GLOBAL WRENCH TECTONICS
- THEORY OF EARTH EVOLUTION
Book review by Min-Xiu GAO,
Geological Inst. Seismological Bureau, Beijing 100029, China.
Published NCGT Newsletter, Dec. 2003
Global Wrench Tectonics - an extension of the author's Our Evolving Planet (1997) reviewed by H.C. Sheth in NCGT Newsletter No.8, 1998 - give a more comprehensive documentation in support of the theory of Wrench Tectonics. In order to place the new global framework in its proper geoscientific perspective, chapters 1 and 2 give a historical account on earlier attempts at phenomenological links in global geology - including Wegenerian-type continental drift an plate tectonics. Chapter 3 affords a snapshot of critical issues relationg to Earth's origin, its internal constitution and suggested energy transfer processes. The principal aspects of the new dynamo-tectonic system are delineated in chapters 4 and 5, while the last two chapters, nos. 6 and 7, enlarge upon the theory's explanatory capability - considering the Alpine age structural development of the Indian Ocean-Antarctica-SE Asia regions (chapter 6), and the changing tectonic pattern from late Archaean to the Middle Palaeozoic (chapter 7).
Professor Storetvedt's theory is intimately associated with global dynamic events - instigated by irregularly distributed degassing and related reorganization of mass - specified by incidents of polar wandering and episodic changes in planetary rotation rate; according to the new theory it is these dynamic turning points that are responsible for the gelogical history. The new global framework is presented in the form of a Theoretical Tree, incorporation the classic corpus of "first order" geological facts along with a range of new phenomenological interconnections.
In this new book, the theory is extended to the planet's very beginning - postulating an initially relatively fast-spinning cold mix of mineral components and gas (primarily hydrogen). It is envisaged that a certain degree of centrifugal sorting led to some concentration of the heavier radioactive elements in the outer geosphere; this early segregation followed by condensation-accretion scenarios bulding up a planetary interior markedly out of thermo-chemical equilibrium. In response to this unstable primordial state, continuing degassing and related reorganization of internal matter has given rise to the Earth's dynamo-tectonic history, gradually turning the body of the Earth into its present, but non-completed, state. According to the new development scheme it follows that the outer regions of the early Earth must have had a high geothermal gradient, and this is in fact consistent with the unusually high temperatures estimated for Archaean magmas. However, subsequent cooling of the relatively hot outer regions gave rise to deep contraction dislocations which originally formed two orthogonal great circle fractures around the globe; remains of these major cooling fractures are to be found in the presently disconnected Benioff zones.
Upwelling silanes, hydrocarbons, and silicon carbides have led to increasing hydrostatic pressure in the outer regions of the Earth, having produced chemical reactions with heat production, resulting in fluid- and gas-rich asthenosperic lenses. The successive increase of hydrostatic pressure in the outer 100 km or so (the present lithosphere) has led to granitization, mineralization, and progressive eclogitization giving rise to gravity-driven thinning, isostatic subsidence, and basification of the felsic-sialic crust. Thus, the provisional crustal end product of the degassing model is the deep oceanic depressions underlain by at thin basaltic incrustation. It follows that eustatic sea-level variations are directly related to the inferred 'oceanization' of the original pan-global continental crust: increase of upper mantle fluid/gas pressure has provided oceanic crustal uplift (and erosion) and sea-level highstand, and in response to subsequent eclogitization and gravity-driven sub-crustal delamination and basin subsidence, sea-level lowstands have developed.
In a rotating Earth, upward fluid flow and associated build-up of upper mantle to lower crustal hydrostatic pressures would have a certain concentration along the (palaeo) equatorial belts producing depressions (geosynclines) in great circle belts around the globe. Then, in response to the globe's variable spatial orientation (polar wander) and/or spin rate, inertia forces will ensue, producing wrench deformation of the entire lithosphere - hence the term Global Wrench Tectonics. In the new dynamo-tectonic system fold belts have developed in two palaeogeographical seatings: a) along time-equivalent equatorial belts and, more sporadically, in b) palaeo-meridional settings.
According to Karsten Storetvedt's narrative, his reconsideration of the Earth's dynamo-tectonic system began, in 1989, as a simple search for an alternative mobilistic system, primarily to account for the Alpine-Himalayan tectonic belt. By employing a combination of palaeomagnetically estimated continental palaeolatitudes and kinematics, polar wandering - based on combined fossil climate evidence and palaeomagnetism - and the global pattern of tectonomagmatic belts, these endeavours eventually led to a comprehensive tectonic framework (outlined in his present book). A major breakthrough in this process came when he realized that inter-continental palaeomagnetic discrepancies do not necessitate lateral motions (as in the drift/plate tectonic model) - in order to fit polar wander paths: only relative Alpine-age in situ rotations of the land masses are reguired. Thus, the author has ingeniously advanced a system of inertia-driven lithospheric rotations, implying that the many longstanding continental fitting problems artificial problems arising from plate tectonics thinking. Further, the presence of deep continental (mantle) roots has become unquestionable facts, and such observations are enigamtic only in the context of theory-inflicted lateral drift (plate tectonics). Within an Earth undergoing irregular degassing, oceanic mantle segments will correspond to sectors with the strongest outward mass/energy transport, giving rise to the reported slowing of seismic velocities, while less affected mantle segments become the seismically faster continental roots.
The inferred ca. 135° of clockwise in situ rotation of India resolves the anomalous orientation of the Deccan palaeomagnetic axis as well as the multiplicity of rock and fossil evidence suggesting that India has always been in close geographic proximity to Asia. Thus, Indian dinosaurs bear a close resemblance to the dinosaur fauna of Asia, and the lack of any faunal endemism - to be expected if India had been an isolated mega-island undergoing large scale northward drifting, as in plate tectonic abstractions- supports the new mobilistic alternative. The fairly large rotation figure of India is ascribed to its nodal tectonic position, at the northern ende of the pronounced 'N-S' trending mega shear zone cutting across the Indian Ocean.
For the North Atlantic the author demonstrates that only ca 25° of in situ clockwise rotation of North America relative to Europe, having principally occurred during the Alpine climax, can explain the descrepancy between the polar pahts for the two continents. Thus, the palaeomagnetic data do not necessitate longitudinal continental separation - only a modest relative in situ rotation is needed for these blocks, thus accounting for the southward fanning of the present North Atlantic. The relative rotations of the adjoining Atlantic continents have given rise to significant wrench deformation within the thin-crusted Atlantic deep sea basins; other deep oceanic regions across the globe were similarly affected by wrench deformation during the Alpine climax. In this process a 'centrally' located deep shear zone - running along the full length of the Atlantic Ocean - developed, serving as the locus for the Mid-Atlantic Ridge whose uplift has taken place only during the last 10 million years or so. In the new degassing Earth model topographic mountain ranges (in oceanic and continental settings alike) are newcomers in Earth history, being buoyantly uplifted through the gradual serpentinization of upper mantle peridotite. Thus, the classical term mountain building, wrongly implying that topographic uplift follows from tectonic thickening of the crust, becomes a misnomer.
The theory of Wrench Tectonics represents a complete uprooting of what, under the reign of plate tectonics, has become conventional thinking in geology. For underpinning and substantiating his alternative geophysical framework, the author has assimilated recent studies and achievements in abundance; as it now stands the new model of the Earth gives a neat coherent presentation of the mass of "first order" geological and geophysical facts, delineating a major prediction-confirmation chain. On the other hand, the predictions of the new ' operating system' will evidently have to be further evaluated, to show how well it will stand up to a closer scrutiny in the light of smaller scale observations and the test of time. Nevertheless, the book is rich in innovation, in challenge, and in enlightenment. To every non-prejudiced Earth scientist Global Wrench Tectonics contains a wealth of new fundamental thinking, and it would provide excellent teaching material at graduate level.
