Sandbox models are a great way to understand how tectonic forces, sedimentation and erosion result in the formation of different geological structures and their surface expressions.
Geological processes are slow; so slow in fact that most of these difficult to comprehend and understand in the natural setting. We see structures as static and sometimes have difficulty visualising how these structures evolved with time. This is where sandbox models can help us: these are simple, easy to design and provide a lot of flexibility for artificially creating the different types of geological settings.
Rift basins are formed due to extensional tectonics. These usually start as a result of mantle convention trying to move two regions of the crust apart from one another. This results in development of normal faults, horst-and-gabbern structures and crustal thinning. If the rifting continues, oceanic crust will start to form along the rift axis in the centre of the zone and with time the setting will change from rift to drift. In some cases the rifting is not complete and stops before the development of oceanic crust. Naramada rift valley in India is an example of such a failed rift system.
Growth faults are also formed in extensional setting. The cause of deformation is usually gravitational instability caused due to sediment loading, for example, in front of a river delta. Normal faults dipping away from the coast are developed and the movement along the fault plane is contemporaneous (i.e. happening at the same time) with the sedimentation. Normal fault motion creates extra accommodation space near the fault plane, which results in an increase in the thickness of the beds closer to the faults. In many cases the main fault is curved with concave side up, and is known as a listric fault.
Formation of In-sequence Thrusts
Thrust faults form in compressional settings. During compression the rocks first undergo folding during which the tightness of the folds increases. Eventually the folds become overturned and with even more compression the rocks start to fail, leading to the start of faulting. Thrust faulting results in crustal shortening and crustal thickening. As the crust thickens, its weight increases, making it difficult far the thrust motion to lift the rocks up. Under such a scenario, the new thrust fault develops away from the thickened crust. With time a sequence of thrusts develop from hinterland (older thrusts) to the foreland (younger) thrusts. This phenomenon is know as ‘in sequence thrusting.‘
Thrusts: Impact of Erosion
In most of the world’s thrust belts, thrusting is associated with erosion. As the thrust belt uplifts the rocks, these start to undergo increased weathering and erosion. For example, the Himalayan front is experience extreme erosion at the same time as the tectonic forces are trying to uplift it. Whether, a mountain belt will increase or decrease in height depends on the balance between the tectonic forces of uplift and the erosive forces.
In sequence thrusting is a result of crustal loading of the hinterland due to increased crustal thickness. If somehow the crustal material from the hinterland is removed (which mostly happens due to erosion and mass wasting), the thrusting in the hinterland can be activated again. This shifting of thrust sequence form the foreland to the hinterland is called ‘out of sequence thrusting.’
This models very nicely shows the evolution of structures associated with some of the famous mountain systems in the world.
Strike Slip Faults
Strike slip faults are not just simple planes where one block horizontally moves past the other. Strike slip motion is usually accommodated by smaller normal or thrust faults. See the video of the sandbox model for more details.
Formation of a Pull Apart Basin: Strike Slip Motion
Pull apart basins develop in strike slip setting. These form at places where there is a bend in the strike slip fault. The orientation of the bend is such that the rocks at the bend move away from one another. This results in the formation of pull apart basins.
Formation of a Pop-up Flower Structure: Strike Slip Motion
Pop-up structures are opposite of the pull apart basins. Pop-up structures, also known as flower structures, develop in strike slip setting. These form at places where there is a bend in the strike slip fault. The orientation of the bend is such that the rocks at the bend move towards one another.
A description of the project and the works presented.
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