Transcript Folds Lecture Geos 304, 2002

Folds
Read Davis and Reynolds, Chapter 7
Why is it important?
•fundamental to deformation of the Earth’s crust;
•structural traps for oil
•ore deposits
Tectonic considerations
Folds, key to covergent margins, aid in resolving past stress regimes, crustal
shortening, closure of sedimentary basins. Sensitive markers to tectonic forces.
Relevance to oil and mining
•Structural traps (anticlines are among the best traps
•Folds control ore geometry especially in strata-bound deposits
Descriptive analysis
•Basic definitions (anticline, syncline,overturned, etc.)
•Geometric analysis (hinge, limb, axial plane, etc)
•Specific geometries
Shapes of folds
younger
older
•Anticlines, synclines
•Antiform, synform
•Anticlinorium, synclinorium
younger
old
Antiforms, synforms, anticlinoria, synclinoria
Do not forget axial surface
Hinge line
Hinge, hinge zone
Axial plane
Inflection point
Recumbent
Equal Area
Fold lies on its sidetypical for poly-deformed
regions
Chevron
Ptygmatic
Cuspate
Geometry, size
Heigth
Median trace
Width
Amplitude
Wavelength
Hinge line and axial plane
Topographic
Surfaces and
folds
Layer thickness may change during folding
Didn’t here- this is a “concentric”
fold
This one did- it is a “similar” fold
Fold tightness
Gentle
Angle is >90 degrees
Equal Area
tight (10ish
degrees opening)
to
isoclinal (0 degrees)
Equal Area
Symmetric, asymmetric
Equal Area
Cylindrical (there is an axial surface) and non-cylindrical
(too complex to have a defined axial surface)
Equal Area
Equal Area
Poles to bedding
Hinge line
Hinge line
readings
Cylindrical or non-cylindrical
More descriptive terms?
Example: Analyze this alpine example:
A second example
A final example