MECHANISM BY CRACK SEAL

This deformation mechanism is the alternation of crystal growth syntectonically in submicroscopic cracks, the process which is healing or sealing. The next crack gets localized at the same region, this being overall mechanically the weakest portion in the rock. The crystal growth again begins and sealing of the crack takes place. This process repeats and a vein of appreciable width may be formed, the crack-seal vein. The veins contains both crystals and within them the inclusions of wall rock. These form inclusion bands parallel to the boundaries of the vein and inclusion trails parallel to crystal fibre long axes. This mecahnism was nicely demonstrated by Ramsay whose figure is reproduced. The fracturing is probably hydraulic, aided by fluid content that makes the rock brittle and eventually fail, followed by drop in this pressure that makes the crystal fibres grow under the influence of ductile stretching coupled with grain boundary migration, diffusion and other allied mechanisms.

The upper part is of a crack seal vein from Proterozoic rocks of Central India and the lower shows the constraints of Coulomb and Naviour criterion for failure that can generate continuously cracking and healing under given PT conditions with fluid pressure mainly influencing the cracking making the rock more brittle as required to generate re-fracturing along pre-existing weak planes.

One of the characteristic feature of the syntectonic veins is that they grow by a mechanism of CRACK-SEAL or by alternate microfracturing and sealing. Once the fracturing occurs, the fracture is immediately filled in by crystal growth in fibrous form. When the fracturing occurs again it occurs again along the same sealed plane since this is the overall weakest surface in the rock and producing another crack may require tremendous amount of mechanical work. The vein goes on fracturing repeatedly and getting filled up by crystalline material again and again, giving a banded appearence to the vein. In fact the individial crystal fibres contain inclusions of wall rock caught up an arranged parallel to the vein boundaries. These are called inclusion bands. Apart from this, each fibre also contains inclusion trails parallel to fibres. Large amount of finite strain can sometimes occur by this process. For details see the figures below and for further information, please see the paper by Ramsay given in the figure. The vein goes on fracturing repeatedly and getting filled up by crystalline material again and again, giving a banded appearence to the vein. In fact the individial crystal fibres contain inclusions of wall rock caught up an arranged parallel to the vein boundaries. These are called inclusion bands. Apart from this, each fibre also contains inclusion trails parallel to its length or growth directions. Large amount of finite strain can sometimes occur by this process.