LATE QUATERNARY TECTONIC MOVEMENTS IN THE MISSISSIPPI DELTA: FAULT MOVEMENTS VERSUS LITHOSPHERIC FLEXURE

Recent studies have reported a 5.2 mm/yr fault slip rate along the Baton Rouge Fault Zone (BRFZ) and a 3 to 6 mm/yr lithospheric flexure-related subsidence rate in the Mississippi Delta (MD), which make coastal restoration in southeast Louisiana all but a losing proposition. However, these numbers have not been evaluated by means of measurements derived from the stratigraphic record in the Lower Mississippi Valley (LMV) and the MD.

Fault slip rate was quantified for a fault scarp in the BRFZ by OSL dating. Stratigraphic correlation across the fault shows that a distinct clay bed, interpreted as a flood-basin deposit, is vertically offset by ~5 m across the fault. This deposit was dated to ~130 to 140 ka (interpreted as MIS 5e) on both sides of the fault. The average fault slip rate is 0.04 mm/yr in the last 130 ka at this location. If slip rates of the other faults in the BRFZ are not significantly different from this location, the aggregate fault slip rate within the BRFZ is probably <0.1 mm/yr on average over this time interval.

Lithospheric flexure was quantified by studying the late Pleistocene long profile of the Lower Mississippi River that is preserved in the Prairie Complex and OSL dated to ~80 ka (MIS 5a). Compared to the present-day long profile, the MIS 5a long profile has been uplifted up to 11 m in the southern LMV and downwarped by more than 20 m in the MD, which is interpreted as driven by lithospheric flexure due to sediment loading. Using the present-day long profile as a proxy for the initial MIS 5a long profile prior to deformation, the bulge in the southern LMV exhibits an uplift rate of <0.14 mm/yr on average, whereas the MD between 30.4 °N and 29.5 °N has subsided at <0.4 mm/yr due to lithospheric flexure during the past 80 ka. The half-wavelength of the flexural bulge predicts a minimum lithospheric thickness for this region of ~60 km.

These findings indicate that fault slip in the BRFZ is geologically slow (by several orders of magnitude) relative to the rapid land-surface subsidence in the MD. The MIS 5a long profile shows no abrupt slope change in the MD as would be expected if tectonic movements were driven by faulting. Therefore, vertical motions in the LMV and MD are driven by lithospheric flexure due to sediment loading. The combined effect of fault movement and lithospheric flexure, while geologically significant, are not a major cause of land-surface subsidence in the MD.