137 / 2014-09-05 11:22:16

Superimposed And Simple Pop-up Structure In Fold-and-thrust Belt And Its Implications: Insight From

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Pop-up structure is widely developed in fold-and-thrust belt and strike-slip tectonic setting (e.g., Sylvester, 1988; Harding, 1990; Rrichard et al., 1995; Dooley and McClay, 1996; McClay and Whitehouse, 2004), and has been paid much attention for its 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). Here, we conducted a series of sandbox modeling to unravel the difference in structural geometry of pop-up structure during the development of fold-and-thrust belt, based on the same initial condition with various shortening velocity (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, would be developed in thrust wedge depending on different shortening velocity.
In general, the geometric pattern of pop-up structure consists of paired thrust and retro-thrust, with their ramp angle showing a versus relationship with increasing shortening velocity before reaching a steady-state eventually, of which ramp angles are 30° ~ 41° for thrusts and 48° ~ 65° for retro-thrusts. In particular, there is significant difference between superimposed and simple pop-up structures. The angle between thrust and retro-thrust (βs) in superimposed pop-up structure is mainly within 100° ~ 108°, and larger than that of simple pop-up structure (βn) between 80° ~ 100° (Table 1). Although displacement of 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 structure. Thus, the superimposed pop-up shows much wider geometry, with low ratio of height/width and more complicated inside deformation (Figure.1). Furthermore, the shortening velocity shows an intimate correlation with structural geometry of thrust wedge. At a high velocity (e.g., 0.3mm/s), the wedge shows largely simple deformation style characterized by simple pop-up structures and increasing deformation propagating continuously towards foreland. However, it is characterized by superimposed pop-up structures with an asymmetrical axial plane (Figure 1, dipped toward hinterland) and more strong deformation localized within the back limb of pop-up structure at a low velocity (e.g., 0.005mm/s).
It should be noted that the growth of superimposed pop-up structure has significant control on the evolution of fold-and-thrust belt and its petroleum accumulation. With the development of new retrothrust fault, pre-existing thrusts were cut and moved backward, resulting into a superimposed pop-up structure characterized with an unsymmetrical axial plane (Figure 1, toward foreland). The deformation has further control on the fault entrapment, spill point of plays located in fold-and-thrust belt.