TRANSIENT CHANNEL RESPONSE TO NEOTECTONICS: OBSERVATIONS FROM ALLUVIAL CHANNELS DRAINING ACROSS THE BATON ROUGE FAULT ZONE, SOUTHEASTERN LOUISIANA

A network of alluvial channels flowing southward over the Pleistocene and Holocene fluvial deposits of the Mississippi Valley crosses the E-W striking Baton Rouge fault zone (BRFZ). The BRFZ is a gravity-driven normal fault system with south-dipping Quaternary scarps. Although BRFZ is a major concern for coastal subsidence, its dynamic behavior is poorly understood. This study aims to investigate recent fault movements and their controls on channel morphology. We quantify transient behavior of the fault-crossing alluvial channels by utilizing stream power-law scaling approach. For this we used LIDAR DEM data with 5 m horizontal resolution.

Quantitative stream-profile analysis of the entire drainage network reveals at least two types of transient features located north of the fault zone (i.e. in the footwall): (1) broad convex zones, ~10-30 km in length, that occur in the smaller catchments, and (2) a series of knickpoints propagating upstream. These irregularities of the channel profiles are likely related to the recent displacement on the BRFZ, where channels are still in a non-steady state condition.

Only smaller channels tend to exhibit the broad convex zones; thus, these convex zones might be related to a threshold drainage area. These smaller channels lack smooth concave-up profiles in the footwall regions indicating that they have yet to adjust to relatively recent fault activity. Generally, relief of these convex zones correlates with fault scarp relief, further supporting that their origins are related to recent faulting. Spatial distribution of knickpoints in the study area suggests two causative processes. The knickpoints within the Pleistocene terraces are potentially linked to the Pleistocene glacioeustatic cycles when many incised valleys were formed during sea-level fall. Whereas, knickpoints marking the northern boundary of BRFZ are likely linked to neotectonic activity along the fault zone. Moreover, the normalized steepness index (k_sn) map of these alluvial channels shows two trends: higher k_sn values characterize the upthrown northern side of BRFZ where the erosion rate is higher, and lower k_sn values reside in the hanging wall. This pattern of k_sn values throughout the drainage network also suggests that the region is experiencing differential erosion.