Deformation of continents Flashcards
Collision of India with Asia
Shallow earthquakes in Asia (<30km) and little subduction. Earthquakes from Himalayas to Mongolia.
Most earthquakes from reverse faults.
India and Asia converged at 150mm/yr during the Mezozoic and since collision 40Ma this has slowed to 50mm/yr
Normal fault
Extensional
Reverse fault
shortening
less than 30 degree dip labels them thrust faults
Strike-slip faults
horizontal slip parallel to strike e.g. San Andreas
Oblique slip
Combination of strike-slip and reverse/normal behaviour
Fault in Asia
Thrust faults bound high mountains e.g. Himalayas
Strike slip faults throughout
Building the himalayas
- Asia and India collide (40Ma)
- Reverse fault line forms due to compressional stress
- Elastic PE released at reverse fault line and hanging wall emplaced on foot wall.
- New reverse fault line forms from compression
- This repeats and topography rises
Currently active thrust faults: Main central thrust, main boundary fault
Structure of Himalayas, from Asia to India
- Kangdese granites - formed while Indian oceanic crust subducted under Asia
- Indus-Tsangpo suture zone - ophiolites and remains of northern India
- High grade terrain of himalayas - High grade metamorphic rocks have been uplifted by reverse faulting.
All from continental India
African northern faulting earthquakes
Follow the trace of the rift valleys East Africa
East African extension
Extension rate is slow (3-4mm/yr) compared to rate e.g. at mid-atlantic ridge 20mm/yr.
Slow rifting, so mantle that upwells can cool partially. Very low temperature eruptions. E.g. Ol Doinyo Lengai which erupts carbonatite. Less silicate chains, so low viscosity. Max temperature of 580 degrees celsius.
Sub-parallel fault scarps that bound basins (graben). Generally since faults in a region dip the same way, you get asymmetrical basins (half-graben)
Hot spot at centre of the rift. Mantle plume creates upwelling throughout the region
Greece
Earthquakes across Aegean, Turkey and Greece
Aegean is the site of a subduction zone where Mediterranean subducts under crete. Earthquakes follow western border of greece and the south of Crete.
North of subduction zone, there’s normal faulting and N-S extension at 35mm/yr. Total amount of extension is 200-300km. Extension started 5Ma.
These extensional faults often tilt to decrease the angle of dip. This increases surface area, creates half-grabem and crustal thickness decreases.
Deformation - what links Greece and Tibet
Deformation spread over such large areas that not described well by rigid plates. Hence, these locations aren’t on a plate but on a regional of deformation separating plates
Limiting the height of mountains
Mountains are in isostatic equilibrium, but this isn’t stress balance. So, they’re supported by the strength of the lithosphere.
Excess thickening causes excess pressure exerted on mantle beneath. This is counteracted by force upwards on the mountain by the mantle.
Mountains can be supported up to 5km unless there is shortening and they spread to make plateaus like Tibet.
Why the continents deform differently than oceanic crust
Significantly more silica rich than the mantle. 40% silica in peridotite. Up to 70% in continental crust. Continental crust has a low enough density that it will not subduct into the mantle.
Weaker than oceanic crust and mantle because closer to its melting temperature - more susceptible to deformation by creep
Continents are formed as early as 3800Ma, much older than oldest oceanic crust which formed at 200Ma. So, continents retain more faults and lines of weakness from the past which can be reactivated.
Differences between continental-continental collision and collisions involving oceanic crust
Continental crust has a density of 2800kg/m^3 compared to 3300kg/m^3 which is the density of the upper mantle. So continental crust doesn’t subduct into the mantle.
The crustal shortening occurs over significantly larger areas, rigid plate model is less relevant.
