Continental Drift 1
Although the theory of the new global tectonics, or plate tectonics, has largely been developed since 1967, the history of ideas concerning a mobilist view of the Earth extends back considerably longer Rupke, 1970 Hallam, 1973a Vine, 1977 Frankel, 1988 . Ever since the coastlines of the continents around the Atlantic Ocean were first charted, people have been intrigued by the similarity of the coastlines of the Americas and of Europe and Africa. Possibly the first to note the similarity and...
A Late Proterozoic supercontinent
Similarities between the Late Proterozoic geologic record in western Canada and eastern Australia Bell amp Jefferson, 1987 Young, 1992 and between the southwestern USA and East Antarctica suggest that these areas were juxtaposed during Late Proterozoic times Dalziel, 1991, 1995 Moores, 1991 Hoffman, 1991 Fig. 11.19a . This seemingly radical suggestion was referred to as the SWEAT South West US and East AnTarctica hypothesis. The widespread Grenville orogenic belts, that immediately pre-date the...
Geologic Evidence For Continental Drift
The continental reconstructions discussed in Sections 3.2.3 and 3.2.4 are based solely on the geometric fit of continental shelf edges. If they represent the true ancient configurations of continents it should be possible to trace continuous geologic features from one continent to another across the fits. The matching of features requires the rifting of the supercontinent across the general trend of geologic features. This does not always occur as the location of the rift is often controlled by...
Pratts hypothesis
Pratt's hypothesis assumes a constant depth to the base of the outermost shell of the Earth, whose density varies according to the surface topography. Thus, mountain ranges would be underlain by relatively low density material and ocean basins by relatively high density material Fig. 2.29b . Equating the weights of columns of unit cross-section beneath a mountain range and a region of zero elevation gives If one substitutes appropriate densities for the crust, mantle, and sea water in these...
Introduction Auq
The phenomenon of isostasy concerns the response of the outer shell of the Earth to the imposition and removal of large loads. This layer, although relatively strong, is unable to support the large stresses generated by, for example, the positive weight of a mountain range or the relative lack of weight of an ocean basin. For such features to exist on the Earth's surface, some form of compensating mechanism is required to avoid the large stresses that would otherwise be generated. Isostasy was...
Magnetostratigraphy
Once the geomagnetic reversal timescale has been calibrated, oceanic magnetic anomalies may be used to date oceanic lithosphere. The method has been progressively refined so that it is now possible to deduce ages back to mid-Jurassic times with an accuracy of a few million years. The Vine-Matthews hypothesis explains the sequence of magnetic anomalies away from ocean ridges in terms of normal and reversed magnetizations of the oceanic crust acquired during polarity reversals of the geomagnetic...
Tests of isostasy
The state of isostatic compensation of a region can be assessed by making use of gravity anomalies. The isostatic anomaly, IA, is defined as the Bouguer anomaly minus the gravity anomaly of the subsurface compensation. Consider a broad, flat plateau of elevation h compensated by a root of thickness r. The terrain correction From the Airy criterion for isostatic equilibrium Substitution of this condition into the equation reveals that the isostatic anomaly is equal to the free-air anomaly over a...
Hotspots
The major part of the Earth's volcanic activity takes place at plate margins. However, a significant fraction occurs within the interiors of plates. In oceans the intra-plate volcanic activity gives rise to linear island and seamount chains such as the Hawaiian-Emperor and Line Islands chains in the Pacific Fig. 5.7 . Moreover, several of these Pacific island chains appear to be mutually parallel. Where the volcanic centers in the chains are closely spaced, aseismic ridges are constructed, such...
Flexure of the lithosphere
Figure 2.30 Flexural downbending of the lithosphere as a result of a two-dimensional load of half-width a, height h, and density ps. between the lithosphere and the behavior of an elastic sheet under load. Figure 2.30 illustrates the elastic response to loading the region beneath the load subsides over a relatively wide area by displacing asthenospheric material, and is complemented by the development of peripheral bulges. Over long periods of time, however, the lithosphere may act in a...
Ambiguity in focal mechanism solutions
It is apparent from Fig. 2.7 that the same distribution of compressional and dilational quadrants would be obtained if either nodal plane represented the actual fault plane. Thus, the same pattern of first motions would be obtained for sinistral motion along a north-south plane as for dextral motion along an east-west plane. Figure 2.8 Ambiguity in the focal mechanism solution of a thrust fault. Shaded areas represent regions of compressional first motions C , unshaded areas represent regions...
Horizontal and vertical tectonics
The origin of the unique dome-and-keel architecture of the Archean cratons Section 11.3.4 is important for understanding the nature of Archean tectonics. In general, interpretations can be divided into contrasting views about the relative roles of vertical and horizontal displacements in producing this pattern. The Eastern Pilbara craton in western Australia illustrates how vertical and horizontal tectonic models have been applied to explain the dome-and-keel structural style. During this...
Deep structure of the central Andes
In 1996, geoscientists working on the Andean Continental Research Project ANCORP '96 completed a 400-km-long seismic reflection profile across the central Andes at 21 S latitude Fig. 10.6 . This profile, together with the results of geologic Allmendinger et al., 1997 McQuarrie, 2002 and other geophysical studies Patzwahl et al., 1999 Beck amp Zandt, 2002 , forms part of a gt 1000-km-long transect between the Pacific coast and the Brazilian craton Fig. 10.7 . Below the central Andean forearc,...
Mechanisms of terrane accretion
Observations from the North American Cordillera, the Appalachians, and many other ancient orogens suggest that the accretion and dispersal of terranes involves processes that are similar to those that occur in modern orogens. The regimes of active arc-continent collision in the southwest Pacific Section 10.5 , for example, offer excellent analogues for how a variety of tectonic and sedimentary terranes originate and are emplaced onto continental margins. In general, as the subduction of oceanic...
Isostatic rebound
The equilibrium flexural response of the lithosphere to loading is independent of the precise mechanical properties of the underlying asthenosphere as long as it facilitates flow. However, the reattainment of equilibrium after removal of the load, a phenomenon known as isostatic rebound, is controlled by the viscosity of the asthenosphere. Measurement of the rates of isostatic rebound provides a means of estimating the viscosity of the upper mantle. Fennoscandia represents an example of this...
General Morphology Of Island Arc Systems
Island arc systems are formed when oceanic lithosphere is subducted beneath oceanic lithosphere. They are consequently typical of the margins of shrinking oceans such as the Pacific, where the majority of island arcs are located. They also occur in the western Atlantic, where the Lesser Antilles Caribbean and South Sandwich Scotia arcs are formed at the eastern margins of small oceanic plates isolated by transform faults against the general westward trend of movement. All of the components of...
The mantle low velocity zone
The low velocity zone Fig. 2.16 is characterized by low seismic velocities, high seismic attenuation, and a high electrical conductivity. The seismic effects are more pronounced for S waves than for P waves. The low seismic velocities could arise from a number of different mechanisms, including an anomalously high temperature, a phase change, a compositional change, the presence of open cracks or fissures, and partial melting. All but the latter appear to be unlikely, and it is generally...
b 1
Egan Range Fault Spring Valley Fault Egan Range Fault Spring Valley Fault Clear Lake Sevier Desert Reflector F Scarps Detachment Figure 7.14 a Shaded relief map of a part of the eastern Basin and Range showing range-bounding faults and locations of seismic reflection profiles black dashed lines and GPS sites white triangles image provided by N. Niemi and modified from Niemi et al., 2004, with permission from Blackwell Publishing . High-angle faults show ball and bar symbol in the hanging wall,...
Autochthonous and allochthonous mineral deposits
The various plate tectonic environments in which many metalliferous deposits are found are shown in Fig. 13.9. The initial rifting of a continent includes the emplacement of alkaline and peralkaline igneous rocks and the establishment of high geothermal gradients Sections 7.4.2, 7.2, respectively . Ore minerals are generated from this magmatism and from the large-scale circulation of hydrothermal fluids that are energized by it. One group of igneous rocks frequently associated with extensive...
Calculation of the ancient radius of the Earth
A rather less involved method of testing the expanding Earth hypothesis entails determining the paleoradius of the Earth using paleomagnetic techniques Egyed, 1960 . The method involves selecting sampling sites of the same age, on the same paleomeridian and differing as much as possible in paleolatitude. They must also be on a landmass that has been stable since the time the sites acquired their primary remanent magnetizations Fig. 12.2 . Determining the paleolatitudes of the sites then...
Airys hypothesis
Airy's hypothesis assumes that the outermost shell of the Earth is of a constant density and overlies a higher density layer. Surface topography is compensated by varying the thickness of the outer shell in such a way that its buoyancy balances the surface load. A simple analogy would be blocks of ice of varying thickness floating in water, with the thickest showing the greatest elevation above the surface. Thus mountain ranges would be underlain by a thick root, and ocean basins by a thinned...
Circulation
The half space model of lithospheric cooling with age predicts that the heat flux through the ocean floor on ridge flanks will vary in proportion to the inverse square root of its age, but across older ocean floor measured heat flow values vary more slowly than this, again favoring a plate model. The GDH1 model of Stein amp Stein 1992 predicts the following values for heat flow, q mWm-2 as a function of age, t Ma Fig. 6.9 Observed depth and heat flow data for oceanic ridges plotted as a...
And Physiography
Continental transforms, like their oceanic counterparts Section 4.2.1 , are conservative plate boundaries where lithosphere is neither created nor destroyed and strike-slip deformation results in lateral displacements across the fault zone. Strike-slip faults generally may occur at a variety of scales in virtually any tectonic setting. Only transform faults represent plate boundaries. In contrast to oceanic fracture zones, which are characterized by a relatively simple linear trough Section...
Cretaceous Superplume
Certain hotspots, as described in Section 5.5, are thought to be the surface manifestation of plumes of hot material ascending from the deep mantle. These are of mod erate size and can be considered to form part of the normal mantle convecting system. It has been proposed, however, that at least once during the history of the Earth there has been an episode of much more intense volcanic activity. The cause has been ascribed to a phenomena termed superplumes, large streams of overheated material...
Backarc Basins
Backarc or marginal basins are relatively small basins of either oceanic or continental affinity that form behind the volcanic arc in the overriding plate of a subduction zone Fig. 9.3 . Oceanic varieties are most common in the western Pacific, but are also found in the Atlantic behind the Caribbean and Scotia arcs. In all of these settings, the basins reside on the inner, concave side of the island arc and many are bounded on the side opposite the arc by a backarc ridge remnant arc . Most of...
Earthquake descriptors
Earthquakes are normally assumed to originate from a single point known as the focus or hypocenter Fig. 2.1 , which is invariably within about 700 km of the surface. In reality, however, most earthquakes are generated by movement along a fault plane, so the focal region may extend for several kilometers. The point on the Earth's surface vertically above the focus is the epicenter. The angle subtended at the center of the Earth by the epicenter and the point at which the seismic waves are...
Geosynclinal Theory
Prior to the acceptance of plate tectonics, the static model of the Earth encompassed the formation of tec-tonically active belts, which formed essentially by vertical movements, on the site of geosynclines. A review of the development of the geosyncline hypothesis and its explanation in terms of plate tectonics is provided by Mitchell amp Reading 1986 . Geosynclinal theory envisaged elongate, geographically fixed belts of deep subsidence and thick sediments as the precursors of mountain ranges...