Tectonics of Fold-Thrust Belts Driven by Plate Convergence and Gravitational Instability

Fold-thrust belts (FTBs) related to plate convergence are found in active margins and foreland of orogenic belts while those related to gravitational failure are found on passive margins. Seismic imaging of the subsurface structure, combined with decades of study and analysis, have resulted in a good first-pass understanding of their tectonics and mechanics, but there are still many significant unresolved issues. Numerical models were used to investigate aspects of thrust belt growth: the interplay between the overall wedge taper, width and height, deformation front, internal movement, and fault position, activity, displacement and dip. Models show that thrust belts grow cyclically, with periods of accretion (rapid thrust-front advance, high displacement and strain rate),and periods of adjustment (slow wedge deformation, low displacement and strain rate). New area balancing methods have been developed to improve the structural restoration and shortening quantification. Application to analogue models and natural examples shows that a higher regional dip results in reduced shortening while a lower regional dip leads to increased shortening. The Northwest Borneo Fold-Thrust Belt (NBFB) are investigated using 3D seismic data. The NBFB contains three fold types: fault-propagation folds (dominant); detachment folds (minor); and fault-bend folds (rare). For each fold, structural style varies along strike, in response to changes in the magnitude of folding, basal décollement strength, and inherited structure and basement topography. The low taper angle (mostly 0.7 of lithostatic pressure). Fold-thrust belts caused by plate convergence are compared with gravity-driven systems. The energy source in gravity-driven systems is the release of gravitational potential energy within the sediment pile, producing upslope extension and downslope contraction. This is resupplied by sedimentation. Episodic sediment input leads to fluctuations in deformation rate. In contrast, the energy source of convergence driven systems is movement of a stressed lithosphere-scale boundary. The rate of shortening across convergence-driven systems is high and generally continuous on a long time scale. Whereas rates are lower across the contractional domain of a gravity-driven system and more variable through time. A plate-driven system is limited by plate motion rate, whereas a gravity driven system is resisted by the strength of the sediments and detachment.