Because this chapter is so extensive, I will only summarize key features of a two dominant models for extension of the lithosphere: (1) pure shear (uniform extension, McKenzie model) and (2) simple shear extensional models (Wernicke).

In all models we have an effective thinning of the crust and an increase in heat flow.

As in plate tectonics, we have two different philosophies in the study of basins caused by extension of the lithosphere.

One says that a thermal plume (hot spot model, or active rifting model) uplifts the lithosphere creating a region of doming. The uplifted lithosphere now has a tendency to spread away from the center of the uplift toward the sides, moving down the dipping and slippery asthenosphere-lithosphere boundary. The crust breaks under its own weight.

The other philosophy would have tension building in the crust caused by some mechanisms such as trench pull to either side and then the hot asthenosphere would upwell to fill the space.

(3.1.1) Observations

East Africa displays signs of an eroded pre-uplift surface possibly caused by doming. Normal faulting then breaks through this eroded pre-rift surface. (See Figure 3.1)

(3.1.3) Deviatoric stresses caused by uplift

Uplift of the lithosphere-asthenosphere boundary occurs wholly within the mantle and creates lateral pressures (Figure 3.5)

(3.1.5) Pure shear extensional model

Figure 3.9

The basic tenets of this model are very simple.

Inside the "rift zone" the extension is the same in the crust and in the mantle.

Beta is the linear deformation. For example a beta of 2 means that the

Original thickness and length underwent a two-fold thinning. At about 3, theoretically we start to produce decompression melting and the creation of new crust. So we can’t have more than about 300% extension of the whole crust and mantle.

Because it is very simplistic, the initial stretching is instantaneous, so that a basin is produced instantaneously (in most cases but uplift can also be produced—see Fig. 3.11). Then, as the asthenosphere cools off the basin subsides a little more in an exponential manner.

The classic pattern of sedimentation is the steer’s head model.

Fault Patterns are symmetric (Fig. 3.21)

Brian Wernicke in 1981 based on studies of the basin and range came up with another model. The basic tenets of this model is that the deformation is concentrated along detachment surfaces that cross the lithosphere. This faulting is asymmetric at depth and leads to an asymmetry in the faulting in the basin. This model works best for areas of prior thick crust and the rift model works well when the crust is originally thinner to begin with.

At convergent margins if the crust is made too thick by stacking many kilometers of crust the weight of the crust upon itself may induce flow and normal faulting, a process know as extensional collapse.

A continental rifted margin becomes passive when seafloor spreading starts. For the rifted margin this is the stage when faulting ceases and thermal subsidence dominates. (sag phase) Before seafloor spreading we have the syn-rift sediments. After the end of rifting and the start of seafloor spreading we have drifting, or post-rift sedimentation.

Usually there is a major unconformity between syn- and post-rift section. The US East coast has been the source of many studies on the behavior of the lithosphere during and after rifting (Figure 3.33)

Specifically, the patterns of coastal sedimentation along the east coast imply an uplift associated with the time of rift-drift transition caused by different rates of extension with depth.

The history of basin subsidence, i.e. the amount of extension, sediment thickness, amount of heat flow and the timing of heat flow can be estimated using quantitative extensional models for basin formation using the history of subsidence shown in borehole data.

This process is known as backstripping. As we remove successive layers of sediment from the geological column we restore the underlying sediments to their true depths at the time of their first deposition. Water depths have to be known well (biostratigraphy). Compaction trends also have to be known. Once these factors are considered the background subsidence not related to the weight of the sediments and water over time can be seen. In some cases the background subsidence can be seen to be very quick as if lying over very weak lithosphere and other times it is seen to occur as if the lithosphere were mildly strong.

We can also calculate extension by looking at the fault geometries or the actual depth to the Moho. (Figure 3.41)

Nowadays that the three-dimensional nature of rift systems is better understood because more seismic data has been collected it is common to note that rift systems are long, about 100to a few hundred km wide at most. The rift systems are composed of sections separated by transfer zones/accommodation zones across which the polarity of faulting changes. In cross-section, do not expect to find full grabens but half-grabens, i.e. asymmetric V-shaped depressions.