ASSESSING THE SPATIAL AND TEMPORAL VARIATIONS IN SUBSIDENCE AND WETLAND LOSS RESULTING FROM ACTIVE FAULTS IN COASTAL LOUISIANA, USA

Active faults affecting coastal Louisiana have been suggested since the pioneering Mississippi Delta studies by Harold Fisk in the 1940’s. With modern seismic reflection data and dense oil and gas drilling the subsurface distribution and geometry of the faults have become well established. However, the modern influence of these faults on Louisiana’s highly anthropogenically altered coastal plain is enigmatic. Wetland conversion to open water (land loss) in coastal Louisiana is 43 sq. km per year, and the cumulative loss with increasing sea-level rise threatens the sustainability of the coast. The causes of wetland loss are numerous, but one contributor is land subsidence. Therefore, fault-driven subsidence is important to understand because of the multi-billion dollar planned investments to reduce wetland loss in Louisiana. The majority of faults in coastal Louisiana are growth faults which had a rapid period of movement and sediment expansion during the Oligocene or Miocene (5 to 20 MBP). Modern reactivation of these faults significantly post-dates their growth phase, and is anomalous. The modern fault rates can far exceed the geologic growth rate suggesting that modern movement and subsidence is episodic. Most faults in coastal Louisiana are listric. The geometric ramifications of creating accommodation space through subsidence has been well studied by fine-element models, rock-mechanics analyses and cross-section reconstructions. The greatest subsidence rates are associated with the shallow/ steepest portion of a listric fault, and are less with deep/ flatter segment of the fault. Most faults in Louisiana include shallower strata that are not fully compacted or lithified. Natural or anthropogenic fluid withdrawal or fluid movement is just one of many drivers that might non-uniformly affect the volume of a fault block. If changes in volume occur at a fault plane, this may manifest as localized fault movement both vertically or laterally. In addition to tectonic drivers, natural compaction, groundwater fluid withdrawal and fossil fuel extraction may all be contributing to fault reactivation, and all have spatial and temporal variability.