphere were depleted of a number of incompatible and other elements, to be correspondingly enriched in the surface layer, resulting in mineralization, plutonism, greenstone belt volcanism, and pervasive potassium (metasomatic) granitization of the crust. These processes were particularly significant in Upper Archaean and earliest Proterozoic times, introducing strong and widespread remobilization and isotopic age resetting of older rocks. As will be discussed later (in chapter 7), it seems that: The Archaean Earth had an orientation very close to its present spatial positioning and, due to the presumed effective outgassing of the outer geospheric layers at that time, the crust must have experienced an overall expansive state. From this pan-global extensional regime, there arose two sets of parallell fractures, oriented in latitudinal and longitudinal directions respectively. Thus originated the basic configuration of the most prevalent fracture network on Earth – the two conjugate near-orthogonal, near-vertical sets of ruptures characterizing bedrock exposures all over the globe. Once formed, these rock discontinuities would be prone to repeated rejuvenation and intensification throughout the planet’s dynamo-tectonic history, thereby continuously implanting the old fracture systems into ever younger surface formations. Apart from the orthogonal fracture network, in places filled with magma, the early Precambrian degassing did not provide planetary swelling of note. Nor does it seem that the recent history of the globe has led to any notable change of the Earth’s dimension.
At around the Archaean-Proterozoic transition, irregularly distributed degassing led to an uneven distribution of mass which, in turn, brought the planet into a dynamically unstable state. In order to regain rotational stability, a significant reorientation of the globe took place at that time – probably the first major phase of the relative polar shift affecting the Earth. In the meantime, due to diminished heating resulting from the reduction in radioactivity and from tidal friction (the latter caused by increasing lunar recession), the outer shell sustained a marked temperature lowering which made the crust much more brittle than before. This change of crustal properties, from a relatively hot and ductile constitution during much of the Archaean aeon to a rather brittle state in the early Proterozoic, had significant importance for the character of tectono-magmatic processes. We shall return to this major geodynamic event in chapter 7; however, in passing, it is appropriate to mention that the major resetting of the relative equatorial bulge – at around the Archaean-Proterozoic boundary – must have enhanced the hydrostatic pressure of upper mantle melt pockets, leading to widespread magmatic activity and initiating the most dramatic and fundamental period of change in the evolution of the continents.
By the time of the late Archaean, accumulating buoyant volatiles had generated a sufficient level of hydrostatic pressure to occasion an effective eclogitization and associated delamination of the felsic crust. Thus started a multi-phase transformation of the presumed thick primitive crust, involving sub-crustal gravitational loss to the mantle with related isostatic subsidence, granitization and plutonism. The outlines of the developing basins were controlled by major dislocations along one of the pre-existing conjugate fracture sets; the elongate shape of many late Archaean basins can be explained readily by the propensity of the predominant development to occur normally along the plane of one of the two fracture sets. It was into such elongated continental depressions, with a basement often consisting of older anhydrous granulitic rocks, that the basic-ultrabasic and granitic magma of the greenstone belts accumulated. However, before the end of the Archaean aeon, the early anhydrous granulites as well as the greenstone belts themselves had been affected by penetrating greenschist and amphibolite facies metamorphism. Seemingly, crustal attenuation did not proceed to an advanced stage during the Precambrian as there is no evidence that deeper basins existed at that time. Surface water was evidently still in limited supply being distributed across a fairly featureless pan-global continental surface. Owing to the suggested relatively low temperatures of the deeper parts of the Earth, degassing of its core and lower mantle may have been a much slower process than outgassing of the much hotter outer levels. However, from the early to middle Proterozoic, we begin to see the surface effect of increasing fluid pressure in the uppermost mantle, demonstrated by continental rifting, basin formation, ‘non’-tectonic magmatic activity and, at around 2 billion years ago, momentous gas pressures giving rise to major blow-out events such as those that produced the Sudbury (Canada) and Vredefort (South Africa) structures. We shall return to the questions of Proterozoic planetary outgassing, crustal mineralization, and associated geological consequences in chapters 5 and 7. The relatively slow outgassing from the core and lower mantle, gradually producing the large heterogeneities currently revealed by seismic mantle tomography, can be regarded as the very reason why the Earth has been a geologically active planet throughout the Phanerozoic. In fact, the fluid pressure needed to trigger effective eclogitization and crustal delamination was apparently not in place much before the late Mesozoic during which time the modern deep oceanic depressions developed.
