Introduction to folds Lecture 12

Sketch me! Describe me!

Folds and folding Fold: a bent or curved planar geologic unit Bedding Dikes Changes in planar fabric orientation (e.g. foliation) Results from ductile deformation (recall- may be brittle on the microscale or plastic!) Scales range from microscopic to 10’s-100’s of km in scale Scale invariance Can be useful to work out overall deformation

Similar to the brittle structures they can range from huge….

To very small (or even at the scale of thin section)!

Folds - Implications for Oil & Mining Folds were first of major importance to man as a structural trap for hydrocarbons. In the early days of oil exploration, the crests of anticlines were nearly a sure thing, as they create a closed, up-dip barrier to oil migration - a trap.

(also gold in a hydrothermal deposit)

4. Basic Deformation Structures

Fold geometry Consider a folded surface in 1D: Several geometric points stand out Crest: “highest” part of the folded surface Trough: “lowest” part of the folded surface Hinge: Point of MAX curvature Inflection point: Point of zero curvature CREST Hinge Inflection point TROUGH Hinge

Fold geometry Consider the folded 2D planar surface: Hinge Consider the folded 2D planar surface: Hinge Line: Connects all points of MAX curvature (sometimes a fold axis, if straight) Inflection line: connects all points of zero curvature Inflection point Hinge line Inflection line Hinge Inflection point

Fold geometry Folds in 3D: Axial surface/Hinge plane: Plane connecting each layer’s hinge line. Axial trace is the 2D line of this plane in cross section. The Axial surface/Hinge plane isn’t always flat. It may curve as the fold behaves differently in each layer! Hinge zone: Area of more gradual curvature adjacent to the hinge proper

Fold geometry Folds in 3D Profile plane: Cross section of a fold perpendicular to the axial surface/hinge plane (our cut-away, in this diagram) Limbs of a fold: Areas of lesser curvature between the hinge line and inflection point of the fold May dip away from the hinge line May dip towards the hinge line

Fold geometry Fold wavelength: Distance between hinges, either crust to crest or trough to trough Fold amplitude: Vertical distance between the hinge and inflection point Interlimb angle: Angle enclosed between two limbs. Connects inflection point on each limb

Hinge lines Limb Limb Limb Limb Inflection pts Interlimb angle

Fold geometry Cylindrical fold Non-cylindrical fold Fold with a straight hinge line Mental image: paper partly wrapped around a cylinder – cylinder axis is parallel to fold axis Non-cylindrical fold Curving hinge line Note! Folds can vary between cylindrical and non-cylindrical depending on scale!

Fold geometry Bluntness: The curvature of the hinge zone Kink and chevron folds: Angular and narrow hinge zone Concentric folds: Well rounded hinge zones; often wide Box fold: multiple hinge planes in the folded area!

(a) What is an important difference between these types? (b) (c)

Fold geometry Interlimb angle acts as a measure of the “severity” of folding Open fold – Large angle between limbs Tight fold – Small angle between limbs

Fold geometry Describing folds by interlimb angles – Tightness of fold Gentle: Low intensity deformation …to… Isoclinal: Intense deformation (parallel limbs)

Hinge lines Limb Limb Limb Limb Interlimb angle: 38 ° = tight fold Inflection pts Chevron? Kink? Concentric? How can you tell?

Fold geometry Antiform – Arch-like fold; limbs dip away from the hinge Synform – Trough-like fold; limbs dip toward the hinge Antiforms and synform frequently alternate in series Why not anticline/syncline??

Fold geometry Anticlines are arched or convex-upward folds with the oldest rocks in the core of the fold Synclines are trough-like or concave-downward folds with the youngest rocks in the core of the fold The axial plane divides a fold into halves, each half being a limb YOUNG OLD OLD YOUNG

Fold geometry To reiterate… Antiform: limbs dip down and away from the hinge zone Anticline: rock layers get younger away from the axial surface of fold – older rocks in the core/center Synform: limbs dip down and toward the hinge zone Syncline: rock layers get older away from the axial surface of fold – younger rocks in the core/center

Fold geometry Geometry depends on where we look… Consider a folded fold as in (h) Erosion at blue line: “normal” fold sequence Erosion to purple line: “opposite” fold sequence

Folded fold (cross section)

Fold geometry Axial plane/hinge line geometry Not all hinge lines are vertical  folds may be asymmetric about the axis!

Fold geometry Folds can be described by hinge line geometry Plunging fold – Has a hinge line that is tilted (plunging) into the subsurface Non-plunging fold – Has a horizontal hinge

Fold geometry Dome - Fold with appearance of an overturned bowl Eroded to expose old rocks in center; younger rocks outside - Result from crustal upwarping. The Black Hills of South Dakota are large oval domes Four way dip closure: Center of dome is a great trap for fluids! Basin - Fold shaped like a bowl Erode to expose young rocks in center; older outside - Results from crustal subsidence. Most of Michigan is a large basin Domes and basins can result from vertical crustal motions.

Fold geometry Monoclines Simple bends or flexures in otherwise horizontal or uniformly dipping rock layers. (think carpet draped over a stair step) Generated by blind faults in the basement rock Blind faults -- They do not cut through to the surface Fault displacement folds overlying sedimentary cover (will see fault propagation folds)

Domes & Basins Chernicoff and Whitney Think of an Egg Carton!

Oil and Gas Concentrate in Domes Figure 9-h-02a: Accumulation of oil and natural gas. Source: Chernicoff and Whitney Text, Houghton Mifflin Chernicoff and Whitney

Grenville Dome: Sinclair, WY S. Gao image Map from Topozone.com Grenville Dome: Sinclair, WY

Syncline- Anticline Pairs + Domes: Zagros Mts, Iran NASA “Earth as Art” web page

Fold symmetry Enveloping surface: Plane tangent to the hinges of a series of folds

Fold symmetry Using the axial surface and enveloping surface to help define vergence M folds: ± 10° of perpendicular Z folds and S folds: Anything else (Recall: clockwise vergence = Z; counter clockwise = S)

Fold symmetry This sense of vergence can help locate ourselves in a larger scale structure!

Fold symmetry 2x generations of folding!

Ptygmatic folds irregular asymmetric folds; usually tightly folded veins or thin layers of strongly contrasting lithology

Folding mechanisms Three main mechanisms of folding  differ based on how stress is applied to folded layers Buckling: Lateral stress Bending: Vertical stresses Passive folding: Shear stress

Folding mechanisms (buckling) High viscosity contrast: Shortening accommodated by folding No viscosity contrast: Shortening by thickening In the brittle deformation regime: Shortening by reverse faulting

Folding mechanisms (buckling) Buckling (Active folding) Instabilities develop when layers of different mechanical properties are subjected to layer-parallel stresses (layer-parallel shortening). Viscosity contrasts are required for buckling! the folding layer is more competent than the surrounding material Buckling typically results in rounded and parallel folds μ: Viscosity symbol

Folding mechanisms (buckling) Shortening direction relative to plane orientation affects the resultant folds Below: When shortening direction is oblique to layer, fold symmetry varies! S M Z

Folding mechanisms (buckling) Fold wavelength: Dependent on layer thickness and viscosity contrast (This example: Strong material between weak layers)

Folding mechanisms (buckling) Identifying viscosity contrasts by contact morphology (in this case, weak material squeezed between stronger) Mullion structures Higher viscosity material Softer, lower viscosity material

Folding mechanisms (buckling) Folding initiates in the thin layers Once the thicker layer starts to fold, the smaller folds in the thin layer become parasitic and asymmetrical (due to flexural flow)

High contrast between layer and matrix Low contrast between layer and matrix

Folding mechanisms (buckling) Layers far apart act as individual layers The closer they get the more they behave as a single layer

Folding mechanisms (buckling) Harmonic folds Disharmonic folds Harmonic folds: A fold which maintains its geometric form, integral wavelength, and symmetry throughout a sequence of layers. Such folds form where the competent layers comprising the sequence are of similar thickness and evenly spaced, and the contrast in competence is constant between each layer and the next. Contrasting properties leading to a lack of symmetry between folded layers  Disharmonic folds!

Folding mechanisms (passive folding) The layering plays no mechanical role in producing the fold and therefore no influence on the fold shape No viscosity contrast involved Passive folds can form in response to any kind of ductile strain

Folding mechanisms (passive folding) Generates Similar folds (recall: Similar folds have changes in bed thickness!) Thickness is constant parallel to axial surface Common in monomineralic rocks (why?) Also common in hot rock (reduces competency contrast)

Folding mechanisms (bending) Bending – passive (no viscosity contrast needed); however, commonly involves contrasting layers! Bending does not yield folds with regular wavelength Common bending folds Fault propagation folds (monoclines) Near magmatic intrusions Near salt domes Between boudins (low viscosity matric fills gaps!) Differential sediment compaction

Folding: mechanisms (bending) Boudins (“sausages”) Form due to layer parallel stretching Mechanical strength contrast: Stronger layer in weak matrix Strong layer necks, then breaks Weak layer passively folds into the “gap”

Folding: Strain accommodation Flexural Slip: Slip along layer surfaces or very thin layers during folding Maintains bed thickness (i.e., parallel folds) Slip increases down the limb from zero at the hinge Evidence for bedding-parallel slip (e.g. slickenlines or shear fractures) Flexural Flow Similar to slip but with distributed shear (ductile deformation)

Folding: Strain accommodation Hinge zone impacts Hinge collapse: Ductile flow out of hinge Ductile flow: material into hinge

Regional Tectonic Fold Mechanisms: Free & Forced Folding

Forced Folding: Here, folds are forced to form as a result of motion upon faults. Beds are not free to fold, and they may or may not have significant layer-parallel stresses. Rather, they are somewhat passively going along for the ride - most often associated with faulting - all types.

2 Main Type of Forced Folding (associated w/ thrusts) Fault Bend Folds (FBF) (ramp anticlines) throughgoing faults - slip is conserved. Fold amplitude is determined by the thickness above the lower hanging wall flat. Once the fold has achieved maximum amplitude, it only grows in width.

2 Main Type of Forced Folding (associated w/ thrusts) 2) Fault Propagation Folds (FPF) fault slip is not conserved. Fault displacement is taken up by folding Fault tip is marked by a ductile ‘process zone’. Ideally, FPF’s evolve into FBF’s