EVALUATING DYNAMIC SUBSIDENCE MODEL PREDICTIONS USING GULF OF MEXICO GEOHISTORY ANALYSIS

Geodynamic models predict that the Gulf of Mexico (GoM) underwent 1 km of Cenozoic dynamic subsidence which swept across the Gulf from southwest to northeast in an 800-1000 km wide depression between 100 Ma and the present as the Farallon slab passed beneath it. We test this model result by examining GoM abyssal plain subsidence through a geohistory analysis of eight ION Gulfspan seismic lines which traverse the northern GoM passive margin from south Texas to Alabama. We show that the abyssal GoM experienced two episodes of rapid subsidence which we evaluate in terms of mantle, lithospheric, and surficial processes. An early Cenozoic (65-49 Ma) episode of enhanced subsidence is evident at all locations west of the Mississippi river. This subsidence was synchronous with the peak of Laramide orogensis and potentially driven by loading of sediments produced during Laramide unroofing. At these locations the sedimentary section is 2-4 times as thick as its equivalent to the east suggesting that it reflects regional flexure resulting from asymmetric filling of the basin. The late Cenozoic event (34 Ma – Recent) is seen in all seismic lines, and correlates temporally and spatially with rapid progradation and aggradation of the sediment wedge. Progradation was facilitated by gravity-slide fault systems linking extension within the wedge to shortening at its toe while aggradation was accompanied by development of numerous salt withdrawal basins wherein mobile salt is replaced by sediment increasing the density of the wedge. We interpret this period of rapid subsidence to be driven by flexure beneath a 10 – 15 km thick wedge of sediment and salt as it advances across the abyssal plain. We find no correlation between abyssal plain subsidence and the sublithospheric passage of a subducted slab. Therefore we suggest that the downward suction force imparted to North America by the sinking Farallon slab could not overcome the flexural rigidity of the lithosphere beneath the northern GoM.