May 15, 2020

A simple rift basin sandbox model with normal faults

Posted by larryohanlon

By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab

Present-day Earth’s deepest lakes (Lake Baikal and Lake Tanganyika) are situated in continental rift basins bounded by normal faults. These basins form due to extensional tectonic movement and are a good visual to check out on Google Earth. Lake Baikal is just over 1 mile (1.6 km) deep at its deepest point, with up to 7 km or so of sediment between the lake bottom and basin floor. It is shown below on Google Earth. The lake itself is about 19 miles (30 km) wide between the opposing yellow arrows.

I think these lakes are interesting because they sit on top of very impressive sediment fills accumulating by the host basin. The simple block diagram shown below illustrates this concept (sourced here). Note the vertical scale compared to the horizontal scale…the diagram is HIGHLY vertically exaggerated to shown representative features, as the real basin is much, much wider than it is deep.

The black lines along the land surface are normal faults; they are projected downward with uncertainty by the dashed lines.

A simple model of a continental rift basin that develops some characteristics of the real thing can be made by constructing a layered sand cake on top of two overlapping sheets of paper, one of which is anchored to the underlying board, etc. This model setup will produce an asymmetric half-graben style of basin, which has a single, high displacement breakaway fault on one side (the left, below) and several smaller faults on the other (the right, below).

“Basement” represents the pre-rift continental crust; the white material under it represents weakened crust along the rift axis. The colorful basin fill layers are cut by normal faults which extend up out of the basement.

This is about as easy as it gets in terms of visually useful sand models. It works in the same way as the real thing. When one sheet is pulled, the principal stress becomes vertical and the sand pack fails along normal faults, allowing it to collapse and form a basin.

When the un-anchored sheet is pulled, the overlying sand cake will begin to collapse along normal faults, producing a basin. Sand layers can be deposited into the basin as it develops to represent sediments filling the accommodation space. This model setup can be made with sand alone, but using a weaker granular material (glass microbeads in this case) to represent weaker basement along the rift axis will allow the model to develop structural characteristics that don’t occur with uniform dry sand.

Watching the surface of the model is certainly illustrative, but if the model can be gelled and sliced it will show interesting normal fault patterns. Use of the weak microbeads under the rift axis typically produces rotation of the overlying stronger basement and the basin’s sediment fill, causing the layers to dip more and more with depth. The microbeads are the white material beneath the thick gray basement in the model section below.

The deepest yellow, orange and blue layers dip towards the deep part of the basin. This model produced interesting conjugate sets of normal faults, along with rotation of blocks of the basin fill (the tilted sections of orange and blue layers)

Real half-graben rift basins tend to have very complex patterns of sedimentation due to the differing topography and river system layouts on each of their flanks, along with large-scale lake level fluctuation and other climatic effects. The portion of the Baikal basin shown below is a good example–the Selenga River Delta on the right introduces very different sedimentary materials from the short, steep rivers entering the lake into much deeper water at the left.

The Selenga delta is the broad, rounded shape pushing out into the lake. The tiny, very steep rivers entering the lake opposite the delta introduce very different types of sediment. I bet the Selenga delta has really bad mosquitoes in the summer…

Different basin fill materials can be added to different parts of the model to illustrate spatially variable sedimentation patterns. Using different fill materials will, however, impact the faulting pattern within the basin if their mechanical properties are different.

My only significant field experience with a rift setting is mapping in part of the Richmond basin, after which the bottom diagram is modeled. Uplift and erosion of this Mesozoic basin has exposed its insides at the land surface, offering a unique perspective on features that are typically buried. Note that these diagrams are stylized and very highly vertically exaggerated, like the first block diagram. Image sourced here.

I tried to add the different material based on the shape of the growing basin and where its deepest point was located. The right side of the model, which is cut by several faults and tapers more gently, received colored layers representing sand-dominated sediment brought in by large rivers. The deepest parts of the basin received the white material, representing deep water muds that accumulate far from sediment inputs on the basin margin. These muds are occasionally covered by lobes of sand that spread out from the right side of basin, representing turbidity currents moving coarse sediment to deeper water. The left side of the basin, along the single breakaway fault, received green sand, representing very coarse fan-type material produced by the short, steep rivers along this steep flank of the rift. Overall, this pattern broadly resembles processes expected of a humid, lower-latitude deep rift lake, like modern-day Tanganyika or the Richmond Basin of the Mesozoic (see bottom diagram above).

This style of model is very simple and leaves out several variables, particularly the uplift of the rift’s flanks resulting from high heat flow beneath the rift. This uplift would be especially noticeable on the footwall of the breakaway fault, which should rise abruptly from the lake or flat basin surface as a narrow and rugged mountain range . The basins produced by these models are also too short and thick. The Baikal and Tanganyika Rifts are up to 10 times as wide as they are deep (or wider), while the materials and deformation style of the models limit their width to 3 or 4 times their depth. Even so, they make a useful visual for connecting horizontal plate movement with vertical surface movement and some of the planet’s most notable freshwater features.

The paper linked here offers some interesting comparison of Baikal and Tanganyika. The image below shows the general location of Baikal’s deepest point, on the downthrown hanging wall of a normal fault bounding the long peninsula and underwater ridge. The Selenga Delta is near the bottom left of the image.

This post was originally published on The Geo Models blog.