Collisional Fold-and-thrust Belts Detached on Salt

Mark G. Rowan, Rowan Consulting, Inc., Boulder, CO

Collisional fold-and-thrust belts detached on salt can be divided into two subsets: those where the prekinematic section is undeformed, and those where diapirs and minibasins were established prior to the onset of shortening. Examples from the Sierra Madre Oriental of Mexico, the Flinders Ranges of South Australia, and the Zagros Mountains of Iran are used to illustrate the very different geometries that result.

In cases where pre-shortening deformation is absent, structural styles are relatively simple, characterized by regular wavetrains of parallel, elongate detachment folds that are broadly symmetrical. Individual folds are typically cylindrical over much of their lengths and then conical and more open where they plunge gently toward their terminations. Thrust development, variations in fold orientation, and anomalously steep plunges on some folds are attributed to the spatial and thickness distribution of the décollement layer.

In fold-and-thrust belts where salt withdrawal and diapirism predated the shortening, the preexisting structural architecture and the consequent variable strength of the overburden control the deformation and yield much more complex geometries. Modelling by B. Vendeville of circular minibasins separated by a polygonal pattern of salt ridges, with diapirs at the ridge intersections, shows that the weak diapirs are squeezed, the intermediate-strength ridges form a complex pattern of contractional structures, and the strong minibasins simply translate and may rotate. The result is a polygonal array of folds, thrusted folds, strike-slip faults, and even extensional structures that typically intersect at, and plunge away from, the diapirs.

If early salt withdrawal and diapirism lead instead to linear salt walls and elongate withdrawal basins, later shortening structures have the same preferred orientation. Moreover, strong strata around the tips of salt walls result in a strike-parallel strain gradient in which the centers of the walls shorten more than the ends. With enough shortening, steep salt welds form that link two remnant diapirs at the tips of the walls. Further shortening leads to folding above the welds, producing geometries where diapirs are located at fold terminations rather than culminations.