Ramps and Flats

Ramps and flats are characteristic of a thin-skinned thrust fault geometry, and they form a step-like pattern. Flat are fault surfaces that form parallel to the strata and usually in weak rock units, such as evaporites and shales. Ramps cut across more resistant rock units, for example sandstone and limestone, forming a dip angle that is typically 30 to 45 degrees.

Ramp-flat geometry forms in both extensional and compressional environments, deforming stratified material. Ramps can occur in a frontal, oblique or lateral position. Frontal ramps intersect the main fault perpendicular to the displacement direction of the fault. Oblique ramps intersect the main fault at an incline to the displacement direction. Lateral ramps have a strike parallel to the displacement direction, and can be considered tear faults. Fault ramps develop due to the amount of stress on the slip surface which will cause the fault to cut up through the more competent layers. With the ramp in place, a new crack can propagate from the base of the ramp and continues until it ramps and joins a higher flat (Boyer and Elliott, 1982). This process is repeated (Figure 1) in a pattern called foreland propagation. This pattern produces anticlinal and synclinal features. The amount of movement along a flat could be 10's of miles and the height of a flat can be 1,000's of feet. The ramp produces an upthrust of older rocks, so there will be a sequence with older rocks on top of younger.

These ramp-flat sequences are important for petroleum geologists because the hanging wall structures can act as hydrocarbon traps, for both oil and gas.

In this oblique view of a sandbox modeling development of structures in a deforming wedge undergoing contraction, an upper wet layer has greater strength. The equivalent of thrust flats form at the contact between wet and dry sand defining a thrust sheet slab that moves mostly intact. The slip surface ramps through the wet sand to the surface.

Reference: Boyer, Steven E. and David Elliott. Thrust Systems AAPG Bulletin Volume 66, Number 9, September 1982, (p 1196-1230).

Julie Welch - 3/27/97