Superimposed and simple pop-up structures in fold-and-thrust belts and their implications: Insights from sandbox models of thrust wedges

摘要

Pop-up structures are widely developed in fold-and-thrust belts and strike-slip tectonic settings (e.g., Sylvester, 1988;Harding, 1990;Rrichard et al., 1995;Dooley and McClay, 1996;McClay and Whitehouse, 2004), and have been of great significance in petroleum exploration.Analogue sandbox modeling has proved to be a powerful visual tool for simulating such complex structures in various tectonic settings (e.g., McClay and Whitehouse, 2004).We conducted a series of sandbox modeling to unravel the differences in structural geometry of pop-up structures during the development of fold-and-thrust belts, based on the same initial conditions with various shortening velocities (0.3mm/s, 0.1mm/s, 0.05mm/s, 0.005mm/s).From our modeling results, two types of pop-up structures, i.e.superimposed and simple pop-up structures, can be developed in thrust wedges depending on different shortening velocities.In general, the geometric pattern of pop-up structures consists of paired thrust and retro-thrust, with their ramp angle showing a inverse relationship with increasing shortening velocity before reaching a steady-state eventually, at which ramp angles are 30°-41°for thrusts and 48°-65°for retro-thrusts.In particular, there are significant differences between superimposed and simple pop-up structures.The angle between thrust and retro-thrust (βs) in superimposed pop-up structures is mainly within 100°-108°, and larger than that of simple pop-up structure (βn) between 80° ～ 100° (Table 1).Although displacements of the thrust and retro-thrust in both types of pop-up structures are similar (30-73.7 mm and 5.2-30.2 mm respectively), there are more faults developed in superimposed pop-up structures.Thus, the superimposed pop-ups show much wider geometry, with low height/width ratios and more complicated internal deformation (Figure.1).Furthermore, the shortening velocity shows an intimate correlation with the structural geometry of the thrust wedge.At a high velocity (e.g., 0.3mm/s), the wedge typically shows a simple deformation style characterized by simple pop-up structures and increasing deformation propagating continuously towards the foreland However, it is characterized by superimposed pop-up structures with an asymmetrical axial plane (Figure 1, dipping toward the hinterland) and more stronger deformation localized within the back limb of the pop-up structure at a low velocity (e.g., 0.005mm/s).It should be noted that the growth of superimposed pop-up structures has significant control on the evolution of fold-and-thrust belts and their petroleum accumulation.With the development of new retrothrust faults, pre-existing thrusts are cut and moved backward, resulting in a superimposed pop-up structure characterized by an asymmetrical axial plane (Figure 1, toward the foreland).The deformation further controls the fault entrapment and spill points of plays located in the fold-and-thrust belt.