CRACK-SEAL MECHANISM

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.