NEW CONSTRAINTS ON RIFT MARGIN DEVELOPMENT THROUGH INTEGRATION OF DEEP-IMAGING SEISMIC INTERPRETATION AND GRAVITY MODELING

Velocity analyses and interpretations of multichannel seismic data combined with modeling of gravimetric data are methods capable of providing accurate, almost-unique solutions to geologic questions. As a result, an iterative integration of deep-imaging seismic interpretations derived from depth- converted PROBE-Study multichannel seismic data, concurrently collected shipboard gravity measurements and proprietary GETECH gravity grids have been used to obtain a better understanding of: 1) the geological meaning of satellite gravity images (GETECH and Sandwell & Smith 1995, and their various derivatives); 2) the geometry and location of ocean-continent boundaries; 3) port-rift margin subsidence; 4) the evolution of the Gabon-Brazil conjugate margin.

PROBE data-derived interval velocities were used to create depth sections and block geometries for gravity modeling. Preliminary density ranges for the derived blocks were established using an Odegard-modified "Gardner/Nafe-Drake velocity density relationship". Deviations between observed and calculated gravity signatures based upon these preliminary blocks were then used to refine both time-depth conversions and density assignments within the given seismic constraints. Detailed models of the gravity field associated with the track lines of profiles 23, 24, and 25 of the PROBE dataset were constructed in order to describe the density distributions of both the deep crustal blocks along the ocean-continent boundary (OCB), and throughout the Gabon salt basin (GSB).

The resulting geologic models are tightly constrained with respect to location and density differences, reflecting different petrologic units. Materials previously identified as oceanic crust appear to consist of higher-density, proto-oceanic crust (POC). Subsidence of POC appears to be decoupled from that of the continental sector. In case of Line 23, the transition occurs simply and abruptly at the POC petrologic crustal boundary. In the case of line 25, blocks of POC with continental affinities occur seaward of the main decoupling zone.

Work in progress uses these constrained models to obtain age estimates of the most landward portion of POC using isostatic and thermal subsidence models.