Paleogeology

Paleogeology, Paleoclimate, in relation to Evolution of Life on Earth

Geologic Features/Processes

Structure
Earth's structureAcasta gneissesasthenospherecore, solid core, liquid corecontinental crust, crust, upper lithospheremantle, inner mantle, outer mantle, inner lithosphere, mantle rocksoceanic crust, rocks of oceanic crust
geologic provinces basincraton large igneous provincesoceanic basinorogenic beltorogenic belts at continental marginsplatformprovincesshieldstructural basin
igneous structures
mantle plumes
oceanic structuresabyssal plaincomposition of oceanic crustcontinental crustcontinental risecontinental shelfcontinental slopecreation/consumption of oceanic crustmid-oceanic ridgesoceanic crustoldest oceanic crustpaleomagnetic recordseamountssubmarine trenchesvolcanic islands
volcanoes

Processes
contact metamorphism
dynamic metamorphism
hydrothermal metamorphism
impact metamorphism
mantle plumes
metamorphism
orogeny
regional metamorphism
subduction subduction zone magmas
tectonics
thermal metamorphism

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geologic provinces

large geological provinces around the globe: orange=shield, pink=platform, turquoise=orogen, blue=basin, purple=large igneous province, cream=extended crust A geologic province is an extensive region with distinctive geologic attributes that differentiate it from surrounding areas. A province may include a single dominant structural element (basin, fold belt) or a number of contiguous related elements. Adjoining provinces may be similar in structure but be considered separate due to differing histories.

Above, left: Map of world geologic provinces Oceanic crust 0-20 Ma 20-65 Ma >65 Ma Geologic province Shield Platform Orogen Basin Large igneous province Extended crust : Source : hi-res image :

oceanic rocks arise in the oceanic ridge system; color scale shows ages in MaThe continental crust is composed of lower density, more ancient felsic rocks of different composition than the mafic rocks of the oceanic crust, the oldest of which are about 180 million years old. Oceanic rocks arise in the oceanic ridge system (left - click to enlarge, hi-res).

A shield is defined as an extensive area of very ancient rocks that have been exposed and levelled by erosion (peneplanation). The exposed Precambrian rocks of a shield are crystalline igneous and high-grade metamorphic rocks within tectonically stable areas lacking active orogenic belts. On a shield the (usually) Precambrian basement rocks of the craton crop out extensively at the surface. A craton is an extensive portion of a continental plate that has remained relatively undisturbed since the Precambrian era, and includes both shield and platform layers.

A platform is that portion of a continent that is covered by horizontal or gently tilted rock and is underlain by very ancient rocks of the crystalline basement. The ancient bedrock was consolidated during deformations that preceded deposition of the sedimentary rocks of the overlying platform layer.

An orogenic belt is that part of the continent where deformation of the Earth’s crust created a mountain range, which may since have eroded down to the roots of the original mountains.

Because shield areas have been little affected by any tectonic events since the Precambrian, they are relatively flat regions in which mountain building, faulting, and other tectonic processes are greatly diminished compared to the activity occuring at shield margins and at the boundaries between tectonic plates. Because of their stability, shields have been flattened by erosion; however, shields commonly have a very gently convex surface and are surrounded by sediment covered platforms. Together, the shield, platform and basement comprise the craton.

Shields are very complex, comprising vast areas of granitic or granodioritic gneisses, usually of tonalitic composition. They contain belts of sedimentary rocks, often surrounded by low-grade volcano-sedimentary greenstone belt sequences containing metamorphosed greenschist, amphibolite, and granulite facies. The marginal areas of stable shields often display complex orogenic sequences occuring during the past few hundred million years. Shield margins generally exhibit tectonic or plate-like dynamic mechanisms.

Shields form the nucleus of most continents and typically are bordered by orogenic belts of folded Cambrian rocks. Continents have expanded through accretion of younger rocks that have undergone deformations through successions of mountain building episodes. Thus, belts of folded rocks were welded onto the borders of the preexisting shields, increasing the area of proto-continents. The margins of shield are subjected to geotectonic forces that remodel the cratons that they partially comprise.

A structural basin is a large-scale formation of rock strata deformed by tectonic warping of previously flat lying strata. Structural basins have affinities with geological depressions, but are distinct from sedimentary basins in which time-dependant aggregation of rocks has in-filled a depression or accumulated within an area. Downwarped strata form synclines, which are concave geological folds, with layers that dip downward toward the center of the structure.

Basins can develop through greater erosion of the fractured, upwarped rocks at the center of an anticline, such that strata that originally lay at the top of the dome are eroded most. Such structural basins are the inverse of domes, which are symmetrically-dipping anticlines. Because a structural basin's strata dip toward the center, the exposed strata in a basin have the youngest rocks in the center.

Oceanic basins commence at continental shorelines, but each continent extends under the adjacent ocean. The continental shelf (or continental platform) comprises the extension of a continent under an ocean. A continental shelf is characterized by a very gentle slope and generally reaches depths of less than 200 metres, at which point there is a steep slope down to the ocean floor.

Shields:
African, or Ethiopian Shield
Amazonian Shield of central South America
The Angaran Shield of West Siberia
Arabian-Nubian Shield
(Western) Australian Shield
Baltic Shield of Scandinavia and Eastern Europe
Canadian Shield AKA Laurentian Shield
The China-Korean Shield containing the North China Craton
The East Antarctic Shield containing the East Antarctic craton
Guiana Shield
Indian Shield

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contact metamorphism

Contact metamorphism typically manifests as heat-induced metamorphic aureoles in the country rock surrounding intrusive igneous rocks (plutons).

The size of the metamorphic aureole depends on the temperature differential between the intrusive magma and the country rock, and on the size of the intrusion. Small intrusions, such as dykes typically have small aureoles with minimal metamorphism, whereas large ultramafic intrusions often display thick aureoles with well-developed contact metamorphism. Contact metamorphic rocks (hornfels) are often fine-grained and show little or no evidence of strong deformation.

The metamorphic grade of an aureole is that of the peak metamorphic mineral which has formed within the aureole – andalusite hornfels, sillimanite hornfels, or pyroxene hornfels. These grades relate to the metamorphic temperatures of pelitic or alumonisilicate rocks and the minerals that they form.

Magmatic fluids derived from the intrusive rock can impact metamorphic reactions, and extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks (metasomatism). Intrusions rich in carbonates produce skarns, fluorine-rich magmatic solutions often form greisens within granites. Metasomatic altered aureoles may include metallic ore minerals and are of economic interest.

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dynamic metamorphism

Dynamic metamorphism results from frictional heat and confining pressures associated with major fault planes.

The metamorphic changes are localised adjacent to the fault plane, and cataclasis is the crushing and grinding of rocks into angular fragments (cataclastic texture) in dynamic metamorphic zones.

Textures of dynamic metamorphic zones depend on the depth of formation where confining pressures determine the predominant deformation mechanisms. At depths of less than 5km, confining pressures are to low to produce dynamic metamorphism, and a zone of breccia or cataclasite is formed instead, with the rock milled and broken into a mélange of random fragments. At greater depths, angular breccias transform into ductile shear textures and mylonite zones.

Pseudotachylites form at depths from 5-10 km, where confining pressures are focused into discrete fault planes and are sufficient to prevent brecciation and milling. The frictional heating at these depths can melt the rock to form pseudotachylite glass or mylonite, and adjacent to these zones, can result in growth of new mineral assemblages.

Within the depth range of 10-20km, ductile deformation conditions prevail and frictional heating is dispersed throughout shear zones, resulting in distributed deformation and a weaker thermal imprint. Here, deformation forms mylonite, with dynamothermal metamorphism observed rarely as the growth of porphyroblasts in mylonite zones.

Overthrusting forces may juxtapose hot lower crustal rocks against cooler mid and upper crust blocks, resulting in conductive heat transfer and localised contact metamorphism of the cooler blocks adjacent to the hotter blocks, often producing metamorphism in the hotter blocks. These metamorphic assemblages are diagnostic of the depth and temperature and the throw of the fault and can also be dated to determine an age of the thrusting.

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Earth's structure

layers within Earth – higher temperature indicated by lighter colorThe Earth has several layers each with its own physical properties (left – higher temperature indicated by lighter color)

1. solid core – high pressures maintain the hot, mostly iron-nickel center in the solid state

2. liquid core – cooler than the solid inner core, but liquid because of lower pressures

3. inner mantle – 1000°- 4,000°C, the asthenosphere, or low velocity zone, is a plastic zone extending from the lithosphere to a depth of 250 km, perhaps as much as 400 km.

4. outer mantle – 500°C-900°C at boundary with crust, the outer mantle is part of the lithosphere that is relatively cool, chemically different than most of the mantle (mafic) and considered more resistant to deformation than the crust, extending from the Mohorovičić discontinuity (moho) to a depth of about 100-250 km

5. crust – the upper lithosphere is relatively light and brittle, composed of less dense felsic rocks, such as granites that have differentiated from melted mantle; typically about 25 miles thick beneath continents, and about 6.5 miles thick beneath oceans. The average thickness of the continental crust is about 35 to 40 km (image below right - click to enlarge), whereas that of oceanic crust is 7-10 km.

average thickness of the continental crustOceanic crust is younger than tectonic plate boundaries at continental margins.

The oldest oceanic crust dates from about 180 Ma (Jurassic) and lies adjacent to continents, while the youngest crust lies adjacent to the mid-oceanic ridge centers. The farther that oceanic crust lies outward from the mid-oceanic ridges, the older the rocks. The paleomagnetic signature of oceanic crust records geomagnetic reversals parallel to the ridge structures.

Because the continental crust is thrust up over oceanic crust at subduction zones, oceanic crust is consumed while continental crust survives. As a result, the Earth's oldest rocks are to be found within the cratonic cores of continents, and the oldest known continental rocks are Canada's Acasta Gneisses in the Slave Craton (Hadean tonalite gneiss, 4.03 Ga, image). A 4.2 Ga zircon xenocryst has been reported within a 3.9 Ga granitic rock of the Acasta Gneiss Complex [r, r2, im2].

Petrology:
The brittle, cool rocks of the crust belong to either the felsic continental crust or the mafic oceanic crust.

Rocks of the oceanic crust are mafic basaltic rocks (sima) with a mean density of about 3.3 grams per cubic centimeter (more dense than felsic continental rocks). With an average thickness of 10 km, the oceanic crust is thinner than the continental crust [image above right, crustal thickness].

The felsic continental crust average to approximately the composition of granodiorite. By virtue of its relative low density, continental crust is rarely subducted or re-cycled back into the mantle, although the collision of continental tectonic plates does cause the crust to thicken, causing melting of the deepest crustal rocks.

Mantle rock that lies shallower than about 400 km (4) comprises mostly olivine, pyroxenes, spinel, and garnets. Typical rock types are believed to be peridotite, dunite (olivine-rich peridotite), and eclogites. As predicted by laboratory investigations replicating high mantle pressures (diamond anvil), olivine is not stable between about 400 km and 650 km depth (upper 3), and is replaced by high pressure polymorphs with approximately the same composition. Olivine polymorphs include wadsleyite (beta-spinel type) and ringwoodite (gamma-spinel structure). Deeper than about 650 km (3), upper mantle minerals become increasingly unstable, so that the most abundant minerals have orthorhombic (pseudocubic) crystal structures (but not compositions) like that of the mineral, perovskite (CaTiO3). At high pressure conditions in the mantle, the pyroxene enstatite (MgSiO3) is a perovskite polymorph that may be the most common mineral in the Earth. The changes in mineralogy at about 400 and 650 km yield distinctive signatures in seismic records of the Earth's interior, and like the Moho are readily detected using seismic waves.

subduction zone magmas

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hydrothermal metamorphism

Hydrothermal metamorphism results from the interaction of a rock with high-temperature fluids, producting metamorphic and metasomatic reactions that depend upon temperature and compositional differences between the country rock and the invading fluid.

The hydrothermal fluid may be:
● magmatic (originating in an intruding magma)
● circulating groundwater
● ocean water

Convective circulation of saline in ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and in other submarine volcanic areas. Patterns of hydrothermal alteration are used as guides in the search for deposits of valuable metallic ores.

[links: images: pyritized rocks (form a halo around the copper deposit) and result from percolation of chemically-reactive hot groundwaters through the strata]

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igneous structures

pahoehoe basaltic lava, HawaiiIgneous rocks comprise 95% of the crust and resulted from the cooling ± crystallization of rock that melted when Earth's radioactivity heated rocks buried deep within the Earth. Intrusive, plutonic rocks comprise the majority of igneous rocks, while extrusive, volcanic rocks are more accessible rocks that initially cooled at the surface. Volcanoes and earthquakes occur at tectonic subduction zones around the "Ring of Fire" or sit above mantle plumes.

Magma is molten rock formed and cooled at depth. Under the pressure of overburden, magma can rise toward the surface through cracks in the overlying rock strata. However, magma may never reach the surface, and if magma cools and solidifies in rock cracks within these strata, then it forms dikes, sills, diapirs, laccoliths, or huge solidified magma chambers called batholiths, any of which may later become exposed at the surface by erosion. Classification as a dike, sill, igneous diapir, laccolith (2, 3, 4, 5, laccolith (l) and set of sills (r), Mt Hillers), bysmalith, or batholith (1, 2, 3, 4, 5, 6, 7) depends upon orientation of the igneous intrusion within the country rock (cold, intruded rock) and size of the intrusion (diagram, diagram 2).

Dikes are tabular or sheet-like bodies of magma that cut vertically or almost vertically through and across strata (discordant plutonic bodies). Hundreds of dikes can invade the cone and inner core of a volcano. If the dike cooled very slowly at great depth, the large crystals of pegmatite dikes have time to form. Sills are tabular slabs or concordant intrusive sheets of igneous rock that have intruded horizontally or nearly horizontally. Laccoliths are moderately large concordant intrusions that cause uplift and folding of the preexisting rocks above the intrusion. Bysmoliths are more or less vertical and cylindrical bodies that crosscut (discordant) adjacent sediments. Batholiths are complex intrusive bodies, that are typically so large that their bases are rarely exposed. The exposed surface area is more than 100 square kilometers. Sometimes batholiths comprise several smaller intrusions. Batholith are usually of granitic composition with minor intermediate varieties. Stocks are smaller structures (less than 100 sq.m) that have probably been fed by deeper level batholiths. Stocks may have been feeders for volcanic eruptions, but because considerable erosion is necessary to expose a stock or batholith, the associated volcanic rocks rarely remain. Xenolithic phenomena produce fragments of metamorphically altered country rock (xenolith) that fell into the melt and became enveloped by the intruding magmatic rock. Xenoliths can be quite large and xenoliths can occur in clusters within the plutonic rock or demonstrate dikelet intrusions by the magma. (excellent, though large bandwidth, illustrations of a xenolith in the Halifax Pluton.)

When melted rock flows at the surface it is called lavaHawaii's and Iceland's basalt lavas fountain or flow freely, while other lavas are sticky and explosive, producing deadly pyroclastic flows (Mount St. Helens, Vesuvius, Pinatubo). See volcanism or volcanoes for more detail.

Large igneous provinces (LIPs) are igneous extrusive structures created by flood basalt vulcanism that emplaced at least 1 million cubic kilometers of lava over periods of a million years. In the distant past, these flows of basalt lava were much more extensive than current flows and not only formed extensive geological provinces, but have been implicated in extinction events.  Flood Basalts and Stratigraphic Boundaries

Geologists debate the postulated origin of LIPs as resulting from newly formed mantle plumes or from continental rifts, though it is possible that both mechanisms have operated. Well known flood basalts include the Columbia River Basalts formed a mere 16 million years ago and are now exposed in the Snake River Gorge in Idaho, the Siberian Flood Basalts that are believed responsible for the Earth's greatest extinction event 249 ± 1 years ago at the Permian-Triassic boundary, and the Deccan Flood Basalts in India, which formed 66 million years ago, give or take a million.)

Interestingly, the Deccan Flood Basalts were formed at roughly the time of the dinosaur extinction at the boundary between the Cretaceous and Tertiary, though this extinction is widely believed (by scientists not creationists!) to have resulted from the Chicxulub meteor impact.

subduction zone magmas

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impact metamorphism

Impact metamorphism involves ultrahigh compression that results either from the impact of an extraterrestrial object, such as a meteorite (bolide) or during an extremely violent volcanic eruption.

Impact metamorphism is characterized by ultrahigh pressure conditions and low temperature, and produces characteristic high-pressure textures and minerals, such as SiO2 polymorphs coesite and stishovite.

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mantle plumes

Mantle plumes are persistent upwellings of hot mantle material that are typically associated with mafic volcanism at deep plume hot spots such as Hawaii. animation . computer simulation of plumes .

Mantle plumes are associated with surface expressions such as large igneous provinces, oceanic islands, seamounts, continental flood basalts, and oceanic and continental linear volcanic chains called "hotspot tracks".

Morgan (1971) first proposed the mantle plume concept (more info) based on Wilson's (1963) ideas that stationary shallow mantle hotspots underlay island/seamount chains in the deep ocean, and motion of crustal plates over these hotspots produced hotspot tracks. Evidence for moving mantle plumes...Is nothing stationary? The Wilson-Morgan hotspot-plume theory comprised a key piece of evidence for the burgeoning theory of global plate tectonics.

"This Dynamic Earth: The Story of Plate Tectonics", U.S. Geological Survey hotspots and mantle thermal plumes (more info) : Bending thoughts about Hawaiian chain : The Hawaiian hotspot debate: an update (more info) : Beneath Yellowstone: Evaluating Plume and Nonplume Models Using Teleseismic Images of the Upper Mantle (more info) : Mantle plumes - both deep and shallow : Is Yellowstone volcanism caused by a deep-seated mantle plume? :

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metamorphism

Metamorphism involves a solid-state recrystallization of a pre-existing rock – igneous, sedimentary, or metamorphic rocks – under the agency of :
● high temperature
● high temperature and pressure (prograde)
● reduced temperature and pressure (retrograde)
● high pressure
● percolating chemical solutions ()

Metamorphic changes in rock are caused by:
regional or Barrovian metamorphism
contact metamorphism
hydrothermal metamorphism
impact metamorphism
dynamic metamorphism
thermal metamorphism

Metamorphic grade reflects systematic sequence of mineralogic and textural changes:

Grade: Low------------Medium----------High

For a shale parent, which is a rock fine-grained sedimentary rock whose original constituents were clays or muds, the sequence is:
-------slate ------phyllite ------ schist -------------gneiss -----------> melt (magma)

For a mafic parent rock such as basalt or gabbro, rock facies follow the series:
greenschist------- ---- amphibolite ------------- granulite facies

A metamorphic facies is defined as "a set of mineral assemblages repeatedly associated in time and space, such that there is a constant and therefore predictable relation between mineral composition and (bulk rock) chemical composition."*

Within metamorphic facies, minerals change so as to accommodate changes in temperature and pressure:
-----chlorites-------------
--------------epidote---------
------no Al-----------------amphiboles----------------Al-
---------------------------------------garnets-----------
-------------------------------------------------pyroxenes--

Another index of metamorphic intensity (beginning with a clay-rich parent rock) depends upon zones in which index minerals appear in sequence:

Zone: chloritesbiotitegarnetsstaurolitekyanitesillimanite index minerals

Facies: greenschist------------- amphibolite -------------- granulite
Grade: ----- Low-----------------Medium--- - ---------High
------slate ---phyllite -------- schist ------------------------gneiss

Zone: index minerals are bold and large:
----------chlorites------
-----------------------quartz/feldspars----------------------------------
--------------------------muscovite---------------
--------------------------biotite--------------------
-----------------------------------
garnets ------
-------------------------------------- --staurolite--
-------------------------------------------kyanite---
-----------------------------------------------------------sillimanite--

Metamorphic facies, grades, and zones are linked. The graded trajectory of mineralogic changes through which a rock passes when subjected to varying degrees of textural change in response to heat/pressure/metasomatism defines the metamorphic facies, and the volume of rock that contains altered index minerals is the metamorphic zone.

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mountains

Mountains rise abruptly above the surrounding region, while mountain ranges comprise closely spaced mountains and mountain belts comprise several mountain ranges that run roughly parallel to each other (Cordillera)

By contrast, orogenic belts may include mountain belts or the eroded remnants of mountain belts. Orogenic belts are typically long, arcuate bands of crystalline rocks in terranes or blocks of deformed rocks separated by dipping thrust faults.

Mountain belts typically comprise multiple layers of sedimentary and volcanic rocks. These accumulations, which can be several kilometers in thickness, were mostly originally deposited in a marine environment. The clastic components of the sedimentary rocks are derived from weathing and transport (erosion) of the continental crust of nearby terrestrial landmasses. These sediments are deposited and lithified to form shales, limestones, and sandstones at the continental shelves, slopes, and rise.

At convergent boundaries, rising magma forms volcanic island arcs and plutonic emplacements. Island arcs typically form in line with the interplate junction, perpendicular to the direction of crustal movement.

Mountains arise through several mechanisms:
  • volcanism at divergent plate boundaries, convergent plate boundaries, or above hotspots (mantle plumes)
  • folding and/or faulting as a result of tectonic forces

  • The general model proposes that mountain building involves three stages:

  • accumulation of sediments, followed by
  • an orogenic episode of compressional rock deformation and crustal uplift, followed by
  • further crustal uplift caused by isostatic rebound and block-faulting.
  • Tectonic convergence can be arc-continent, ocean-continent, or continent-continent:

  • Arc-continent convergence involves collision of an island arc with a continental margin, in which the oceanic plate area between the arc and the continent is subducted into the asthenosphere, accreting the island arc volcanic rocks and sediments onto the continental margin.
  • Ocean-continent convergence involves accretion of marine sedimentary deposits onto the continental margin.
  • Continent-continent convergence occurs when an ocean basin closes, bringing two continental plates into collision and thrusting huge mountain systems upward.
  • Tectonic convergence folds and elevates the sedimentary strata from the original ocean basin, and creates faulting when the compressional forces exceed the ability of rocks to deform. The high compressional forces typically cause reverse and overthrust faulting.


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    oceanic structures

    elevations of oceanic and continental crustThe Earth's crust is divided into continental crust and oceanic crust.

    abyssal plain : composition of oceanic crust : continental crust : continental rise : continental shelf : continental slope : creation/consumption of oceanic crust : mid-oceanic ridges : oceanic crust : oldest oceanic crust : paleomagnetic record : seamounts : submarine trenches : volcanic islands

    Structurally, moving outward from continental structures at sea level to those most distant from shore or at the greatest depths (image at left - elevations - click to enlarge image - mid-res, hi-res versions):

    continental shelf (continental crust) – the terrigenous-relict sediment-covered, flooded continental margin extends varying distances from shore (average 80 km), and slopes gradually from shore to the shelf break at a fairly uniform depth of 140 m. Sediments are coarser closer to shore and finer at greater distances from shore.

    continental slope (oceanic crust begins) – steeper than the sediment covered continental shelf, the continental slope extends from the shelf break to depths of 2-3 km, often cut by submarine canyons.

    continental rise – of shallower gradient than the continental slope, the rise extends up to 500 km from the slope and comprises thick sediments deposited by turbidity currents from the shelf and slope.[9]

    abyssal plain – gently sloping or flat regions that cover approximately 40% of the ocean floor, reaching depths between 2,200 and 5,500 m.

    submarine trenches – deep trenches formed by the subduction of oceanic crust beneath continental crust at convergent boundaries of tectonic plates.

    volcanic islandsbasaltic shield volcanoes like the chain of Hawaiian islands above mantle plumes, oceanic rift islands (Atlantic Iceland, Jan Mayen), or volcanic arc islands at subducting margins (Mariana Islands, Aleutians, Mauritius, Tonga in the Pacific Ocean; Lesser Antilles, South Sandwich Islands in Atlantic Ocean). ◙ subduction zone magmas

    seamounts – submarine mountains that typically are extinct volcanos.

    mid-oceanic ridgesspreading center mountain ranges with a total total length of about 60,000 km that ring the globe at the divergent boundaries of tectonic plates. Oceanic crust is replenished at the mid-ocean ridges and destroyed at continental margins.

    age of oceanic crust that has spread laterally from its line of formation in the mid-oceanic ridge spreading centers; color scale indicates agesOceanic crust (right - age of oceanic crust -click to enlarge image - mid-res, hi-res) is younger than continental crust and is replenished at spreading centers. Oceanic crust is consumed when it subducts at convergent tectonic plate boundaries at continental margins. The deepest ocean lies in the Challenger Deep of the Pacific's Mariana Trench at a depth of 10,911 m below sea level [image: CD: mid-res, hi-res; MT: mid-res].

    The oldest oceanic crust dates from about 180 Ma and lies adjacent to continents, while the youngest crust lies adjacent to the mid-oceanic ridge centers. As oceanic crust is followed outward from the mid-oceanic ridges, its age gradually increases (and its paleomagnetic signature records geomagnetic reversals).

    Rocks of the oceanic crust are mafic basaltic rocks (sima) with a mean density of about 3.3 grams per cubic centimeter (more dense than felsic continental rocks). With an average thickness of 10 km, the oceanic crust is thinner than the continental crust [image crustal thickness].

    subduction zone magmas

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    orogeny

    cross-section diagram depicting Taconic Orogeny of eastern North America from 543-440 MaOrogeny is the tectonic process by which mountain chains (orogenic belts, orogens) are/were built.

    (image at left - click to enlarge - Taconic Orogeny of eastern North America from 543-440 Ma).
    Whereas a mountain chain is a geographic structure, an orogen may have been completely destroyed by erosion, exhuming ancient mountain-root rocks that display the folding, faulting, and metamorphic traces of the orogeny.
    Orogenic belts are typically long, arcuate bands of crystalline rocks in terranes or blocks of deformed rocks separated by dipping thrust faults. Long parallel strips of rock exhibit similar characteristics along the length of the belt, but differ across the belt. The crystalline metamorphic rocks may lie below overthrust younger sediments that dip away from the orogenic core. Thin nappes (slices) of sediments from the sea bottom and near shore are thrust from the margins of compressing, alpinotype orogens toward the core, so are intimately associated with folds and metamorphism.

    Orogeny can be associated with continental collision and volcanic activity. Where orogeny results from continental-continental collision, very high mountain chains such as the Himalayas and Alps can be thrust upward. Orogenic belts are associated with subduction zones at continental-oceanic collision margins, at which oceanic crust is consumed, producing volcanoes and island arcs. Over the tens of millions of years of orogenic activity, island arcs and their associated ocean trenches may be accreted to the collisional continental margin.

    The topographic height of orogenic mountains is compensated (principle of isostasy) by a thickening of the underlying continental crust. Isostacy operates to balance the weight of upthrust mountains chains as they 'float' atop the denser mantle.

    Characteristic sub-types of orogeny:

    Alpinotype (ocean trench style);
    Relatively narrow orogens with rapid, high uplift and predominant nappe structures including abyssal sedimentary rocks (black shale, chert, etc.) sitting atop deep, high pressure metamorphic zones with many facies. Abundant ophiolite complexes with ultramafic rocks, and few granites or migmatites.

    Hercynotype (back-arc basin type):
    Very wide orogens with minimal, slow uplift and shallow, low-pressure/high temperature metamorphism in thin metamorphic zones with abundant granite and migmatite. Rare nappe structures, few ophiolites, and almost no ultramafic rocks.

    Cordilleran (arc) type;
    Low-pressure/low geothermal gradient metamorphism with moderate uplift, dominated by calc-alkaline igneous rocks, andesites, and granite batholiths. Structures display a lack of nappes and migmatites, and lack of ophiolite and abyssal sedimentary rocks.

    Variations:
    Collisional tectonics modified by a transform plate boundary (New Zealand). Island arc orogenies occuring at a distance from the continental backstop (New Guinea). Proterozoic continent-continent collisional orogens (Musgrave Block in Australia).

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    plate tectonics

    Earth's tectonic platesThe lithosphere is divided into tectonic plates that move, albeit very slowly, in relation to each other – converging, diverging, riding over/under one another, and sliding past one another.

    Tectonic plate movements are responsible for the Pacific "Ring of Fire" and for the enormous mid-ocean ridge systems, as well as for the rock cycle, magmatism, and continental motion from the late Archaean to the present.  Tectonic Plate Boundaries 


    Diagramatic cross section illustrating the main types of plate boundaries.  The East African Rift Zone is a good example of a continental rift zone. Courtesy of USGS. Cross section by José F. Vigil from This Dynamic Planet – a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.) Illustration of the Main Types of Plate Boundaries



    Phenomena associated with the four main types of plate boundaries include:


  • Convergence between arc-continent (ocean-ocean), ocean-continent, or continent-continent plate boundaries. Collision (convergence) generates the compressional forces associated with accretion, folding and shear zone faulting (thrust), orogenic mountain building, subduction, magmatic emplacement, regional metamorphism, seismic activity, and volcanism.

  • Divergence of plates generates spreading centers where plates are separating or rifting apart under tensional stresses. New crust is created by magma welling up from the mantle in continental rift zones (c-c) or at mid-ocean ridges (o-o).

  • Subduction zones and associated submarine trenches occur where denser oceanic plates sink below less dense continental plates in the special case of subduction at converging arc-continent or ocean-continent plate margins. In subduction zones, a cool slab of oceanic lithosphere sinks beneath an accretionary sedimentary prism, which, in turn, often lies beneath a forearc basin. At subduction zones, the oceanic lithosphere, layers of marine sediments, and trapped water are recycled into the deep mantle. Subduction zones give rise to a range of magmas in magmatic arches.

  • Transform (strike-slip) fault boundaries, or transform boundaries occur along the margins of plates that are sliding past one another, producing shallow earthquakes as accumulated stresses are released suddenly. Transform boundaries are large faults or fracture zones that connect two spreading centers (divergent plate boundaries) or, less often, trenches (convergent plate boundaries). Most transform faults occur on the oceans, though the San Andreas fault zone in California is an example on land.


    relationship between tectonic plates and the circum-Pacific The Pacific 'Ring of Fire' (image at right) is a concentration of volcanic island arcs, oceanic trenches, and Cordilleran stratovolcanoes (and associated earthquakes and tsunamis), which encircles the Pacific Basin.

    The Nazca and Cocos Plates are subducting beneath the South American plate. The Juan de Fuca and a section of the Pacific Plate are being subducting beneath the North American Plate (Aleutian Islands, Kamchatka—Kurile Islands arcs). The San Andreas Fault zone is a transform fault that offsets a portion of the East Pacific Rise under southwestern United States and Mexico.


    diagram of ocean cycle with new oceanic rocks formed where mantle convection cells rise beneath mid-oceanic ridges and consumed where oceanic plates subduct at ocean trenchesThe exact mechanism by which radioactive heating and resultant convection currents (image below left) is coupled to plate motion is not yet fully elucidated. Nevertheless, plate tectonic theory revolutionized understanding of important geological processes.

     Tectonic Plate Boundaries 



































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    regional metamorphism

    mineralogical metamorphic facies along trajectories of contact and regional metamorphic temperature-pressure conditions; c=contact, a=Abukama or Buchan, b=Barrovian, s=Sanbagawa, f=Franciscan; click on image to toggle to description of metamorphic facies Regional metamorphism involves large volumes of rock subjected to heat and intense compression during tectonic events such as subduction and orogeny.

    Four mineral assemblage series are recognized as resulting from temperature-pressure environments in regional metamorphism: Buchan, Barrovian, Sanbagawa, and Franciscan.

    (image at left – click image to toggle mineralogical metamorphic facies –
    (di = diagenetic alteration)
    (h = hydrothermal metamorphism)
    (c = contact metamorphism [VLPHT])
    regional metamorphism:
    a = Abukama or Buchan Facies Series [LPHT];
    b = Barrovian Facies Series [MPHT];
    S = Sanbagawa Facies Series [HPMT];
    F = Franciscan Facies Series [HPLT].

    The arrows trace conditions at : 'c' = contact aureoles around plutonic intrusions; a = island arcs, oceanic ridges, contact aureoles; b = continental orogenic belts; s = accretion zones; f = subduction zones.)

    In Paired Metamorphic Belts, circumpacific belts of oceanic-side belts of high pressure/low temperature metamorphism are found associated with continent-side belts of high pressure/high temperature metamorphism. Two examples of paired metamorphic belts are seen in the Sanbagawa Belt/Ryoke-Abukuma Belt of Japan and in the Franciscan complex/Klamath Mts.-Sierra Nevada roof pendants of California. Other paired metamorphic belts are seen in New Zealand, Indonesia, Washington State, Chile, Jamaica, the Alps, and the northern coast of South America.

    Most areas with paired metamorphic belts show evidence of convergent plate subduction associated with low geothermal gradients that generated the oceanic-side high pressure/low temperature metamorphism. Evidence of many such belts has probably been lost because the hydrous minerals of blueschist facies metamorphism are overprinted by facies resulting from normal geothermal gradients. Overprinting occurs when circulating fluids convert blueschist minerals to greenschist and amphibolite facies mineral assemblages.

    Continental-side high pressure/high temperature belts form beneath the island arc or continental margin volcanic arc. These regions are, during emplacement of the arc, subject to higher than normal geothermal gradients, producing Buchan and Barrovian Facies Series metamorphic Rocks. These belts of high T/high P metamorphism can be subequently uplifted and exposed at the surface by emplacement of batholiths and isostatic adjustment.

    The Japanese paired belts probably remained adjacent to one another because subduction moved farther off the coast after compressional tectonics accreted the island arc and trench complex to Japan at the end of the Mesozoic. The paired California belts are separated because the oceanic ridge was subducted, and then the margin of the Farallon plate converted from compression-subduction to a transform fault margin dominated by strike slip faulting. Isostatic rebound of the highly deformed Franciscan Complex has resulted in its exposure at the surface.

    Regional metamorphic changes are classified according to metamorphic facies, which are recognizable terranes or zones. Metamorphic facies display an equilibrium assemblage of key minerals that were in equilibrium under specific temperature/pressure conditions throughout the orogenic terrane.

    Metamorphic Facies:
    low T - low P : Zeolite Facies
    mod to high T - low P : Prehnite-Pumpellyite
    low T - high-P : Blueschist Facies
    mod to high T - mod P : Greenschist - Amphibolite - Granulite
    mod to high T - mod P : Eclogite

    ---Buchan or Abukama Facies Series develops at moderate pressures due to either regional heating by plutonic intrusion at shallow to moderate depths, plate collisions at convergent margins, or crustal thinning. The moderate pressures of Buchan metamorphism are lower than that of the aluminum silicate triple point, producing a critical sequence of aluminum silicates that differ from higher pressure Barrovian rocks in the Amphibolite Facies, where andalusite and cordierite appear:
    kaolinite → pyrophyllite → andalusitesillimanite

    ---Barrovian Facies Series develops at higher pressures than the Buchan series, and are found in Paleozoic and some Precambrian mountain belts. Barrovian pressures are higher than that of the aluminum silicate triple point, producing the mineral series:
    kaolinite → pyrophyllite → kyanitesillimanite

    ---Sanbagawa Facies Series are named for the Sanbagawa Belt, which is part of a complex that was accreted to Japan at higher temperatures than the Franciscan complex (Mesozoic), but lower temperatures than Buchan/Barrovian. The Facies sequence is:
    Zeolite → Prehnite-Pumpellyite → Blueschist → Greenschist → Amphibolite Facies

    ---Franciscan Facies Series develop along low geothermal gradients, and are exhibited in the mélange Cretaceous Franciscan Complex of California. [wp] Franciscan metamorphism results in a series of rocks that pass through the Facies sequence
    Zeolite → Prehnite-Pumpellyite → Blueschist → Eclogite Facies.

    subduction zone magmas

    more: Contact Metamorphism Dynamic Metamorphism Regional Metamorphism and Regional Metamorphism Metamorphic Rock Identification

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