AS Structural Geology Flashcards
Bed
a unit of sedimentation, which can vary considerably in thickness.
Bedding planes
mark a break between beds. They represents a break in sedimentation, a change in the composition or grain size, or a change in colour of the sediments.
Dip
the maximum inclination of a bed measured from the horizontal using a clinometer.
True dip
the actual angle of dip measured at right angles to the strike.
Apparent dip
a dip that is measured to be less than the maximum inclination.
Strike
the horizontal line on a bedding plane, measured as a bearing from north using a compass. It is at right angles to the dip.
Fault
a fracture in a rock along which there has been an observable amount of displacement.
Fault plane
a plane of fracture, along which the rocks have been displaced.
Upthrow
the side of the fault where the movement is upwards, in relation to the other side.
Downthrow
the side of the fault where the movement is downward, in relation to the other side.
Throw
the vertical displacement of rocks along the fault plane.
Footwall
the side of the fault that lies below the fault plane, if the fault is not vertical.
Hanging wall
the side of the fault that lies above the maximum inclination of the fault plane as measured from the horizontal.
Slickensides
the striations and polishing found on a fault plane indicating the direction of relative movement.
Fault breccia
composed of fragments produced by rocks fracturing during faulting. It is found along the fault planes.
Fault gouge
composed of very finely ground particles, produced by grinding of rock during faulting. These are often fused together due to frictional heat and found along fault planes.
Mylonite
a rock produced by dynamic recrystallisation of minerals on a fault plane.
Anticline
an upright fold with the oldest rocks in the core.
Antiform
an upward closing fold.
Syncline
a downward pointing fold with the youngest rocks in the core.
Synform
a downward closing fold.
Fold limb
the section of a fold between one hinge and the next, forming the side of a fold.
Hinge
the line along which there is a change in the amount and/or direction of dip, forming the most sharply curved part of the fold.
Crest
the highest point of a folded bed.
Trough
the lowest point of a folded bed.
Axial plane
a plane that joins the hinges of all the beds. It bisects the fold.
Axial plane trace
the outcrop of the axial plane at the Earth’s surface.
Plunge
the angle of dip of the axial plane from the horizontal.
Dome
an anticline which dips away from the centre in all directions.
Basin
a syncline which dips towards the centre from all directions.
Overfolds
folds which have both fold limbs dipping in the same direction but by different amounts.
Recumbent folds
have axial planes and fold limbs which are close to horizontal and always less than 30 degrees.
Nappes
recumbent folds that are broken along thrust planes.
Isoclinal
folds have parallel limbs that are nearly vertical and in very tight folds.
Stress
the force applied to rocks.
Strain
a change in the shape or volume of a body as a result of applied stress.
In cases where vertical stress is applied to perfectly elastic materials:
Strain = the change of length of line ÷ the original length of line
Competent
rocks are strong and brittle and tend to joint and fault.
Incompetent
rocks are weak and plastic and tend to fold and develop cleavage.
Tension
the force trying to pull rocks apart, the Earth’s crust will be lengthened.
Compression
the force trying to push rocks together, the Earth’s crust will be shortened.
Shear forces
are those which act along a plane in the rock and promote sliding along that plane.
Joint
a fracture in competent rocks along which no observable movement has occurred.
Tectonic joints
form as a result of folding and cause tension joints parallel to the axial plane and cross joints on the limbs.
Cooling joints
form as a result of contraction on cooling of igneous rocks.
Unloading joints
are often horizontal as well as vertical and form as a result of lower pressure near to the surface.
Cleavage
planes of weakness in incompetent sedimentary rocks (e.g. shale, made from clay minerals) and low-grade metamorphic rocks (e.g. slate) along which these rocks will split. It forms perpendicular to o max in folds, so is at an angle to bedding planes.
