Analysis of Fault Network and Rock Mass Connectivity in Pull-Apart Basins

Sims, Darrell W.¹; Morris, Alan P.¹; Ferrill, David A.¹; Waiting, Deborah ^1
1 Department of Earth, Material, and Planetary Sciences, Southwest Research Institute, San Antonio, TX.

Fault system geometry of pull-apart basins is often complex and difficult to interpret from two- or even three-dimensional seismic data. This is particularly the case for the interpretation of steeply dipping to vertical faults. Here we examine lateral and vertical fault network and rock mass connectivity of a pull-apart basin developed over a ductile detachment. We used analogue methods so that pull-apart development in plan view could be closely observed and documented. Our scaled models consisted of a horizontal color-layered sandpack over a thin (0.5 cm) layer of silicone putty. Final subsurface fault geometry was obtained by vertical, closely spaced (< 1cm) serial cross-sections coupled with map view images of the developing model upper surface. We use the following method to perform the analyses. The cross-sections and map view are photographed and converted to digital images. The cross-section and final plan-view images are co-registered to a common reference frame and imported into three-dimensional seismic interpretation software. Fault traces and contacts between colored sand layers are converted to two-dimensional fault and horizon surfaces to create a three-dimensional visualization of layers and faults throughout the pull-apart system. Fault trace maps are taken from sequential vertical and horizontal slices through the three-dimensional visualization model. Fault network connectivity and rock mass connectivity are then calculated for each slice. Our results indicate that basin fault systems become increasingly complex with increased depth, producing an overall increase in horizontal fault network connectivity with depth and, conversely, a decrease in rock mass connectivity. Lateral variability in connectivity at depth is observed along basin strike, and is not predictable from surface fault trace maps.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009