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 Anticlines, synclines Antiform, synform Anticlinorium, synclinorium old younger older younger
Antiforms, synforms, anticlinoria, synclinoria
Hinge, hinge zone Limb Inflection point Hinge line Do not forget axial surface Axial plane
Recumbent Fold lies on its side- typical for poly-deformed regions
Chevron Ptygmatic Cuspate
Geometry, size Median trace Wavelength Amplitude Width Heigth
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
tight (10ish degrees opening) to isoclinal (0 degrees)
Symmetric, asymmetric
Cylindrical (there is an axial surface) and non-cylindrical (too complex to have a defined axial surface) Hinge line Poles to bedding Hinge line readings
Cylindrical or non-cylindrical
More descriptive terms?
Example: Analyze this alpine example: Eocene Paleocene
A second example
A final example