Plate Margins Where Stuff Happens Divergent Convergent Transform

Plate Margins - Where Stuff Happens…. . Divergent Convergent Transform

Relative Movements of Plates a) Divergent, (b) Convergent-continent/ocean basin, © transform, (d) convergent-continent/continent

Continent Divergent Margin n ACTIVE - Red Sea began ~10 Ma INACTIVE - East African Rift Zone; parts up to 20 Ma Example of stable triple junction - 3 breaks. One arm fails http: //earthview. sdsu. edu/trees/tecqest. htm

Types of Convergence Ocean-Ocean, Ocean-Continent, and Continent-Continent

How Do Divergent Margins Develop? DOMING n High heat flow, thermal expansion of crust - few km high / 1000’s km across. n High, dry climate RIFTING n Normal faults accommodate stretch n NO ocean crust n Possible formation of triple junction n High, dry climate: non-marine sandstones, conglomerates in basin n East African Rift Zone DRIFTING n Crust thins and initial intrusion of basalts as new seafloor n Shallow ocean; high evaporation n Later “normal” marine sediment n Drainage reverses in to the sea. n Gulf of California (5 Ma) n Red Sea (10 Ma)

Components: • Oceanic plate • Trench • Fore-arc ridge (melange) • Fore-arc basin • Continental plate with volcanic mountain range (e. g. Andes)

Mountain Building-e. g. the Andes

Components: • • Oceanic plate Trench Fore-arc ridge (melange) Fore-arc basin, Island arc Back-arc basin (down-going plate melts / accom. of spreading) • Oceanic Plate

Volcanic arcs may develop. . . n n within oceanic lithosphere, where ocean floor subducts beneath ocean floor, and an island arc results, e. g. Lesser Antilles arc or at the edge of a continent, where oceanic lithosphere subducts beneath continental lithosphere, and a continental margin magmatic arc forms, e. g. Andes

Basins related to volcanic arcs n fore-arc n back-arc n intra-arc n All may be either submarine or subaerial, or may have marine & subaerial parts n Much sediment is supplied from active arc.

Fore-arc basins n Lie in the arc-trench gap, between volcanic arc and submarine trench n range from small basins on trench slope to large basins (50 to 100 km wide, and > 500 km long) with thick fills (several km) n Basins tend to become wider and shallower with time, partly because of accretion at trenches

fore-arc basins Sediment sources: n volcanic arc n outer arc n longitudinally from a continent Tectonic style varies: n compressional n extensional n strike-slip

Back-arc basins n lie behind the magmatic arc n often the site of extension & thinning of crust n may overlie either ocean or continental crust n oceanic back-arc basins are eventually subducted and destroyed, or preserved in thrust complexes related to ocean closure. n back-arc basins on continental crust - more varied facies, because of terrigenous input; higher preservation potential.

Intra-arc basins n Sedimentary basins within magmatic arcs, between volcanoes, or between older and younger belts of the arc n Some are fault-bounded and subside rapidly. Faulting due to extension within arc, or flexure of lithosphere due to weight of volcano. n With time, position of the arc migrates, and basins may change between intra-arc, backarc and fore-arc.

Sediment supply and transport n Sediment supply varies according to volcano behaviour, governed by magma viscosity and gas content. n In deep water, explosive activity is suppressed by hydrostatic pressure. n More silicic magmas in more evolved arcs therefore greater explosive activity, more supply of pyroclastic sediment.

Sediment transport and deposition is controlled by: n topography - both subaerial and submarine n volcanic processes, especially eruption column height, direction of pyroclastic flows n sediment transport systems - e. g. rivers, prevailing winds

Subduction Zones

Subduction zones n n also termed convergent or consuming plate margins occur where adjacent plates move toward each other and relative motion is accommodated by one plate over-riding the other. These zones are classified as either oceanic or subcontinental, depending on the overriding plate. If the "subducting" plate is continental, subduction will cease and a mountain belt will form within a collision zone.

Where do subduction zones occur? n n n along the "Ring of Fire" around the Pacific Ocean. Two short subduction zones occur at the Lesser Antilles, at the eastern side of the Carribean plate and the South Sandwich Island plate. Three short segments of the Alpine Himalayan system involve subduction of oceanic lithosphere. – the Calabrian and Aegean boundaries in the Mediterranean Sea – Makran boundary along the SW boundary of the Iran plate.

Physiography n n n Outer Swell Outer Trench Wall Trench Forearc (Arc-Trench Gap) Volcanic Arc Back-Arc

Physiography 2 n Outer swell – Low topographic bulge (a few hundred meters of relief) – develops just outboard of where subducting plate bends down into the mantle. Outer Trench Wall – Slope on ocean floor between the outer swell and the trench floor. – Slope dip is typically -5 degrees

n Trench n - A trough upto 11 km deep infront of arc and final repository of most arc derived sediments. – Deep valley that develops at the plate boundary. – Continuous for 1000 s of km – Turbidites and pelagic typically are present here but are deformed and overriden by subduction complexes. Many trench are essentially empty of sediments because of the absence of any adjacent sediments supply.

Forearc (Arc-Trench Gap) – Consists of region between trench and the arc. – steep inner trench wall (lower trench slope) n dips of - 10 deg – flattens into a gentle slope termed the forearc basin (upper trench slope). – The inner trench wall is usually separated from the forearc by the outer ridge. – The accretionary prism underlies the inner trench wall, the outer ridge and part of the forearc basin.

Volcanic Arc n Active arc built on a topographically high region of older rocks, the arc basement n may be a shallow marine platform or an emergent region of older rocks. n In continental arcs, the basement is continental crust standing a few kms above sea level. n Volcanoes in island arcs are usually 1 - 2 km above sea level. Volcano elevation in continental arcs is strongly influenced by continental crust thickness.

Back-Arc n Area behind the volcanic arc. n In island arcs this region consists of basins with oceanic crustal structure and abyssal water depths. n Sometimes remnant arcs are preserved behind the island arcs. n On continents this region is the continental platform which may be subaerially exposed, or the site of a shallow marine basin.

Gravity n Typically, similar free-air gravity profiles – 50 m. Gal gravity high associated with the outer bulge – 200 m. Gal low associated with the trench and accretionary prism – 200 m. Gal high associated with the arc. n n n Isostatic anomalies have the same polarity as the free-air gravity Suggests that the gravity anomalies are caused by the dynamic equilibrium imposed by the system by compression. Compressional forces cause the trench to be deeper and the arc to have less of a root than they would be if only isostatic forces were at work.

Structure from Earthquakes n n Subduction zones are characterized by dipping seismic zones termed Benioff zones or Wadati-Benioff zones Result from deformation of the downgoing lithospheric slab. The zones have dips ranging from 40 to 60 deg

n Because, the slab is colder and more dense than surrounding asthenosphere, it's position can be mapped seismically as high velocity anomalies and as high "Q" (little attenuation of seismic waves) zones in the mantle. High Q, and high velocity are thought to correspond to relatively high density, cold material

earthquake hypocenters related to their position within the slab n n n Shallow depths predominantly thrust faults within the upper part of the downgoing plate or in the adjacent overriding plate. Down to depths of 400 km, down-dip extension. – In some slabs, down-dip extension is found in the upper part of the slab, accompanied by down-dip compression at the base of the slab. The extension probably results from the lithosphere being pulled into the mantle by the weight of the downgoing portion.

n Deep slabs usually show down-dip compression – may result from increased viscous resistance at depth. – deeper part of the slab will feel a push from the weight of the shallower portion of the slab. n n Between 70 - 300 km, faulting may occur due to dehydration of serpentinite. From 300 - 700 krn may also be due to the sudden phase change of olivine to spinel which may be accommodated by rapid shearing of the crystal lattice along planes on which minute spinel crystals have grown.

Structural Geology- Trenches normally contain flat-lying turbidites deposited by currents flowing down into the trench from the overriding plate or along the axis of the trench. The outer swell is probably caused by elastic bending of the subducting plate.

Forearc n n n may be underlain either by the accretionary prism or arc basement rocks covered by a thin veneer of sediments or both. Where there is little sediment accumulation on the subducting plate, island arc or continental basement may extend all the way to the lower trench slope and little or no accretionary prism may occur. Forearc basement may draped by a thin veneer of sediment, and is commonly cut by normal faults toward the trench.

Accretionary Prism n n n wedge of deformed sedimentary rocks the main locus of crustal deformation Rocks are typically cut by numerous imbricate thrust faults that dip in the same direction as the subduction zone. As more material is added to the toe of the wedge, the thrusts are moved upwards and rotate arcwards. Rocks within the accretionary prism are derived from the downgoing and/or overriding plates.

Accretionary Prism n n n At the toe of the wedge, sediments are added thru offscraping propagation of the basal thrust into undeformed sediments on the subducting plate. This process results in progressive widening of the wedge, and eventually a decrease in dip on the subduction zone.

Accretionary Prism n n When sediments on the downgoing plate are subducted without being disturbed they can still be added to the prism thru underplating propagation of the basal thrust into the downgoing undeformed sediments to form a duplex beneath the main part of the prism.

Subduction Erosion n n erosion and subsequent subduction of rocks from the toe of the prism. Sediment on the subducting plate that is not added to the overriding plate thru these processes may descend into the mantle and contribute to the generation of arc magmas.

Forearc Basin n Wide sedimentary basin – develops above irregular basement on the upper part of the arc-trench gap. – Sediments from the active arc or arc basement rocks n deposited by turbidity currents traveling along the basin axis or perpendicular to the arc. n asymmetric basin – inner part of the upper slope basin subsides – outer edges rises due to accretion at the toe of the wedge. n high-P, low-T metamorphism – increases in grade toward the inner forearc region – in the direction of subduction

Arc n Arc basement – older more deformed and metamorphosed rocks in platform on which the modem arc is built. – oceanic rocks – On the continents, complex continental basement. n Volcanic arc – a chain of largely andesitic stratovolcanoes spaced at fairly regular intervals of 70 km. – The structural environment of these arcs is commonly extensional (numerous normal faults) – volcanoes in grabens termed volcanic depressions. – underlain by large plutonic bodies (e. g. the Sierra Nevada).

Arcs n Metamorphism – common and suggest a high geothermal gradient. – Much of the lower crust may be at the melting temperature of granite. n Sediments – debris from active volcanoes. – deposited as turbidites. – In tropics, settings these volcanogenic sediments may interfinger with carbonate reefs. – In continental arcs, sediments are often deposited subaerially.

Back-arc n n n extensional tectonics and subsidence. In oceans arc-derived sediments are deposited in an ocean basin behind the arc termed the back-arc basin. In continents, sediments are deposited into basins on the continental platform and are termed foreland basins or retro-arc basins.

Foreland Fold and Thrust Belts n n n Relation between foreland fold and thrust belts and subduction not understood not all continental arcs display these features. Possible explanations if there is a relation – Thrust belt caused by compression at margin of overriding plate due to subduction of hot, buoyant lithosphere. – Thrust belt associated with shallow dip of a downgoing slab. – Thrust belt associated with subduction of an aseismic ridge.

Models of thermal processes in subduction zones n Rate of Subduction – The faster the descent of the slab, the less time it has to absorb heat from the mantle. n Slab Thickness – The thicker the descending slab, the more time it takes to come into equilibrium with the surrounding lithosphere. n Frictional Heating – occurs at top of slab due to friction as slab descends and is resisted by the lithosphere.

n Conduction – n heat into slab from the asthenosphere Adiabatic Heating – associated with compression of slab with increased pressure at depth. n Heat of Radioactive Decay – decay of radioactive minerals in the oceanic crust (minor) n Latent Heat of Mineral Phase Transitions – olivine-spinel transition at 400 km is exothermic. Spinel-oxide transition at 670 km could be either exothermic or endothermic.

n All thermal models show that the downgoing slab maintains its thermal identity to great depths (e. g. contrasts of 700 deg C can still exist at 700 krn depth).

If the slab is so cold, how do we get enough heating to cause arc magmatism? n Melting of Slab in Presence of Water – Partial melting may take place at lower temperatures due to presence of water as slab dehydrates. Water is released by transition of amphibolite to ecologite, and dehydration of serpentinite at depths of - 100 km. n Corner Flow and Melting of Mantle – Downgoing slab may cause flow of hot mantle into the comer of the overriding mantle where it impinges on the downgoing slab. This may provide enough heat to cause melting.

Origins of back-arc basins n Entrapment of previous oceanic crust – Change of plate motion may lead to abandonment of a fragment of oceanic crust behind the arc. (e. g. , Aleutian Basin and West Philippines Basin ) n Formation of new crust - behind the arc. 3 models o Spreading caused by forceable injection of a diapir rising from the downgoing slab. o Spreading induced in the overriding plate by the viscous drag in the mantle wedge caused by the motion of the downgoing plate (comer flow). o Spreading induced by the relative drift of the overriding plate away from the downgoing slab (slab fixed with respect to mantle). This is also termed roll-back.

Ocean-ocean plate convergent boundary.

Structure of a continent-ocean convergent boundary.

Continent-continent collision.

Mid-ocean ridge divergent boundary showing transform faults.

Two stages of an opening rift.

slab-pull due to its greater density

Reconstruction @ 250 million years ago

Concept of a Basin n n Three dimensional architecture of basin fill. Affected by spatial and temporal pattern of tectonic subsidence: – Lithospheric deformation process. – Three basic causes of subsidence: n Loading and flexure (like an elastic plate). n Thermal and density changes - isostasy. n Faulting - isostasy. n n Sea level changes. Sediment supply rates and source position (drainage basin outlets).

Basin Classification n Extensional: – Half graben/graben - Rift Basin, e. g. Basin and Range, U. S. A. – Mature oceanic speading - e. g. Atlantic margin (Passive Margin). – Syn-rift, post-rift megasequences. n Compressional: – Foreland basin - flexural loading of the Earth's lithosphere. – Two types: n Collisional, e. g. Himalaya. n Back arc, e. g. Andes/Precordillera. – Piggy-back basin (thrust sheet top basin). n Strike-Slip Basins: – E. g. Dead Sea, Israel.

Basin Classification Cont. n Passive Margins: – E. g. Atlantic Margin. n Subduction Related: – Oceanic trench, e. g. Marianas Trench. – Fore-arc basin, e. g Taiwan or Median Valley in Scotland. – Back-arc basin, e. g. Sea of Japan. n Cratonic "sag" Basins: – E. g. Chad Basin, Africa. n n Abyssal Plains. Predictive models of facies distributions: useful for subsurface exploration of oil or understanding dispersal of pollutants.

Benioff Zones n n n Earthquakes occur at shallow, intermediate and deep levels beneath subduction zones The earthquakes define a plane which begins at the trench and dips at about 45° beneath the arc This dipping plane of earthquake foci is called the Benioff Zone The Benioff Zone follows the upper part of the descending oceanic plate Shallow earthquakes also occur through the arc

Island Arcs n n n Island arcs are of chains of volcanically active islands arranged in a curved arc An ocean trench occurs on the oceanwards side Island arcs first develop on oceanic crust The crustal thickness in an arc is intermediate between oceanic and continental Volcanic activity begins abruptly at a Volcanic Front about 200 - 300 km in from the trench The volcanic front and trench are separated by an Arc-Trench gap with no volcanism

Island Arc Volcanism n n Volcanic rocks in island arcs are mostly of andesitic composition The magmas originate mostly by partial melting of subducted oceanic crust and overlying mantle Melting begins when the slab descends to about 100 km depth, forming the volcanic front Partial melting of basaltic ocean crust Rising magmas Volcanic eruptions

Chemical Differentiation n n Mid-Ocean Ridge: Partial melting of Mantle basalt magma Subduction Zone: Partial melting of Basalt crust andesite magma Mature Arcs: Partial melting of Andesite crust rhyolite magma All of this is an irreversible chemical differentiation of the mantle in several stages Continental crust grows by accumulation of increasingly silica-rich rocks

Ocean trench Sedimentation n n Unconsolidated sediment from the ocean floor is scraped off the descending plate at the trench Slices of the oceanic crust may be included as ophiolite belts These rocks form a complex rock mass called an Accretionary Wedge The Accretionary Wedge is buckled upwards as new material is pushed beneath its base The chaotic jumble of rocks in the Accretionary wedge is called a Tectonic Mélange Accretionary Wedge

Metamorphic Rocks and Subduction n High Temp - Low Pressure Metamorphism Occurs in the core of volcanic arcs – Abnormal heating of the crust – thermal effects of subduction-related magmatism n n High Pressure-Low Temp Metamorphism – occurs in the accretionary wedge – cold rocks are dragged to great depths and then upthrust again Granites

Basin Analysis n Basins – – – Topographically low places where sedimentary materials accumulate. Basins are characterized by accomodation space. Accomodation is created by… Eustatic sea-level rise Subsidence

Basin Analysis Basin analysis is the detailed investigation of the processes that – – – Form basins Fill basins Alter basins Uplift (invert) basins Destroy basins Requires sedimentology, stratigraphy, hydrogeology, petroleum geology, seismology, geophysics, geochemistry, paleontology, etc.

Types of Basins n n Intracratonic Rift related Strike-Slip related Collision / Subduction related

n n _Intracratonic – Basins that form within continental crust Rifts – Divergent plate boundaries that eventually develop into spreading centers.

n n Rifts – Divergent plate boundaries that eventually develop into spreading centers. _Initial rift sediments are arkosic sandstones interbedded with basalts. _Rifts then flood and deposit a thick sequence of evaporites. _Then marine sedimentationtakes over.

Basins & basin types Basin: a region of depressed crust, typically with greater thicknesses of sediment accumulation than surrounding regions. – Basins form by tectonic processes that cause the crust to subside, and so to create large amounts of accommodation space. Rapid Aspace growth translates into abundant and wellpreserved organic matter within the sediment. – The abundance of organic matter combined with the low grade burial metamorphism that sediments in thick accumulation experience translate into major oil and gas accumulations. – Basins provide the most complete successions of the geological record.

Tectonic setting for the majority of basins Constructive (spreading) margins: This setting produces a genetic series of basins from localized rift-related basins through narrow oceans (Red Sea phase) to voluminous passive margin basins. Destructive (convergent) margins: This setting produces basins associated with subduction zones (foredeep basins), intra-arc spreading basins, back-arc spreading basins (Sea of Japan, Great Basin), and back-arc thrust (foreland) basins. Transform (strike-slip) margins: mostly localized pullapart basins such as the Gulf of California and Dead Sea. Grades into rift-type and thrust-type tectonics where relative plate motion is not fully strike-slip (i. e. , transtensional and transpressional regimes). Intra-cratonic basins. Fairly mysterious, concentrically subsiding basins formed on continental crust well away from plate boundaries. The cause of subsidence is unknown, but is generally considered to involve some combination of density-driven stress and in-plane stress.

Rifting and passive margin n n Continental rifting begins with dome-formation. Produced by hot-spot volcanism (bimodal: dominantly basaltic plus some rhyolite from partial melting of the crust by basaltic magma and decompression melting). Much extension by normal-faulting. Individual domes link up to form a more or less continuous series of rift valleys (grabens, as in the East African Rift zone). Extension results in thinning of the upper crust. The lower crust thins by ductile flow. High heat flow produces thermal uplift; uplift leads to further thinning of the upper crust by erosion. Continued extension produces oceanic crust between the newly formed, thinned continental margins. Eventually (over 107 years), sea floor spreading and thermal subsidence yields a distinct mid ocean ridge, open communication with the oceans (Red Sea phase) to form a permanent seaway. Continued seafloor spreading carries the new continental margins away from the active tectonic zone. The muchextended crust cools and subsides over the next 108 yrs. as a passive continental margin ("passive margin, " for short).

Rift basins. n n n Smallish volumes of sediment (100’s to a few 1000 m of accommodation space) in localized, fault-bounded basins. Dominantly terrestrial (subaerial and lacustrine) and arid as result of uplifted margins that cast rain-shadows into the basins as well as their small drainage capture area. Early rift sediments typically heterogeneous with basement clasts and volcanics dominant. Grains with low physical and chemical maturity. Late rift sediments often include extensive evaporites from episodic oceanic invasions (reflects variable rates of subsidence and sea level rise and fall). Remobilized salt becomes important in passive margin history later (see below).

Passive margin basins. n n Lots of accommodation space (10, 000 to 15, 000 m or more) from combination of thermal and sediment loaddriven subsidence. As oceans widen, marine conditions come to dominate. Early phases often still arid and dominated by carbonate deposition and fringing reefs along the shelf edge (at low latitude sites) With continued subsidence the fringing mountains diminish, drainage basin size grows, and clastic sediment supply rates increase. In humid regions or times of low sea level, deposition switches to the clastic off-lap suite with formation of the familiar shelf environments: typical association of deltas, beaches, and sub-tidal clastic shelf deposits grading outward to deep sea fans and contourites in continental rise settings. Prograding sediments build a wide continental shelf upon a voluminous accumulation of clastic or carbonate (or some mix of clastic and carbonate). The clastic off-lap suite is well developed along the present Atlantic coast of North America, South America, Africa, and Europe. The southern passive margin of Laurentia (what is now the eastern margin of North America) was dominated in the Cambrian and Ordovician entirely by carbonates. Both are major source of petroleum.

Destructive (convergent) margin basins n n n Foredeep Basins Foreland Basins. Facies patterns.

Foredeep Basins n n n Basin forming mechanisms: dominantly a combination of load-related and density-related subsidence. Loading leads to flexure of the crust. Subsidence near the load is matched by a compensating (but smaller amplitude) upbend at the periphery of the down-bend. At destructive plate boundaries, dense, old oceanic crust subducts beneath continental crust or younger, lessdense oceanic crust, forming an elongate, deep oceanic trench. Interaction between the down-going plate and the overriding plate generates thrust slices (slabs of rock and deformed sediment) sheared off of one or the other plate, which are thrust ocean-ward, overtop of the down going plate. The subduction zone may also include abundant sediment supplied to the trench (especially in humid climates; ex. central Andean margin of South America). Thrust and sediment load causes subsidence of the crust, deepening the basin and creating large amounts of additional accommodation space. This is a foredeep basin: a basin that form from a combination of trench (density driven) and load-driven subsidence in the downgoing (lower) plate. It is located ocean-ward of the volcanic arc and is rimmed distally by an uplifted forebulge. Ex: north Australian shelf-Banda Arc collision.

Foreland Basins. n n Thickened crust of an island arc complex or continental crust at the margin of the upper plate leads to intense interaction between the downgoing plate and the upper plate. This generates considerable additional thickening and deformation of the upper plate via thrust slices that stack up to form (together with the volcanoes) a tall, complex mountain system and its isostatic roots. Ex: Andes Mountains. Thrusts migrate away from the collision (subduction) zone toward the "foreland. " Thrust and sediment loads causes subsidence of the crust forming a deep flexural basin (8 -10 km accommodation space) followed by an uplifted forebulge and a much smaller second order basin (a few 100’s of meters A-space at most). Ex: Amazon Foreland Basin.

Facies patterns. n n n Load-driven flexure is rapidly accommodated by subsidence/uplift. Rapid growth of accommodation space in the foreland or foredeep basin leads to trapping of sediments in the source area and strong transgression and sediment starvation as relative water depth increases over a period of 104 - 105 years. Rapid uplift on the foreland or peripheral bulge results in loss of accommodation space, but facies response varies greatly with local conditions the state of eustatic sea level. Possibilities range from subaerial exposure to prograding carbonate or siliciclastic facies. Unlikely to exhibit transgression. Basin fill shows strong progradation of sediments filling deep basin. Sediments grade upward and proximally from black shales to flysch (mixed shale and turbiditic litharenites), to near shore clastics (deltaic and shelf-like sediments) to alluvial plane deposits (fans & rivers, etc. ). Early basin fill with abundant, well preserved organic material. As the system ages, the mountains erode and the sediments become less volcanic, more plutonic and metamorphic (granitic) source-rock dominated, and become more mature both physically and chemically. These late phase deposits are sometimes referred to as mollasse.