Structural Style of Passive-Margin Thrust Belts Compared to Orogenic Thrust Belts: Lessons from Namibia and Elsewhere

Until recently, knowledge of thrust systems was based almost exclusively on onshore thrust belts, driven by plate convergence. Established paradigms of thrust systems included the belief that sections can be balanced on the basis of bed-length conservation, the dominance of in-sequence thrusting, and the possibility of ultra-low-angle triangle zones (intracutaneous wedges).

Passive margin thrust belts have many similarities to their conventional orogenic counterparts, but we also see significant differences in style, rate and process. Established thrust-belt paradigms do not universally apply to passive-margin thrust systems.

The relative timing of deep water thrust systems is commonly recorded by the deposition of synkinematic sediments, which reveal that a simple in-sequence model is rare in passive margin thrust belts; more commonly, there is evidence of simultaneous movement on multiple thrusts, with overall foreland directed progression.

The most common structural style in passive margin toethrust belts is that of an imbricate fan, in which displacement ramps up to sea floor from a single main decollement. Ramp-flat geometries are rare, and duplexes exist only in special circumstances. Previously unrecognized classes of structural styles, such as ploughshare-shaped ramps, are identified.

Wherever we see complete linked systems of updip extension and downdip contraction, it is clear that bed length is not conserved. The amount of seismically visible extension is typically greater than the amount of visible contraction; a significant amount of downslope transport is accommodated by significant lateral compaction. Incorporating lateral compaction in to classic thrust belts may be necessary, it this may resolve some improbable geometric interpretations.

Orogenic and passive margin thrust belts need to be recognized as related but different members of the same family, with both shared and unique characters. Understanding the differences should lead to a better understanding of both types.