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Abstract

Scaled analog models are powerful tools for dem-onstrating the structural styles of extensional fault systems. Analog models were constructed to simu-late deformation in a sedimentary sequence above three principal kinds of extensional detachments- horizontal and tilted, uniformly extending detach-ments; simple listric faults; and ramp/flat listric faults. For each detachment geometry, models were made of homogeneous sand, sand/mica, and sand/clay mixtures in order to simulate, respectively, deformation of isotropic sediments, of aniso-tropic sediments, and of sedimentary sequences with competency contrasts. Horizontal and tilted, uniformly extending detachments produced asym-metrical graben structures bounded by steep planar faults and containing internal arrays of domino faults. Planar faults that extend into the synrift sequence become listric. The experimental results are directly comparable with the geometries of ini-tial rift structures such as those in the East African rift system.

Simple listric extensional detachments are char-acterized by rollover anticlines with geometrically necessary crestal-collapse graben structures. With progressive deformation, crestal-collapse grabens show hanging-wall nucleation of new faults. Com-plex and apparently conjugate fault arrays result from superposition of successive crestal-collapse grabens. At large extension values, a fan of listric growth faults forms in the synrift sequence. The structures formed in simple listric detachment experiments are closely matched by those found in prograding deltaic systems.

Ramp/flat listric extensional fault systems gener-ate a rollover anticline and a crestal-collapse graben system associated with each steepening-upward segment of the detachment, and a hanging-wall syncline and complex deformation zone associated with the ramp. Hanging-wall synclines are charac-teristic of ramp/flat detachments. Differences in ramp geometries produce dramatic changes in the types of hanging-wall structures that are produced. Similar ramp/flat geometries are found in deltaic environments and on continental margins, such as the Murre fault system off Newfoundland.

The models demonstrate that the geometry of the detachments exerts a fundamental control on the evolution of extensional fault systems.

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