Monday, February 29, 2016

Primary and Tectonic Folds in the Context of Exploration and Mining

Folds are the most familiar, and commonly the most spectacular, manifestation of bulk heterogeneous deformation in rocks. They occur at all scales from the microscopic to the scale of the whole lithosphere. Folds form under a range of conditions in igneous, sedimentary and metamorphic rocks.

A necessity for identification of folds is the presence of some planar layering that can define the fold geometry. Massive rocks with no planar layering can undergo the same deformation processes at the same conditions but a lack of marker horizons means that the processes of folding may not be identified.

Folds commonly occur in conjunction with cleavages. Cleavages and folds form in response to applied stresses and show coeval evolution as the deformation progresses. It is common to see rocks that show several stages of folding and cleavage development. Cleavages that form at the same time as folds will show develop in an axial planar orientation that broadly bisects the fold hinge. However, the angle between the cleavage and the folded layer will be variable depending on fold shape.

Features that are commonly folded include bedding, tectonic foliations and igneous intrusions. Successive stages of folding can lead to complex fold geometries and the rocks are said to contain superposed folds.

The majority of fold geometries are produced in response to tectonic stresses that deform the rocks. However, it is not uncommon to encounter primary folds that are produced in response to such things as soft sediment deformation (e.g. slumping) in sediments and flow folds that form in lavas and crystallising magmas.

Tectonic folds develop in response to an applied stress and form in many structural environments and virtually all tectonic regimes. They are dominantly products of contractional deformation associated with mountain building processes (orogenesis) in convergent plate margin environments.

Less common folds will develop in extensional environments as the hanging wall block or upper plate undergo necessary deformation to accommodate shape changes on faults or volume changes induced by extension. One such example is that of rollover antiforms that form in hanging wall blocks that move down on curved faults. Folds are common in association with faults, examples are :
  • Rollover antiforms on normal faults
  • Fault propagation folds on thrust (reverse) faults
  • Curved fibre geometries due to variation in extension direction in a fault
  • Drag folds
tectonic folds
Figure tectonic fold indication.

Other folds occur in response to volume changes during the processes of igneous and diapiric emplacement. In this case the folds are products of volume compensation processes.

The folds form above and adjacent to the bodies being emplaced and often have very curviplanar axial planes. Movement of material as the folds evolve equates to a mass transfer process that allow space to be created for such bodies as igneous intrusions and salt diapers.

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