USING DIFFERENTIAL COMPACTION MODELLING FOR IDENTIFYING FAULT ORIGIN: AN EXAMPLE USING THE LAS VEGAS VALLEY FAULT SYSTEM, NV, USA

Differential compaction is suggested as a cause of fault development in basins with abrupt lateral facies variations in basin-fill sediments. In most cases, a large burial depth is required to maximize the difference in compaction between two adjacent lithologic units and form a fault between them. But when it comes to shallow compaction in young developing basins, could sediment compaction be large enough to form high scarps on the surface or is tectonism required? We address this issue using a case study of the Quaternary Las Vegas Valley Fault System (LVVFS), southern Nevada. The LVVFS consists of six normal faults with surface escarpments up to 45 m high whose origin is controversial. The most common theories are development through tectonic rupture like other faults the western USA, differential compaction, or a combination of both. Notably, in addition to the significantly different seismic hazard each fault development model provides, knowing the origin of the LVVFS contributes to understanding of Cenozoic extension in southern Nevada, and hence, the central Basin and Range Province. In this study, we evaluate the role of differential compaction in the LVVFS development and whether differential compaction could conceal tectonic activity. We use porosity data from neutron, resistivity and lithologic well logs, and thin sections to construct a basin-wide compaction model for Las Vegas basin utilizing porosity variations with depth. Decompaction techniques are used to calculate the depositional stratigraphic thickness and compare it to present-day thickness to determine the differential compaction and tectonic components that created the present-day escarpments. Our results suggest that (1) none of the differential compaction triggers, including abrupt facies/thickness change at depth, are observed in the basin; (2) early calcite cementation hinders compaction of fine-grained sediments which typically occur in the LVVFS hanging wall, while little cement was observed in the coarse-grained sediments that dominate the footwall, and thus, compaction increases opposite to the LVVFS throw direction; and (3) differential compaction contributed to escarpment destruction rather than building. These three points suggest that tectonic rupture is essential for the development of the LVVFS escarpments.