前陆盆地形成与演化砂箱物理模拟启示--以四川盆地西部龙门山为例

Analogue modeling insights to foreland basin growth:A case study on the Longmenshan thrust belt in western Sichuan Basin

通过砂箱物理模拟实验揭示冲断带-前陆盆地系统形成演化过程中,单一构造剥蚀、沉积作用和剥蚀-沉积耦合过程对其盆-山系统形成演化的控制影响作用,尤其聚焦于龙门山-川西前陆盆-山系统沿走向变化的构造剥蚀-沉积作用对其盆-山系统的控制作用。单一构造剥蚀作用过程模拟揭示浅表剥蚀作用强度与冲断带楔形体楔长、楔高呈负相关性;构造剥蚀作用使冲断带后缘逆冲断层更加易于发生断层再活化与无序冲断,制约着冲断带前陆向扩展。浅表沉积作用导致楔形体前缘冲断层上覆物质加载、更易于发生断层闭锁,促使冲断带楔形体前陆向/前展式扩展生长。沿走向变化的剥蚀-沉积作用过程导致冲断带楔形体后缘深部层系剥蚀走向变化、断层产状变陡,前陆盆地中前陆向断层明显减小、其倾角减缓,且反向冲断作用加强(乃至导致主断层反向冲断),尤其走向差异性导致前陆盆地发育斜向断层。受控于龙门山冲断带-川西前陆盆地系统晚三叠世以来走向差异性剥蚀-沉积作用,川西前陆盆地中南段发育晚白垩世—新生代再生前陆盆地,可能形成前陆盆地前缘斜向断层-龙泉山断层。

Analogue modeling of the formation and evolution of Longmenshan belt and foreland basin system in western Sichuan Basin reveals a strong impact of tectonics,erosion,and sedimentation as well as of the interactions between the later two processes,especially those changing along the the strike of the system.The modeling shows a negative relationship between wedge geometry(i.e.,length and height)and surface erosion magnitude.Heavier erosion makes it easier for faults at the back of the wedge to thrust in a random sequence and be reactivated,thus stopping the wedge from further forelandward propagating.While surface sedimentation,that adds to the layers overlying the thrusts and increases the possibility of fault locking,serves to faciliate the forelandward propagation of the wedge.Changing(e.g.,increasing)erosion-sedimentation process along the strike of the wedge results in a direction shift of the strike of deep strata and steeper faults at the back of the wedge,some evidently shrinking forelandward faults with smaller dips and increasing backthrusting and the formation of oblique ramps in the foreland basin.We therefore conclude that it is this changing or differential erosion-sedimentation process in the Longmenshan belt and foreland basin system since the Late Triassic that controls the development of a rejuvenated foreland basin with a possible oblique ramp(Longquanshan fault)from the Late Cretaceous to Ceonozoic.