Author Topic: SURGE TECTONICS  (Read 439 times)

Admin

  • Administrator
  • Hero Member
  • *****
  • Posts: 555
    • View Profile
Re: SURGE TECTONICS
« on: March 22, 2017, 05:17:23 pm »
SURGE TECTONICS
Chapter 6 Magma Floods, Flood Basalts, and Surge Tectonics
6.1 Introduction
... The old term, "plateau basalt", had functioned with comparative efficiency and illustration, but Tyrrell's "flood basalts" gave an immediate and striking image of basalts poured out in broad areal effusion. "Plateau basalt" has continued in the literature to a considerable degree, but "flood basalts" has become by far the preferred term in mafic volcanology.
 The origin of flood basalts has sparked controversy since they were first identified in the last century [the 1800s]. The purpose of this chapter is to re- examine the critical data, including descriptions of many flood-basalt provinces, to introduce the new term "magma floods" for flood basalts--a term that we consider more appropriate and encompassing--and to propose an explanation of our own in terms of surge tectonics.

6.1.1 SIGNIFICANCE OF FLOOD BASALTS
Some 63% of the ocean basins are covered flood basalts. At least 5% of the continents are likewise covered with flood basalts. Thus 68%---a minimum figure--- of the Earth's surface is covered with these basaltic rocks. Flood basalts, then, are not the oddities that many suppose them to be. In spite of this, they receive little attention among the scientific community. We examined nearly twenty geologic textbooks and reference works published since 1969, and found only two with more than three paragraphs on flood basalts. ... Such treatment---or lack of treatment---seems unusual, out of place, if one considers that flood basalts are the most important rock exposed at the Earth's surface (..., 1986...).
 Engel et al. (1965) long ago demonstrated that deep ocean-floor tholeiitic basalts are the oceanic equivalent of the continental flood basalts. The Basalt Volcanism Study Project (1981) differentiated between the continental flood basalts and "ocean-floor basalts," while recognizing that the principal differences were the abundance of minor and rare-earth elements. Press and Siever (1974...) recognized the fact that the ocean-floor basalts and continental flood basalts are nearly the same, and that their differences are explained readily by contamination in the continental crustal setting. Yoder (1988), one of the world's authorities on basaltic magmas, stated essentially the same thing.
 In fact, as increasing numbers of basalts are analyzed, the difference between the oceanic and continental floods blurs even further. For example, ... (1991) found groups of samples from the Siberian Traps that are essentially indistinguishable from midocean ridge basalts. Fitton et al. (1991) found numerous Great Basin basalts that are chemically indistinguishable from midocean ridge basalts, and Sawlan (1991) observed a complete chemical continuum from midocean ridge basalts to the flood basalts in the Baja California, Gulf of California, and Mexican basin- and-range province.
 These extremely close--in places identical--genetic relationships are well established. In a subsequent section of this chapter, we shall present geochemical data to support this statement.

6.1.2 CLASSIFICATION
Continental flood-basalt provinces are geometrically of two types. The first is broadly ovate, or even round, with the maximum diameter ranging from about 500 km (Columbia River Basalt) to more than 2,500 km (Siberian Traps). The second is distinctly linear, with a width of 100 to 200 km and lengths up to and even exceeding 3,000 km.
 Oceanic flood-basalt provinces at first appearance are difficult to classify. However, as more ... data ... become available, it is possible to distinguish the same two types of geometries there as well. Ovate to semi-ovate shapes characterize many oceanic submarine plateaus. The maximum diameters of these plateaus, excluding the Kerguelen Plateau, are in the order of 1,200 to 1,600 km.
 Linear ridges are of two types. The larger is the midocean-ridge system with widths between 1,200 and 3,600 km; the smaller is exemplified by the various linear island and seamount chains with widths of 100-200 km and lengths of thousands of kilometers.
 Ovate flood-basalt provinces include [over 13 places]....
 Linear flood-basalt provinces include [over 14 places]....
 Tectonism and metamorphism can severely disrupt any flood-basalt province after its formation. For example, ... the Antrim Plateau Volcanics of northern Australia ... parts ... have been removed by erosion. ... Similarly, only very scattered, strongly flooded, and metamorphosed remains of the Willouran Mafic rocks are preserved in ... South Australia, but their distribution shows that [it] is a linear flood-basalt province.

6.1.3 THE PETROGRAPHIC CHARACTER OF FLOOD-BASALT PROVINCES
To judge from the geological literature, many earth scientists assume that flood- basalt provinces are composed mainly of basalt and little else. This characterization is justified for some provinces but it is incorrect for many more. For example, the Columbia River flood-basalt province consists nearly 100% of tholeiitic basalt with small volumes of basaltic andesite and minuscule amounts of dacite and rhyolite (..., 1979-1988). In contrast the Snake River flood-basalt province on the southeastern side of the Columbia River province consists more than 50% of rhyolite and siliceous (rhyolitic) ignimbrites (..., 1989). A second example is the Lebombo monocline region of the Karroo flood-basalt province in southern Africa. Here are thick sequences of rhyolite (and perhaps ignimbrite) which, for most of the length of the monocline--at least 600 km--comprise 30 to 55% of the volcanic section (..., 1983). Yet another example is the Keweenawan (Midcontinent) flood-basalt province where every region has large volumes of rhyolite associated with the basalt. Our first point is that many flood-basalt provinces are bimodal, and the volume of associated silicic extrusive (or intrusive) rocks can be substantial.
 A second common assumption is that tholeiitic basalt and related tholeiitic rocks constitute the principal mafic rock types. Here again, the field evidence proves the assumption is incorrect. It is true that the Columbia River flood-basalt province consists 99% of tholeiitic mafic rocks. Yet the huge, 3,000-km-long Arabian flood-basalt province consists mainly of alkalic basalt. In fact, Camp and Roobol (1981) and Camp et al. (1991) refer to this example as the "Arabian continental alkalic basalt province." Thus our second point is that many types of basalts may be present in flood-basalt provinces. Tholeiitic basalt is just one of those types.

6.2 Descriptions of Selected Continental Flood Basalt Provinces
We present here some brief geological descriptions of representative ovate and linear continental flood-basalt provinces in order of decreasing age. Many additional continental provinces could have been added to this list, but we believe that those selected adequately illustrate the points we wish to make. Undoubtedly, some earth scientists will not agree that all of our examples are, in fact, flood- basalt provinces. Therefore, we include data on areal extent, volume, thickness, composition, and age which led us to conclude that we were dealing with flood- basalt provinces. Data concerning the ages, areal extent and volume of these provinces and others are summarized on Table 6.1.

6.3 The Use of Geochemistry in Identifying Flood Basalts
6.3.1 INTRODUCTION
Geochemical/petrochemical studies of igneous rocks for many decades were restricted to (1) studies of the bulk chemistry (major compounds only) of each rock type, and (2) deviations from the "norm" determined for each rock type. High- pressure ... and high-temperature studies were conducted in the search for the chemical phases and eutectics of rock melts. Such studies were invaluable in determining the origins of various rock types, and led to many classical papers, especially the Yoder and Tilley (1962) and Yoder (1976) treatises on the origin and generation of basalt.
 With the advent of plate tectonics, petrochemistry was used increasingly as a supplement to traditional methods of identifying tectonic environments. The assumption was made that each tectonic environment had its own petrochemical "signature." When major-element studies failed to bear out this assumption, however, increasing attention was given to minor (trace) and rare earth elements. Regrettably, nearly all large-scale petrochemical research concentrated on the basalts (e.g., the NASA-sponsored Basaltic Volcanism Study Project published in 1981), and other rock types have failed to receive anything like the attention that the basalts received. As an inevitable consequence, many conclusions were made on the basis of basalt geochemistries alone. Our points are: (1) that a great deal of research---many decades, in fact---will be necessary before sound conclusions regarding the chemical "signature" of tectonic environments will or can be soundly based; and (1) even though the more silicic magma types are in very large part aggregates of crustal compounds and processes, they too have important scientific "messages" to impart. It is too early to reach final conclusions based only on basalt data.
 The results of minor and rare-earth element studies, however, have been helpful, for they document in part the history of each sample with the use of spidergrams (Fig. 6.16). They also discriminate easily between midocean-ridge basalts and other basalts, although this already was possible from major element data alone. However, as we document below, the ability of spidergrams to discriminate among most tectonic settings is doubtful without much additional information, partly from isotope data and, in the long run, with the aid of actual field data.
 An important step that must be taken now is to standardize the order in which the trace and rare-earth elements appear on a spidergram (Fig. 6.16). Second, there is no consistency about which elements are included or excluded (Fig. 6.16), and this problem also must be resolved. Too often elements important to an interpretation are omitted on spidergrams. Finally, there is no consistency about which material is used for "normalizing" element plots. Currently some are chondrite-normalized; some are normalized against an idealized midocean-ridge basalt composition; and many are normalized against the composition of an hypothetical primordial mantle, a practice which, as Thompson et al. (1983) have noted, introduced unnecessary subjectivity into interpretations.

6.3.2 BASALT MAGMAS
... [Skipping 3 paragraphs]
It is important to be aware that the concentration of incompatible trace elements* [those most likely to be transported by melts and other fluids passing through the mantle and therefore most likely to preserve evidence of mantle enrichment and depletion processes in their relative abundances] changes greatly in this basaltic liquid, depending on their relative partition coefficients, initial concentrations, and dilution rates. In the midocean-ridge basalts, the volume of incompatible minor elements is very small, a fact that suggests that the parental material has already undergone some partial melting and loss of liquid, but still retains parts of all major melt phases (..., 1988).
 Several processes involved in the emplacement of magmas in the crust complicate the above picture. The composition of surface samples from rocks that were molten and under high pressure is not necessarily that of the parental liquid at depth. This is true because (1), as the liquid rises, internal reaction relations take place that successively eliminate olivine and orthopyroxene (..., 1967-1988). Hence the composition of the basalt may be altered considerably during its rise from ca. 130 km; (2) of heat loss; (3) the change in pressure further changes the liquid composition; and (4) the rise of the melt produces a change in the stable phases within the liquid.
 The reasons for the differences between continental flood basalts and midocean-ridge basalts are related in part to the above factors, but differences in the thickness of the lithosphere clearly must exert an important influence as well (..., 1988). The penetration of an old, thicker, continental massif by basalt melt is clearly more difficult than that of the much thinner oceanic lithosphere, although the rising magma rises in the same way under both lithospheres, following the Peach-Kohler climb force (Newton's Law of Gravity; ..., 1964-1989) and stops when the level of neutral buoyancy has been reached (..., 1989). The longer---or slower---the rise beneath the continental crust, the greater the fractionation, as reflected in the more iron-rich character of the continental lavas (..., 1981- 1988). Deep-seated magma segregation beneath the continents provides for more alkalic parental magmas, a greater range of enrichments, and a greater variation that depends on repose time, interactions with the continental crust, and the rates of ascent. The bimodal character of so many continental flood basalts implies the presence for periods of time of multiple magma chambers.

6.3.3 STUDIES OF MINOR AND RARE EARTH ELEMENTS
When studies of major elements and compounds revealed difficulties in discerning chemical signatures peculiar to each tectonic environment, research began to focus on studies of minor (trace) elements, rare-earth elements, and chemical isotopes. Although a high degree of success has been claimed for such studies, the facts tell quite a different story. Indeed, it is a poor reflection on the state of current geoscientific resaerch that the eagerness of some researchers to satisfy preconceived hypotheses and models has led some into publishing material that is scientifically sound [unsound?]. Minor (trace) element, rare-earth element and chemical isotopes studies are summarized for the following environments.
 Midocean-Ridge Basalts (Ocean-Floor Volcanism) ...
 Ocean-Island Basalts (Oceanic Intraplate Volcanism) ...
 Continental Flood Basalts (Continental Intraplate Volcanism) ...
 Volcanic Arc Basalts ("Subduction" Basalts) ...
 Island Arc Basalts ...
 Continental Margin Volcanic Arcs ...

6.4 Geochemical Comparisons among Basalts Erupted in Different Tectonic Settings
... 6.4.7 CONCLUSION
Our examination of the literature on basalt rocks has led us to conclude that geochemistry is useful in distinguishing between midocean-ridge basalts and other basalts. This is true of bulk geochemistry, major-element geochemistry, and minor (trace) element and rare-earth element tectonic settings other than that of the midocean ridge. Exceptions to this statement do exist, but only in areas where the investigator has exceptional knowledge of the field relations among the various igneous units that he/she is investigating. Geochemical techniques are useful, however, in deciphering the chemical histories of the various igneous units, subject once again to the proviso that field relations among the various units being studied are well understood.

6.5 Duration of Individual Basalt Floods
6.5.1 INTRODUCTION
The length of time during which a particular basalt flooding episode lasts differs greatly among the various flood-basalt provinces. Some, such as the Siberian flood-basalt province, have been active more than 200 Ma. Others---the Wrangellian province, for example---probably completed their flood activity in 5 Ma or less. Even in flood-basalt provinces of long duration, the largest volume of basalts may have been extruded in one, or perhaps two or three relatively short bursts. A close relationship seems to exist between times of tectogenesis and times of major basalt flooding.

6.5.2 FLOOD-BASALT PROVINCES OF LONG DURATION
Radiometric and/or paleontologic constraints are available for only a few flood- basalt provinces. Therefore, we mention only places where good dating is available. The radiometric data are summarized on Table 6.1.
[2.5-283 Ma are indicated.]

6.5.4 CONCLUSION
We have discussed several flood-basalt provinces which were active during periods that ranged from more than 210 Ma (long duration) to less than 12 Ma (short duration). We have found no evidence to suggest that there are any time controls or any rules of thumb that guide the length of time during which a flood-basalt province may remain active. Nor is there a relationship between type of flood- basalt province may remain active. Nor is there a relationship between type of flood-basalt province and the duration of its extrusion. For example, the Columbia River province is ovate while the Wrangellian province is linear; yet the two endured for approximately the same lengths of time. Reports that the Deccan and Siberian flood-basalt provinces were in fact of very short duration are based on a lack of information. In fact, information adequate to determine the "lifespans" of most flood-basalt provinces, including Siberian province, is not yet available.

6.6 Flood-Basalt Provinces and Frequency in Geologic Time
As we observed near the beginning of this chapter, the commonly used textbooks of physical geology, structural geology, and geotectonics rarely list more than 10 to 20 flood-basalt provinces. However, the magnificent review of basalts by the participants in the Basalt Volcanism Study Project (1981) mentions or figures not less than 56 flood-basalt provinces and 45 additional provinces of dike swarms which the project participants thought might have fed flood-basalt provinces that have since been removed by erosion.
 The participants in the Basalt Volcanism Study Project (1981) concurred on a large number of phenomena that characterize flood-type volcanism. However, they showed considerable confusion, ambivalence, and lack of agreement on which, and what type of, provinces should or should not be described as flood-basalt volcanism. This confusion and ambivalence manifest themselves with respect to the differences between flood-basalt provinces and continental rift-related provinces. Additionally, they used interchangeably the terms "flood basalt," "plateau basalt," "continental rift volcanism," and "hot-spot volcanism."
 We summarize here briefly their overall remarks on the ages of flood-basalt activity. They wrote that (1) most flood provinces are less than 200 Ma; (10 no major flood-basalt activity took place in the interval 1,100-200 Ma (yet ... they list eight provinces within this time span, two of which are huge---the Siberian Flood-Basalt Province and the Emeishan Flood-Basalt Province); (3) reasonably well-preserved remnants of flood provinces are known from the time interval 2,150- 1,100 Ma, and (4) a few poorly preserved remnants are present in the geological record to 3,760 Ma (..., 1981, ...). Yet, on page 41, the same authors state that flood-basalt provinces older than 1,200 Ma are unknown.
 The participants ... have for the first time, to the best of our knowledge, provided solid evidence that flood-basalt volcanism is a phenomenon that has persisted since the beginning of--or since very early in---the Earth's history. However, we have not seen any convincing evidence to support the claim by Rampino and Stothers (1988), and a similar claim by White and McKenzie (1989), that flood- basalt volcanism is periodic, with large outpourings every 32 to 30 Ma. We suspect, but cannot prove, that flood volcanism is triggered by tectogenic (orogenic) pulses that are episodic. In our opinion, the available evidence all but demonstrates an endogenic origin. Which of the various possible endogenic causes is the correct one must await the careful sampling and dating of thousands of more carefully located igneous-rock samples in every major flood-basalt province.
 Yoder (1988, ...) wrote that "Great basaltic 'floods' have appeared on the continents throughout geologic time (Table 1)," but showed on his Table 1 none older than 1,200+/- 50 Ma. He also ... made it clear that he regards midocean-ridge and other oceanic basalts as flood basalts, as have a number of earlier workers (..., 1974). We concur absolutely with their interpretation. We also concur with the participants of the Basalt Volcanism Study Project (1981) that evidence of the existence of flood provinces extends back in time to at least 3,760 Ma, and very likely to the Earth's earliest (but nowhere preserved) history. Interestingly, most of what Press and Siever (1974), Yoder (1988), and we concur in what was anticipated by the pioneer work of Engel et al. (1965).

6.7 Non-Basalt Flood Volcanism in Flood-Basalt Provinces
The bimodal nature of many flood-basalt provinces has been known and stressed for many years (..., 1981). Time seems not to be a major factor (the idea being that, the longer an underlying magma chamber is present, the more the magma will interact with the continental crust above it). The most important factor may be the crustal stress state.
 Estimates of the volume of non-basaltic rocks in a given flood-basalt province are difficult to find. Accordingly in Table 6.2 we have left many blank spaces and the percentages that we have supplied are poorly documented except in local areas.
 ... [Skipping 6 paragraphs]
 We believe that the evidence from these examples demonstrates convincingly that there is a complete gradation from all-basalt and basaltic andesite flood provinces to bimodal provinces containing mainly rhyolite and ignimbrite. Hence, there are basalt floods and rhyolite floods.
 ... [Skipping most of 1 paragraph] The volumetric predominance of these ash-flow tuffs has led to recognition of the [Sierra Madre Occidental] as the world's largest rhyolite-dominated volcanic province" (Fig. 6.28).
 ... [Skipping one paragraph]
 Thus, from 38 Ma until 17 Ma, a truly bimodal column of extrusive rocks accumulated in northern Mexico and adjoining parts of the United States, with rhyolite at one end, basaltic andesite at the other, and very little rock of intermediate compositions. ... [Skipping remainder of paragraph]
 We believe that these basalts of the "southern cordilleran basaltic andesite" suite are flood basalts. And if they are flood basalts, then we have demonstrated that the same mechanism that leads to continental and oceanic basalt outpourings also produces the "orogenic andesite suite".
 The Okhotsk-Chukotka Volcanic Belt, a linear belt of Cretaceous volcanics, is similar to the Sierra Madre Occidental. It extends 3,000 km from the mouth of Uda Bay (northwestern Sea of Okhotsk) to the Bering Sea almost at St. Lawrence Island. It seems to have every type of volcanic from andesitic through rhyolite. Basalts are scarce. Soviet geologists either ignore it or say that it is the remnant of a volcanic arc.
 
6.8 Flood Basalts or Magma Floods?
Although we advocate the continued use of the term "flood basalt," it is clear that another term is needed to describe floods of andesite, dacite, and rhyolite. For future studies, we suggest the all-encompassing term magma floods. In this way, we can include all of the various lava types, dikes, necks, and sills. It is a term that even embraces situations such as the Ferrar Dolerite of Antarctica and the network of sills and dikes of the Amazon basin.

6.9 Surge-Tectonics Origin of Magma Floods
In the preceding pages we have referred to the presence of several flood-basalt provinces around the world, and have shown that some flood provinces include large volumes of silicic rocks, usually rhyolite and/or dacite. We have also shown by the northern Mexican example that flood basalts can interfinger with the andesite orogenic suite. In addition, we have presented evidence that spidergrams are not more effective at identifying the tectonic setting than bulk chemistry. The available evidence has led us to the conclusion that the same mechanism causes volcanism in the midocean ridges, linear island and seamount chains, oceanic plateaus, island arcs, and continental interiors. We next attempt an explanation of our conclusion.
 Many attempts have been made to explain flood volcanism in the framework of the plate-tectonics hypothesis. The two principal explanations involve (1) hot spots, or mantle plumes and (2) an extraterrestrial cause (e.g., an asteroid impact).
 Extraterrestrial causes have been proposed by Alt et al. (1988), who applied this hypothesis to the Columbia River flood-basalt province. A major problem with this concept is that it does not explain linear flood-basalt provinces such as the Keweenawan (Mid-Continent) rift and Wrangellia. Furthermore, Mitchell and Widdowson (1991) pointed out that impact and shock phenomena should be present in the area surrounding the Columbia River province if it resulted from extraterrestrial action, but they are entirley absent.
 Mantle diapirism or asthenosphere upwelling constitutes the hot-spot or mantle- plume hypothesis (..., 1971) used widely in tectonic models today. Recent literature on mantle plumes include works by ... (1988-1991). Hot spots are often portrayed as diapiric bodies that are essentially cylindrical, mushrooming plumes. While this might account for isolated volcanoes, it does not account for the massive ovate and linear flood basalt provinces found in many parts of the world.
 Mantle upwelling also has been invoked by many writers to explain the presence of long, linear continental rifts (..., 1983), which are, for the most part, similar to one another. ... [Skipping remainder of paragraph listing widths and lengths of numerous linear rifts etc]
 As we noted in Chapters 3 and 4, Mooney et al. (1983) observed that all active rifts studied by them have an anomalous lower crust with P-wave velocities in the 7.0 to 7.7 km/s range (Fig. 6.36). [Others] obtained the identical result.... Fuchs (1974) believed that this pod of anomalous lower crustal material houses the mechanism that causes rifting. It is interesting to note that all midocean ridges have a pod of 7.0-7.7 km/s as well (..., 1959-1965). (Furthermore, each island arc and foldbelt also has a pod of 7.0-7.7 km/s material that pinches out from the center of the arc or foldbelt (..., 1987-1989 ... for the Japan arc ... [and] for the Appalachians.)
 Figure 3.6 is a cross section across the Baykal rift, from Krylov et al. (1979) and Sychev (1985). Years of refraction work have shown the Lake Baykal is underlain at about 32 km by a pod that is connected to the deeper asthenosphere. The shallow pod contains a low-velocity zone that presumably is a partial melt. The pod extends the full length of the rift. It is, in short, a channel containing partly molten magma and an excellent example of one of our surge channels. Were it to burst, we believe that it would produce another linear flood-basalt province.
 According to our surge tectonic hypothesis, magma in surge channels moves both vertically and horizontally. When two surge channels come in contact, their magmas join together. If they are oriented at an appreciable angle to one another, we believe that the result is a "collision". These5 "collisions" are responsible for the eruption of round or ovate flood-basalt provinces worldwide.

CHAPTER 7
CONCLUSIONS
We have proposed a new hypothesis of global tectonics in this book, one that is different and will be considered unorthodox by many scientists and non-scientists alike. However, we believe that current tectonic hypotheses cannot adequately explain the increasing volume of data being collected by both old and new technologies. We believe that the hypothesis of surge tectonics does explain these data sets, in a way that is simple and more accurate.
 The major points of the surge-tectonics hypothesis can be summarized as follows:
 1. All linear to curvilinear mesoscopic and megascopic structures and landforms observed on Earth (and similar features seen on Mars, Venus, and the moons of Jupiter, Saturn and Uranus), and all magmatic phenomena are generated, directly or indirectly, by surge channels. The surge channel is the common denominator of geology, geophysics, and geochemistry.
 2. Surge channels formed and continue to form an interconnected worldwide network in the lithosphere. They contain fluid to semifluid magma, or mush, differentiated from the Earth's asthenosphere by the cooling of the Earth. All newly differentiated magma in the asthenosphere must rise into the lithosphere. The newly formed magma has a lower density and therefore, is gravitationally unstable in the asthenosphere. It rises in response to the Peach-Kohler climb force to its level of neutral buoyancy (that is, to form a surge channel).
 3. Lateral movements in the Earth's upper layers are a response to the Earth's rotation. Differential lag between the more rigid lithosphere above and the (more) fluid asthenosphere below causes the fluid, or mushy, materials to move relatively eastward.
 4. Surge channels are alternately filled and emptied. A complete cycle of filling and emptying is a geotectonic cycle. The geotectonic cycle takes place along this sequence of events:
 a. Contraction of the strictosphere is always underway, because the Earth is cooling;
 b. The overlying lithosphere, which is already cool, does not contract, but adjusts its basal circumference to the upper surface of the shrinking strictosphere by large-scale thrusting along lithosphere Benioff zones and normal-type faulting along the strictosphere Benioff zones.
 c. Thrusting of the lithosphere is not a continuous process, but occurs when the lithosphere's underlying dynamic support fails. When the weight of the lithosphere overcomes combined resistance of the asthenosphere and Benioff zone friction, lithosphere collapse begins in a episodic fashion. Hence, tectogenesis is episodic.
 d. During anorogenic intervals between lithosphere collapses, the asthenosphere volume increases slowly as the strictosphere radius decreases and decompression of the asthenosphere begins.
 e. Decompression is accompanied by rising temperature, increased magma generation, and lowered viscosity in the asthenosphere, which gradually weakens during the time intervals between collapses.
 f. During lithosphere collapse into the asthenosphere, the continentward (hanging wall) sides of the lithosphere Benioff zones override (obduct) the ocean floor. The entire lithosphere buckles, fractures, and founders. Enormous compressive stresses are created in the lithosphere.
 g. When the lithosphere collapses into the asthenosphere, the asthenosphere- derived magma in the surge channels begins to surge intensely. Where volume of magma in the channels exceeds volumetric capacity, and when compression in the lithosphere exceeds the strength of the lithosphere that directly overlies the surge channels, the surge-channel roofs rupture along the cracks that comprise the fault-fracture-fissure system generated before the rupture. Rupture is bivergent and forms continental rifts, foldbelts, strike-slip zones, and midocean rifts. We call such bilaterally deformed belts kobergens.
 h. Once tectogenesis is completed, another geotectonic cycle or subcycle sets in, commonly within the same belt.
 5. Movement in the surge channel during the taphrogenic phase of the geotectonic cycle is parallel with the channel. It is also very slow, not exceeding a few centimeters per year. Flow at the surge-channel walls is laminar as evidenced by the channel-parallel faults, fractures, and fissures observed at the Earth's surface (Stoke's Law). Such flow also produced the more or less regular segmentation observed in tectonic belts.
 6. Tectogenesis has many styles. Each reflects the rigidity and thickness of the overlying lithosphere. In opcean basins where the lithosphere is thinnest, massive basalt flooding occurs. At ocean-continent transitions, eugeosynclines with alpinotype tectogenesis form. In continental interiors where the lithosphere is thicker, either germanotype foldbelts or continental rifts are created.
 7. During the geotectonic cycle, and within the eugeosynclinal regime, the central core (crest of the surge channel) evolves from a rift basin to a tightly compressed alpinotyhpe foldbelt. Thus a rift basin up to several hundred kilometers wide narrows through time until it is a zone no more than a few kilometers wide that is occupied by a streamline (strike-slip) fault zone (e.g. the San Andreas fault). Then as compression takes over and dominates the full width of the surge-channel crest, the streamline fault zone is distorted, until it and the adjacent rocks are severely metamorphosed. If the underlying, and now severely deformed surge channel still contains any void space, the overlying rocks may collapse into it, and through this process of Verschluckung (engulgment) become a Verschluckungzone.
 8. The Earth above the strictosphere resembles a giant hydraulic press that behaves according to Pascal's Law. A hydraulic press consists of a containment vessel, fluid in that vessel, and a switch or trigger mechanism. In the case of the Earth, the containment vessel is the interconnected surge-channel system; the fluid is the magma in the channels; and the trigger mechanism is worldwide lithosphere collapse into the asthenosphere when that body becomes too weak to sustain the lithosphere dynamically. Thus tectogenesis may be regarded as surge-channel response to Pascal's Law.
 9. Surge channels, active or inactive, underlie nearly every major feature of the Earth's surface, including all rifts, foldbelts, metamorphic belts, and strike-slip zones. These belts are roughly bisymmetrical, have linear surface swaths of faults, fractures, and fissures, and belt-parallel stretching lineations. Aligned plutons, ophiolites, melange belts, volcanic centers, kimberlite dikes, diatremes, ring structures and mineral belts are characteristic. Zoned metamorphic belts are also characteristic. In some areas, linear river valleys, flood basalts, and/or vortex structures may be present. A lens of 7.8-7.0 km/s material always underlies the belt.
 10. Active surge channels are most easily recognized by the presence of high heat flow (Fig. 2.26), microseismicity, lines of thermal springs, small negative Bouguer gravity anomalies, and a 7.8-7.0 km/s lens of material that is transparent in the center or throughout.
 11. Inactive surge channels possess a linear positive magnetic anomaly, a linear Bouguer positive gravity anomaly, and a linear, lens-shaped pod of 7.8-7.0 km/s material that is reflective throughout.
 12. A surge-tectonics approach to geodynamics provides a new means for determining the origin of the Earth's features and their evolution through time, for analyzing regions prone to earthquakes and volcanism, and for predicting the location and formation of mineral deposits throughout the globe.
« Last Edit: March 29, 2017, 07:45:29 pm by Admin »