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Sequence of Deformation in Thrust-Fold Belts: Implications for Cross-Section Balancing


The complex deformational history of thrust-belt structures has implications for the validity of cross-section balancing. 1) The first structures to form are regional fracture sets generated by prevailing plate tectonic stresses. 2) As the thrust wedge grows, the tectonic load flexes the foreland crust leading to extensional fractures sub-parallel to the thrust belt. 3) With continuing thrust advance, the foreland basin rocks experience a third phase of fracturing induced by near-field thrust-tectonic stresses. 4) Perhaps overlapping with stage 3, dominant units, such as carbonate and coarse clastic rocks, undergo layer-parallel shortening (LPS) by pressure-solution cleavage, wedging, and tectonic compaction. Bedding plane detachments accommodate contraction within the LPS zones to produce fold-, cleavage-, and fault-duplexes. 5) The various detachments may link to form through-going thrusts, generating ramp anticlines (fault-bend folds), detachment (lift-off) folds, and fault-cored anticlines (fault-propagation folds). 6) As thrust-related folds evolve, fractures and subsidiary faults initiate in response to constantly changing stress fields in the dominant members. This complex and protracted sequence of deformation raises conflicts with the underlying assumptions of cross-section balancing. Zones of LPS, joined by bedding parallel detachments, produce cryptic global shear strain throughout individual thrust sheets and the thrust wedge as a whole. Cross-section balancing relies on measuring line-lengths and/or areas between established pin-lines, lines perpendicular to bedding in regions presumed to have experienced only plane strain. If rocks have experienced extensive global shear, this assumption is invalid. The validity of balancing is further compromised by an inability to predict the magnitude and distribution of sub-resolution strain produced by LPS in stage 4. Finally, geometric and kinematic relations between thrusts and folds may not be as simple as portrayed in common balancing approaches. Thrust-belt cross sections usually show continuous, through-going master faults (step 5 above), which originate at great depth within the hinterland and step up section toward the foreland to terminate in triangle zones or emerge at the earth's surface. However, at various stages in their evolution, discontinuous thrust segments may be linked by single folds and fold duplexes, leading to alternative interpretations for many “balanced” sections.