Deformation dynamics of the retro-wedge foreland: Seismic interpretation and numerical modeling study of the Llanos Basin, Colombia

Abstract

Combining reflection seismic interpretation with numerical modeling, this paper studies the deformation dynamics of the foreland plate overridden by the retro-wedge of an advancing subduction zone. The post-middle Eocene Llanos foreland basin serves as the natural laboratory. The reflection seismic interpretation identifies five elongated clusters of flexure-driven normal-faults, centered on the forebulge axis. Clusters have different stratigraphies of their syn-tectonic strata. They are located foreland-ward of thickness maxima of the coeval foreland basin fill. Normal faults are mostly planar, steep and die out with depth without being detached. Both fault clusters and infill thickness maxima are characterized by the SW-to-NE migration in time. Those sub-sets of normal faults, which are adjacent to retro-wedge, underwent tectonic inversion. They became reverse faults in a short time interval, likely less than 1-2 million years. The modeling demonstrates that the inversion requires a stress regime with sub-horizontal sigma 1 stress affecting the foreland crust located between the forebulge and retrowedge. This regime does not occur when the foreland plate is only loaded by the retro-wedge and foreland basin fill. It requires both vertical loads plus horizontal compression affecting the foreland plate. The modeling also indicates that both aforementioned loading scenarios result in the development of forebulge axis-centered clusters of flexure-driven normal faults. The modeling also indicates that the orogen-parallel Jurassic-Early Cretaceous rift-related crustal thinning zone located in the foreland plate results in its narrow flexure mode. Instead, the lack of crustal thinning results in a broad flexure mode of the foreland plate.