Summary

Heat flow variations within both continental and oceanic crust play an important role influencing hydrocarbon maturation rates. Therefore, mapping of the continentaloceanic boundary (COB) is very important in offshore oil and gas exploration. In some areas of the world such as at active margins, the COB is very distinct, while in other areas, such as the passive margin Santos Basin in offshore Brazil (Figure 1), the position of the COB is less certain and more open to interpretation. Thick sediments, evaporites and volcanic rocks present within the stratigraphic sequence tend to obscure both seismic and gravity signals making it more difficult to distinguish crustal types along this margin. This study analyzes the most up-to-date satellite altimeter-derived gravity dataset and seismic data available. This integration has helped to distinguish, and better constrain, areas of rifted and extended continental crust from areas of true oceanic crust.

Introduction

Based on crustal type and tectonic framework, most passive margins can be divided into three distinct zones: continental crust, rifted or extended/attenuated continental crust, and oceanic crust. Typical continental crust thickness at the passive margin ranges from 20 to 40 km and formed by igneous or metamorphic rocks. During crustal attenuation (rifting), continental crust thins forming extensional features such as normal extensional faults, horst-and-grabens, and half-graben structures. Sediment supply to these active extensional grabens is sourced, not only from the erosion of proximal rift shoulder uplifts, but also from more distal hinterland areas. This hinterland-derived sedimentation typically enters the rift system via strike-slip or wrench systems that connect the graben systems together. This active sedimentation produces sediment thickening towards the centre of the rift. As crustal attenuation proceeds, eventually oceanic crust starts to form. This crust is significantly thinner and is formed usually by a gabbroic complex overlain by basalts. These rocks, as well as shallow upper mantle peridotites, have higher densities than typical continental crustal rocks and can be quite distinct. Basaltic lava flows comprising the top layer of the oceanic crust often form very distinctive seismic patterns known as Seaward Dipping Reflectors (SDRs). Examples of these distinct reflectors are well-known from Offshore Norway (Vöring Basin) and offshore Brazil. Oceanic fracture zones are also one of the most prominent features of the oceanic crust and can be used as a distinguishing criterion. Unlike sediment deposited in active continental rifts, sediments deposited on oceanic crust usually have constant thickness (no active uplift/tectonism) and tend to onlap pre-existing basement topography. Density/velocity contrast, crustal thickness differences and basement/sediment relationships are all geologic criteria that can help distinguish crustal types. Integration of gravity and seismic data aids this determination.

Methodology

Satellite altimetry-derived gravity (SADG) data are commonly used in offshore exploration to map regional tectonic features such as extension of the fracture zones, identification of major rifts, and delineation of the COB. Reprocessing of the SADG is constantly conducted by various groups to improve the data resolution. SADG data used in this study were extracted from the global Sandwell and Smith dataset, version 15.1 released in 2006.

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