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The Cordilleran Orogeny The earths lithosphere can be divided into two major structural features: ocean basins and continents. Ocean basins, as we discussed in another essay, are formed at spreading ridges and consumed at subduction zones. The continents are a different story. How did the continents originate, and how have they changed over time? The origin of the earliest continental crust is still relatively obscure, but the evolution of the continents over time is not: the continents have clearly grown laterally over time via the process of accretion.
To understand how this process works, we can consider the Pacific plate and its interactions with surrounding oceanic and continental margins. The Pacific plate is created by seafloor spreading at the Pacific rise and moves slowly northwest in a conveyor belt fashion at about 8 - 11 cm per year. As a result of this movement, the Pacific plate converges with surrounding plates. In areas where the Pacific plate is being sub ducted, the overriding plate is characterized by periodic deep-focus earthquakes and a ring of felix / andesites volcanoes a few hundred kilometers inland. The vast majority of all earthquakes occur at the three major types of plate boundaries: spreading ridges, sub-duction zones and transform faults. Scattered throughout the Pacific plate are a variety of topographic features rising hundreds to thousands of meters from the ocean floor.
These include island arcs and seamounts, such as the Hawaiian islands-Emperor Seamount chain, extinct spreading ridges, uplifted regions of ocean crust, and apparent continental fragments, such as the Onto Java plateau. Nur et al estimate that are approximately 100 such features are present on the modern sea floor, covering approximately 10 % of its surface (p. 7). Reader and Schubert (1984) estimate that approximately 1. 40 km 3 of new sialic crust is created annually in the form of new volcanic arcs and seamounts. As the Pacific plate continues to move northwest, these structures arrive at subduction zones located at the margins of the Pacific plate. There they are fated to become detached from the sub ducting plate and accreted onto the margin of the overriding plate. Often a portion of oceanic crust will be sandwiched between the accreted terrain and margin.
This process of lateral accretion at subduction zones has obviously played a major role in the growth and evolution of the Circum Pacific margins. Ben-Avraham writes: Recent investigations of the geology of the Pacific Ocean margins have shown that many sectors of these margins are composed of hundreds of pieces called tectonostratigraphic terrains Terrains are fault-bounded packages of rocks of regional extent, each characterized by a geological history that is different from the histories of neighboring terrains (Jones et al. , 1983)... The Circum-Pacific terrain map (Howell et al. , 1983) defines more than 300 different terrains within the Pacific borderlands. Some of the terrains are truly exotic to their present surroundings and came from remote locations, while others came to their present location from nearby areas.
The evolution of the Pacific Ocean margins involved accretion -collision tectonics, which prevailed along most of its sectors for a long time. As a result of this process, lengthy sectors of the margins of the continents surrounding the Pacific Ocean were built from bits and pieces that came from the sea. The collison al events are quite complex and are usually associated with intense folding, thrust and strike-slip faulting, penetrative deformations, and recrystallization. (Dimattia W. and Oder B 1989, p. 3) Accretionary processes have added a significant volume of material to the margins of North America and Eurasia during the Phanerozoic. Howell and Jones estimate that the total area of accreted terrains added to Circum-Pacific margins during the past 200 my is approximately 33 x 10 ^ 6 km 2 (p. 38).
Condie (1989, p. 256) writes: "The conclusion that much of the Cordillera in western North America is composed of a collage of accretionary terrane's is now well established by geological, pale ontological, and paleo magnetic evidence (Coney et al. , 1980; Jones et al. , 1986). More than 200 terrains that lie west of the Precambrian craton have been recognized in the Cordillera. Most of these terrane's have been added to North America during the Mesozoic and Cenozoic, during which time the continental margin was extended by as much as 800 km. Most Cordilleran accreted terrains appear to represent fragments of continents, oceanic plateaus, or portions of arc systems, some of which traveled greater than 5000 km from their sources. From late Proterozoic to early Jurassic time, the western border of North America was a passive margin somewhere in the vicinity of the Nevada-California border. Sediments deposited during this time were in the form of westward-thickening wedges of marine deposits (miogeocline).
These typically grade from open marine in the west to shallow marine in the east. Sediment sources were to the east. In overall structure the Cordilleran miogeocline was very similar to the Phanerozoic Atlantic continental margins. Beginning in the late Paleozoic, there was a reorganization of plate movement, leading to the formation of a subduction zone along the western margin of North America.
By Jurassic time, several accretionary events led to the emergence of highlands at the western margin, effectively blocking the sea from transgressing eastward over the craton. These events are reflected in changes in the geology of the Cordilleran foreland, where Proterozoic through Paleozoic open marine deposits with sediment sources to the east are replaced in late Paleozoic/Early Mesozoic time by terrestrial, fluvial, restricted marine and eolian deposits with sediment sources to the west. Paleomagnetic evidence shows that, as expected, many of the Terrances now found in the Cordillera and the rest of the circum Pacific margins, including China, Japan, Siberia, Alaska, and Canada, have traveled large distances before reaching their present positions. In some case, a previously unified terrain has become fragmented and its fragments widely dispersed. One example is the Wrangellia terrain, pieces of which are found in British Columbia, Oregon, and southern Alaska. Though fragments of Wrangellia are currently spread over about 24 degrees latitude, paleo magnetic evidence shows that the original spread was probably less than 4 degrees (Howell and Jones, p. 37).
How are paleo magnetic data explained on the basis of Noahs flood theories? Since large Phanerozoic basalts and plutons would have remained well above their curie temperatures for weeks or months after the flood catastrophe, they should possess remnant magnetization consistent with their present latitudes. Assuming that the dipole geomagnetic field has existed since creation, and that the only significant latitudinal displacement of continents occurred during the weeks or months of the flood, then it seems that all igneous rocks, Precambrian or Phanerozoic, should document only a creation latitude or a post-flood latitude. There is ample evidence that Phanerozoic style accretionary tectonic processes were operating well before the Cambrian, at least since the early Proterozoic, and that these processes played a dominant role in the growth and evolution of the continents (see below). All continents document an extensive Precambrian/Pre flood tectonic history involving rifts, transforms, and collisions, along with their characteristic associations of rock types. Tectonic processes occurring during the Phanerozoic were just a continuation of tectonic processes which had already been in operation for well over a billion years.
To illustrate this point, we can compare the Phanerozoic evolution of Eurasia to the Proterozoic evolution of the North American craton. It is evident in both cases that these landmasses were formed by a process of lateral accretion of smaller landmasses. In Eurasia this accretion occurred in several stages throughout the Phanerozoic and latest Proterozoic, whereas in North America, this process of accretion was largely completed during the Proterozoic, the major exception being the Cordilleran belt on the western margin of North America, which is composed of terrane's which were accreted during the Phanerozoic, and a few Paleozoic terrane's accreted to east coast. Regarding the structure of Eurasia, Maruyama et al.
note: The Eurasian plate is the largest in the world but is not composed simply of a large craton surrounded by younger organic belts like the North American plate. Instead, the present Eurasian landmass is a collage of six, once separated, major craton's, cemented by a number of Phanerozoic orogenic belts of various ages (p. 75). The major Asiatic Phanerozoic fold belts, of decreasing age, are termed Caledonian (early to middle Paleozoic), Variscan (late Paleozoic), Indosinian (early Mesozoic), Yenshanian (late Mesozoic), and Himalayan (Cenozoic)... All of these fold belts contain oph iolite and / or high P/T metamorphic rocks and are thought to be formed during accretion (p. 78). Wherever they are found, ophiolites are evidence of convergent tectonic processes. Ophiolites are slivers of oceanic crust which have been scraped off or objected from the top of a subjecting oceanic plate and accreted onto the margin of an overriding plate.
From the top down, they consist of oceanic sediments such as chert's, followed by pillow basalts, sheeted dikes, gabbros, and certain ultra mafic rocks (e. g. , serpentinized harzburgite and lherzolite). Average thickness is about 5 km. The distinctive sheeted dike complex [diabase dikes, > 30 cm thick, chilled margins] of ophiolites are generated by successive magma intrusions at spreading (divergent) plate boundaries, which of course implies plate motion.
Ophiolites are often abundant in fold belts sandwiched between two separate terrane's, craton's or continents. Each of the Asian craton's (the Russian, Siberian, Indochina, Sino-Korean, Tarim, and Yangtze, etc. ) are flanked by ophiolites, and / or accretionary prisms and island arc assemblages. The craton's themselves are much older than the Phanerozoic fold belts separating them. For instance, the Indian Platform contains rocks well over 3 by, but the fold belt separating India from mainland Asia is composed of Jurassic-Cretaceous age ophiolites, ophiolitic melange, and deep-ocean sedimentary rocks (Gansser, 1980; Le Fort, 1997; Cornfield et al. , 1999). The Indian craton had a long and varied history prior to its accretion to Eurasia. Paleomagnetic evidence suggests that India, Australia and Antarctica were part of the same super continent during most of the Proterozoic (Condie, 1989, p. 324).
Young-earth creationism attempts to explain the large-scale structure of the earths surface in terms of two events described in the book of Genesis - the 2 nd and 3 rd creation days, a less-than 48 hour event which occurred approximately 4500 BC (e. g. R. V. Gentry, 1988; K. Wise, 1992), and Noahs flood, which occurred about 2500 BC, and which is said to have lasted less than one year.
Roughly speaking, the boundary between creation strata and "flood strata" corresponds to the conventional Cambrian/ Precambrian boundary. On this view, all Precambrian geologic formations were created instantaneously and ex nihil, by direct divine intervention, and have no actual prior sediment logical or tectonic history whatsoever. This is actually an overgeneralization, since there appears to be wide disagreement amongst creationists as to which strata are flood deposits and which arent. Several different pre-flood / flood and flood / post -flood boundary schemes have been proposed. Some creationists see many Precambrian strata as flood deposits, whereas others believe, as we said, that they were created instantaneously. Some creationists see all Phanerozoic strata as flood deposits, whereas others place the flood / post -flood boundary as early as the Permian (Steven J.
Robinson, Can...
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