Comparison of fold-thrust belts driven by plate convergence and gravitational failure

Abstract Submarine Fold-thrust belts (FTBs) are predominantly formed by the deformation of sedimentary sequences as a result of subduction of oceanic plates at active margins, gravitational failure at many passive margins, or a combination of these two at both types of margin. A key question is: Is the FTB driven by gravitational failure basically the same as the FTB driven by plate convergence or are there fundamental differences? Deepwater FTBs in the toes of gravity-driven systems display many elements of structural style that are similar or even identical to those in FTBs driven by plate convergence. However, some structural elements and elements of fold-thrust belt development history differ between the two systems. To address this, we used examples from various tectonic settings (end members and hybrid systems) to conduct detailed structural analysis in terms of geometry, structure, strain distribution, shortening rate, deformation history, and tectonic process. The new results suggest: (1) The energy source in gravity-driven systems is the gravitational potential energy within the sedimentary material itself that is being deformed to create the updip extension and downdip contraction, whereas it is the movement of stressed lithospheric-scale tectonic plates, which lie outside the local sediment pile and within the broader undeformed crust and lithosphere; (2) as a result, the thickening is attained largely by deposition of the syn-kinematic sequence in the gravity-driven system, whereas it is attained by shortening and stratigraphic repetition by thrusting in the plate convergence-driven system; (3) the energy in gravity-driven systems is resupplied by sediment input from large river deltas and therefore deformation tends to be episodic, linked to major episodes of sediment input. Whereas in a system driven by plate motions, the energy is resupplied by movement of a boundary upon which force is acting, and tends to be continuous, and less episodic; (3) the thrust faults of both systems are predominantly basinward-verging thrust faults. Backthrusts and back rotation appear to be only observed in the purely or predominantly plate-convergence driven systems; 4) the rate of shortening across plate convergence-driven systems is high (e.g. 43–48 mm/yr in Hikurangi, 37–46 mm/yr in the Makran), and generally continuous on a long timescale. Whereas across the contractional domain of gravity-driven systems, shortening is slow (e.g. 1.4–2.0 mm/yr in the Niger delta, 0.8 mm/yr in the Para-Maranhao basin) and more variable through time; (5) for both driving mechanisms, the fold-thrust belt propagates forward with new thrust initiation at the toe, but fault activity differs slightly. Focused activity at the toe of the FTB is more common where driven by plate convergence but not observed in gravity-driven systems. Activity across much of the FTB is observed for both systems, but activity focused in the rear to middle of the FTB is only observed in gravity-driven systems; (6) the plate-driven system is primarily limited by rate of plate motion, i.e., the rate at which the plate is fed into the FTB, whereas in a system driven by gravity, the movement is limited (resisted) by the strength of the sediments and detachment.