Rifting: Seismic Examples from the Western Florida Margin and around the World

Long-offset seismic reflection data (10 km streamer, 40 km record collection) over the western Florida shelf and slope provide new insights for rifting. This setting benefits from (1) the occurrence of Jurassic salt, which constrains paleo-depth at the rift-drift transition; (2) oceanic crust at the COB whose backstrip indicates accretion at 2.5 to 3 km paleo-depth; and (3) reduced sediment overburden due to the relative isolation from terrigenous input, which allows good reflectivity to 24 km depth.

The northwest Florida slope has SDR packages, landward-dipping upper crustal listric faults, and autochthonous and mobilized salt. The southwest slope has no observed SDRs or salt, and upper crustal faults dip basinward. In the north especially, a “sub-salt” unconformity (SSU) is ubiquitous and overlies peneplaned rift basins and SDR packages. The surface is only gently faulted, even in today's slope environment, indicating that crustal extension or plate accretion had moved elsewhere by the time of salt deposition. However, it descends basinward to the depth of oceanic crust at the COB. The subaerial SSU and overlying salt must have subsided rapidly to 2.5 km paleo-depth as ocean crustal accretion began. Also, the distal rifted margin always overlies a landward dipping detachment defining the base of rifted crust that appears continuous with Moho. This detachment accommodates rapid basinward-increasing subsidence of the outer margin, and represents the primary means by which continents are sloughed off of rising plastic mantle during rifting.

Seismic data at other margins were examined to generalize the inferred processes for Florida. Interpretations from Brazil, Africa, and Greenland demonstrate that the model holds. Literature suggests that rapid (<2 Ma), large (~2.5 km), tectonically controlled subsidence at outer margins as seafloor spreading begins is largely overlooked. In early models for SDR development by Hinz and Mutter, no explanation was offered for the transition from subaerial to abyssal conditions, and this process remains murky today. These data provide a means, timing and rate by which that subsidence is documented.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California