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What are Fractures and Faults ?

What is Fractures and Faults?

Fractures and faults are prominent in the upper part of the continental and oceanic lithosphere, where they are often associated with earthquake activity. The above Google Earth images show a fracture field in southern Pakistan (N25º35/E62º10). Open Google Earth and check for yourself how many phases of faulting you can argue for.


The Concept of Structural Level

The concept of Structural Level is based on the observation that the style of deformation changes with depth due to changes in temperature and confining pressure (the product between density, depth and gravitational acceleration). With depth, strain fabrics become more pervasive, syn-metamorphic, and often subhorizontal.


The Upper Structural Level (0-15km) is the domain of brittle deformation (faults and fractures), upright isopach folds, extensional fractures and stylolitic joints. Bedding and early fabrics are always recognizable.


The Intermediate Structural Level (10-25km) starts at the schistosity front, where cleavage begins to form. The style of folding includes: similar folds, tight folds and overturned folds with a strong axial planar schistosity. Pressure solution is the dominant cleavage-forming mechanism, quartz veins testify for dewatering of rocks due to metamorphism. Ductile deformation dominates over fracturing. Metamorphic grade does not exceed the greenschist to mid-amphibolite facies.


In the Lower Structural Level (>20 km) the style of deformation style includes: metamorphic nappes, large scale recumbent folds and shallow dipping ductile shear zones. Deformation is intense and pervasive, tectonic transposition makes the mapping of early fabric difficult if not impossible, fabrics are sub-horizontal. Metamorphic grades ranges from amphibolite facies to granulite facies, and partial melting is often present.


Stylolitic Joints

Joints: Planar discontinuities involving no relative displacement of the adjacent blocks. Joints develop during the exhumation of rocks following erosion of the overburden. Joint result from contraction and expansion due to cooling and decompression respectively.


Stylolites and stylolitic joints: Stylolitic joints are discontinuities that result from a deformation mechanism called "pressure-solution". They form through stress-induced dissolution along an irregular surface. Dissolution is triggered by stress concentration at the contact between grains. This process puts into solution molecules detached from minerals and clastic grains.


Stylolitic joints are often darker than their host rocks as iron-coated insoluble particles accumulate in the joint. In section, stylolitic joints are made of tooth-like cones. The cones axes can be either perpendicular to the joint or can make an angle, but they always point in the direction of the maximum stress at the time of their formation.