Thrust sheath fold

Sheath folds are distinctive curvilinear folds in which the hinge actually wraps around on itself.  In three-dimensions sheath folds look much like their name implies, a sheath that might holster a sword (or in Oman, the traditional khanjar!).  When eroded, the tubular-shape of a sheath fold displays a characteristic eye-shape in cross section—that’s what we see on the slopes above Wadi Mayh.

Sheath folds were first recognized in the late 1970s and early 1980s, but, in my opinion, not properly appreciated until the 1990s.  They form when layers are strongly sheared and early formed fold hinges are rotated into cone-like shapes; the long-axis of the sheath fold parallels the direction along which the rocks were most stretched.

In 2007 Mike Searle and Ian Alsop published an excellent article in the journal Geology on mega-sheath folds from the Wadi Mayh area. The sheath folds are developed in shallow marine carbonate rocks of Permian and Triassic age that are in tectonic contact with underlying high-pressure metamorphic rocks formed when the Oman ophiolite was obducted onto the Arabian margin.  The folds in the photo are actually subsidiary folds of an even larger mega-sheath fold about 15 km in length!

For me, sheath folds, regardless of the scale, dramatically illustrate that solid rocks are capable of flow, often in complex, but enticingly beautiful ways.

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