E-Lær Flashcards
What are the three fundamental mechanisms of folding?
Buckling, bending and passive folding.
What is “buckling”?
Buckling or active folding can occur when a layer is shortened parallel to its length.
Buckling requires:
- A layer with higher competence or viscosity than its surroundings (matrix)
- Layer parallel shortening
- Plastic deformation.
What is passive folding?
Passive folding involves passive flow of rock. Which means that there is no competence contrast, so layers serve as visual markers that passively flow with the rest of the rock.
Passive folds are similar (Class 2) folds, which maintain a constant thickness parallel to their axial surface.
What is “bending”?
Bending can occur when forces act across the layering (bedding or tectonic foliation).
Bending is passive in the sense that folding does not initiate and grow as a result of competency contrast (no buckling instability). However, bending commonly involves layers of contrasting competence.
Ex.: Basement fault buried under sediments, and fault is activated - resulting in a monocline (forced folding).
Can also occur when salt or magma rise toward the surface. Badinage also generate forces that act across the layering.
Name the important parts of folds:
Hinge line, fold axial plane, fold limbs, amplitude, wavelength, interlimb angle.
What are foliations?
Planar structures that penetrate metamorphic rocks, and make rocks split into slabs and give deformed rocks exiting expressions.
Define “tectonic foliation”.
Tectonic foliation is a general term about penetrative and cohesive planar structures in rocks that involve shortening across the structure. This is the general meaning of foliation as used by structural geologists.
Define “primary foliations”.
Primary foliations are non-tectonic penetrative planar structures in rocks, such as sedimentary layering or lamination in sedimentary rocks, flow banding in volcanic rocks, and cumulate layering in intrusive rocks.
Define “cleavage”.
Cleavage is foliation in (very) low grade (up to middle greenschist facies) metamorphic rocks. The word implies that the rock is easily split or cleaved.
Define “schistosity”.
Schistosity is tectonic foliations in more coarse-grained and recrystallized rocks such as quartz-schist and mica-bearing mylonitic gneisses that were deformed under uppermost greenschist to amphibole facies conditions.
Name different types of cleavages.
Compaction, pencil, slaty, phyllitic, crenulation (microfolding).
What is schistosity?
When recrystallization is rapid and widespread the minerals get coarse, as does the foliation. In mica-schists the foliations gets less planar and more wavy, disturbed by minerals such as garnet, amphibole, kyanite, etc.
The aligned micas define the schistosity.
Define “gneissic foliation”.
The banding or layering seen in gneisses are generally referred to as gneissic foliation. I consists of layers of contrasting compositions, and the layers are rotated and flattened structures, such as dikes and primary layering whose origin may be difficult du define.
Gneissic foliations may be migimatitic where partial melting is involved, and mylonitic if strain is very high.
What is “transposition foliation”?
Transposition refers to the process where shearing and flattening rotate various structures into (sub)parallelism so that they together constitute a foliation or bading.
A transposition foliation typically contains isoclinal fold hinges with thinned limbs. Both coaxial and noncoaxial deformation can result in transposition, although simple shear is commonly involved.
Define “lineations”.
Tectonic lineations are linear structures in deformed rocks whose formation relate to strain and deformation. They are common structures, and many lineations are related to kinematics (shear direction or transpiration direction).
Name the three principal types of lineations.
Penetrative lineations (penetrates metamorphic rocks, similar to foliations, linear fabrics).
Geometric lineations (linear features that are defined by the geometry of (other) structures, e.g., fold hinges).
Surface lineations (lineations restricted to surfaces such as faults, reflects movement of slip).
What does “contraction” signify?
Contraction is a strain-related term, signifying shortening in a direction set by the acting stress field (tensor).
What are contractional faults?
Contractional faults are faults that accommodate contraction or shortening, and in most cases correspond to reverse or thrust faults, which accommodate shortening in the horizontal direction.
What is the difference between reverse- and thrust faults?
Thrust faults are reverse faults with shallow dip, i.e., less than ca. 30 degrees, while reverse faults have steeper dip.
Explain “nappe complexes”.
Thrust nappes consists of one or several subordinate thrust sheets that possess a common displacement history. Horses designate the smallest tectonic units originating within thrust nappes or sheets.
Nappes, sheets, and horses are bounded at the base by a sole thrust or floor thrust and at the top by a roof thrust.
Nappes are thin compared to their lateral extent and commonly exhibit a wedge or lense shaped geometry in cross-section.
Series of stacked nappes form nappe complexes.
Define autochtons, allochthons, paraautchthons and décollement.
Allochthon refers to nappe rocks that have been translated away from their original position.
Autochthon designates in situ rocks that have not been transported, i.e., basement rock.
Locally transported rocks are referred to as paraautochthons.
The low-angle fault or shear zone that separates the allocation and paraautochthon from the underlying autochthon is called décollement.
Define “hinterland” and hinterland characteristics.
The hinterland defines the central mountainous region of the orogen.
Hinterland characteristics:
- Thick-skinned deformation, meaning that both basement and cover (wedge) are involved.
- Penetrative deformation.
- Formation of large metamorphic nappes.
- Extencive nappe-internal folding is common.
Define “foreland” and foreland characteristics.
The foreland occupies the margins of a orogen.
Foreland characteristics:
- Thin-skinned contractional tectonics.
- Very localized deformation.
- Formation of nappe-systems.
- Local basement not involved.
- Deposition of sediments following erosion of the hinterland.
What are “ramps”, and how are they categorized?
Thrusts typically climb toward the surface in a stepwise manner that gives rise to ramp-flat geometries. Flats form along soft, incompetent layers, whereas ramps develop where the thrust cuts across relatively stiff layers.
Ramps are categorized according to their orientation relative to the main transport direction.
- Frontal ramps are perpendicular to the transport direction and display reverse dip slip.
- Lateral ramps are parallel to the transport direction and produce vertical transfer fault with strike-parallel slip.
- Oblique ramps form oblique slip faults.
What are imbrication zones?
Imbrication zones result from ramp formation and constitute a series of fault blocks )horses) thrusted up along more or less parallel ramp faults with dominantly reverse dip slip.
Imbrication zones form preferentially in the foreland, where the basal décollement climbs toward the surface.
