TECTONICS AND OROGENESIS

Key facts about tectonic areas:

a) tend to be elongate; long, narrow, linear
b) lie along or parallel to margins (or former margins) of continents
c) orogenic zones show distinct belts of metamorphism and/or deformation:

 1a) stable platform (craton): slightly / undeformed,  thin sedimentary "veneer"; sometimes with gentle  arches, dome, intracratonic basins

 1b) rift belts in some areas linear normal fault  bounded basins( horsts and grabens)

 2) foreland  basin: wedge of sediments of similar  type thickening toward mountain belt

 3) foreland fold and thrust belt; (part of foreland  basin) thicker section of unmetamorphosed but  strongly deformed rocks: folded (into anticlines- synclines)  and/or faulted with compressional,  thrust  faults, directed toward the foreland (craton)

 4) hinterland metamorphic belt: strongly  deformed rocks of distinctive types (commonly  thick siliciclastics plus volcanics); slightly to strongly  metamorphosed; increasing temperature toward the  igneous/metamorphic "core"

 5) "core complex"; igneous batholiths, high  grade  metamorphic rocks
 

 6) belt of strongly deformed (melange) deep sea  sediments and igneous pressure metamorphic belt;  associated with ophiolites on "oceanward" side of  mountain belt

 Note: notion of paired metamorphic belts:  outer  high pressure/low temperature and inner zoned  high-low temperature belt; was not readily

d) mountain belts experience episodes of compressional deformation : Orogenies

Some areas experience episodes of extensional rifting sometimes called Taphrogeny
 

Notions of tectonic upheaval and a link with Earth internal processes go back at least to Strabo;

Early attempts to formulate notions of Earth movements largely assumed predominant vertical  uplift  rather than lateral compression; although some lateral folding was assumed in old "wrinkled apple" models

Predominant theory of later 1800s to 1960s was geosynclinal theory. conceived by James Hall, named by James Dwight Dana and amplified by Swiss geologist H. Stille:

Hall was impressed by the thickening of sediments into the deformed mountain belt (New England) and he postulated that mountain belts started as great troughs along the sides of continents: geosynclines, that became filled with sediments from some outside source area; eventually the sediment pile became so thick that it collapsed upon itself deforming and partly melting the sediment pile:

Explained the linear trends, parallel to continent margin and the thickening did not satisfactorily explain the lateral compression nor the source of the sediments and volcanics:
Dana said it was a theory of mountain building with mountain building left out!

Stille working in the Alps recognized two major divisions of the geosyncline (terms still used by some):

miogeosyncline: middle, foreland part, between platform and main mountain belt; area of thickening wedge of sediments of shallow marine-non-marine; similar type to the platform; high subsidence

eugeosyncline: (generally represented by hinterland deformed, metamorphosed clastic rocks and volcanics);
a deeper trough bordered by volcanoes

also Stille and others recognized a tectonic-sedimentary-metamorphic cycle that differed in the foreland (miogeosyncline-platform) vs. the eugeosyncline This was an important contribution and remains a key observations:

Phase     Foreland (mio-)   Hinterland (eu-)

late-orogenic  molasse*      [uplift, erosion]
    subgraywacke

syn-orogenic  flysch**      [inversion of topo,        graywacke     metamorphism]
              granites,

early syn-oro.  black shale     thick flysch-                                         volcanics;                  andesites

pre-orogenic  orthoquartzite-   thin flysch;
  passive    carbonate     deep sea shales,                 cherts, etc.

taphrogenic   [erosion]     thick arkosic flysch       extension   local arkose-    basalts
       basalt graben-fills
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*  flysch:  interbedded thin sandstones (turbidites)  and shales; represents a turbidite wedge

** molasse (from Fr. millstone grits) shallow marine to non-marine siliciclastics; typically with redbeds

All of these things had to be accounted for in a model of tectonics- mountain building

Older models accounted for some, but plate tectonics revolutionized the whole understanding of mountain building , rifting and more
 
 
 
 
 
 

PLATE TECTONIC MODELS OF RIFTING-OROGENESIS

Plate tectonics revolutionized our understanding of the processes:

Return to different types of plate margins:

Transform Boundaries

Not as fully integrated into tectonic models; minor aspects of continents; are areas where two plates slide past one another

Produce a mixture of extensional (pull-apart basins) and local compressional deformation; fragment formerly continuous rock strata

Divergent Boundaries: two plates moving apart; extensional, rifting; associated with normal faulting; horsts and grabens; mafic igneous rocks’ minor local thermal metamorphism

Begins with thermal doming of lithosphere: swollen asthenosphere, mantle plumes below; may occur near centers of continents where thermal blanketing initially allows heat to buildup; must be dissipated, eventually breaks to surface

Lithosphere (crust) eventually splits, typically breaks in a triple junction (common in brittle materials) three rift arms at ~ 120 to one another; e.g. Afar Triangle; Red Sea-Gulf of Aden, East African Rift belt

arms of triple junctions may become interconnected to form spreading center that could evolve into MOR;
 NB:  all three arms may continue as active spreading ridges BUT, more often one of the three becomes inactive and may be left on a continent as a dormant failed arm or aulacogen: these form long narrow, normal-fault bounded basins, may subside and collect sediments; famous oil basins and also many rivers flow through these aulacogens  (e.g. Mississippi and Miss. Embayment), Amazon, Niger River)

There may also be failed collateral rifts that lie parallel to an active spreading center (ocean basin), e.g. Connecticut River Valley graben

Sedimentology:  arkose-basalt suite

when rift valleys develop in continental crust, they may lead to rapid erosion of uplifted (horsts) of granitic basement/sediment cover; tends to deposit immature sediments in adjacent grabens; hence commonly arkosic redbeds, fanglomerates, etc. may get non-marine sediments giving way to marine, sometimes with a period of evaporite deposition in narrow restricted basin

These may be associated with basalts that are extruded/intruded along rifts; hence: an arkosic-basaltic suite of rocks may accumulate

Passive Continental Margins

Following successful rifting to form an active spreading center an ocean basin develops and begins to open

Rift shoulders on either side of ocean basin cool down and subside to form passive continental margins or trailing edges; but active subsidence due to thermal contraction may continue (at gradually decreasing rate) for tens of millions of years

Robert Dietz pointed out in late 1960s that these actively subsiding outer continental shelves provide a realistic model for the pre-orogenic "miogeosyncline"

Initially, a good deal of immature, siliciclastic sediment derived from adjacent continents may pour off the shelf edges onto the rifted continental slopes; this forms a thick prism of arkosic-graywacke sediment turbidite fans and muds the rise prism; Dietz felt that this slope and rise prism was a partial analog of the "eugeosyncline" ; it may later become deformed IF the passive margin converts to an active one; hence this model explains several aspects of the tectonic cycle better than the older "geosynclinal" model

Sedimentology: orthoquartzite-carbonate suite

As the passive margin evolves seas encroach up onto the cratons ; craton becomes increasingly denuded and peneplaned SO:  deep weathering will take place and supply of terrigenous siliciclastics will gradually decline; clean, mature, quartz rich clastics may give way to a carbonate (intrabasinal) regime; continental slope will become increasingly starved of sediments (thin flysch)

 hence the typical sediments of a quiescent time will be an orthoquartzite-carbonate suite
 

Convergent Boundaries:

Most critical boundaries for compressional tectonics (orogeny) are convergent boundaries, the leading edges or active boundaries, where plates come together and one overrides the other; most important sites of orogeny are one continental leading edges

Several types of plate convergence are possible:

A) Ocean Floor-Ocean Floor convergence; Marianas Type

where a subduction zone develops on the ocean floor and one slab of oceanic lithosphere ("seafloor") begins underriding another

several zones will develop

a) trench--b) accretionary wedge--c) forearc basin--d) volcanic island arc--e) backarc basin

trench is the beginning of the subduction zone at which cold slab descends below another

accretionary wedge  is thrusted, often chaotically deformed (melange)  deep sea sediments, trench turbidites and mafic-ultramafic rocks; material is obducted (back flow) and off scraped in an underplated stack (most recently scraped material is at the base (opposite normal superposition)

in the melange may be obducted slabs of oceanic lithosphere; ultramafics-sheeted gabbro dikes-pillow basalts--deep sea sediments (e.g. radiolarian cherts):
these have been recognized for years and were termed:
ophiolites ("snake rocks", because of the abundance or greenish serpentines, etc.)

cold, water saturated rocks are subjected to high pressures but relatively low temperature metamorphism; yields distinctive types of metamorphic rocks:

chlorite grade metasediments,
 ultramafics may become metamorphosed to blueschists (with distinctive amphibole glaucophane)
pillow basalts may undergo serpentinization; greenish colors of chlorite and epidote, chrysotile

shales develop scaly cleavage from overpressured pore water; develop "broken formation" textures; accretionary wedge may rise above sea-level in some cases

forearc basin:  (do not confuse with foreland basin): a subsiding area between accretionary prism and island arc
(it is in front of or seaward off the island arc-hence forearc)
collects siliciclastic debris eroded from accretionary. wedge and sediments and volcanics from adjacent  island arc; width of the forearc is related to arc-trench gap, a function of  dip of the subducting plate; steep dip= narrow a-t gap

island arc:  magmas are generated at depth by partial melting of downgoing slab; generates magmas of intermediate composition (andesitic); these form magma chambers and break through to form a chain of volcanoes parallel to trench;  sediment and other rock around the magma chambers will be metamorphosed at high temperature

backarc basin may develop due to loading and extension of crust behind the arc; also fills with volcanoclastic sediments

 NOTE:  trench-accretionary wedge and island arc explain pattern of paired metamorphic belts

B) Island Arc (Accretionary Wedge) Continent Collision (Timor type Margin)

if subduction of an ocean plate beneath an island arc continues, the oceanic plate on the leading  (downgoing) slab may be consumed and then the edge of a continent may encounter the trench; continental crust will not subduct far because of its low density , but may underride the accretionary wedge slightly;

in this case, the off scraped oceanic sediments and ophiolites  as well as continental slope and rise sediments may become obducted or thrusted up onto the margin of the continent;

But the system will eventually jam and subduction will stop;

the immense load of the accretionary wedge will cause continent edge to subside  as the trench becomes a low angle overthrust zone

subsidence of the continental edge creates a special basin called a peripheral foreland basin  (foreland basin is any area of tectonic loading subsidence on continental crust);

p. foreland basin then fills with sediment eroded from the accretionary wedge and island arc; jamming of the plate and  isostacy may also cause an uplifting welt of continental crust or peripheral bulge on the continent ward side of the foreland basin. now the set up is:

 a) peripheral bulge--b) peripheral foreland basin (=trench)--c) accretionary wedge--d) forearc basin-- e) island arc

arc will eventually become extinct as subduction stops

C) Ocean Floor--Continent Convergence (Andean type Margin) this scenario will take place when a slab of ocean floor underrides the margin of a continent   (rather than  continent partially overridden by an ocean floor plate as in last case); situation is similar to ocean floor-ocean floor collision in that will get:

trench-accretionary wedge-forearc basin-arc;
also get paired high P/low T and high T metamorphic belts

but, here get magmatic arc on the craton, instead of volcanic island arc;  magmas  intermediate to somewhat more granitic in composition if partial melting of continental slab takes place,

also, behind the magmatic arc get a zone of craton directed compression and gravity spreading (from high arc region) to form a foreland fold and thrust belt;  and the loading of the congenital crust forms a subsiding basin analogous to backarc basin:  here it is called a retroarc foreland basin; will fill with sediment from the adjacent fold and thrust belt and magmatic arc;  now, the set up is:

a) trench--b) accretionary wedge--c) forearc basin--d) magmatic arc--e) fold and thrust belt--f) retroarc foreland basin

D) Continent-Continent Collision (Himalayan Type Margins)

In case for which subduction exceeds rate of ocean basin opening, entire ocean floor may be subducted beneath one continent; this will bring one continent into collision with another continent; one may slightly override the other as is the case with Indian which is overriden by Asia

In this cases a suture zone will be created; the downgoing (overridden) continent will develop a peripheral foreland basin;  magmatic arc and retroarc basin will develop on the overriding continental plate; forearc basin (in between will be destroyed, uplifted and deformed

the total cross section will include:

              Continent 1 (Downgoing Plate)
a) periph. bulge--b) peripheral foreland basin--
  Continent 2 (Over riding Plate)
c) accretionary wedge-- d) forearc basin--e) magmatic arc--f) fold and thrust belt--g) retroarc foreland basins

accretionary wedge and remnants of compressed forearc basin are wedged in between the two continents;
as a result ophiolite belt forms suture zone;
basin-filling flysch to molasse succession develops in both foreland basins (peripheral and retroarc):

basic pattern:

6) [folding and thrusting may affect synorogenic flysch and molasse sediments; as late phases of  orogeny affect its own sediments]

5) molasse: shallowing into non-marine sands and muds:  Overfilled foreland basin; sedimentation off marginal mountain belt outstrips subsidence and basin gets filled

4) flysch: Synorogenic sedimentation; rapid erosion and dumping of turbidites (graywackes) and mudrocks, off prograding clastic wedge

3) black shales (often with K-bentonites) : Underfilled foreland basin; pulse of thrust loading subsidence

2) deeper muddy carbonates, commonly with K-bentonites: Initial subsidence due to thrust loading;  shelf is changing to foreland basin carbonate ramp  gives way abruptly to:

1) orthoquartzite-carbonate: Non-orogenic; passive continental shelf sea) succession passes gradationally upward into 2.

Also, as thrust loading takes place foreland basin rolls back (migrates) toward craton; the basin is filling with siliciclastic wedges; on the inner or cratonic side a forebulge may be developing due to isostatic uplift; erodes as basin fills

Eventually after basin is filled-load is redistributed and the proximal side of the basin and mountain belt pops up isostatically and is beveled

Later the foreland basin may roll back toward hinterland as load relaxes
TYPES OF BASINS

Note the different types of sedimentary basins that may develop due to tectonics:

aulacogens and rift basins: tend to fill with arkose, basalt

forearc and backarc basins around volcanic arc; tend to be rich in graywackes and volcanics

peripheral and retroarc foreland basins (due to loading of continental crust by thrust loads)

pull apart and sag basins  associated with transpression

intracontinental basins  uncertain origin

different types of uplifts also; magmatic arcs, accretionary wedges. thrusts and folds; forebulge and peripheral bulges; intracratonic arches and domes

WILSON CYCLES

Process of subduction eventually stops; ocean floor spreading elsewhere tends to die out; supercontinent is formed; major uplift will result if one continent under thrusts other (e.g. Himalayas and Tibetan Plateau)

Millions of years later new rifting tends to occur near to, but not at the rift zone (that becomes annealed, thickened and is not prone to open again); this leaves remnants of one continent attached to the other (e.g. Boston, Florida)

Continent-Continent Collision completes a megacycle of rifting, ocean basin opens, ocean basin closes,  suturing of continents, renewed rifting

This cycle of ocean basin opening then closing has been termed a Wilson cycle (from J.T. Wilson)

Largest scale cycle in Earth’s history involves a grand scale pattern of continental convergence and collision and supercontinental rifting and divergence

LARGEST SCALE TECTONIC-CLIMATIC CYCLES

Wilson Cycle is a Manefestation of Alternating Pangeic and Rifting ("Anti-Pangea") states

Why??  When land mass clumped on one side of planet thermal blanketing may lead eventually to buildup of mantle heat; plumes split the mass apart; then continents gravitationally move "downslope" into low basin on back side of earth; eventually they aggregate again on opposite side of Earth

May relate to a large scale cycle of convection in mantle; leads to times of high volcanism and rapid spreading and times of greater quiescence and lower spreading rates, Pangea

This, in turn may lead to a series of other effects:

High tectonism ("Anti-Pangea")-- rapid SFS--high sea level (transgression)-- high atmospheric  CO2; , also subduction and metamorphism of carbonate sediments releases stored CO2 back to atmosphere:  Greenhouse Climates

Low tectonism (Pangea)--slow SFS--low sea level (regression), extensive weathering of exposed rocks (draws down CO2), also extensive growth and burial of land plants and limestone precipitation takes CO2  out of atmosphere-- Icehouse Climates

Greenhouse- Icehouse supercycles are key aspect of global climatic history; this shows the integration of mantle cycles-lithosphere--hydrosphere and atmosphere; all feed into changes in biosphere, as well

Much more on integrated atmosphere, hydrosphere, lithosphere, biosphere evolution coming up!.............