Passive margin

A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-ocean ridge and the locus of extension moves away from the continent-ocean boundary. The transition between the continental and oceanic lithosphere that was originally created by rifting is known as a passive margin.

Rifting to Spreading Transition
Rifting-to-spreading transition
Passive Contiental Margin
Passive continental margin

Global distribution

Globald
Map showing the distribution of Earth's passive margins (yellow swaths).

Passive margins are found at every ocean and continent boundary that is not marked by a strike-slip fault or a subduction zone. Passive margins define the region around the Atlantic Ocean, Arctic Ocean, and western Indian Ocean, and define the entire coasts of Africa, Greenland, India and Australia. They are also found on the east coast of North America and South America, in western Europe and most of Antarctica. East Asia also contains some passive margins.

Key components

Active vs. passive margins

This refers to whether a crustal boundary between oceanic lithosphere and continental lithosphere is a plate boundary or not. Active margins are found on the edge of a continent where subduction occurs. These are often marked by uplift and volcanic mountain belts on the continental plate. Less often there is a strike-slip fault, as defines the southern coastline of W. Africa. Most of the eastern Indian Ocean and nearly all of the Pacific Ocean margin are examples of active margins. While a weld between oceanic and continental lithosphere is called a passive margin, it is not an inactive margin. Active subsidence, sedimentation, growth faulting, pore fluid formation and migration are all active processes on passive margins. Passive margins are only passive in that they are not active plate boundaries.

Morphology

Bathmetry
Bathymetric profile across a typical passive margin. Note that vertical scale is greatly exaggerated relative to the horizontal scale.

Passive margins consist of both onshore coastal plain and offshore continental shelf-slope-rise triads. Coastal plains are often dominated by fluvial processes, while the continental shelf is dominated by deltaic and longshore current processes. The great rivers (Amazon. Orinoco, Congo, Nile, Ganges, Yellow, Yangtze, and Mackenzie rivers) drain across passive margins. Extensive estuaries are common on mature passive margins. Although there are many kinds of passive margins, the morphologies of most passive margins are remarkably similar. Typically they consist of a continental shelf, continental slope, continental rise, and abyssal plain. The morphological expression of these features are largely defined by the underlying transitional crust and the sedimentation above it. Passive margins defined by a large fluvial sediment budget and those dominated by coral and other biogenous processes generally have a similar morphology. In addition, the shelf break seems to mark the maximum Neogene lowstand, defined by the glacial maxima. The outer continental shelf and slope may be cut by great submarine canyons, which mark the offshore continuation of rivers.

At high latitudes and during glaciations, the nearshore morphology of passive margins may reflect glacial processes, such as the fjords of Norway and Greenland.

Cross-section

PMfinal
Transitional crust composed of stretched and faulted continental crust. Note: vertical scale is greatly exaggerated relative to horizontal scale.
Volcanic passive margin
Cross-section through transitional crust of a passive margin. Transitional crust as a largely volcanic construct. Note: vertical scale is greatly exaggerated relative to horizontal scale.

The main features of passive margins lie underneath the external characters. Beneath passive margins the transition between the continental and oceanic crust is a broad transition known as transitional crust. The subsided continental crust is marked by normal faults that dip seaward. The faulted crust transitions into oceanic crust and may be deeply buried due to thermal subsidence and the mass of sediment that collects above it. The lithosphere beneath passive margins is known as transitional lithosphere. The lithosphere thins seaward as it transitions seaward to oceanic crust. Different kinds of transitional crust form, depending on how fast rifting occurs and how hot the underlying mantle was at the time of rifting. Volcanic passive margins represent one endmember transitional crust type, the other endmember (amagmatic) type is the rifted passive margin. Volcanic passive margins also are marked by numerous dykes and igneous intrusions within the subsided continental crust. There are typically a lot of dykes formed perpendicular to the seaward-dipping lava flows and sills. Igneous intrusions within the crust cause lava flows along the top of the subsided continental crust and form seaward-dipping reflectors.

Subsidence mechanisms

Passive margins are characterized by thick accumulations of sediments. Space for these sediments is called accommodation and is due to subsidence of especially the transitional crust. Subsidence is ultimately caused by gravitational equilibrium that is established between the crustal tracts, known as isostasy. Isostasy controls the uplift of the rift flank and the subsequent subsidence of the evolving passive margin and is mostly reflected by changes in heat flow. Heat flow at passive margins changes significantly over its lifespan, high at the beginning and decreasing with age. In the initial stage, the continental crust and lithosphere is stretched and thinned due to plate movement (plate tectonics) and associated igneous activity. The very thin lithosphere beneath the rift allows the upwelling mantle to melt by decompression. Lithospheric thinning also allows the asthenosphere to rise closer to the surface, heating the overlying lithosphere by conduction and advection of heat by intrusive dykes. Heating reduces the density of the lithosphere and elevates the lower crust and lithosphere. In addition, mantle plumes may heat the lithosphere and cause prodigious igneous activity. Once a mid-oceanic ridge forms and seafood spreading begins, the original site of rifting is separated into conjugate passive margins (for example, the eastern US and NW African margins were parts of the same rift in early Mesozoic time and are now conjugate margins) and migrates away from the zone of mantle upwelling and heating and cooling begins. The mantle lithosphere below the thinned and faulted continental oceanic transition cools, thickens, increases in density and thus begins to subside. The accumulation of sediments above the subsiding transitional crust and lithosphere further depresses the transitional crust.

Classification

There are four different perspectives needed to classify passive margins:

  1. map-view formation geometry (rifted, sheared, and transtensional),
  2. nature of transitional crust (volcanic and non-volcanic),
  3. whether the transitional crust represents a continuous change from normal continental to normal oceanic crust or this includes isolated rifts and stranded continental blocks (simple and complex), and
  4. sedimentation (carbonate-dominated, clastic-dominated, or sediment starved).

The first describes the relationship between rift orientation and plate motion, the second describes the nature of transitional crust, and the third describes post-rift sedimentation. All three perspectives need to be considered in describing a passive margin. In fact, passive margins are extremely long, and vary along their length in rift geometry, nature of transitional crust, and sediment supply; it is more appropriate to subdivide individual passive margins into segments on this basis and apply the threefold classification to each segment.

Geometry of passive margins

Rifted margin

This is the typical way that passive margins form, as separated continental tracts move perpendicular to the coastline. This is how the Central Atlantic opened, beginning in Jurassic time. Faulting tends to be listric: normal faults that flatten with depth.

Sheared margin

Sheared margins form where continental breakup was associated with strike-slip faulting. A good example of this type of margin is found on the south-facing coast of west Africa. Sheared margins are highly complex and tend to be rather narrow. They also differ from rifted passive margins in structural style and thermal evolution during continental breakup. As the seafloor spreading axis moves along the margin, thermal uplift produces a ridge. This ridge traps sediments, thus allowing for thick sequences to accumulate. These types of passive margins are less volcanic.

Transtensional margin

This type of passive margin develops where rifting is oblique to the coastline, as is now occurring in the Gulf of California.

Nature of transitional crust

Transitional crust, separating true oceanic and continental crusts, is the foundation of any passive margin. This forms during the rifting stage and consists of two endmembers: Volcanic and Non-Volcanic. This classification scheme only applies to rifted and transtensional margin; transitional crust of sheared margins is very poorly known.

Non-volcanic rifted margin

Non-volcanic margins are formed when extension is accompanied by little mantle melting and volcanism. Non-volcanic transitional crust consists of stretched and thinned continental crust. Non-volcanic margins are typically characterized by continentward-dipping seismic reflectors (rotated crustal blocks and associated sediments) and low P-wave velocities (<7.0 km/s) in the lower part of the transitional crust.

Volcanic rifted margin

Volcanic margins form part of large igneous provinces, which are characterised by massive emplacements of mafic extrusives and intrusive rocks over very short time periods. Volcanic margins form when rifting is accompanied by significant mantle melting, with volcanism occurring before and/or during continental breakup. The transitional crust of volcanic margins is composed of basaltic igneous rocks, including lava flows, sills, dykes, and gabbro.

Volcanic margins are usually distinguished from non-volcanic (or magma-poor) margins (e.g. the Iberian margin, Newfoundland margin) which do not contain large amounts of extrusive and/or intrusive rocks and may exhibit crustal features such as unroofed, serpentinized mantle . Volcanic margins are known to differ from magma-poor margins in a number of ways:

  • a transitional crust composed of basaltic igneous rocks, including lava flows, sills, dykes, and gabbros.
  • a huge volume of basalt flows, typically expressed as seaward-dipping reflector sequences (SDRS) rotated during the early stages of crustal accretion (breakup stage),
  • The presence of numerous sill/dyke and vent complexes intruding into the adjacent basin,
  • the lack of significant passive-margin subsidence during and after breakup, and
  • the presence of a lower crust with anomalously high seismic P-wave velocities (Vp=7.1-7.8 km/s) – referred to as lower crustal bodies (LCBs) in the geologic literature.

The high velocities (Vp > 7 km) and large thicknesses of the LCBs are evidence that supports the case for plume-fed accretion (mafic thickening) underplating the crust during continental breakup. LCBs are located along the continent-ocean transition but can sometimes extend beneath the continental part of the rifted margin (as observed in the mid-Norwegian margin for example). In the continental domain, there are still open discussion on their real nature, chronology, geodynamic and petroleum implications.[1]

Examples of volcanic margins:

  • The Yemen margin
  • The East Australian margin
  • The West Indian margin
  • The Hatton-Rockal margin
  • The U.S East Coast
  • The mid-Norwegian margin
  • The Brazilian margins
  • The Namibian margin
  • The East Greenland margin
  • The West Greenland margin

Examples of non-volcanic margins:

  • The Newfoundland Margin
  • The Iberian Margin
  • The Margins of the Labrador Sea (Labrador and Southwest Greenland)

Heterogeneity of transitional crust

Simple transitional crust

Passive margins of this type show a simple progression through the transitional crust, from normal continental to normal oceanic crusts. The passive margin offshore Texas is a good example.

Complex transitional crust

This type of transitional crust is characterized by abandoned rifts and continental blocks, such as the Blake Plateau, Grand Banks, or Bahama Islands offshore eastern Florida.

Sedimentation

A fourth way to classify passive margins is according to the nature of sedimentation of the mature passive margin. Sedimentation continues throughout the life of a passive margin. Sedimentation changes rapidly and progressively during the initial stages of passive margin formation because rifting begins on land, becoming marine as the rift opens and a true passive margin is established. Consequently, the sedimentation history of a passive margin begins with fluvial, lacustrine, or other subaerial deposits, evolving with time depending on how the rifting occurred and how, when, and by what type of sediment it varies.

Constructional

Constructional margins are the "classic" mode of passive margin sedimentation. Normal sedimentation results from the transport and deposition of sand, silt, and clay by rivers via deltas and redistribution of these sediments by longshore currents. The nature of sediments can change remarkably along a passive margin, due to interactions between carbonate sediment production, clastic input from rivers, and alongshore transport. Where clastic sediment inputs are small, biogenic sedimentation can dominate especially nearshore sedimentation. The Gulf of Mexico passive margin along the southern United States is an excellent example of this, with muddy and sandy coastal environments down current (west) from the Mississippi River Delta and beaches of carbonate sand to the east. The thick layers of sediment gradually thin with increasing distance offshore, depending on subsidence of the passive margin and the efficacy of offshore transport mechanisms such as turbidity currents and submarine channels.

Development of the shelf edge and its migration through time is critical to the development of a passive margin. The location of the shelf edge break reflects complex interaction between sedimentation, sealevel, and the presence of sediment dams. Coral reefs serve as bulwarks that allow sediment to accumulate between them and the shore, cutting off sediment supply to deeper water. Another type of sediment dam results from the presence of salt domes, as are common along the Texas and Louisiana passive margin.

Starved

Sediment-starved margins produce narrow continental shelves and passive margins. This is especially common in arid regions, where there is little transport of sediment by rivers or redistribution by longshore currents. The Red Sea is a good example of a sediment-starved passive margin.

Formation

Formation of passive margins

There are three main stages in the formation of passive margins:

  1. In the first stage a continental rift is established due to stretching and thinning of the crust and lithosphere by plate movement. This is the beginning of the continental crust subsidence. Drainage is generally away from the rift at this stage.
  2. The second stage leads to the formation of an oceanic basin, similar to the modern Red Sea. The subsiding continental crust undergoes normal faulting as transitional marine conditions are established. Areas with restricted sea water circulation coupled with arid climate create evaporite deposits. Crust and lithosphere stretching and thinning are still taking place in this stage. Volcanic passive margins also have igneous intrusions and dykes during this stage.
  3. The last stage in formation happens only when crustal stretching ceases and the transitional crust and lithosphere subsides as a result of cooling and thickening (thermal subsidence). Drainage starts flowing towards the passive margin causing sediment to accumulate over it.

Economic significance

Passive margins are important exploration targets for petroleum. Mann et al. (2001) classified 592 giant oil fields into six basin and tectonic-setting categories, and noted that continental passive margins account for 31% of giants. Continental rifts (which are likely to evolve into passive margins with time) contain another 30% of the world's giant oil fields. Basins associated with collision zones and subduction zones are where most of the remaining giant oil fields are found.

Passive margins are petroleum storehouses because these are associated with favorable conditions for accumulation and maturation of organic matter. Early continental rifting conditions led to the development of anoxic basins, large sediment and organic flux, and the preservation of organic matter that led to oil and gas deposits. Crude oil will form from these deposits. These are the localities in which petroleum resources are most profitable and productive. Productive fields are found in passive margins around the globe, including the Gulf of Mexico, western Scandinavia, and Western Australia.

Law of the Sea

International discussions about who controls the resources of passive margins are the focus of Law of the Sea negotiations. Continental shelves are important parts of national exclusive economic zones, important for seafloor mineral deposits (including oil and gas) and fisheries.

See also

References

  1. ^ Norwegian volcanic margin Archived June 22, 2012, at the Wayback Machine
  • Hillis, R. D.; R. D. Müller (2003). Evolution and Dynamics of the Australian Plate. Geological Society of America.
  • Morelock, Jack (2004). "Margin Structure". Geological Oceanography. Archived from the original on 2017-01-10. Retrieved 2007-12-02.
  • Curray, J. R. (1980). "The IPOD Programme on Passive Continental Margins". Philosophical Transactions of the Royal Society of London. A 294 (1409): 17–33. Bibcode:1980RSPTA.294...17C. doi:10.1098/rsta.1980.0008. JSTOR 36571.
  • "Diapir". Encyclopædia Britannica Online. Encyclopædia Britannica. 2007.
  • "Petroleum". Encyclopædia Britannica Online. Encyclopædia Britannica. 2007. | http://www.mantleplumes.org/VM_Norway.html
  • "UNIL: Subsidence Curves". Institute of Geology and Palaeontology of the University of Lausanne. Retrieved 2007-12-02.
  • "P. Mann, L. Gahagan, and M.B. Gordon, 2001. Tectonic setting of the world's giant oil fields, Part 1 A new classification scheme of the world's giant fields reveals the regional geology where explorationists may be most likely to find future giants". Archived from the original on 2008-02-09.
  • Bird, Dale (February 2001). "Shear Margins". The Leading Edge. 20 (2): 150–159. doi:10.1190/1.1438894.
  • Fraser, S.I.; Fraser, A. J.; Lentini, M. R.; Gawthorpe, R. L. (2007). "Return to rifts - the next wave: Fresh insights into the Petroleum geology of global rift basins". Petroleum Geoscience. 13 (2): 99–104. doi:10.1144/1354-079307-749.
  • Gernigon, L.; J.C Ringenbach; S. Planke; B. Le Gall (2004). "Deep structures and breakup along volcanic rifted margins: Insights from integrated studies along the outer Vøring Basin (Norway)". Marine and Petroleum Geology. 21–3 (3): 363–372. doi:10.1016/j.marpetgeo.2004.01.005. | http://www.mantleplumes.org/VM_Norway.html
  • Continental Margins Committee, Ocean Studies Board, National Research Council, eds. (1989). Margins: A Research Initiative for Interdisciplinary Studies of the Processes Attending Lithospheric Extension and Convergence (PDF). The National Academies Press. doi:10.17226/1500. ISBN 978-0-309-04188-1. Retrieved 2007-12-02.CS1 maint: uses editors parameter (link)
  • Geoffroy, Laurent (October 2005). "Volcanic Passive Margins" (PDF). C. R. Geoscience 337 (in French and English). Elsevier SAS. Retrieved 2007-12-02.
  • R. A. Scrutton, ed. (1982). Dynamics of Passive Margins. USA: American Geophysical Union.
  • Mjelde, R.; Raum, T.; Murai, Y.; Takanami, T. (2007). "Continent-ocean-transitions: Review, and a new tectono-magmatic model of the Vøring Plateau, NE Atlantic". Journal of Geodynamics. 43 (3): 374–392. Bibcode:2007JGeo...43..374M. doi:10.1016/j.jog.2006.09.013.
1933 Baffin Bay earthquake

The 1933 Baffin Bay earthquake struck Greenland and the Northwest Territories (now Nunavut), Canada with a moment magnitude of 7.7 at 18:21:35 Eastern Time Zone on November 20.

The main shock epicenter was located in Baffin Bay on the east coast of Baffin Island. Shaking was only felt at the small town of Upernavik, Greenland. The event is the largest recorded earthquake to strike the passive margin of North America and is the largest north of the Arctic Circle. No damage was reported because of its offshore location and the small population of the nearby onshore communities.

Angola Basin

The Angola Basin is located along the West African South Atlantic Margin which extends from Cameroon to Angola.

It is characterized as a passive margin that began spreading in the south and then continued upwards throughout the basin.

This basin formed during the initial breakup of the supercontinent Pangaea during the early Cretaceous, creating the Atlantic Ocean and causing the formation of the Angola, Cape, and Argentine basins.

It is often separated into two units: the Lower Congo Basin, which lies in the northern region and the Kwanza Basin which is in the southern part of the Angola margin.

The Angola Basin is famous for its "Aptian Salt Basins," a thick layer of evaporites that has influenced topography of the basin since its deposition and acts as an important petroleum reservoir.

Champlain Thrust

The Champlain Thrust is a 200-mile long fault extending from southern Quebec, down through western Vermont in the Champlain Valley, and into eastern New York in the Catskills/Hudson Valley. This east dipping thrust fault transports Cambrian-Ordovician passive margin shelf rocks westward by about 30–50 miles (48–80 km) and places them on top of Middle Ordovician rocks. The Middle Ordovician accretion of the one or more island arcs terranes drove the initial thrusting during the Taconic Orogeny, though reactivation of the fault may have occurred during the middle Devonian Acadian Orogeny. The Champlain Thrust marks the most westerly thrust of the Taconic Orogeny.

Chilhowee Group

The Chilhowee Group is a sedimentary body composed of early Cambrian siliciclastic sedimentary rocks which crop out along the eastern margin of the Blue Ridge province in Alabama, Maryland, Tennessee, North Carolina, Virginia, and West Virginia. They represent a rift to passive margin sequence, with mostly coarse, feldspathic sandstones and conglomerates in the lower member and shales and phyllite in the upper members.The Chilhowee Group contains four formations; the Loudoun Formation, Weverton Formation, Harpers Formation and Antietam Formation. Another name for the Harpers formations is the Hampton formation, and the Antietam Formation is also known as the Erwin Formation. The Hampton Formation has minor economic importance in the area near the James River Face Wilderness. As of 1982 there were three quarries operating near the James River Face Wilderness. Those quarries produced roofing shale, light weight aggregate, and various materials for brick making. The Antietam Formation also had a minor economic importance, particularly from 1945 up until 1966. There were three quarries producing crushed quartzite, which was used to produce concrete aggregates, road metal and railroad ballast (Brown 1982).

Clastic wedge

In geology, clastic wedge usually refers to a thick assemblage of sediments--often lens-shaped in profile--eroded and deposited landward of a mountain chain; they begin at the mountain front, thicken considerably landwards of it to a peak depth, and progressively thin with increasing distance inland. Perhaps the best examples of clastic wedges in the United States are the Catskill Delta in Appalachia and the sequence of Jurassic and Cretaceous sediments deposited in the Cordilleran foreland basin in the Rocky Mountain region.Not all clastic wedges are associated with mountains. They are also characteristic of passive continental margins such as the Gulf Coast; these are quiescent environments, where sediments have accumulated to great thickness over a long period of time. These passive margin continental shelf sediment sequences are termed miogeoclines.Clastic wedges are often separated into one of two distinct types: flysch, mostly dark shales that originate from moderate to deep marine water; and molasse, which is composed mainly of red sandstones, conglomerates and shales that were deposited in terrestrial or shallow marine environments.

Colorado Basin, Argentina

The Colorado Basin (Spanish: Cuenca del Colorado) is a sedimentary basin located in northeastern Argentina. The basin stretches across an area of approximately 180,000 square kilometres (69,000 sq mi), of which 37,000 square kilometres (14,000 sq mi) onshore in the southern Buenos Aires Province and the easternmost Río Negro Province extending offshore in the South Atlantic Ocean.

The basin comprises a sedimentary succession dating from the Permian (pre-rift stage) and Early Cretaceous (rift stage) to the Quaternary, representing the passive margin tectonic phase of the basin history. The Mesozoic rifting in the basin resulted from the break-up of Pangea and the formation of the South Atlantic. Long hiatuses exist in the succession.

The basin is of paleontological significance for hosting fossiliferous stratigraphic units dating to the Late Miocene. The Arroyo Chasicó Formation defines the Chasicoan South American land mammal age and contains a rich mammal and other vertebrate fauna. The contemporaneous Cerro Azul Formation has provided fossil rodents, armadillos and opossums. The Middle to Late Miocene Gran Bajo del Gualicho Formation contains vertebrate fossils of the cetacean Preaulophyseter gualichensis. The Río Negro Formation has provided fossils of the glyptodont Plohophorus figuratus. The Permian succession in the basin has provided flora microfossils.

Contrasting with the South Atlantic passive margin basins to the north (Santos Basin in southern Brazil) and south; Golfo San Jorge and Austral Basins, the Colorado Basin does not produce hydrocarbons. Exploration for petroleum started in the 1940 with the drilling of two onshore wells and several onshore and offshore wells have been drilled in the 1960s, 1970s and 1990s. The main source rocks are found in the Permian succession, with reservoir rocks the Colorado Formation. Seals are provided by the Early Paleocene Pedro Luro Formation.

Continent-ocean boundary

The continent-ocean boundary (COB) or continent-ocean transition is the boundary between continental crust and oceanic crust on a passive margin. The identification of continent-ocean boundaries is important in the definition of plate boundaries at the time of break-up when trying to reconstruct the geometry and position of ancient continents e.g. in the reconstruction of Pangaea.

Geology of East Timor

The geology of East Timor has been studied onshore and with offshore seismic studies. The region experienced rifting between the Permian and early Cretaceous. Shallow water sediments shifted to deep water sediments by the Triassic. The region was a subsiding passive margin from the Early Cretaceous through the Eocene, experiencing deep water carbonate and shale deposition.

During the mid-Eocene, the Australian Plate collided with a subduction zone, generating folds and thrusts plus emplacing sheets of ophiolite and continental rock on top of Mesozoic sedimentary rocks. Some eroded sediments were shifted into a neighboring foredeep and these tectonic conditions produced oil-forming conditions in Triassic shales. Slow subsidence and carbonate deposition were typical of the region from the Eocene through the Miocene, until a second phase of thrusting uplifted onshore structures up to one kilometer. Offshore thrust sheets were buried beneath Pliocene and Pleistocene sediments.

Geology of North America

The geology of North America is a subject of regional geology and covers the North American continent, third-largest in the world. Geologic units and processes are investigated on a large scale to reach a synthesized picture of the geological development of the continent.

The divisions of regional geology are drawn in different ways, but are usually outlined by a common geologic history, geographic vicinity or political boundaries. The regional geology of North America usually encompasses the geographic regions of Alaska, Canada, Greenland, the continental United States, Mexico, Central America, and the Caribbean. The parts of the North American Plate that are not occupied by North American countries are usually not discussed as part of the regional geology. The regions that are not geographically North American but reside on the North American Plate include parts of Siberia (see the Geology of Russia), and Iceland, and Bermuda. A discussion of North American geology can also include other continental plates including the Cocos and Juan de Fuca plates being subducted beneath western North America. A portion of the Pacific Plate underlies Baja California and part of California west of the San Andreas Fault.

Geology of the Death Valley area

The exposed geology of the Death Valley area presents a diverse and complex set of at least 23 formations of sedimentary units, two major gaps in the geologic record called unconformities, and at least one distinct set of related formations geologists call a group. The oldest rocks in the area that now includes Death Valley National Park are extensively metamorphosed by intense heat and pressure and are at least 1700 million years old. These rocks were intruded by a mass of granite 1400 Ma (million years ago) and later uplifted and exposed to nearly 500 million years of erosion.

Marine deposition occurred 1200 to 800 Ma, creating thick sequences of conglomerate, mudstone, and carbonate rock topped by stromatolites, and possibly glacial deposits from the hypothesized Snowball Earth event. Rifting thinned huge roughly linear parts of the supercontinent Rodinia enough to allow sea water to invade and divide its landmass into component continents separated by narrow straits. A passive margin developed on the edges of these new seas in the Death Valley region. Carbonate banks formed on this part of the two margins only to be subsided as the continental crust thinned until it broke, giving birth to a new ocean basin. An accretion wedge of clastic sediment then started to accumulate at the base of the submerged precipice, entombing the region's first known fossils of complex life. These sandy mudflats gave way about 550 Ma to a carbonate platform which lasted for the next 300 million years of Paleozoic time.

The passive margin switched to active margin in the early-to-mid Mesozoic when the Farallon Plate under the Pacific Ocean started to dive below the North American Plate, creating a subduction zone; volcanoes and uplifting mountains were created as a result. Erosion over many millions of years created a relatively featureless plain. Stretching of the crust under western North America started around 16 Ma and is thought to be caused by upwelling from the subducted spreading-zone of the Farallon Plate. This process continues into the present and is thought to be responsible for creating the Basin and Range province. By 2 to 3 million years ago this province had spread to the Death Valley area, ripping it apart and creating Death Valley, Panamint Valley and surrounding ranges. These valleys partially filled with sediment and, during colder periods during the current ice age, with lakes. Lake Manly was the largest of these lakes; it filled Death Valley during each glacial period from 240,000 years ago to 10,000 years ago. By 10,500 years ago these lakes were increasingly cut off from glacial melt from the Sierra Nevada, starving them of water and concentrating salts and minerals. The desert environment seen today developed after these lakes dried up.

Maracaibo Basin

The Maracaibo Basin, also known as Lake Maracaibo natural region, Lake Maracaibo depression or Lake Maracaibo Lowlands, is a foreland basin and one of the eight natural regions of Venezuela, found in the northwestern corner of Venezuela in South America. Covering over 36,657 square km, it is a hydrocarbon-rich region that has produced over 30 billion bbl of oil with an estimated 44 billion bbl yet to be recovered. The basin is characterized by a large shallow tidal estuary, Lake Maracaibo, located near its center. The Maracaibo basin has a complex tectonic history that dates back to the Jurassic period with multiple evolution stages. Despite its complexity, these major tectonic stages are well preserved within its stratigraphy. This makes The Maracaibo basin one of the most valuable basins for reconstructing South America's early tectonic history.

Miogeocline

A miogeocline is an area of sedimentation which occurs along the passive margin of a continent. The deposits occur as typically shallow water clastic sediments which thicken seaward to form a clastic wedge parallel to a tectonically quiescent coast. Modern examples include the continental shelf of the northern Gulf of Mexico and the Atlantic coast of North and South America.

The term was coined in 1966 by Dietz and Holden from the miogeosyncline concept of the outdated geosynclinal theory. Dietz and Holden modified the term to miogeocline as the sedimentary deposits described were not synclinal in form.Ancient miogeoclines such as the Neoproterozoic to Cambrian Cordilleran miogeocline of the southwestern U. S., the Paleozoic Appalachian miogeocline, the Precambrian Belt Supergroup of Montana and Idaho and the Huronian sediments of Canada which were involved in the Grenville Orogeny. The Devonian to Mississippian northern Cordilleran miogeocline of northern Yukon and Northwest Territories of Canada represents an area of current research in Arctic geology. The ancient miogeoclinal sediments become attached to or accreted onto the adjacent continent following later continental collisions or orogenies. Thus the sediments of the Appalachian miogeocline became part of the Appalachian Mountains during the Appalachian orogeny.

Non-volcanic passive margins

Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins (VPM). Transitional crust welds continental crust to oceanic crust along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rifts to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.

Otway Basin

The Otway Basin is a northwest trending sedimentary basin located along the southern coast of Australia. The basin covers an area of 150,000 square kilometers and spans from southeastern South Australia to southwestern Victoria, with 80% lying offshore in water depths ranging from 50-3,000 meters. Otway represents a passive margin rift basin and is one of a series of basins located along the Australian Southern Rift System. The basin dates from the late Jurassic to late Cretaceous periods and formed by multi-stage rifting during the breakup of Gondwana and the separation of the Antarctic and Australian plates. The basin contains a significant amount of natural gas and is a current source of commercial extraction.

Permian Basin (North America)

The Permian Basin is a large sedimentary basin in the southwestern part of the United States. The basin contains the Mid-Continent Oil Field province. This sedimentary basin is located in western Texas and southeastern New Mexico. It reaches from just south of Lubbock, past Midland and Odessa, south nearly to the Rio Grande River in southern West Central Texas, and extending westward into the southeastern part of New Mexico. It is so named because it has one of the world's thickest deposits of rocks from the Permian geologic period. The greater Permian Basin comprises several component basins; of these, the Midland Basin is the largest, Delaware Basin is the second largest, and Marfa Basin is the smallest. The Permian Basin covers more than 86,000 square miles (220,000 km2), and extends across an area approximately 250 miles (400 km) wide and 300 miles (480 km) long.The Permian Basin lends its name to a large oil and natural gas producing area, part of the Mid-Continent Oil Producing Area. Total production for that region up to the beginning of 1993 was over 14.9 billion barrels (2.37×109 m3). The cities of Midland, Texas, Odessa, Texas and San Angelo, Texas serve as the headquarters for oil production activities in the basin.

The Permian Basin is also a major source of potassium salts (potash), which are mined from bedded deposits of sylvite and langbeinite in the Salado Formation of Permian age. Sylvite was discovered in drill cores in 1925, and production began in 1931. The mines are located in Lea and Eddy counties, New Mexico, and are operated by the room and pillar method. Halite (rock salt) is produced as a byproduct of potash mining.

Sioux Quartzite

The Sioux Quartzite is a Proterozoic quartzite that is found in the region around the intersection of Minnesota, South Dakota, and Iowa, and correlates with other rock units throughout the upper midwestern and southwestern United States. It was formed by braided river deposits, and its correlative units are thought to possibly define a large sedimentary wedge that once covered the passive margin on the then-southern side of the North American craton. In human history, it provided the catlinite, or pipestone, that was used by the Plains Indians to carve ceremonial pipes. With the arrival of Europeans, it was heavily quarried for building stone, and was used in many prominent structures in Sioux Falls, South Dakota and shipped to construction sites around the Midwest. Sioux Quartzite has been and continues to be quarried in Jasper, Minnesota at the Jasper Stone Company and Quarry, which itself was posted to the National Register of Historic Places on January 5, 1978. Jasper, Minnesota contains many turn-of-the-century quartzite buildings, including the school, churches and several other public and private structures, mostly abandoned.

Tectonic evolution of Patagonia

Patagonia comprises the southernmost region of South America, portions of which lie either side of the Chile–Argentina border. It has traditionally been described as the region south of the Rio Colorado, although the physiographic border has more recently been moved southward to the Huincul fault. The region's geologic border to the north is composed of the Rio de la Plata craton and several accreted terranes comprising the La Pampa province. The underlying basement rocks of the Patagonian region can be subdivided into two large massifs: the North Patagonian Massif and the Deseado Massif. These massifs are surrounded by sedimentary basins formed in the Mesozoic that underwent subsequent deformation during the Andean orogeny. Patagonia is known for their vast earthquakes and the damage.

The rocks comprising Patagonia occurred along the southwestern margin of the ancient supercontinent of Gondwana. During a period of continental rifting in the Cambrian period, a portion of Patagonia was separated from Gondwana, and the resulting passive margin that formed was a site of extensive sedimentation throughout the early-middle Paleozoic era. During the Devonian period, a transition to convergence resulted in the eventual collision of the Patagonian landmass in the late Paleozoic, with contact first occurring in the mid-Carboniferous. Several theories exist for the origin of the Patagonian landmass, though there are two that have greater consensus. The first of these theories cites an allochthonous origin of the Patagonian landmass from Gondwana during the Paleozoic, while the other argues that Northern Patagonia is an autochthonous component and that only the southern portion is allochthonous. The collision of Patagonia was succeeded by the rifting and eventual breakup of Gondwana during the early Mesozoic, a process which invoked large-scale rotation of the Patagonian landmass. Further extension through the Jurassic and Cretaceous periods formed the Rocas Verdes back-arc basin, while a transition to a compressional tectonic regime in the Cenozoic concurrent with the Andean orogeny resulted in formation of the foreland Magallanes basin.

Tectonics

Tectonics (from Latin tectonicus; from Ancient Greek τεκτονικός (tektonikos), meaning 'pertaining to building') is the process that controls the structure and properties of the Earth's crust and its evolution through time. In particular, it describes the processes of mountain building, the growth and behavior of the strong, old cores of continents known as cratons, and the ways in which the relatively rigid plates that constitute the Earth's outer shell interact with each other. Tectonics also provides a framework for understanding the earthquake and volcanic belts that directly affect much of the global population. Tectonic studies are important as guides for economic geologists searching for fossil fuels and ore deposits of metallic and nonmetallic resources. An understanding of tectonic principles is essential to geomorphologists to explain erosion patterns and other Earth surface features.

Volcanic passive margin

Volcanic passive margins (VPM) and non-volcanic passive margins are the two forms of transitional crust that lie beneath passive continental margins that occur on Earth as the result of the formation of ocean basins via continental rifting. Initiation of igneous processes associated with volcanic passive margins occurs before and/or during the rifting process depending on the cause of rifting. There are two accepted models for VPM formation: hotspots/mantle plumes and slab pull. Both result in large, quick lava flows over a relatively short period of geologic time (i.e. a couple of million years). VPM's progress further as cooling and subsidence begins as the margins give way to formation of normal oceanic crust from the widening rifts.

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