Lecture 6 - Fabric 2: Veins and Porphyroblasts Flashcards
Why are veins and strain fringes of significance?
They can capture progressive deformation, the crystals grow while the vein or fringe is opening. Not only the final stage of deformation can be seen but these structures tell us something about deformation history
What happens to crystals when a) veins open faster than crystals can grow b) veins open quickly and in fast steps but some crystals reach the wall rock c) veins open slower than crystals?
a) crystals develop facets, blocky growth b) Crystals cannot go freely, they become streched c) Crystals reach wall rock, if rough the grain boundaries track the opening history, fibrous growth
Describe :
antitaxial growth, vein is made up of material that is different than host-rock, growth from the centre outwards
Describe:
syntaxial growth, vein contains same material than host-rock, growth is from host-rock inwards, vein opens always in the centre.
Describe:
ataxial growth, vein contains same material than host-rock but the fracture surface where the vein opens varies - random fracturing leading to stretched crystals.
Describe:
antitaxial growth, crystals in fringes are different from the central object, grow in the host rock towards the fringe. Opening surface is at the object.
Describe:
syntaxial growth, crystals in the fringe are the same as the central object, crystals grow on the object towards the host-rock.
What happens during irrotational strain?
During irrotational strain the veins open as simple mode I extension fractures parallel to 1 or they originate from shear fractures that open later on. Fringes develop next to rigid objects where strain shadows form that lead to local extension sites. These strain shadows fill with material and become strain fringes that are being pulled away from the rigid object
How do ‘tension gash’ geometries form?
During rotational strain veins form “tension gash” geometries. The vein will open in the extensional ISA direction (like an extension crack). During progressive deformation the centre of the vein (old part) will rotate (behave ductile). At the same time the vein grows in length opening in the direction of the extensional ISA. The new parts of the vein are found at the tips and these tips indicate the orientation of the ISA and thus the shear sense. Tension gashes are very good shear sense in indicators and can be found quite often next to large faults.
How do boudins develop?
Boudins develop when there is a competence contrast between layers. Small irregularities in the layer are amplified and the more competent layer is torn apart by flow or by fracturing.
How does a bone-type boudin form?
In this case veins form during the early extension stage of the boudin. The vein material is more competent than the layer so that the strain later on localizes in the layer and not the veins. This leads to the bone-shape
This is an example of a porphyroblast growing in a crenulation cleavage. Explain what is happening
The porphyroblast grows over the crenulation and preserves it. After the growth of the porphyroblast the younger foliation tightens and the crenulation disappears outside the porphyroblast. Only the porphyroblast shows evidence of an older folation and a crenulation cleavage!
Describe with note to the diagram how a pre-tectonic porphyroblast appears
A pretectonic porphyroblast simply grows before deformation occurs. Foliations then form around the rigid object. One metamorphic event (growth of porphyroblast) is followed by tectonic events (foliations).
What is meant by intertectonic growth?
Intertectonic growth means that we have a tectonic event (or more) that is followed by a metamorphic event. During this metamorphic event there is no further tectonic deformation. After the growth of the porphyroblast we have another tectonic event.
What is meant by syntectonic growth?
Syntectonic growth means that a tectonic event starts and at some time the porphyroblast starts to grow while the rock is still deforming. The centre of the porphyroblast will capture the older deformation whereas the outside will show more complex and younger deformation.
What is meant by a post tectonic growth porphyroblast?
A post-tectonic porphyroblast simply grows when the deformation is over.
