Incorporating Erosion Into Structural Forward Models: Reconstructing Burial and Deformational Histories From Angular Unconformities in Growth Strata

Abstract

Fault-related structures in onshore and nearshore basins often show signs of regional subaerial erosion. In seismically imaged growth strata, syn-deformational erosion is evidenced by angular unconformities within hangingwall strata and missing section within footwall strata. Erosion complicates correlation across faults and has significant implications for burial history. This study presents a new approach to structural forward modeling that parameterizes model surfaces by age as well as depth. By including surface age, we can define complex footwall and hangingwall burial histories that include periods of erosion. The modeled fold geometry depends on fault shape, shear angles, and horizon slip, according to established kinematic theories (specifically inclined shear fault bend folding and trishear fault propagation folding). Where younger model surfaces intersect older fold surfaces, the younger surfaces erode and truncate the older surfaces. The models are fully interactive, allowing us to continually modify footwall to hangingwall correlations and fault shape until the computed horizon shape and unconformity geometry match the observational data.

In this presentation we apply the new modeling technique to seismic examples of extensional and contractional structures with complex burial histories indicated by hangingwall unconformities. The first example is a basin-bounding growth fault within the Bohai Bay, South China Sea where over 5 km of syn-extensional erosion has removed the entire footwall section. By interactively modeling the observed hangingwall angular unconformities, we quantitatively reconstruct both the eroded footwall and burial history for the growth fault. The second two examples are inversion structures from the Subandes in Peru and the Junggar Basin in China. Both inversion structures feature multiple types of angular unconformities that independently reflect periods of extension and contraction. Forward modeling these structures refines the timing and magnitude of each deformational phase as well as providing the burial history. A final example from the Outeniqua Basin in South Africa shows how complex hangingwall unconformities can arise solely from movement along fairly simple faults. For each example, quantitative animations show the sequential development of the structures including periods of burial, erosion, and changes in deformation style.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018