ppt-5

An F1 Fold; 2 limbs & hinge surface.

N

B

A

An F1 Fold; 2 limbs & hinge surface.

N

B

A

B

A

pole to S0 pole to S1

Flattening onto a plane in D2

N

B

A

B

A

The red dashed great circle is the bulk flattening plane

pole to S0 pole to S1

Flattening onto a plane in D2

N

B

A

B

A

pole to S0 pole to S1 If limbs A & B of F1 fold can deform independently, each will be cylindrically folded about a different axis whose orientation is the same as the Xn of the limb with the D2 flattening plane

Flattening onto a plane in D2

N

B

A

B

A

pole to S0 pole to S1 If limbs A & B of F1 fold can deform independently, each will be cylindrically folded about a different axis whose orientation is the same as the Xn of the limb with the D2 flattening plane.

Flattening onto a plane in D2

N

B

A

B

A

pole to S0 pole to S1 Poles to bedding in the nose of the F1 fold will lie somewhere in the field between the great circles defined by the cylindrical folding of the limbs.

Flattening onto a plane in D2

N

B

A

B

A

pole to S0 pole to S1 Cleavage, S1, does not contain either of the B2 fold axial directions. It is therefore folded conically, about one or other of the new axes.

Small circles (approx – just sketched freehand)) about the fold axis in the A limb of the F1 fold.

N

Any plane that is conically folded about this axis will lie on just one of these small circles.

Conical Folding N

The plane selected here (poles are the blue circles) contains the axis. It is folded about a small circle at 90 degrees – thus a great circle, and a cylindrical fold.

Conical Folding N

Poles to planes on a small circle at 70 degrees to the axis. These look very like poles on a great circle, and would generally not be recognizable as conical folds based just on their pattern.

Conical Folding N

Poles to planes on a small circle at 70 degrees to the axis. These look very like poles on a great circle, and would generally not be recognizable as conical folds based just on their pattern.

Conical Folding N

This problem is even worse if the rotation takes the pole to the plane through the primitive (i.e., if the plane itself is rotated through the vertical).

Conical Folding N

This problem is even worse if the rotation takes the pole to the plane through the primitive (i.e., if the plane itself is rotated through the vertical).

Conical Folding N

S2

It is departure of S2 from the field of the poles to the earlier fabric (or the marked variations in the attitude of the S2/S intersection lineation – which amounts to the same thing) which really suggest a conical fold.

Conical Folding N

Conical folds with small apical angles are hardly folds at all.

N

B

A

B

A

pole to S0 pole to S1

A lineation parallel to B1 would be rotated on a small circle about the relevant B2 fold axis.

Other Possibilities: 1. One limb, such as the longer limb in an asymmetrical fold, dominates the kinematic scheme. It will fold cylindrically, and the shorter limb may be constrained to fold conically about the axis defined by the long limb.

2. The first-phase cleavage (S1) is the mechanically active fabric (e.g., in a slate). S1 will then be folded cylindrically, and the bedding will be folded conically.

A Strong Cautionary Note: These kinematic schemes are approximations to the real-life situation, given the strong likelihood of late-stage flattening.

Late-stage flattening will not render a cylindrical fold noncylindrical, but it will render a conical fold non-conical.

Domains

Data D t off any kind ki d ffrom thi this region i would ld yield i ld a very complex l pattern on a stereonet, unless the pattern were due to perfectly coaxial folding.

Domains

JJudicious di i choice h i off d domains i reduces d thi this problem. bl R Removall off d data t th thatt represent nose regions of early folds is the simplest procedure, if a pattern is already evident. Otherwise, data must be handled very cautiously.