Lecture Thirty Five - Tectonic basins as sedimentary resting places Flashcards
What is a basin?
A low area on the Earths surface relative to the surroundings.
Explain the diversity of basins.
Basins may develop on:
- Oceanic crust.
- Continental crust.
- Island arc crust (uncommon).
Basins may be small (km^2) or very large (thousands of square km’s).
Basins may change in size and shape due to: - Erosion. - Sedimentation. - Tectonic activity. - Eustatic (global) sea level changes). (These can occur concurrently).
Basins may include multiple environments:
- E.g. continental rift basins include alluvial fans, river deltas and lake environments.
What controls basin evolution?
Sedimentary processes and influenced at the largest scales (time and space) by:
Tectonics:
- The primary architects - topography/relief and accommodation space.
- Sediment transport rate (e.g. stable = slow).
- Types of environments (terrestrial/oceanic environments).
Climate:
The primary determinant of:
- Weathering rates.
- Precipitation and run off.
- Local depositional environments (e.g. glacial fjords vs tropical reefs (I.e. tropical reefs will move more sediments due to more tropical storms etc, where as glaciers are very slow moving).
- Organic productivity (I.e. carbonates will be produced more quickly and much more readily in tropical environments as opposed to in glacial environments).
What are basin classifications based on?
Type of underlying lithosphere (oceanic, continental, transitional).
Position with respect to plate (intracratonic, plate margin).
Type of plate margin nearest to basin (divergent, convergent, transform).
Explain divergent tectonic basins with relation to basin settings.
Narrow continental rift basins.
Narrow oceanic basins.
Passive margin continental shelf and slope basins.
Basin evolutionary stages:
Occurs due to regional thermal mantle convection beneath continental crust.
Stage A: Regional thermal up-doming.
- Regional uplift and erosion.
Stage B: The rift stage.
- The continental crust is stretched and thinned due to mantle upwelling.
- The upper crust is cool and behaves in a brittle manner when stretched it fractures and fats (normal).
- -> results in horst/graben architecture.
- The lower crust is hot and ductule or plastic.
- -> It flows when stretched.
- Both brittle/plastic mechanisms cause the crust to think when extended/streched.
- Upper crustal blocks subside in the zone of extension, but the whole region is uplifted.
- -> A depression or divergent basin at this stage is called a continental rift basin.
An example elf a narrow continental rift basin is the East African Rift (EAR).
This is a elongated rift lake. In a few million years it will either open up into an oceanic basin or be a failed rift which will not go anywhere.
Narrow continental rift basins can have alluvial fans, fluvial (rivers and lacustrine (often carbonate deposits (shell beds ‘coquinas’).
Volcanism - flood basalts may or may not be present in initial refitting.
Bimodal - both mafic and felsic.
Stage C: Breakup phase. - Continued extension leads to crustal/continental breakup. - Then a new sea floor spreading ridge would form in a narrow oceanic spreading basin. - Initially narrow (e.g. Red Sea). --> May evolve into open oceanic basin). - Origin: --> Narrow continental rifts evolve. --> Oceanic spreading ridge develops. --> Oceanic crust in acial basins. --> Continental crust at basin margin. - Environments and facies: --> Same as continental rifts. --> Fan deltas. --> Shore line. --> Shelf and slope. --> Abyssal plain. (Listed in early stages to late stages).
Stage D: Sag phase.
- As the new ocean widens, and the continental margin moves further from the convecting ridge the continental margin subsides = thermal subsidence/sag.
- It is no longer supported by the buoyant convective system.
- Think MOR and how the source of new lava is topographically higher, and as you move away from this point, the topography gets progressively lower.
Stage E: Drift phase.
- The ocean widens.
- Sediment derived from the continent loads the margin.
- -. Continental margin undergoes ‘isostatic subsidence.’
- A continental shelf and slope sedimentary basin prism forms.
- The sea transgresses onto the continental margin.
Summaries what basins form at divergent plate boundaries.
Initial rifting of continental masses = continental rift basins.
–> Alluvial fans, river, lake environments and volcanoes.
Then become continental margin environments as rifts widen and seafloor spreading ridges form.
They become deep marine environments as oceans widen via ongoing seafloor spreading, forming deep abyssal plain basins.
Explain basin inversion.
Stage F (can sometimes happen, sometimes will not): Basin invasion.
- -> The beginning of the ned.
- Transition from extension to compression.
- Normal faults reactivated into reverse faults.
- Uplift and folding.
- Basin sediments are eroded.
- -> Source of next cycle of basins.
- Remnants of original basin preserves.
- Smalled, but distinct basins form.
- Logical progression to the final stage of basin formation.
- Extensional basin is compressed against another plate/continent.
- This creates a lot of uplift and source material for the next cycle.
Explain basins associated with convergent plate boundaries.
Convergent orogenic belts:
- Primary causes of uplift and erosion.
- -> Earth’s primary sediment source.
- Can have extremely wide sedimentary distribution.
- Localised but distinct basins of their own.
Basins associated with convergent plate boundaries:
1) Oceanic plate under continental margin subduction setting.
- -> Continental arc E.g. Andes.
2) Continental collision zones - oceanic closure E.g. Himalayas.
3) Oceanic plate under oceanic plate subduction settings = island arc settings E.g. Marianas and Tonga (This setting are far less common and less significant).
- Tectonically very active.
- Compressional faulting, folding; earthquake prone.
- -> Reverse faults, thrust faults and tight folding.
- Compressional tectonics lead to:
- -> Lagre amounts of uplift and erosion.
- -> Localised lowlands for sediment accumulation.
Explain continental margin arc subduction basins.
Oceanic plate being subducted under continental margin.
Depositional environments range from continental to deep marine e.g. Andes and Cascades.
–> Intra-arc, trench and forearc and foreland (backarc) basins.
** See diagram in notes **
Subduction of oceanic lithosphere produces deep sea trench basin and volcanic arc.
Trench basin:
- Sedimentary succession: 500-2000m thick.
- Forms between the down going oceanic plate and the accretionary prism.
- Sediments: a mi of deep and shallow marine breccia and volcaniclastic facies.
Forearc basins:
- Environments and faces.
- -> Shelf and fluvial/deltaic - includes carbonates.
- -> Deep marine - turbidites and pelagic sediments.
- Sediment sources (aka provenance_:
- -> Volcanic arc sediments (voluminous), carbonates, accretionary prism detritus.
- -> Minor contribution from continental derived material.
- Tend to be overfilled (sediment influx is greater than the accommodation space).
- Average thickness 3-6 km.
Retro-arc foreland basin (or backarc basins):
- Form along the continental interior of orogenic belts (for land fold and thrust belts).
- Thickening of crust drives subsidence - isostatic subsidence.
- Forms wide (100’s of km) and long (more than 1000’s of km) basins.
E.g. North American Cordillera, Andes.
- Sediment sources (aka ‘provenance’):
–> Detritus derived from volcanic arc and thrust belt.
–> Mixed: volcaniclastics, meta-sedimentary, metamorphic, plutonic.
- Environments and facies:
–> Alluvial fan, fluvial (rivers), lacustrine (lakes).
–> May become submerged if global sea levels are high (e.g. shallow marine).
Describe continental collisional basins.
– Long term subduction of oceanic plate under continental
margin.
– Bring “passenger” continent into collision with arc host
continent.
– Oceanic basin closes during collision.
– Subducting continent under thrusts over-riding continent.
– Uplift, mountain range, doubles continental crust thickness.
E.g. Himalayas, European Alps.
- Uplift of the collisional mountain chain (orogen). --> High relief erosional mountain environments. --> Alluvial fan and braided rivers. --> Very thick sediments in collisional “foreland basin” on one or both sides of the orogenic belt. E.g. Indo-Gangetic Plain/Basin at foot of Himalayas. --> Sediments up to 15 km thick. - Basin and crust subside due to both thrust loading (Fold Thrust Belt) and sediment loading.
May also generate giant “escape corridor” passive margin submarine fan systems.
E.g. Bengal and Indus
submarine fans.
- Foreland Basin (at foot of Fold and Thrust Belt).
- Large volumes of sediment flux.
- Environments and facies.
–> Continental, alluvial fans, fluvial, lakes.
–> Escape corridors: submarine fans (turbidites).
E.g. Ganges river bed.
