Depth-Dependent Lithospheric Extension: Supporting Evidence, Structural and Depositional Characteristics, and General Applicability

Garry Karner
ExxonMobil Upstream Research Company ([email protected])

Recent drilling results and reflection and refraction seismic interpretations from the ultra-deepwater regions of the Iberian-Newfoundland margin have seriously challenged current paradigms concerning the mechanisms responsible for the rifting, stretching, and thinning of continental lithosphere. The Iberian-Newfound-land conjugate system is characterized as non-volcanic margins exhibiting a deficit of synrift magmas, a zone of exhumed subcontinental mantle in the continent–ocean transition zone, and an apparent extension discrepancy (i.e., depth-dependent extension). Depth-dependent lithosphere thinning is the process by which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust.
The required extensional balance between the lower and upper plate can be accomplished by the exhumation and emplacement of thinned lower crust and continental mantle adjacent to a zone of syn-extensional sagging. The existence of exhumed serpentinized peridotite of continental affinity in the transition zone is an extremely important observation as it is a direct indicator of depth-dependent extension.
Second, these continental peridotites are both faulted and associated with magnetic anomalies. Apparently, the serpentinization process can create sea-floor spreading magnetic anomalies during continental mantle exhumation, thus challenging the long-held view that the existence of correlatable magnetic anomalies unambiguously defines ocean crust. Kinematic reconstructions of the highly faulted Iberian Abyssal Plain, the same region characterized by extensional allochthons, the S-reflector, and top-basement detachment systems (e.g., Hobby High), constrained using ODP Leg 103 and 179 data results in a pre-faulting crustal thickness 6-12 km overlain by shallow-water Tithonian carbonates.
In this scenario, the exhumation of continental mantle necessarily predates this last phase of extreme brittle deformation across the margin in such a way as to maintain isostatically a shallow water environment while preferentially thinning the lower crust and lithospheric mantle. A paradoxical situation presents itself of apparent upper crustal tectonic quiescence while the lower crust and mantle is undergoing extreme extension.
The purpose of this presentation is to review the evidence for depth-dependent lithosphere extension across the Iberian-Newfoundland conjugate margin and discuss the evidence for extension discrepancies characterizing the structural and stratigraphic development of other continental margin systems around the world, notably the Exmouth Plateau of northwest Australia, the Marion Plateau of northeast Australia, and the Congo and Angolan margins of West Africa.

AAPG Search and Discovery Article #90073 © 2007 AAPG Foundation Distinguished Lecturer Series 2007-2008