HR: 1330h
AN: G43A-01    [Abstracts]
TI: Geological implications of recently derived vertical velocities of benchmarks of the south-central United States of America
AU: * Dokka, R K
EM: rkdokka@c4g.lsu.edu
AF: Center for GeoInformatics and Dept. of Civil & Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803 United States
AB: It has been long-recognized that the south-central United States of America bordering the Gulf of Mexico (GOM) is actively subsiding, resulting in a slow, yet unrelenting inundation of the coast from south Texas to southwestern Alabama. Today's motions are but the latest chapter in the subsidence history of the GOM, a region that has accommodated the deposition of over 20 km of deltaic and continental margin sediments since mid Mesozoic time. Understanding the recent history of displacements and the processes responsible for subsidence are especially critical for near-term planning for coastal protection and restoration activities. Documentation of the true magnitude and geography of vertical motions of the surface through time has been hampered because previous measurement schemes did not employ reference datums of sufficient spatial and temporal precision. This situation has been somewhat improved recently through the recent analysis of National Geodetic Survey (NGS) 1st order leveling data from >2710 benchmarks in the region by Shinkle and Dokka (NOAA Technical Report 50 [2004]). That paper used original observations (not adjusted) and computed displacements and velocities related to NAVD88 for benchmarks visited during various leveling surveys from 1920 through 1995. Several important characteristics were observed and are summarized below. First, the data show that subsidence is not limited to areas of recent sediment accumulation such as the wetland areas of the modern delta (MRD) of the Mississippi River or its upstream alluvial valley (MAV), as supposed by most current syntheses. The entire coastal zone, as well as inland areas several hundred km from the shore, has subsided over the period of measurement. Regionally, vertical velocities range from less than -52 mm/yr in Louisiana to over +15 mm/yr in peripheral areas of eastern Mississippi-Alabama. The mean rate is ~-11 mm/yr in most coastal parishes of Louisiana. In the Mississippi River deltaic plain, subsidence was 2-3 times higher than estimates based on long-term geologic measurements. The data also indicate that adjacent alluvial ridges where the population is concentrated have been similarly affected. In the Chenier plain of southwest Louisiana, a region previously thought to be subsiding at slowly, rates of sinking are similar to those of the deltaic plain. Second, spatial patterns suggest that motions at most locations may have both long (10-100 km) and short (<5 km) wavelength components. Gross aspects of some long wavelength motions can be explained by flexure produced by late Quaternary sediment loads such as the MRD and the MAV. Short wavelength spikes in motions correlate well with areas of fluid withdrawal, faults, and salt structures. Third, motions at many benchmarks have not been linear through time. For example, subsidence in ~10-30 km wide zones surrounding some active normal faults of south Louisiana declined as faulting has slowed (and vice versa). Subsidence in these areas reached a peak in 1970 and declined thereafter. Some local changes also correlate with changes in human-related activities (e.g., reduced groundwater pumping and slower subsidence in the Lake Charles area beginning in the late 1980s).
DE: 1208 Crustal movements--intraplate (8110)
DE: 8107 Continental neotectonics
SC: Geodesy [G]
MN: 2005 Joint Assembly