Abstract: Control of Stratigraphy on Thrust Vergence and Formation of Triangle Zones Within Thrustbelts

Brent A. Couzens, Bruno C. Vendeville, David V. Wiltschko

In thrustbelts, triangle zones form where there is a loss of systematic thrust vergence, and back- and forethrusts form. We reviewed the stratigraphy of thrustbelts worldwide and conducted tectonic experiments varying the mechanical stratigraphy to investigate why certain thrustbelts lose vergence while others do not. In experiments, dry sand modeled strong brittle rocks, glass beads modeled strong plastic decollements, and viscous silicone modeled very weak decollements.

Our survey distinguishes between Type I triangle zones, with just a single decollement, and Type II triangle zones, with two or more decollements. Thrust vergence is lost and Type I triangle zones typically form above a very weak, single decollement, such as evaporites. By contrast, thrust vergence remains systematic if the decollement is relatively strong. Our survey suggests that Type II triangle zones have a stratigraphy that consistently includes an upper, bendable, but otherwise unfaulted roof sequence, underlain by a roof-detachment zone with backthrust motion and a lower duplex above a floor detachment Vergence is lost and backthrusts form on the roof detachment where the roof sequence is weaker and more anisotropic. Experiments on Type II triangle zones show that the strength f both floor and roof detachments also controls thrust vergence. Where only the roof detachment is weak, the relative motion can still remain forward. However, if both floor and roof detachments are weak, thrust vergence is lost, the relative motion is backwards, and shortening in the lower duplex is accommodated in the roof sequence directly above horses in the lower duplex.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France