Stratal fold patterns adjacent to normal faults: observations from the Gulf of Mexico

Abstract

Abstract This paper describes the study of a subsurface array of young growth faults from the Gulf of Mexico, using a high-resolution three-dimensional (3D) seismic data set. The seismic data have resolved all of the faults within the survey area in detail, enabling the full three-dimensional structure and displacement patterns of the faults to be accurately mapped. In this paper, special attention is paid to the magnitude and orientation of small ‘drag’ folds that occur in the wall-rock of these faults, ubiquitous throughout the data set and well resolved by the seismic data. The results indicate that in both the footwalls and the hangingwalls the folds occur with variable amplitude, and also that they are small in relation to the dimensions of the respective faults. That is, they are spatially impersistent, often exhibiting rapid changes from normal drag to reverse drag over distances as short as 100 m. Furthermore, strong discordance with the recorded patterns of throw (the vertical component of displacement) suggests that no simple apparent correlation exists with the respective displacement distributions. By comparison with a number of field and theoretical examples, various mechanisms are discussed to explain the probable origin and variability of these folds. In general, however, evidence for the cause of the folds is limited and remains largely inconclusive. It is suggested that one potential explanation is stratal deformation in the vicinity of points of overlap and linkage between faults in the dip direction (dip linkage). In an earlier study of the fault throw distributions it is has been argued that this is the preferential mechanism by which faults in this part of the Gulf of Mexico enlarged their surface area during growth.