Sheath folds

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In regions of large differential flow, such as in thrust zones, some parts of the fold axial regions may advance relative to other parts.
 
 
 
 
 
 
 
 

The fold axes then become folded, in some cases so markedly that a fold may look like the finger of a glove or the sheath of a knife.
 
 
 
 

When eroded, cross sections resemble eyes and are commonly called eye folds.
 
 


Tongue, north coast of Scotland

This area is behind the Moine thrust and shows a mappable sheath fold where the older Precambrian has "intruded" the Cambrian Moine schist. Smaller scale sheath folds are illustrated here that mimic the geometry of the large Archaean cored sheath.

Skullomie cove, on the east side of Tongue loch


85_335. View of Skullomie cove with sheath folds -- not obvious in this photo. The axes plunge generally gently to right. The island is composed of Devonian Old Red Sandstone. Grid ref. NC 627 630. LatLon 58.519538,-4.377849


85_321. In a larger sheath, minor folds will have Z or S profiles on different sides of the main fold. These have Z. Grid ref. NC 627 630. LatLon 58.519538,-4.377849


85_322. Cross sections of sheath folds -- eye folds. Grid ref. NC 627 630. LatLon 58.519538,-4.377849


85_334. Eye fold. Grid ref. NC 627 630. LatLon 58.519538,-4.377849


85_333. The eye of a sheath with steep axis on the left, and shallow on the right. Grid ref. NC 627 630. LatLon 58.519538,-4.377849


85_328. The axes are regular on the flank of the sheath fold and are mullions. Grid ref. NC 627 630. LatLon 58.519099,-4.376020


85_323. Lineation on straight limb of sheath fold. Grid ref. NC 627 630. LatLon 58.519538,-4.377849


85_331. An outcrop sized sheath. The fold turned to the right of the hammer and curves around behind the coat. Grid ref. NC 627 630. LatLon 58.519538,-4.377849


85_327. Another view. The lineations are parallel to the hammer and the long edge of the notebook, and horizontal in between. Grid ref. NC 627 630. 58.519538,-4.377849


85_325. Curved lineation of a sheath fold. Grid ref. NC 627 630. LatLon 58.519538,-4.377849


85_324. Curved lineation of a sheath fold. Grid ref. NC 627 630. LatLon 58.519538,-4.377849


85_332. The core of the large sheath. Grid ref. NC 627 630. LatLon 58.519538,-4.377849

Strathan and Portvasgo, on the west side of the Kyle of Tongue


85_303. S folds. Strathan. Grid ref. NC 573 649. 58.549014,-4.450250


85_309. The core of a large eye fold. Portvasgo. Grid ref. NC 588 651. LatLon 8.550799,-4.433548


85_315. Eye folds. Portvasgo. Grid ref. NC 588 651. LatLon 8.550799,-4.433548


85_314. Eye folds. Portvasgo. Grid ref. NC 588 651. LatLon 8.550799,-4.433548


85_305. Moine flags with lineation running down dip of the foliation. There are numerous sheath folds to the left.  Portvasgo. Grid ref. NC 588 651. LatLon 8.550799,-4.433548

Oykell Bridge

This is a classic locality that presented the problem of reconciling the strongly linear fold axes having the same orientation as the movement direction of the Moine Thrust -- i.e. to the northwest. Intuitively, we might expect associated fold axes to be perpendicular to the movement direction. Recognition of sheath folds resolves this dilemma. At this locality, we see only one highly linear limb of the sheath.


85_214. The folds are strongly mullioned; for the most part they are bedding cleavage intersections. Oykell Bridge. [NC386008; 57.967111, -4.730087]


85_216. A close up of mullion folds. Oykell Bridge. [NC386008; 57.967111, -4.730087]
The lineation is very regular at these outcrops -- Trend 120, plunge 20, i.e. down the dip of the Moine Thrust.


85_215. Fold mullions. Oykell Bridge. [NC386008; 57.967111, -4.730087]


85_219. More fold mullions. Oykell Bridge. [NC386008; 57.967111, -4.730087]


85_217. Cleavage mullions in Moine schist. The cleavage, sloping shallow to right, intersects bedding in a very regular lineation. Oykell Bridge. [NC386008; 57.967111, -4.730087]

North coast of Spain


86_098. Eye fold viewed down the stretching lineation; in the basal shear of the Mondonedo nappe.  West of Foz.


86_097. Nose of sheaf fold with stretching lineation. West of Foz.


86_115. Tight isocline traced for 30 m as part of an eye fold.  Carino, Cedeira.


86_119. Oblique view of sheath folds.  Cannon point, Cedeira.


86_120. View of foliation (S2) surface showing some variation of fold axis on the limb of a larger sheath fold.
Cannon point, Cedeira.

South Australia


67_083. Tectonic fish, one of several sheath folds in older Precambrian gneiss.   Weekeroo, South Australia. -32.192291,140.070763

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