TECTONICS &

SEDIMENTATION
 The relationship was first emphasized more than a century back

 The concept underwent a revolution since 1960 when plate tectonic

theory became the ruling paradigm

 In pre plate tectonic days sedimentary facies and their assemblages

were used to be correlated with only “ancient geosynclines’ or “orogenic
belts”

 Modern sediments and sedimentary rocks outside orogenic belts were

neglected

 However, this overemphasis on orogenic belts reflects the perception-

tectonics plays the key role in sedimentation

 Tectonics creates relief and erosion-deposition conjointly tend to

remove the relief

 Diastrophism creates the source as well as the sink for the sediments
 Nature of interplate-contact, convergent, divergent or transform, further
determines the slope of the depositional surface

 While slope is gentle along the divergent margin, it tends to be steep

along convergent margin

 Hence, tectonics has a direct influence on the rates of erosion, which in

turn has important control on mineralogical maturity of sediments

 Textural maturity of sediment depends on the extent and energy

condition during transportation, chiefly governed by the tectonic-
topography

 The rate of sedimentation is a function of both rate of supply and rate of

subsidence; both function of tectonism

 The balance determines largely deposition below and above sea level
and that, in turn, determines texture, structure and geometry of
sedimentary units
 The places around the globe where sediments accumulate into stratigraphic
successions are collectively referred to as Sedimentary basins
 Sedimentary basins are morpho-tectonic depressions accommodating
sediments
 The major control on the formation of basins is plate tectonics and basins are
classified in relation to interplate and intraplate stress fields, responsible for
formation and evolution of the basin
 Before the advent of plate tectonics, basins were considered in terms of
Geosynclines
 In the 19th century to explain the huge thickness of accumulations and
subsequent deformation of the strata in mountain belts some mechanism has
been inferred to produce basin and then deformed the strata
 The large downfolds were termed Geosynclines and the upfolds were
Geoanticlines
 A geosyncline was considered to be a site for sediment accumulation which
gradually sink, at least partly, under the weight of added material; eventually
leading to deformation, at least of the lower part, igneous intrusion and
extrusion
 Geosyncline scheme considered two kinds of basins viz., eugeosycline and
miogeosyncline and two kinds of sedimentary assemblages flysch and
molasse
 Pre-plate tectonic classification of sediments:

o Preorogenic o Flysch

o Euxinic o Molasse

 Preorogenic

o This stage is characterised by an quartz arenite-carbonate facies

association; sands are craton-derived and are interbedded with
limestone and dolomite

o This in turn may give way to the euxinic facies-flysch-molasse

o The facies corresponds to passive margin, also termed trailing,

inactive or “atlantic type” in terms of plate tectonics
 Euxinic

o If subsidence is marked but sediment supply is restricted, the basin is

starved and collects only a thin black shaly residuum or becomes the
locus of chemical deposition

o Typical products of such environment are chert, siliceous pelagic

limestone or phosphorite

 Flysch

o Flysch basins are strongly negative areas (geosynclines or trenches)

covered by deep marine water

o Continuous deposition of mud is interrupted time and again by

turbidity underflows, often leads to the development of grading and
rhythmic bedding of sheet or tabular geometry
 Molasse

o If the depositional surface is raised to sea level or above, either by

accelerated sedimentation or decreased subsidence, the sand and mud
are distributed by different processes

o The sand collects in the loci of maximum turbulence (bar and

channels); mud collects in the areas of minimum turbulence
(floodplains)

o Hence sands form linear, textural mature bodies intercalated in a thick

accumulation of silt and clay

o Thus the petrology and geometry of these deposits (molasses) are

radically unlike flysch even though bulk composition is essentially
same

 Though geosynclines are not all alike, they are initiated, filled and
commonly, though not always, deformed
 Filling of different stages have different character

o Early stage: euxinic facies

o Middle stage: flysch facies comprising of alternations between

greywacke and shale; sediments are not entirely derived from craton

o Late stage: flysch gives way upward to molasse facies in which

sandstone become coarser, cleaner and cross-bedded; reducing
condition is replaced by oxidizing environment, red sandstone and
mudstone become abundant; sediments are mostly craton derived

 In some geosyncline successions, the quartz arenites and carbonates of

pre-orogenic stage may occupy the basal part
 So the geosynclinal cycle , in general, upward coarsening, richer in
terrigenous material and transition from reducing marine

 Although this cycle reflects change in source, greywackes are

tectonogenic and quartzarenites are cratogenic (Kay, 1949); the source
shift can also takes place because of change in relief that, in turn, is
related to tectonism

 Flysch refers to deep-water clastic sediments deposited in synorogenic

stage of geosyncline; it commonly passes up into molasses, a shallow
marine to nonmarine deposit formed under late-orogenic to post
orogenic stage

 The so-called flysch and molasses occur in what is now known to be a

peripheral basin associated with collision setting; however, both flysch
and molasses can occur in many tectonic settings, including some that
are not orogenic in the original sense.
 The classical geosynclinal cycles of (1) pre-flysch, (2) flysch, (3)
molasses can now be interpreted in plate tectonic term as a result of
Wilson cycle of (1) oceanic opening by sea floor spreading (2) oceanic
closure by subduction and (3) continental collision

 Present-day sedimentary models indicates that facies of ancient

mountain belts, i.e. preflysch, flysch and molasses may have formed in
several plate-tectonic settings

 Euxinic facies has formed either within an oceanic environment or

within a back-arc basin; a similar but not identical facies because it
lacks volcanics may have formed in the distal basins of a strike-slip
junction.
 The problem with the geosyncline theory is that the forces which
cause subsidence and later deformation of the layers of strata are not
adequately explained

 The Geosyncline scheme could not relate cause of basin subsidence

and evolution, failed to bring in all kinds of basins within its
ambit and was abandoned later

 Plate tectonic scheme of classification of basins was originally

forwarded by Dickinson (1974) and was later improved upon by
Ingersoll (1988), Allen and Allen (1990), Busby and Ingersoll (1995)
and Miall (2000)
 Three major criteria considered in this classification are

o Lithospheric substratum: oceanic versus continental

o Proximity of the basin to a plate margin

o Type of plate margin nearest the basin i.e., convergent, divergent,

conservative (similar to Bally and Snelson, 1980)

 Basic mechanisms for basin subsidence include

o Mechanical stretching: Causes crustal thinning and normal faulting.

o Thermal subsidence: Cooling of the lithosphere

o Loading: a) Tectonic loading or b) sedimentary or volcanic loading

 Crustal thinning and normal faulting dominate in case of mechanical

stretching (divergent margin) , whereas thrust faulting dominates in
case of tectonic loading (convergent margin).
 Sedimentary loading cannot form a sedimentary basin on its own,
but magnifies the effect of tectonic subsidence (a factor of 2.2 to 2.5)

 Areas of high sediment influx, such as deltas, thus subside relatively

rapidly

 Cooling is most important in intraplate setting which originates at

divergent plate boundaries, i.e., mid-oceanic ridges (e.g. passive margin
basins)

 For strike-slip setting, basins may form either due to crustal extension
beacause of mechanical stretching and subsequent thermal subsidence
or due to tectonic loading (because of development of thrust belts in
areas of compression) depending on the local stress regime
 Three other, relatively less important, mechanisms are

o Subcrustal loading: Lithospheric flexure during underthrusting of

the dense lithosphere, important for basins close to the subduction
zone

o Asthenosphere flow: Dynamic effects of the asthenospheric flow,

commonly due to descent of subducted lithosphere provides minor
component of subsidence in many basins

o Crustal densification: Increased density of the crust due to

changing pressure/temperature conditions and/or emplacement of
higher-density melts into lower density crust

 In reality, a complex combination of these processes contributes to the

total subsidence of a basin, although one process may dominate
depending on the plate tectonic setting
 Considering all these complexities, sedimentary basins can be
classified in following five major types

o Basins associated with divergent settings

• Terrestrial Rift Basins

• Proto Oceanic Basins

o Basins in intraplate settings

• Pasive Continental Margin Basins

• Intracratonic Basins

• Oceanic Basins

• Dormant Oceanic Basins

o Basins associated with convergent settings
• Trench Basins • Retroarc Foreland
Basins
• Trench-Slope Basins
• Remnant Ocean Basins
• Forearc Basins
• Peripheral Forland
• Backarc Basins
Basins
• Intra-arc Basins
• Piggyback Basins
o Basins in transform and transcurrent fault settings

• Pull-apart Basins

o Basins in hybrid settings

• Intercontinental Wrench Basins

• Aulacogen
• Impactogen
• Successor Basins
Classification of sedimentary basins in terms of tectonic setting
Basins associated with Divergent Settings
 Basins are produced either due to ‘active rifting’ (asthenospherically driven)
or ‘passive rifting’ (lithospherically driven)
 In case of active rifting, the impingement of a thermal plume (or hot spots) at
the base of the lithosphere causes crustal domal uplifting, extension and
thinning
 There is increased heat flow associated with this type of rifting and this results in
voluminous bimodal volcanism
 In case of ‘passive rifting’ the extensional stresses in the continental
lithosphere cause thinning and passive upwelling of hot asthenosphere
 Subsidence history of active rift basins is complicated, but the subsidence
behavior of passive rift basins is well understood
 In this model rift basins are characterized by a relatively rapid, fault controlled
initial subsidence (also known as syn-rift subsidence), due to crustal and
lithospheric thinning followed by a slow thermal subsidence (also known as
post-rift subsidence or sagging) as the crust and lithosphere cools
 If rifting continues to critical values of stretching then proto oceanic rift trough
and passive margins develop
 Major types of basins includes Terrestrial Rift Basins and Proto Oceanic
Basins
Terrestrial Rift Basins
 Active rift basins, e.g., East African Rift System, form in areas of domal upwarps
and are characterized by early volcanism, while passive rifting because of
lithosphere stretching results in fault-bounded terrestrial rift valley (e.g. Gulf of
Suez)
 The axis of the basin lies, more or less, perpendicular to the axis of extension
 Eolian, fluvial, alluvial and lacustrine deposits are common and marine
sediments are rare within the terrestrial rift valleys
 Basin bounding faults may be planar or listric and commonly form half grabens
 In case of continental rift the major drainage should be parallel to valley axis,
while tributaries should flow along the gentler valley flank, at high angle to the
main drainage
 In case of half grabens, because of progressive subsidence towards the steeper
flank the main drainage may gradually shift closer towards the steeper flank
 Sediments are derived usually from the gentler flank

 As a consequence sedimentation rate is generally low and sediment

grain-size does not exceed sand grade

 Nonetheless, coarse grained sediment may form sedimentary cones on

the steeper flank

 Deltas that form on the gentler flank are generally of mouth-bar type,
while those forming on the steeper flak are likely to be of Gilbert-type

 Dominance of vertical movement leads to unroofing of silicic plutonic

bodies
 As a result, sand is quartzofeldspathic tending to be arkosic; however,
paleoclimate also has a role in determining mineralogical composition of
sand; K-feldspar should dominate over plagioclase feldspar

 Volcanics and volcaniclastics , silicic or bimodal, may occupy a substantial

part of the sequence that may develop, depending on the tectonic nature of
the rift setting

 In Maritime rifts, which are those flooded by the sea, coarse sediment is
deposited as fan deltas at the basin margins and distributed by wave, tide,
storm and gravity currents

 In the absence of terrigenous clastic material, carbonate sedimentation may

be dominant within such rift basins, consisting of reefs on the basin flanks
and deep water carbonates in the centre
Proto-oceanic rift troughs (the transition from rift to ocean)

 Continued extension in the continental crust eventually leads to complete

rupture

 Basaltic magma usually rises along the axis of the rift and oceanic crust begins to
develop

 When there is a thin strip of basaltic crust in between two halves of a rift system
the basin is called Proto-oceanic trough

 Thermal subsidence associated cooling of freshly erupted basaltic magma is the

main cause of subsidence in this basin

 Sediment supply comes from flanks of the trough which will be uplifted
 Rivers will feed sediment to shelf areas

 Connection to the open ocean may be intermittent and the areas of high
evaporation may periodically desiccate

 Evaporites may form part of the succession

 Red sea is the only modern example of proto oceanic rift acting as a
narrow seaway between continental blocks

 Rivers feeding the Red Sea from Arabian and African margins supply
clastic detritus and carbonate sedimentation occurs in warm waters

 In the early stage of the basin formation when connection with the open
ocean was intermittent evaporation deposition was important
Basins in Intraplate Settings

 Once an ocean is fully open during Wilson cycle, sedimentary basins

may be present in a variety of settings along the margin within both
continental and oceanic plates

 Plate margin processes influence development of these intraplate basins

 Sedimentation in a passive margin commences after completion of

rifting and as a new ocean basin has begun to form by sea floor
spreading

 Eventually, the plate boundary or the spreading ridge far seaward of the
basin, locking the basin into a relatively stable platform at the edge of
the rifted continent

 Major types of basins includes Passive Continental Margin Basin,

Intracratonic Basins, Active Ocean Basins and Dormant
Ocean Basins
Passive Continental Margin Basin

 A passive margin is the transition between oceanic and continental

crust which is not an active plate margin

 The term passive is used as the opposite to the active margins between
oceans and continents where oceanic crust is being subducted

 The continental crust is commonly thinned in this region and there may
be a zone of transitional crust before fully oceanic crust is encountered

 Subsidence of the passive margin is mainly due to continued cooling of

lithospere as the heat source of the spreading centre becomes further
away, augmented by the load on the crust due to the accumulated
sediment

 Morphologically the passive margin is the continental shelf and slope

 They include Continental rises and terraces (wedge-shaped sediment

pile on seaward-side of continental slope) and Continental platforms
 They include Continental rises and terraces (wedge-shaped sediment pile on
seaward-side of continental slope) and Continental platforms
 Continental platforms are stable cratons covered by thin, laterally extensive
sedimentary strata
 Basins developed on these stable platforms are referred to as cratonic basins
 They are commonly bowl shaped (ovate)
 Tectonism being subdued, subsidence rate remains virtually constant, but
increases progressively basinward
 In a passive margin basin depositional slope gradient decreases to a minimum
 As a result, rate of sedimentation and terrigenous sediment grain size decreases
 On account of a much wider shelf coming within wave influence, sands are likely
to be mature, both texturally as well as mineralogically
 In deeper parts mud deposition takes place; in case of curtailment of terrigenous
supply, carbonate deposition is encouraged
 In the quartz population plutonic grains dominate; K-feldspar dominates over
calcic feldspar
 Sediments commonly thicken towards the basin interior
Intracratonic Basins

 Intracratonic basins are relatively large, commonly ovate downwarps that occur
within continental interiors away from plate margins

 Often floored by remnant rifts

 Formed over continental crust

 Subsidence in intracratonic basins are largely due to mantle-lithosphere cooling

and sedimentary or volcanic loading

 After rifting within the continental crust a change in thermal regime takes place

 Continental crust extended and thinned brings hotter mantle material closer
 After cessation of rifting geothermal gradient of the area reduced,
leading to cooling of the crust; as cold rocks are denser than hot rocks,
cool lithosphere sinks, resulting thermal subsidence and in turn produce
intracratonic basins

 Intracratonic basins are typically broad but not very deep and the rate of
subsidence due to the cooling of the lithosphere is slow

 Fluvial and lacustrine sediments are commonly encountered in these

basins although flooding from the adjacent ocean may result in a broad
epicontinental sea

 Intracratonic basin a wholly continental settings are very sensitive to

climate fluctuations as increased temperature may raise rates of
evaporation in lakes and reduce the water level over wide area
Intracratonic Basin, Chad Basin, central
Africa

Schematic cross-section of the

Intracratonic Michigan Basin, North
America
Active Ocean Basins

 Basaltic crust formed at mid-oceanic ridges is hot and relatively buoyant

 As the basin grows in size new margins created along the spreading
ridges, older crust moves away from the hot mid-oceanic ridges

 Cooling of crust increases its density and decreases relative buoyancy as

it moves away from the ridges

 The depth of the basin increases away from the ridges (around 2500m)
to 4000-5000m where basaltic crust is old and cool

 The ocean floor is not a flat surface

 Spreading ridges tend to be irregular, offset by transform faults; isolated
volcanoes and linear chains of volcanic activity form submerged
seamounts

 In addition to the formation of volcanic rocks, shallow water

environment may be a site for carbonate deposition and formation of
reefs

 In the deeper part of the ocean basin sedimentation is mainly pelagic,

consisting of fine grained biogenic detritus and clays

 Nearer to the edges of the basins terrigenous clastic material may be

deposited as turbidites

 Sediments deposited in such basins adjacent to active margins may

eventually be subducted into a trench and consumed during an episode
of ocean closing

 Alternatively, they may be offscraped in trenches

Dormant Ocean Basins
 These basins differ from active basins in the sense that these ocean basins are
neither spreading, nor subducting

 There are no active plate boundaries in near vicinity

 Floored by oceanic crust

 Dormant ocean basins form in two ways

o Spreading ridges of proto oceanic rifts or small ocean basins stop activity due
to plate margin rearrangement (e.g., Gulf of Mexico)
o Remnant ocean basins are not closed, but subduction stops, leaving an oceanic
gap in the suture belt (e.g., Black Sea, North Caspian Sea)
 The dominant mode of subsidence is sediment loading

 If the oceanic lithosphere is young (e.g., Gulf of Mexico) then thermal

subsidence also becomes significant

 Sediment nature is similar to active ocean basin but lacks volcanic activities
Basins associated with Convergent Settings
 At convergent setting two lithospheric plates converges towards each other
leading to the subduction of oceanic plates and ultimate collision and suturing of
continental plates, after total subduction of the oceanic plate
 Different types of basins, related to either subduction of oceanic plate or
collision between two continental plates, can be produced within this setting
 Basins developed under such setting can further be grouped according to the
nature of convergent margin
o Basins related to subduction
o Basins related to continental collision
Basins related to Subduction

 In this setting, the downgoing ocean plate descends into the mantle
beneath the overriding plate (subduction) which may be a oceanic or
continental plate

 As the downgoing plate bends to enter the subduction zone a trough is

created at the contact between two plates; this is Ocean trench

 The descending slab is heated as it goes down and partially melts

 The magmas generated rise to the surface through the overriding plate to
create a line of volcanoes, a volcanic arc

 Arc-trench systems are the regions of plate convergence, yet there may be
local extension as well as compression in the upper plate
 The amount of extension is governed by the relative rates of plate
convergence and subduction and this is in turn influenced by the angle
of subduction

 If the angle of subduction is steep then convergence is slower than

subduction at the trench, the upper plate is in net extension

 Steep subduction occurs if the downgoing plate consists of old, cold

crust

 However, not all backarc areas are under extension: some are ‘neutral’
and others are sites of the formation of a flexural basin due to thrust
movements at the margins of the arc massif (retroarc basins)

 Major types of basins includes Trench Basins, Trench-Slope

Basins, Forearc Basins, Intra-arc Basins, Backarc Basins,
Retroarc Foreland Basins and Remnant Ocean Basins
Trench Basin

 Ocean trenches are elongate, gently curving troughs that form where an oceanic
plate bends as it enters a subduction zone
 Deepest (up to 11km, average depth is about twice of the average ocean bottom)
basins of the world
 They are also narrow, sometimes as little as 5 km across, although they may be
thousands of kilometres long
 Subduction occurs beneath either an oceanic lithosphere (Mariana type) or
continental lithosphere (Andean type)
 Peru-Chile trench system is an ideal example

 Deposits include pelagic deposits and the arc derived sediments

 Trenches formed along margins flanked by continental crust tend to be

filled with sediment derived from the adjacent land areas

 Intra-oceanic trenches are often starved of sediment because the only

sources of material apart from pelagic deposits are the islands of the
volcanic arc

 Transport of coarse material into trenches is by mass flows, especially

turbidity currents that may flow for long distances along the axis of the
trench
Trench-Slope Basin

 The sedimentary pile accumulated on the ocean crust and in the trench is not
necessarily subducted along the destructive boundary

 The pile of sediment may be wholly or partially scarped off the downgoing plate
and accrete on the leading edge of the of the over-riding plate to form an
accretionary prism or complex

 Small, flat-floored basins restricted within such accretionary prism and bounded
between thrust sheets are known as Trench-Slope Basin

 These prisms or wedges are best developed where sediment thickness in the
trench is thick

 Deposits include oceanic sediments

only, mainly pelagic sediments and
turbidites, bringing sediments to
overriding plate

 Such sediments are usually deformed

Forearc Basins
 Occur between trench-slope break and the parallel volcanic arcs

 Width of a fore-arc basin varies from 50 to 300km depending on arc-trench

gap controlled by subduction angle

 The basin may be underlain by either oceanic crust or a continental margin

 The thickness of sediments that can accumulate in a forearc setting is partly

controlled by the height of the accretionary complex

 Subsidence in the forearc region is due only to sedimentary loading

 The main source of sediment to the basin is the volcanic arc

(volcaniclastics) and, if the arc lies in continental crust, the hinterland of
continental rocks

 Intraoceanic arcs are commonly starved of sediment because the island-arc

volcanic chain is the only source of detritus apart from pelagic sediment
 Even fore-arc basins may be altogether absent in intraoceanic arcs, if
the subduction of oceanic sediment predominates and in consequence,
no accretionary prism can form

 Given sufficient supply of

detritus a forearc succession
will consist of deepwater
deposit at the base
shallowing upward to
shallow marine, deltaic and
fluvial sediment at top

 Volcaniclastic debris is
likely to present in almost
all cases
Sunda trench of Sumatra Island
Intra-arc & Backarc Basins

 Extensional intra-arc or backarc basins form where the angle of

subduction of the downgoing slab is steep and the rate of subduction is
greater than the rate of plate convergence

 Rifting occurs in the region of the volcanic arc where the crust is hotter
and weaker

 Initially the arc itself rifts and splits into two parts, an active arc with
continued volcanism closer to the subduction zone and a remnant arc

 At this stage an intra-arc basin forms, a transient extensional basin

that is bound on both sides by active volcanoes
 As extension between the remnant and active arcs continues, a new
spreading centre is formed to generate basaltic crust between the two

 The region of extension and the crustal formation is the backarc

basin

 This backarc basin continues to grow by spreading until renewed rifting

in the active arc leads to the formation of a new line of extension closer
to the trench

 Once a new backarc basin is formed the older one is abandoned

 The lifespan of these basins is relatively short

 Extensional backarc basins can form in either oceanic or continental

plates

 Continental backarc basins forms behind continental-margin arcs are

similar to foreland basins, but are not associated with foreland fold
thrust belt
 Intra-arc basins are the site of accumulation of mainly volcanically derived
sediment, that is, volcaniclastics
 Distinction from backarc sediment pile is difficult in rock record
 The principal source of sediment in a backarc basin formed in an oceanic plate
will be the active volcanic arc

 Among sandstones, litharenites

and felsarenites are common
 The majority of lithic fragments
are volcanic in origin
 Plagioclase feldspar dominates
over alkali feldspar
 More abundant supplies are
available if there is continental
crust either or both sides of the
basin
 Backarc basins are typically
under-filled, containing mainly
deep water sediment of
volcaniclastic and pelagic origin
The Samisu rift is an example of a
Backarc Basin, south of Japan
Retroarc Foreland Basins

 At ocean–continent convergence settings, shortening in the overriding

continental plate and subduction related magmatism can create a
mountain belt

 Thrust belts on the landward side of mountain chains in these settings

result in loading and the formation of a ‘retroarc’ foreland basin

 While a ‘back-arc’ basin is subjected to extensional or neutral stress, a

‘retro-arc’ basin is subjected to compressional stress

 The loading of the crust on the opposite side of the arc to the trench
results in flexure, and the formation of a basin

 These basins are called ‘retroarc’ because of their position behind the
arc and the ‘foreland’ because the mechanism of formation is by
flexure of the loading edge of the continent in a similar way to peripheral
foreland basins
 The continental
crust will be
close to sea
level at the time
the loading
commences so
most of the
sedimentation
occurs in
fluvial, coastal
and shallow
marine
environments

 Main source of detritus is from the mountain belt and the volcanic arc

 The Andes have been uplifted by crustal thickening and the intrusion of
magma associated with subduction of the Pacific Plate at Peru-Chile
trench
Remnant Ocean Basins
 These are sinking ocean basins flanked by at least one convergent margin
 As a rifted continental margin approaches a subduction zone, coastal
promontories, resulted from hot spots, failed rifts and transform faults, collide
first
 Diachronous orogenic uplift and erosion result
 Adjacent remnant ocean basins thus become natural repositories of voluminous
sediments derived from the growing orogenic belts
 Progressively increasing sedimentation rate leads to shallowing of the basin and
eventually lead to continental sedimentation (molasse stage)
Basins related to Continent Collision

 When an ocean basin completely closes with the total elimination of

oceanic crust by subduction two continental margins eventually
converge

 Where two continental plates converge subduction does not occur

because the thick, low-density continental lithosphere is too buoyant to
be subducted

 The result is collision between two converging plates

 Collision of plates involves a thickening of the lithosphere and the

creation of an orogenic belt, a mountain belt formed by collision of
plates

 The two continental margins which collide are likely to be thinned,

passive margins
 Shortening initially increases the lithosphere thickness up to ‘normal’
values before it over-thickens

 As the crust thickens it undergoes deformation with metamorohism

occurring in the lower parts of the crust and faulting and folding at
shallower levels in the mountain belt

 Thrust faults form a thrust belt along the edges of the mountain chain
within which material is moved outwards, away from the centre of
orogenic belt

 Major types of basins includes Peripheral Foreland Basin and

Piggyback Basins
Peripheral foreland basins

 The thrust belt moves material on either side of the orogenic belt

 Under this load the crust flexes to form a peripheral foreland basin

 The width of the basin will depend on the amount of load and the
flexural rigidity of the foreland lithosphere

 Rigid (typically older, thicker) lithosphere will respond to form a wide,

shallow basin, whereas younger, thinner lithosphere flexes more easily
to create a narrower, deeper trough

 Increasing the load increases the basin depth

 In the initial stages of foreland basin formation the collision will have
only proceeded to the extent of thickening the crust (which was formerly
thinned at a passive margin) up to ‘normal’ crustal thickness
 Although this results in a load on the foreland and lithospheric flexure in the
orogenic belt itself will not be high above sea level at this stage and little detritus
will be supplied

 Early foreland basin sediments will therefore occur in a deepwater basin, with
the rate of subsidence exceeding the rate of supply

 Turbidites are typical of this stage

 When the orogenic belt is more mature and has built up a mountain chain there
is an increase in the rate of sediment supply to the foreland basin outpacing the
increased rate of subsidence

 Foreland-basin stratigraphy typically shallows up from deep water to shallow

marine and then continental sedimentation, which dominates the later stages of
foreland-basin sedimentation

 In foreland basins formed during the tertiary in Europe as part of the Alpine
orogeny, the early, turbidite-rich stage of sedimentation is referred to as the
‘flysch’ stage and later , shallow marine to continental deposits are referred to
as the ‘molasse’ stage
 Foreland basin stratigraphy is often complicated by the deformation of the
earlier basin deposits by later thrusting

 The basin will tend to become larger with time as more load is added, and the
later deformation at the margin will include some of the earlier basin deposits

 Erosion and reworking of older basin strata into the younger deposits are
common

 Sometimes thrusting may subdivide the basin to form piggy-back basins

which lie on top of the thrust sheets and which are separate from the foredeep,
the basin in front of all the thrusts

 These basins form and get

filled up by sediment
while being carried on
moving thrust sheets
 Sediments share
character with those of
foreland and trench-slope
basins
Basins in Transform and Transcurrent Fault Settings

 Transform faults are strike-slip faults that define plate boundaries

and penetrate the entire crust, whereas transcurrent faults are
restricted to intraplate areas and penetrate only the upper crust

 If a plate boundary is a straight line and the relative plate motion purely
parallel to that line there would be neither uplift nor basin formation
along strike-slip plate boundaries

 However, such plate boundaries are not straight, the motion is not
purely parallel and they consist not of a single fault strand but of a
network of branching and overlapping individual faults
 Strike-slip faults tend to curve, split into branches and are frequently
offset in an en echolen fashion. These complex patterns give rise to
localized extension and compression

 The combination of strike-slip motion and extension is known as

transtension, and the combination of strike-slip motion and
compression is known as transpression

 Transtensile regions exhibit normal faulting, basin extension and

volcanism

 Transpressive regions develop thrust faulting, folding and uplift

 The basins that result from strike-slip faulting are named as

“Transtensional Basins”
 These basins can form by three main mechanisms

o The overlap of two separate faults can create regions of extension

between them known as pull-apart basins; typically rectangular or
rhombic in plan with widths and lengths of only a few kilometres or
tens of kilometres; unusually deep, especially compared with rift
basins

o Where there is a branching of faults a zone of extension exists

between the two branches forming a basin

o The curvature of a single fault strand results in bends that are either
restraining bends (locally compressive) or releasing bends (locally
extensional); releasing bends form elliptical zones of subsidence

 These basins are generally short-lived; subsidence rate is usually rapid

resulting in unusual thickness of the sediment pile
 Typically the margins are sites of deposition of coarse facies (alluvial
fans and fan deltas) and these pass laterally over very short distances to
lacustrine sediments in continental settings or marine deposits

 Basin-margins accumulate coarse and immature (both mineralogically

and texturally) sediments in coalesced fans

 The conglomerates are likely to be polymictic and poorly sorted. Lith-

arenites are likely associates within which supracrustal, rather than
plutonic rock fragments would dominate

 Facies transitions, vertical and lateral would be frequent because of

supply of sediment from both the basin flanks
Basins formed at strike-slip plate boundaries and in the regions of
intra-continental strike-slip tectonics
Basins in Hybrid Settings

Intracontinental wrench basins

 These basins form due to collision of continents of varying shapes and sizes

 They are diverse in nature and share unusual complexities

Aulacogens

 Aulacogens are long-lived, deeply subsiding, fault-bounded sedimentary basins

that extend at high angles from the margins towards the interior of continents

 Aulacogens are believed to form as the result of evolution of a rift triple junction

 As plate separation proceeds, one of the arms of the triple junction becomes
inactive, and remains preserved in the continent as a failed rift or aulacogen
Impactogens

 Rift basins caused by stresses transmitted from convergent plate

margins or impact of some extraterrestrial objects

Successor basins

 These basins form in intermontane settings on top of inactive fold thrust

belts

 Successor basins owe their origin to end-orogenic activity

 As these basins develop in near-absence of any tectonic activity,

sediment-water loading turns to be the significan mechanism for
subsidence
Wilson Cycle

 The periodicity of ocean

formation and closure is
known as Wilson cycle
 It approximated to about
500My tectonic cycles for
formation and destruction of a
ocean basin
 The Wilson cycle predicts that
at any place tectonic setting
changes through time
 A piece of continental crust
may experience rift extension
initially and then become the
passive margin to an ocean;
during closure of the ocean
basin a passive margin may
become a fore-arc region
related to a subduction zone
and finally be incorporated
into an orogenic belt upon
continental collision