Folds II Geometry and classification Lecture 23 Gly 326

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

YOUNG OLD YOUNG OLD

Fold geometry Folds can described by hinge geometry Plunging fold – Has a hinge line that is tilted (plunging) into the subsurface Non-plunging fold – Has a horizontal hinge Measure the plunge of the hinge line as a lineation!

Fold geometry Dome - Fold with appearance of an overturned bowl Erode 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 - Result from crustal subsidence. Most of Michigan is a large basin Domes and basins can result from vertical crustal motions and/or from doubly plunging structures!

Richat structure (“Eye of the Sahara”): A dome! From Wiki: The Richat Structure, also known as the Eye of the Sahara and Guelb er Richat, is a prominent circular feature in the Sahara desert near Ouadane, west–central Mauritania. This structure is a deeply eroded, slightly elliptical, 40 km in diameter dome. The sedimentary rock exposed in this dome ranges in age from Late Proterozoic within the center of the dome to Ordovician sandstone around its edges. The sedimentary rocks comprising this structure dip outward at 10°–20°. Differential erosion of resistant layers of quartzite has created high-relief circular cuestas. Its center consists of a siliceous breccia covering an area that is at least 3 km in diameter.[1][2][3]

Michigan basin: A basin Michigan basin: A basin! Note - does not mean that is topographically a “bowl”

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 Recall: Blind faults! They do not cut through to the surface! Fault displacement folds overlying sedimentary cover (remember fault propagation folds??)

Fold classification For a fold in cross section (profile plane): fold classification by interlimb angle (Tightness) Recall: Interlimb angle encloses hinge and connects inflection points Works when we have the profile plane (cross section perpendicular to hinge plane/fold axis

Fold classification Classify this fold based on tightness 1) Pick a folded layer to use 2) ID the hinge and inflection points 3) Draw your lines 4) Measure the angle

Fold classification Classification based on hinge line plunge vs. axial surface orientation Axial surface dip (Recall: Axial surface connects hinge lines for multiple layers!) 90-80°: Upright 80-10°: XXX inclined ≤10°: Recumbent Fleuty method, 1964

Fold classification Classification based on hinge line plunge vs. axial surface orientation Hinge line plunge (Recall: Hinge line can be curved or straight, and also inclined relative to the ground) 0-10°: Horizontal 10-80°: XXX plunging ≤80°: Vertical Fleuty method, 1964

Fold classification N 25° Axial surface dip: 75° E Classify this fold by the Fleuty (1964) method! N 25° Axial surface dip: 75° E Is it an anti or syn- form? 2) Is the fold plunging? 3) What angle is the hinge line plunging at? 4) What is the dip of the Axial surface?

Gently plunging, Steeply inclined Antiform! Fleuty method, 1964 Strata dip away from fold axis Plunging 25° at 180 ° Axial surface dip: 75° E Gently plunging, Steeply inclined Antiform! Fleuty method, 1964

Fold classification Ramsay’s (1967) classification Dip isogons: lines connecting points of identical dip for vertically oriented folds (Upright or steeply inclined) 3 Classes, illustrated at the right:

Fold classification Class 1: isogons converge towards inner arc Class 2: isogons parallel axial trace (similar folds) inner arc = outer arc (variable thickness) Class 3: isogons diverge towards the inner arc

Fold classification Class 1A: hinge thickness < limb We’ll just consider “similar” and “parallel” Class 1A: hinge thickness < limb Class 1B: uniform thickness (parallel folds) Class 1C: intermediate between Class 1B and Class 2

Fold classification Parallel or similar? Which is which? (Uniform bed thickness or variable?) A B PARALLEL SIMILAR

Fold classification and symmetry Folds can be symmetric or asymmetric in cross-section Symmetric: when looking at a cross-section perpendicular to the axial surface (profile plane!), the two sides are mirror images of each other Sometimes called M-folds Asymmetric folds are sometimes called S- or Z-folds Z-folds have short limbs that appear to have been rotated clockwise (S-folds: counter-clockwise) Nothing to do with curved vs. angular shape!

Fold symmetry Fold systems consisting of folds with a consistent asymmetry are said to have a vergence Given by (a) the sense of displacement of the upper limb relative to the lower limb, or (b) the rotation of the inclined short limb  clockwise rotation implies a right-directed vergence

Fold symmetry Large folds tend to have smaller folds occurring in their limbs and hinge zones (first order folds These smaller folds (second-order) are also called parasitic folds 2nd order fold asymmetry or vergence indicates their position on the large-scale structure!

Even though layers retain their original thickness, they nonetheless experience shortening and extension. outer & inner arc & neutral surface

Inner-arc compression Outer-arc extension & Inner-arc compression