Abstract

Rift basin exploration has provided the oil and gas industry with almost one third of discovered global hydrocarbon resources. The maturity of prolific passive margin sequence plays has necessitated a shift in technical emphasis to understanding petroleum geology of the deeper precursor rift basin megasequences. Modelling of these petroleum systems relies extensively on theoretical stretching hypotheses for whole-crust evolution and heat flow prediction. Several alternatives have emerged to account for anomalous rift-related thermal stress, asymmetry of structural styles and origin of thick syn-rift ‘sag-basin’ subsidence patterns at passive continental margins. These observations cannot be predicted by the pure shear uniform stretching model first introduced by McKenzie in 1978. Newly acquired, 2D long cable deep seismic records provide empirical evidence that supports more complex, polyphase, depth-dependent stretching origins for rift-basin formation. Heterogeneities are believed to be inherent in the brittle upper crust, in the ductile lower crust and lithospheric mantle and so result in the complex distribution of accommodation space recorded by syn-rift megasequence deposition during episodes of orthogonal or oblique extension. No single stretching model uniquely describes the varied structural response of the anisotropic crust to plate-scale extension. A hybrid of phased simple and pure shear deformation mechanisms separated by regional lateral, and possibly vertical crustal discontinuities, may explain the apparent paradox of along-strike co-existence of uniform versus depth-dependent stretching structural geometries.

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