Historical Geology - Part V - A brief history of Earth

Formation of the Layered Earth

Dating interstellar hydrogen suggests that the Universe is about 10 billion years old, and the current estimate of the age of our solar system is about 4.6 billion years. The planets began to form nearly immediately, but as coalescing balls of atoms and molecules. Within the protoplanets, the particles clumped together under their own mutual gravitational attraction. As the clumps got larger and larger, they had more gravity, and so swept up more particles. Continued rotation differentiated the particles and gases, with the heaviest materials (uranium, lead, iron, nickel etc.) forming the center of the protoplanet and the lighter materials (hydrogen , helium and others) at the outer layer. In between the heavy solids and the light gases are the rock formers: silicon and aluminum combined with oxygen. There are several earth formation models which generally sort into two classes: (1) formation as a homogenous body, and stating that at some point equilibrium had been reached; or (2) formation as a layered or stratified body which implies that equilibrium was never attained during fractionation. In either case, the formation was essentially as a solid body, for if the earth had melted completely (it is suggested) that all the iron and metals would have sunk to the core and the crust would therefore be undersaturated with heavy materials.

Evidence for earth formation comes from meteorites which come in two general types stony silicates and irons with up to 20 percent nickel. Irons comprise 7 percent of the total meteorites recovered. The different natures of stonies and irons, and their ancient ages (usually around 4 billion years) suggest that the early solar system differentiated quickly. So, the early earth may have layered into an iron nickel core, a silicate/metallic mantle and a metallic/silicate/aluminum crust with an atmosphere rich in hydrogen and helium. The early atmosphere was largely lost. It has been suggested that the solar wind deprived the inner planets of the lightest fraction of their gases and added them to the "gas giant" planets. This would explain why the terrestrial planets (Mercury, Venus, Earth and Mars) are depleted in the light gases and the large gas giants (Jupiter, Saturn, Neptune and Uranus) have such enormous atmospheres compared with their solid cores.

At this point about 4 billion years ago, the early earth might have looked like present day Venus, with darker lowlands and lighter highlands, scarred and pocked with meteor craters and giant volcanos releasing heat from the mantle. Gases were provided to the earth's atmosphere by outgassing from the mantle. It has been suggested that some of the outgassing was a result of impact. As the impacting body (the bolide) struck the thin metallic/silicate/aluminum crust, the mantle would be revealed and particles and gases released. These gases in present day volcanos with mantle sources include mostly water vapor and carbon dioxide with hydrogen, nitrogen, ammonia, methane and chlorine being emitted in lesser quantities. In this model, the water vapor condensed and formed the early oceans, leaving the atmosphere composed of carbon dioxide and methane. In addition, bolide impacts weakened the crust which permitted flow of hotter mantle material up towards the surface, raising the isotherm, and perhaps providing an initial push to mantle convection.

Any discussion of the early earth must include a look at the formation of the earth's moon. There are several theories:

1. it formed elsewhere and was captured by earth gravity;
2. it formed by fission of the cloud of gas which then gave rise to both the earth and the moon;
3. it was part of the earth, but was fractured out of it by some unknown method; or
4. it was a blob of earth material blasted out by the impact of a very large bolide.

Evidence which must be reconciled in any theory of lunar formation includes the angular momentum of the system and the composition of each body. The moon's average density of 3.3 is below the average density of the terrestrial planets and it has a different composition of common elements than the earth, but its density and composition can be understood if the large bolide struck at an oblique angle and only sheared off crustal material. The large impact theory is the one most commonly accepted today.

Studying the moon samples brought back by Apollo missions reveals that the lunar highlands are Anorthite with from 0.2 to 4.0 iron while the mare basalts contain from 7 to 8.5 percent iron. This supports the so called "magma ocean hypothesis," which suggests that early crust formed on top of an ocean of magma. Such an early crust would have areas of lighter material and areas of heavier material. Some geologists have speculated that the lower layer would be heavier than the upper layer and that it is impact remelting of the lower layer that we are seeing in the iron enriched mare flood basalts. This heavier lower crust, they suggest is represented best by a sample called the "low K⁺ Fra Mauro basalt" which is abbreviated (LKFM). Another heavier lunar sample was named KREEP which stands for "K⁺, rare earth element, phosphorus rocks." These are the two categories of lunar rock most often shown in diagrams, usually without the abbreviations explained.

The oldest rocks on earth date to about 4 billion years and therefore are the bottom of the geologic history of the earth. The Archean period begins then, and continues to 2.5 billion years before the present (bybp) when the Proterozoic Era begins. The Proterozoic ("before life") continues until 600 million years ago (mya) when the present Era, the Phanerozoic ("obvious life") Era began. Some writers call the time before the Phanerozoic the "Pre-Cambrian" since the Cambrian is the first period of the Paleozoic.

The Phanerozoic is divided into the Paleozoic ("ancient life"), Mesozoic ("middle life), and Cenozoic ("recent life") periods on the basis of fossils found in each. The Paleozoic ran from 600 mya to 250 mya, the Mesozoic from 250 mya to 65 mya and the Cenozoic from 65 mya to the present. Most of the crust we have to study dates from these periods. Crust from the Precambrian is rare and the oldest crust is most rare.

A widely accepted theory of the development of earth's crust states that the early earth would have had a protocrust formed from ultramafic, mafic and felsic layers. Some suggest this was evenly distributed like a layer cake while others prefer a "pod" system of felsic blobs in surrounding mafic materials. As shown on the moon where there is little erosion, bolides were common in the early solar system. A large bolide impact could have broken through the crust into the mantle. Immediately, a huge crater would be excavated and the rocks around the crater heated. Instantaneous pressure release by the removal of the "overburdening" crust would cause partial melting of the rocks below the crater. Mantle isotherms would rise and blobs of hot rock rise through the protocrust. If the rising rock is trapped plutons form, if released - lava flows would mark the event. A new crust covers the crater; the heat continues to be released by volcanos.

The area under the impact would remain low relative to the rest of the crust, and gradually accumulate breccias, sandstones and tuffs composed of locally-derived volcanic material. The additional weight of sediment on the weakened base would push it down into the mantle which would permit more sediments to accumulate in the basin.

The Archean: Gneiss, greenstones and granulites

Most Precambrian exposures ("shields" or "cratons") contain metamorphic rocks and granitic intrusives showing that even older rocks had been metamorphosed or melted. Some Precambrian shields may be the roots of ancient mountain ranges - the mountains were eroded to a flat surface (peneplain) long ago. Erosion of the earliest mountains provided sediments which were themselves reworked and recycled over the eons to form the continental masses we see today. Study of the earliest sedimentary rocks reveals their environment of formation while study of metamorphic rocks sometimes tells us what kind of rock was changed but always gives us the dates at which their atomic clocks were reset - thus the date of deformation. Dating zircons found in all types of rocks shows the oldest age of inclusions in igneous, metamorphic or sedimentary rocks.

Archean age sedimentary rocks are very similar to each other. They formed in marine environments and most have been subsequently deformed. We will explore each type of Archean formation, the greenstone belts, banded iron formations, and Archean granitic rocks. The oldest known rock is a granitic gneiss which implies that differentiation (at least local differentiation) was complete, and that the rock was metamorphosed at 4 bybp.

Greenstone belts were so named as a physiographic province because the rocks are greenish in color and the outcrops occur in wide linear or curving linear structures. Greenstone belts are composed of several kinds of initial rocks - all of which have subsequently been deformed. Some parts of the metamorphic suite are more highly metamorphosed than others. From bottom to top the rocks are:

• Igneous rocks including deformed pillow lavas and gabbros which may represent the original crust formed after the initial crust, but yet in a spreading center or Hawaiian-style igneous environment.
• Greywackes and shales occur in thin beds of less than 1 meter. The graded bedding suggests deposition from turbidity currents. This produces rocks called " turbidites" which were originally fine sediments swept offshore into deep water.
• Sandstones, shales and coarse conglomerates. The sandstones indicate fluvial or coastline deposition, the shales were deposited in deeper water and the massive conglomerates so typical of greenstone belts may have formed by massive debris flow deposits (lahars) from Archean volcanos.

So, from bottom to top in a greenstone sequence, we have ultramafic and mafic igneous, covered by turbidites, and other sedimentary rocks grading to conglomerates which represent intermediate to felsic flows and primarily felsic sandstones and shales. The oldest belts form one complete sequence, greenstone belts younger than 3 bybp usually have several repeats of the sequence.

Banded Iron Formations (BIFs) are the source of most of the world's iron. Archean BIFs are referred to as "Algoma-type" BIF and were deposited during volcanic and tectonic quiet in shallow and deep marine environments. They are composed of alternating layers of iron oxides and jaspers (quartz with iron impurities). Both magnetite (Fe⁺³) and hematite (Fe⁺²) are present in BIFs.

Archean granitic rocks surround and intrude the greenstone belts. They were deposited as huge batholiths late in the greenstone belt deposition and now form the stable continental interiors (ex: pink granite in Illinois part of the granite-rhyolite complex). The granulite-gneiss rocks which surround the greenstones are calc-alkaline felsic (Ca⁺/Na⁺ granitic) rocks which have been deformed to the granulite facies. These types of rocks, particularly trondhjemite and tonalite, form in island arc and Andes-type environments. The amount of deformation (granulite facies) shows that temperatures in the rock deformed were high, while pressure could be from two to ten times surface normal. Inclusions in these bands may be amphibolite, mica schist, marble (especially in southern India where it outcrops 250 meters thick and 30 kilometers long), quartzites and BIFs including the Isua formation in West Greenland.

Combining the granitic metamorphic bands and the greenstone belts in a tectonic setting, various geologists have proposed that the greenstone belts represent back arc spreading environments, and the felsic bands the actual subduction arc. Later compression metamorphosed both structures and resulted in the arcuate or basin-like deformed features we see today.

The Archean had two high grade metamorphic events, one seen in Greenland, Labrador and the Limpopo belt in Africa from 3.8 to 3.6 bybp and the second from 3.1 to 2.8 bybp. The oldest metamorphism resulted in gneisses of the amphibolite facies, while later Archean gneisses were raised to higher temperatures and are of the granulite facies. The mountains raised at the end of the Archean were comparable to the modern Andes and Himalayas, but erosion soon produced an unconformity surface which marks the end of the Era at about 2.5 bybp.

Metamorphism resets atomic clocks and therefore much of our knowledge of Archean formations is limited by our inability to accurately date events. This massive metamorphic event may represent the first supercontinent accretion.

A diagram showing the temperature/pressure relationship of igneous melting and the metamorphic grade is a common feature of Igneous Petrology texts.

The Proterozoic: New processes, new rocks

At the beginning of the Proterozoic (about 2.5 bybp), the stable continental interiors were shedding sediments into the oceans. In contrast with the repetitious turbidites of the Archean, Proterozoic sediments reflect a variety of environments of deposition. Proterozoic sedimentary rocks look like Phanerozoic sedimentary rocks - but there are no fossils in the older sequences. The primary marine environments of Proterozoic deposition are platforms, basins and geosynclines. Platform deposits represent epicontinental seas, basins are weak spaces in the crust which collect sediment, and geosynclines are offshore marine environments. There are several kinds of geosyncline, basically the near shore environment on the continental shelf is the miogeosyncline and the offshore environment is the eugeosyncline.

Miogeosynclinal sedimentary rocks include shallow-water limestones, dolomites, shales, and clean well-sorted sandstones. All of these formed in tectonically stable settings with little igneous activity. Eugeosynclinal sedimentary rocks are turbidites and pelagic sediments formed on the continental rise and slope as well as in the abyss. They are usually shales and greywackes or other poorly-sorted sandstones. Sometimes bedded cherts are found in these deposits. Both forms of geosyncline form only on passive continental margins. From 2.5 to 1.5 bybp, swarms of dolerite (diabase) dykes intruded into Archean terrains. These swarms are interpreted to represent igneous and tectonic igneous activity which may not have existed previously - the rifting of Archean continental masses or the first known hotspots. That the magmas were derived from mantle sources can be seen in their composition; they are tholeiitic, like Hawaiian basalts.

Whether these dyke swarms are failed rifting arms (aulacogens) or merely outbursts of the still superheated, young mantle is not yet known. What is known is that the mega-continent formed at the end of the Archean did split (although along other lines) and that sediments were shed from the continents into the oceans and into sedimentary basins such as those in the Witwatersrand region of South Africa.

The Svecokarelian Orogeny built mountains in southern Finland and Sweden from 2.2 to 1.7 bybp. Ophiolites and metamorphic deposits show that a major suture was created and that calc-alkaline plutons rose in the are from 1.85 to 1.75 bybp. In North America the contemporary Hudsonian Orogeny includes the Circum-(Lake) Superior belt and the Wopmay Orogen in northwestern Canada. Halite and gypsum were laid down in drying oceans during this period. At the end of this time, Canada, South Greenland, northwest Scotland and the Baltic shield were combined into a landmass which lasted until 1.2 to 1.0 bybp.

Some new rock associations formed at this time show that fractionation and recycling of older crust continued to develop. Anorthositic plutons are composed of more than 90 percent plagioclase (Ca⁺ feldspar) and have associated gabbros, norites and troctolites. This shows reworking of mafic sea floors combined perhaps with limey sediments. To the outside of these plutons are found monzonites and granites. Rapakivi granites have big crystals of K⁺ feldspar surrounded by a rim of oligoclase (Na⁺/Ca⁺ feldspar) in a matrix of quartz and mafic minerals. [625 N. Michigan, Chicago sports and "orbicular granite" on its face and lobby.] The plutons rose along fault planes and formed mushroom-shaped magma chambers. Sedimentary red beds, red clastics and amygdaloidal lavas (e.g. Keewenaw Group) show that the erosion of older uplands proceeded, and rifting - or attempted rifting - continued.

If rifting of the early continents produced an Atlantic-style ocean, Wilson's Cycle suggests that the continents being separated are pushed away by newly formed sea floor. When an Atlantic-style ocean has spread as far as it can, subsidence occurs along the edge of the seafloor plate, subduction begins on one or both margins and the subsequent compression results in folds, faults and subduction zones. These active margins leave characteristic deposits called mobile belts. The subduction of former seafloor sedimentary rocks produces Ca⁺ and Na⁺ rich granites and intermediate to felsic lava. During subduction, if a spreading center arrives at the continental margin part may be smeared off on the continent to form an "ophiolite suite" deposit. The suite maintains stratigraphic order with serpentinized peridotite, gabbro and basalt pillow lavas overlain by cherts and turbidites deposited on them while in situ.

The Proterozoic deposits also include the first record of glacial deposits. These ancient tillites are found unconformably over scratched and striated rock and look just like end-Paleozoic or modern glacial tills. In some works, these deposits are called boulder-conglomerates or boulder-clays. There appear to have been two major glacial episodes during the Proterozoic: one from 2.2 to 2.3 bybp and the second late in the Era after continental breakup created the margin of the western U.S. about 700 mya.

Most of the world's BIF deposits date from 2.6 to 1.8 bybp. Proterozoic BIFs are called "Superior-type" iron formations after the type locality around Lake Superior. The iron and silica in these BIFs was carried in solution and precipitated directly at the site of deposition. The process does not occur widely in modern oceans because iron released by weathering is oxidized when it contacts oxygenated surface waters or the atmosphere. Neither does silica precipitate from ocean water today, except at hot springs and near mid-ocean ridges, because silica-secreting plankton like diatoms and radiolarians draw down much oceanic silica and keep the ocean undersaturated in silica.

Precambrian oceanic and atmospheric chemistry can be inferred from the rock record. The oceans were water then as now, but were more acidic (pH <7) than today (ph ^~8). The early ocean contained more iron than it does today since vast quantities of iron were removed from the system and deposited in BIFs. In the middle and late Proterozoic, carbonate rocks (mostly dolomites) were deposited. This is inferred to represent a higher percentage of magnesium in the ancient oceans.

In contrast to the relatively stable ocean chemistry, the Archean and Proterozoic atmosphere was vastly different than the one we know today. In the Archean and early Proterozoic, there was no free oxygen (O₂) in the atmosphere. Free oxygen is too reactive to be maintained unless replenished, and until the rise of bacteria and algae there was no way to recharge the supply.

BIFs, therefore, show us that they were laid down before the earth had developed its oxygen atmosphere. Some geologists have proposed that the iron layers in Superior-type BIFs were formed by planktonic blue green algae which bonded iron to oxygen waste products of metabolic activity; the resulting iron oxides sank to the bottom of the Proterozoic oceans. The banding indicates, in this hypothesis, seasonality, upwellings or reduction/expansion in environment of the algae. The widespread outcrops of BIFs show that this process was occurring all over the world, not in some peculiar microhabitat.

Early fossils are known from the 3.2 bybp Fig Tree Series of South Africa, equivalent formations in Australia and in the Gunflint Chert near Lake Superior in North America. The most abundant Proterozoic fossils are actually structures produced by blue green algae, not the algae themselves. These structures are called stromatolites and are composed of carbonate laminae produced by the algae. The end of the Proterozoic contains the Ediacara fauna of southern Australia. Animals present in the Ediacara include jellyfish, sea pens and segmented worms as well as some forms that seem to have no living relatives. The Ediacara fauna first appears in the fossil record about 570 to 600 mya. At about the same time, the first shell-making invertebrates are found. Surprisingly, the earliest fossils are not simple, but are representatives of families known from the Paleozoic record including trilobites and brachiopods. The major question facing paleontologists about this "Cambrian explosion" of fossilized animals is whether these species suddenly arrived on the scene or whether their ancestors had been around for a long time but only began to be preserved after they developed hard shells. From an environmental biology point of view, there's no real reason to evolve a cumbersome shell unless there is something out there with teeth big enough to eat you if you are shell-less. The sea pens (ammonoids) may have been the evolutionary trigger for the production of shelly fauna. Many Proterozoic midcontinental sedimentary deposits were lost to erosion before the Cambrian, so we may never know the full extent of the fauna in the ancient oceans.

Paleozoic Tectonics: Trilobites to Pangaea

Because the Cambrian begins on all continents with an unconformity over the Precambrian, it appears that at the end-Proterozoic (600 mya), the continental masses formed a second megacontinent (Laurentide Orogeny). One of the few cross-boundary sections, the Burgess Shale of western Canada, was deposited in a shallow sea which spanned the Proterozoic-Paleozoic Eras. Following the apparent highstand of the continents relative to the sea, the continents slowly flooded in the Cambrian. The megacontinent began rifting, forming the first proto-Atlantic ocean for which we have hard evidence. This ocean is called the Iapetus and separates the North American Plate from Europe and Africa which were still joined. The Iapetus Ocean reached its maximum in the late Cambrian and began to close in the Middle Ordovician. Subduction was on the American margin. At about the same time, a piece of the North American plate was rifted and transform transported away from its craton. This piece was subsequently welded onto South America and the place where it was ripped out forms the Mississippi embayment. The Silurian period, while stable in the midcontinent reefs of the Niagaran escarpment, was marked by continued subduction of the Iapetus, raising the Taconic mountains and producing vast amounts of ash.

In the late Devonian, the Iapetus was closed and the Acadian foldbelt mountains rose from Iapetus geosynclinal deposits. The Appalachians were raised again in the final collision between eastern North America and the western European/African plates. Huge amounts of sediment were shed westward onto the midcontinental region. The fused North American and European plates closed downward from the pole to the equator, rotating around their combined Euler pole. Africa transformed westward during the Mississippian and Pennsylvanian, colliding with North America during the end of the Carboniferous. This collision further elevated the Appalachians in North America and created the Atlas Mountains in Africa. During the Carboniferous, the southern megacontinent, "Gondwanaland," composed of South America, Africa, India, Australia, Antarctica and parts of southeast Asia and other small bits had moved over the south pole.

Continental glaciation scoured off all Paleozoic sedimentary records from Gondwana continents and emplaced tillites, boulder-clays and boulder-conglomerates. As water was alternately taken up and discharged by the glaciers, sea-level fell and rose with respect to the continents. This cyclical oscillation in sea-level resulted in the deposition of cyclothem deposits on continental lowlands from Kansas, across "Laurasia" through to what is now Russia. The end-Paleozoic collision of Russia with Siberia uplifted the Urals from geosynclinal sediments. At 250 mya, when the formation of the megacontinent "Pangaea" was essentially complete, flood basalts flowed over vast areas of Siberia.

There were two oceans at the end of the Paleozoic, the Tethys (closing Atlantic-type) which was an equatorial ocean with only eastern outlets and the world ocean (Pacific type) named Panthalassa. The continents were ringed by subduction zones. Continuing compressive force uplifted the continental masses relative to sea level and exposed the continental shelves to erosion.

The Assembly of North America

Geologists are fond of regional maps composed by stripping away the surface soils and sediments and showing the lateral extent of underlying formations. Maps are available with the various provinces which make the current North American Plate outlined and dated. The "Canadian Shield" is similar to the Superior Archean Greenstone/granite/gneiss province. South of that is the 1.3 to 1.0 Ga crust known as the Grenville Province. Below that is the midcontinent region composed of the following:

• "Lower Huronian" now Huronian Supergroup showing diabase intrusion, sedimentation and volcanism associated with rifting, about 2.2 to 2.0 Ga
• "Upper Huronian" now Marquette Range Supergroup and the Animikie Group recording the Penokean Orogeny. Tonalitic and granitic plutons indicate collision and subduction volcanism, also sediments dating from 2.0 to 1.8 Ga
• Unconformity from 1.8 to 1.2 Ga; elsewhere this time is recorded as a period of cooling. This time contains the Penokean Province and the Eastern Granite-Rhyolite Province.
• "Keweenawan" now the Keweenawan Supergroup which records mafic volcanics in a rifting environment and dates to about 1.2 to 1.0 Ga . Huge volumes of red sediments and conglomerates were deposited and native copper/silver deposits emplaced.
• Unconformity from 1.0 to 0.6 Ga (600 mya)
• "Cambrian" same as now, the first period in the Paleozoic.

This then was Laurentia, the continent assembled from collided, rifted and sutured microplates. In the early Paleozoic, it rifted along the "pC Cratonic Margin." Its passive margin was deformed and covered by Appalachian/Ouachita thrust sheets during the assembly of Pangaea at the end of the Paleozoic. In the Mesozoic the same edge was rifted and developed the new passive margin we see today.

Cambrian rifting created zones of weakness in the crust. Passive margin sediments deposited along the continental edges, epicontinental sediments deposited in down-faulted areas like the Mississippi Valley graben. Note the relationships between fault trends and areas of Cambrian tectonic activity. Crust deforms both perpendicular to and parallel to planes of rifting and transform faulting. Note also that the Blue Ridge margin of preCambrian basement rock may be the anchor which stops later deformations of passive margin sediments into the Appalachian Mountains.

As North America (Laurentia) and Northern Europe slowly closed the Iapetus Ocean between them from the Ordovician through the Devonian, two major orogenic events occurred. The first, the Taconic orogeny dates to the early Ordovician. Massive volcanism of a type associated with subduction indicates that the Iapetus oceanic plate descended below the Laurentian margin. Also an island arc or continental fragment called "Avalonia" was welded onto the coast. Sediments shed into the closing ocean were deformed during the Silurian Caledonian/Acadian orogeny and the continental landmasses joined are renamed "Laurasia."

In the southern hemisphere, South America, Africa, India, Australia and Antarctica are joined and drift over the south pole. This results in glaciation in the southern hemisphere and rapidly changing sea levels in Laurasian epicontinental seas. The Klamath Mountains rose in the Antler orogeny along the western margin of Laurasia. The Urals mark the suture where eastern Russia collided with Laurasia during the Carboniferous. The continued closure of the gap between Laurasia and Gondwana resulted in the Hercynian orogeny. The joined Laurasian landmass slowly drifted into the Gondwanan land mass, first colliding with Africa and building the Appalachians and the Atlas mountains. A second collision betweeen North and South Americas built the Ouachitas. All the continents were joined in a mega continent named by Wegener "Pangaea." For over 10 million years Pangaea was uplifted relative to sea level, perhaps by heat from the mantle building up under the continental crust. Then it broke apart along rifts.

Figure 34 shows how Pangaea broke apart along midcontinental rifts and their associated aulacogens. This created the beginning of a passive margin along the Atlantic coast.

During the late Paleozoic, the Western U.S. became a more active tectonic environment, subducting plates of the proto-Pacific Sea. Subduction along the western margin resulted in the Antler Orogeny and the emplacement of the Klamath/North Sierran island arc. Continued subduction of oceanic plates and their associated basalt platforms or continental fragments resulted in a series of suspect terranes along the western margin of North America. As the Northwestern coastline has received new material, stress has been transmitted into the continental interior. These collisions would result in compressive force. Meanwhile, changes along the Southwestern coast have resulted in tensional forces being applied to the interior. The alternating smash and pull has created the Basin and Range province where parallel rows of thrust fault mountains are interspersed with basins containing their shed sediments.

One of the most succinct descriptions of western North American tectonics was written by Howell (1989): "In summary, the principal post-Paleozoic tectonic events along the southwest margin of North America involve the initiation of a subduction regime as a consequence of a global reorganization of plate motions responding to the breakup of Pangaea. In Middle Jurassic, left-slip transform faulting cut into the continent and moved crustal fragments toward the south into the gap being formed between North and South America. By the middle of the Cretaceous, plate motions in the Pacific possessed a component of north-directed slip relative to the North American plate and the then-young continental magmatic arcs and associated terranes were displaced to the north by varying amounts as far as British Columbia. By at least 15 mya, this coastwise dispersion was accompanied by crustal stretching that has extended 1500 km into the cratonal core. This in contrast to Alaska and British Columbia, where accretion tectonics has effected a net growth to the continent, along the southwest margin there has been truncation and crustal attenuation. Most of the Cretaceous and younger material that has accreted to this margin lies to the west of the San Andreas fault system, indicating how dispersion tectonics continues today as the principal modus operandi; furthermore, a new ocean basin could likely form somewhere in the area of the modern Basin and Range. The basalts that are erupting through fissures in the Rio Grande rift are similar in composition to midocean-ridge basalts, indicating that primitive melts emanating from the mantle have direct access to the surface. In the restricted area of the fissures, even now there seems to be no continental crust at all..."

Obviously, in the space of these few pages, it is not possible to adequately discuss all aspects of the assembly of North America. It is my intention that these notes, like those from the classes previous, will provide sufficient information including orogenies and currently accepted dates, as well as correlate pre-Plate Tectonic and post-Plate Tectonic theory acceptance terminology so that the student can read popular and college level publications on the subject. Much geology is unnecessarily confused by overlapping and extra terminology which resulted from many workers making many observations in the absence of a unifying paradigm which would associate events with their remaining deposits. For nearly 50 years, North American geology was hampered by a refusal to accept continental drift or plate tectonics. Even now, it is difficult for some American geologists to accept that the Earth is active - 24 hours a day and 365 days of the year. All areas of the Earth are active - even those considered to be "stable continental interiors." Four billion years of previous events have created folds and faults which underlie the rocks we consider "basement" and through which only a few holes have been drilled. All our understanding of the regions below are based on remote sensing. Expect change and accept it as the science advances. What we know today about Plate Tectonics (and other sciences) is only a fraction of what is available to know.

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