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

Abstract

Fold-thrust belts (FTBs) related to plate convergence are found in active margins and in the foreland of orogenic belts, while those related to gravitational failure are typically 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. After a new thrust initiates at the wedge front, the entire wedge shortens and thickens to re- attain critical taper, with significant frontal thrust activity and minor activity on older thrusts. 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). Detailed observation of the zone in front of the thrust front indicates that deformation in that region is a critical component of system advance. New area balancing methods, involving evaluation of the role of the regional slope, have been developed to improve the accuracy of structural restoration and shortening quantification. Application to analogue models and natural fold-thrust belts highlights the importance of regional slope in area balancing restoration: a higher regional dip results in reduced shortening while a lower regional dip leads to increased shortening. Accuracy of the shortening estimate requires independent constraint of parameters, particularly the initial regional slope. The tectonics of the Northwest Borneo Fold-Thrust Belt (NBFB), offshore Brunei, 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. Fault spacing responds to basement topography and topography also blocks forward propagation of the fold-thrust belt. Fault dip increases as the fault and fold matures. The low taper angle (mostly <6) implies a high basal fluid pressure (>0.7 of lithostatic pressure). Evidence of two distinct stages of fault (fold) activity, wide distribution of active contractional deformation across the entire belt (rather than just at the toe), and present-day extensional inactivity, suggest that the NBFB results from a combination of primary gravitational tectonics and secondary plate convergence. 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. Plate movement is continuous, so thrust belt deformation is less episodic. Back-thrusts and fault back rotation are more common in convergence-driven systems and landward-vergent thrusts can be present. 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, i.e. the rate at which the plate is fed into the FTB, whereas a gravity driven system is resisted by the strength of the sediments and detachment.

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ORCID for Lisa Mcneill: orcid.org/0000-0002-8689-5882

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