Publication
B. Couzens-Schultz,
B. Vendeville, and D. Wiltschko, 2003, Duplex style and triangle
zone formation: insights from physical modeling: Journal of Structural
Geology, v. 25, p. 1623-1644 [PDF]
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Duplex
Style and Triangle Zone Formation:
Insights from Physical Modeling
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Duplexes are
a common feature in thrust belts at many scales. Their geometries
vary significantly from antiformal stacks with significant forethrusting
in the cover (e.g. southern Pyrennes, Spain) to triangle zones (e.g.
foreland Canadian Rockies) to low-displacement individually spaced
ramp-anticlines (e.g. Sub-Andean thrust belt, Bolivia). We present
a series of physical experiments demonstrating that the strength
of the de´collements relative to that of the intervening and
overlying rock layers plays a significant role in controlling the
duplex style. The models comprise brittle layers made of dry quartz
sand and de´collements made of two types of viscous silicone
polymers. The strength of the de´collements in the models
is a function of the shortening rate applied to the model. The relative
strength of the de´collements and surrounding rocks affects
the development of active- or passive-roof duplexes (triangle zones).
It also affects the amount of translation of
individual thrust blocks and the spacing of thrust ramps, which
in turn determine if a duplex evolves into an antiformal stack or
into individually spaced ramp-anticlines. Model results indicate
that specific associations of structural features form systematically
under similar rheological and boundary conditions. The presence
of relatively strong de´collements promotes local underthrusting
of the cover, individual ramp-anticlines, internal deformation of
thrust sheets, low early layer-parallel shortening, and sequential
towards-the-foreland propagation of structures. Weak de´collements
promote forethrusting of the cover, antiformal stacks, coeval growth
of structures, and low internal strain, with the exception of significant
early layer-parallel shortening. No underthrusting at a regional
scale occurred in any model.
Brent A. Couzens-Schultz,
Shell International
Exploration and Production, Inc. P.O. Box 481, Houston, TX 77001,
USA.
David V. Wiltschko,
Department of Geology and Geophysics, Texas A&M University,
College Station, TX 77843, USA
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