Looking at different types of folds on seismic data Introduction to folds Looking at different types of folds on seismic data

These folds are asymmetrical. One limb is longer than the other.

This fold is symmetrical. The limbs are the same length.

Folds can be gentle, symmetrical and rounded. 1.5 km N CMP 100 110 120 130 140 150 160 170 180 190 200 210 220 230 600 100 200 300 400 500 700 800 900 1000 600 100 200 300 400 500 700 800 900 1000 Fold axial plane TWT (msec) TWT (msec) Folds can be gentle, symmetrical and rounded.

Or inclined, angular and asymmetric. Look at this horizon Fold axial plane Or inclined, angular and asymmetric.

1.5 km N ⇃ ↾ ⇃ ↾ There may be other deformation features associated with a fold. Can you spot the faults? What sort of faults are they?

S 2 km CMP 800 900 1000 1100 1200 1300 1400 1500 1600 1700 800 1000 1200 1400 600 400 200 800 1000 1200 1400 600 400 200 TWT (msec) TWT (msec) ⇃ ↾ ⇃ ⇂ Reverse faults often occur with folds as both are caused by compressive forces. But not all the faults are reverse faults. This normal fault was re-activated.

When is an antiform an anticline? (…and when is a synform a syncline?)

antiform anticline anticline antiform anticline antiform syncline synform synform syncline Remember this? To be an anticline or a syncline the relative age of the rocks must be known. In this series of folds the rocks have not been inverted, therefore the youngest overlie the oldest rocks.

⤚ ⤙ ⤙ ⤚ anticline youngest youngest oldest oldest syncline Look at the anticline: From the centre of the structure the age of the rocks decreases towards the outside: the oldest rocks are at the core. Now look at the syncline: From the centre of the structure the age of the rocks increases towards the outside: the youngest rocks are at the core.

Overview The individual properties of a rock and the physical setting (e.g. temperature and confining pressure) determine how it will react under stress. Some layers may be more competent than others and so deform in different ways, i.e. buckling, flexural slip or shearing (see links for more details). There are many causes of stress in rocks that can cause folds: Plate movements e.g. orogenies Igneous intrusions e.g. granite batholiths Salt diapirs (where large quantities of low density salt rise towards the surface causing deformation). The time over which these stresses are applied can also affect the deformation e.g. low strain rates over extended time periods produces plastic deformation. Folds vary in size from microscopic to many km. Only the large scale folds can be seen on seismic data.