Effects of Fault Throw and Mechanical Stratigraphy on Fault Architecture in the Lenghu Fold-Thrust Belt, Qaidam Basin, NE Tibet

Abstract

Understanding the detailed fault architecture at an outcrop scale of reverse faulting is critical for understanding the processes involved in fold-thrust belts as well as predicting the degree of fault compartmentalisation, the relationship between folds and faults, the distribution of strain and sub-seismic faulting deformation. Although previous studies have investigated the relationship between faults and folds developed within compressional systems, the detailed fault architecture of thrust faults remain poorly constrained, particularly at an outcrop scale. Not only are sub-seismic faults poorly imaged but even seismically resolvable structures are poorly imaged because of either steep dipping nature of reflections or complexity of fault architecture at an outcrop scale, which makes it difficult to predict the fault zone geometry precisely. The detailed fault architecture analysis of an exceptionally well-exposed fold-thrust belt in the Qaidam Basin is reported.

The Lenghu fold-thrust belt, provides an exceptionally well-exposed outcrop example of a reverse fault-propagation fold, which enables high-resolution fieldwork to collect multiple scale data, including stratigraphy, strikes/dips, shallow subsurface profiles and detailed fault outcrop maps. Detailed stratigraphic logging coupled with high-resolution profiles provides a unique insight into the 3D geometry of a thrust fault at both regional and an outcrop scale. Integrating the high-resolution fault system mapping, we observe that 85-90% of the estimated throw is accommodated on the main fault zone, which has sufficient throw to be imaged on a seismic profile, while 15-20% of the throw is accommodated on smaller scale folds and faults that are beyond seismic resolution. The exceptionally exposed outcrops of the Lenghu thrust fault was mapped in detail, which allows us to understand the fault architecture patterns regarding to the fault throw (ranging from tens of centimeters to tens of meters). Fault outcrops with comparable fault throw but different level of mechanical heterogeneity also allow us to evaluate the control of mechanical stratigraphy on fault zone deformation. By coupling the structural observations within a stratigraphic context, we can demonstrate that although the main fault controls the overall strain in the system, the local stratigraphy plays a critical role in how the strain is accommodated and whether it is partitioned into single faults, multiple-fault splays or folding.

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