THE EFFECTS OF ACTIVE NORMAL FAULTING ON THE MODERN-DAY SUBSIDENCE OF SOUTHERN LOUISIANA

Analysis of 1st order class 2 geodetic level data suggests that faulting contributes to subsidence in two ways. The first is a local effect and occurs by downward translation of the hanging wall near the fault, and broad (<5 km) downwarping of the hanging wall that diminishes gradually away the surface trace. Hanging wall deformation is due to volume compensation mechanisms, i.e., reverse drag antiforms and antithetic faults that form in association with curviplanar normal faults. Isostatic uplift and tilting of footwall areas near normal faults due to unloading is minimal because of contemporaneous sedimentation in half graben. Sedimentation on the hanging wall has the effect of preventing the footwall from being unloaded and thus deformed. Active faulting plays a more important role in regional subsidence by weakening the lithosphere, which in turn changes the way that the lithosphere bears the load of the Mississippi River delta (MRD). The regional component of subsidence in south Louisiana is the result of sediment loading on an elastic lithosphere that is cut by faults that move episodically. As the locus of faulting shifts, so does the locus of subsidence. During intervals when faults are locked or less active, the lithosphere is more intact and bears the entire load. Deflection of the surface can be approximated by an elastic plate of infinite length that deforms under a load. During times of greater fault activity such as 1969-1971, however, the partially severed lithosphere subsides more broadly and at higher rates. Faulting has the effect of reducing the elastic thickness of the lithosphere. Movement along a major fault will segment temporarily the MRD into separate loads which then operate on each broken plate independently, deflecting them downward. Areas on opposite sides of the fault tilt inward forming a vee-shaped zone of deformation and surface subsidence.