Injection Folding

Abstract

If a thick suite of rocks of high plasticity occurs at a certain depth below the surface in a sedimentary series, the rocks of this suite may flow in certain conditions and be squeezed out of one place and injected into others. In the area of squeezing the plastic series becomes thinner and increases in thickness in the area of injection. The less plastic sedimentary rocks lying above the plastic ones are deformed, yielding to the latter’s movement. These strata subside above the zone of pinching and are raised above the areas of injection of plastic material. The regions of pinching usually occupy broad areas and the strata lying above them are not in the main disturbed, while the regions of injection are sites of concentrated swelling of the plastic material and local convex structures are formed above them in the overlying sedimentary series, either dome-shaped or brachyanticlinal depending on the form of the injection. The convex structures may vary in degree of expression from very gently sloping uplifts to pronounced folds with steep, and even overturned bedding of strata on the limbs, broken up by numerous faults. As for the rocks that occur below the plastic series, they may not be involved at all in the deformations, or movements in the “basement” may affect the location of the zones of pinch and swell in the plastic series to some extent. The mechanism of this effect will be dealt with later.