By Philip S. Prince, Virginia Division of Geology and Mineral Resources

In this model, I tried to reproduce a structural style that it is clearly expressed in some very interesting landforms–the Santa Cruz and Don Figuerero Mountains of southern Jamaica.

The very geometric Santa Cruz and Don Figuerero Mountains reflect structural control. Intense karst development limits surface channel networks, preserving the “outline” of fault bounded blocks. The Santa Cruz Mountains are south of Lacovia Tombstone; Mandeville is located atop the plateau that is the Don Figuerero Mountains.

While the Blue Mountains are by far the most topographically significant features on the island of Jamaica, the Santa Cruz and Don Figuerero Mountains on the island’s south coast are equally-eye catching when viewed as digital topography. These elevated areas are highly geometric in their appearance, forming parallelogram-like shapes in map view.

The edges of these mountain blocks are conspicuously straight and show minimal dissection by surface streams, presumably due to sub-surface karst flow. Large landsides have occurred along the scarps, which are up to 800 meters (~2700 feet) high.

This large slide is located northwest of Mandeville on the hanging wall scarp of the Spur Tree Fault. It is an impressive feature, and descends nearly 800 meters.

The shape of the mountain blocks is important because it likely communicates an important aspect of their underlying geology. The “blocky” shape, particularly that of the Don Figuerero Mountains, is not consistent with buckle-type folding occurring above a low-angle detachment or thrust. Instead, the shape of the uplifted areas suggests movement of steeply dipping normal faults which have reactivated and are now undergoing reverse motion. Evidence for this interpretation is discussed in detail in Benford et al. (2014), which is a really cool paper and definitely worth a read (https://pubs.geoscienceworld.org/gsa/lithosphere/article/7/1/21/145722/interaction-of-reactivated-faults-within-a).

The main idea is that an extensional phase produced a series of basins which were filled and subsequently buried under nominally flat-lying post-rift sediments during a phase of tectonic quiescence. The onset of a compressional stress field opposite to the previous extensional regime reactivated the pre-existing fault network, deforming the overlying post-rift strata. The geometric appearance of the Santa Cruz and Don Figuereros is thus an expression of the buried fault structure coming back to life.

This conceptual model was the basis for the analog setup.

Compare this cross section to the model sections below, and keep in mind that only 1 km of subsurface is shown here…the model can be thought of as extending from the surface down to the brittle-ductile transition (13-15 km or so).

There is some evidence for buckle folding related to slip on a detachment just southwest of Santa Cruz, where the blocky shape transitions to a more elongated, round-topped ridge. Broadly speaking, however, these mountain masses are clearly not typical of what would be produced by detached deformation of a sedimentary package.

This interpretation is based purely on topographic expression, but the elongated ridge style o on the left certainly looks like buckle folding of a thin rigid layer above a weak decollement.

I attempted to reproduce this structural style with a model employing the basic sequence of events suggested by Benford et al. (2014). I used an elastic base between solid panels to produce a zone of stretching and thinning within the layer pack. This “basin” was filled with growth sediment, and then covered with a post-extension sequence of one weak layer beneath two very brittle layers. Motion sense was then reversed, shortening the filled basin(s) and producing reverse faults.

 

The end result was interesting. The fault pattern is very distinct from what would be produced by an undisturbed, flat-lying layer pack with no inherited architecture. Aspects of the extensional structure geometry can be seen in the pattern of reverse faulting, which appears to involve both steep reactivation-type faults and some low-angle slip on the shallow detachment to produce buckling. It’s obviously not a perfect reconstruction, and the magnitude of mechanical contrast between strata strength and fault weakness in these models can not approach reality. Even so, the final geometry of this model is very distinct from what would be produced by deforming a flat-lying layer pack without an inherited fault network (see below).

Reverse faults are too steep for “typical” thrusts developed under pure compression in these materials. These sections are also not perfectly orthogonal to dip, so the faults are actually a bit steeper than what is seen here (apparent dip).

Compare this internal geometry and its surface expression to a basic restraining bend model developed from flat-lying layers with available decollements. The flat-lying architecture is comparatively boring, but some of the serial sections illustrate how fault orientation resulting from restraining bend compression might match up nicely with old, buried normal faults, particularly the ones that rotated into lower dips with progressive extension.

 

This post was originally published on The Geo Models blog.