Автор: Hajiyev Rufat Akif
Место работы: Azerbaijan republic compony Socar,
Azneft İB, 28 May NQCI
название тезиса: Structural geology
Structural
geology is
the study of the three-dimensional distribution of rock units with
respect to their deformationalhistories. The primary goal of
structural geology is to use measurements of present-day rock geometries to
uncover information about the history of deformation (strain) in the rocks, and ultimately, to
understand the stress field that resulted
in the observed strain and geometries. This understanding of the dynamics of
the stress field can be linked to important events in the regional geologic
past; a common goal is to understand the structural evolution of a particular
area with respect to regionally widespread patterns of rock deformation (e.g., mountain building, rifting) due to plate tectonics.
The study of geologic structures has been of prime
importance in economic geology, both petroleum geology and mining geology.Folded and faulted
rock strata commonly form traps for the accumulation and
concentration of fluids such as petroleum and natural gas. Faulted and
structurally complex areas are notable as permeable zones for hydrothermal fluids and the resulting concentration areas
for base and precious metal ore deposits. Veins of minerals containing various
metals commonly occupy faults and fractures in structurally complex areas.
These structurally fractured and faulted zones often occur in association with intrusive igneous rocks. They often also occur around
geologic reef complexes and collapse features such as ancient sinkholes. Deposits ofgold, silver, copper, lead, zinc, and other metals,
are commonly located in structurally complex areas.
Structural geology is a critical part of engineering
geology, which is concerned with the physical and mechanical properties of
natural rocks. Structural fabrics and defects such as faults, folds, foliations
and joints are internal weaknesses of rocks which may
affect the stability of human engineered structures such as dams, road cuts, open pit mines and underground
mines or road tunnels.
Geotechnical risk, including earthquake risk can only be investigated by inspecting a
combination of structural geology and geomorphology.In addition areas
ofkarst landscapes which are underlain by underground
caverns and potential sinkholes or collapse features are of importance for
these scientists. In addition, areas of steep slopes are potential collapse or
landslide hazards.
Environmental
geologists and hydrogeologists or hydrologists need to understand structural geology because
structures are sites of groundwater flow and penetration, which may affect, for
instance, seepage of toxic substances from waste dumps, or seepage of salty
water into aquifers.
Plate tectonics is a theory developed during the 1960s which
describes the movement of continents by way of the separation and collision of
crustal plates. It is in a sense structural geology on a planet scale, and is
used throughout structural geology as a framework to analyze and understand
global, regional, and local scale features
The inclination of a planar structure in geology is
measured by strike and dip. The strike is the line of
intersection between the planar feature and a horizontal plane, taken according
to the right hand convention, and the dip is the magnitude of the inclination,
below horizontal, at right angles to strike. For example; striking 25 degrees
East of North, dipping 45 degrees Southeast, recorded as N25E,45SE.
Alternatively, dip and dip direction may be used as this is absolute. Dip
direction is measured in 360 degrees, generally clockwise from North. For
example, a dip of 45 degrees towards 115 degrees azimuth, recorded as 45/115.
Note that this is the same as above.
The term hade is occasionally used and is the
deviation of a plane from vertical i.e. (90°-dip).
Fold axis plunge is measured in dip and dip direction
(strictly, plunge and azimuth of plunge). The orientation of a fold axial plane
is measured in strike and dip or dip and dip direction.
Lineations are measured in terms of dip and dip direction,
if possible. Often lineations occur expressed on a planar surface and can be
difficult to measure directly. In this case, the lineation may be measured from
the horizontal as a rake or pitch upon the surface.
Rake is measured by placing a protractor flat on the
planar surface, with the flat edge horizontal and measuring the angle of the
lineation clockwise from horizontal. The orientation of the lineation can then
be calculated from the rake and strike-dip information of the plane it was
measured from, using astereographic
projection.
If a fault has lineations formed by movement on the
plane, e.g.; slickensides, this is recorded
as a lineation, with a rake, and annotated as to the indication of throw on the
fault.
Generally it is easier to record strike and dip
information of planar structures in dip/dip direction format as this will match
all the other structural information you may be recording about folds,
lineations, etc., although there is an advantage to using different formats
that discriminate between planar and linear data.
The convention for analysing structural geology is to
identify the planar structures,
often called planar fabrics because this implies a textural formation, the linear structures and, from analysis of these, unravel deformations.
Planar structures are named according to their order
of formation, with original sedimentary layering the lowest at S0. Often it is
impossible to identify S0 in highly deformed rocks, so numbering may be started
at an arbitrary number or given a letter (SA, for instance). In
cases where there is a bedding-plane foliation caused by burial metamorphism or diagenesis this may be enumerated as S0a.
If there are folds, these are numbered as F1,
F2, etc. Generally the axial plane foliation or cleavage of a fold is created during folding, and the
number convention should match. For example, an F2 fold should have an S2 axial foliation.
Deformations are numbered according to their order of
formation with the letter D denoting a deformation event. For example, D1,
D2, D3. Folds and foliations, because they are formed by
deformation events, should correlate with these events. For example, an F2 fold, with an S2 axial plane foliation would be the
result of a D2 deformation.
Metamorphic events may span multiple deformations.
Sometimes it is useful to identify them similarly to the structural features
for which they are responsible, e.g.; M2. This may be possible by
observing porphyroblast formation in cleavages of known deformation
age, by identifying metamorphic mineral assemblages created by different
events, or via geochronology.
Intersection lineations in rocks, as they are the
product of the intersection of two planar structures, are named according to
the two planar structures from which they are formed. For instance, the
intersection lineation of a S1 cleavage
and bedding is the L1-0 intersection
lineation (also known as the cleavage-bedding lineation).
Stretching lineations may be difficult to quantify,
especially in highly stretched ductile rocks where minimal foliation
information is preserved. Where possible, when correlated with deformations (as
few are formed in folds, and many are not strictly associated with planar
foliations), they may be identified similar to planar surfaces and folds, e.g.;
L1, L2. For convenience some geologists prefer to
annotate them with a subscript S, for example Ls1 to differentiate them from
intersection lineations, though this is generally redundant.