BACKTHRUSTS

Fold and thrust belts are complex areas found at the leading edge of a mountain belt. They have an overall wedge geometry due to thin skinned crustal shortening. The tip of the wedge has a thrust fault geometry that is evident by a stepped structure: ramps and flats. Flat faults connect the ramps. Fig. 1.

The formation of backthrusts occurs during foreland propagation. Foreland propagation occurs in fold and thrust belts and is due to thin skinned crustal shortening and determined by wedge geometry. Internal deformation is determined by strength of the basal detachment, strength of material, amount of erosion and isostatic subsidence. This, in turn, will cause many of the features we see: duplexes, forethrusts and backthrusts to name a few. Backthrusts are thrust sheets that are directed back in the opposite direction of foreland propagation. Forethrusts are the most common formation, backthrusts are rare.

One idea on how backthrusts are created is the formation of shear fractures in conjugate sets with opposite dip. 1) A ramp is formed in two competent layers, 2) a fault connects the two ramps and there is some fracturing within one of the competent units in the opposite direction, 3) the fault propagates until it meets an existing flat fault and the backthrust is created (fig 2). This is called a pop-up structure. The materials need to be highly cohesive and have signficant internal friction to form backthrusts. They also seem to form where a very efficient detachment horizon (salt) pinches out and the thrust deformation has a tendency to lock-up.

Backthrusts are probably more common than we realize. They are common as a hangingwall flat on a footwall flat with no stratigraphic separation (Eisenstadt, De Paor, 1987). This makes them difficult to recognize in the field. Other reasons that explain the lack of field recognition are erosion of the feature or that backthrusts only occur at certain points in thrust belt formation.


View of small backthrust generated in sandbox model.


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Kirk Heim - 3/27/97