Physical Modeling of Fold-and-Thrust Belt Evolution and Triangle Zone Development: Dabashan Foreland Belt (Northeast Sichuan basin, China) as an Example


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Triangle zones, generally found in foreland fold-and-thrust belts, serve as favorable objects of petroleum exploration. Taking the Dabashan foreland belt as an example, we studied the formation and development of triangle zones, and investigated the effect of décollements and the mechanical contrast of lithology by employing the method of physical modeling. Four experimental models were conducted in the work. The results showed that ‘sand wedges’ grew episodically, recorded by deformational length, height and slope angle. The height versus shortening rate presented an S-shape curve, and uplifting occurred successively in the direction of the foreland belt. During the formation of the triangle zone, layer-parallel shortening took place at the outset; deformation decoupling then occurred between the upper and lower brittle layers, divided by a middle-embedded silicone polymers layer. The upper brittle layers deformed mainly by folding, while the lower sand layers by thrusting. As shortening continued, the geometry of a triangle zone was altered. We consider that the triangle zone in the Dabashan foreland belt was modified from an early one based on available seismic profiles and the experimental results. In addition, décollements and mechanical contrast impose significant influence on structural development, which can directly give rise to structural discrepancies. More décollements and obvious mechanical contrast between brittle layers can promote the coupling between the upper and lower brittle layers. Basal décollement controls the whole deformation and decreases the slope angle of the wedge, while roof décollement determines whether a triangle zone can be formed.