Gavin Lewis¹, Edward Mcallister², James R. Porter², Robert J. Knipe², Charles F. Kluth³

(1) Chevron, Oslo, Norway
(2) 2) Rock Deformation Research, Earth Sciences, Leeds University, Leeds, United Kingdom
(3) Chevron, San Ramon, CA

ABSTRACT: Linked Extensional and Compressional Systems: A Field and Analogue Modeling Study

Linked extensional and compressional systems are commonly observed on seismic data from the Gulf of Mexico, Niger Delta and Angola. They occur at various scales from decimeter systems formed by slumping or small scale growth faulting to kilometre scale systems involving the entire post rift sequence. Upslope sedimentary loading results in evacuation of a ductile medium (salt/shale) and extension of the carapace. The formation of a thrust belt or development of salt sheets/nappes compensates the extension. This poster presentation is concerned with the formation of thrust structures at the toe of the detachment systems. Examples from field outcrops in the Eastern Kentucky Coalfield, USA and analogue modeling experiments demonstrate the structural architecture and progressive structural development.

Roadcuts through the Eastern Kentucky Coalfield expose deltaic sediments of Mississippian to Pennsylvanian age. Metre scale listric growth faults are observed soling onto marine shale units. Downslope shortening takes the form of synthetic and antithetic thrust stacks and box folds. The formation of structural relief has controlled the location of subsequent extensional structures when the foldbelt was buried. The outcrops have been scanned using a high-resolution section scanner to provide 3D visualization of the structures.

The structural development can be recreated in the laboratory, a layered polymer and glass bead pre-kinematic state, loaded with a wedge of fine. Extensional structures were infilled to regional at regular intervals to provide a record of the extensional history in vertical sections. The lengths of the shelf and toe regions were measured through time to ascertain the percentage of the extension compensated for by shortening. As extension became negligible on the shelf a second load was prograded over the foldbelt, analysis of the serial sections shows the relationship between the compressional and post dating extensional structures.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado