Traps directly related to thrusting include those within a single thrust sheet, trai1ing, 1eading edge and intraplate folds, and multiple structures within duplex fault zones, imbricate fans, and triangle zones. Lateral hanging wall and footwall ramps produce the strike closure for these structures.
Thrust and fo1d belt traps must be discussed within broader context which incorporates trap styles within foreland areas. Petroleum generation, migration, and entrapment in many foreland structures can be linked indirectly to the thrusting process. Tectonic loading by the thrust belt initiates foreland structures, which continue to grow during additional crustal shortening.
Recent exploratory drilling and seismic profiling of Cenozoic to recent convergent plate margins have demonstrated that many of the principles of ancient fold-thrust belt geometry are app1i cab1e to these younger fold-thrust belts and accretionary prisms. Similarities include the presence of shingled imbricate thrusts, duplexes overlain by dominant thrust sheets, strike-parallel extensional features, and kink-style folds.
The past thirty years of oil and gas exploration in the Canadian and U.S. Rockies has revealed a wealth of information bearing on the generation, migration, and entrapment of hydrocarbons within thrust belts. As the vo1ume of seismic and well data has grown, a 1arge range of trap styles have been identified. These range from simple anticlinal structures within a single sheet to complexes of multiple sheets. Papers by Bally, Gordy and Stewart (1966), Dahlstrom (1970), Royse, Warner and Reese (1975), and Lamerson (1982) discuss the various structural types of hydrocarbon-bearing structures that occur within Rocky Mountain thrust belts. Selected papers compi1ed by Perry, et a1. (1984) serve as a good review of what is known about North American thrust-faulted terranes.
Al though these works discuss to some degree the lateral variations that occur within thrust belts, little mention has been made of the specific effect that these features have on trap formation. On most structures, simple four-way closure would not exist but for the presence of 1ateral ramps. Recent Utah-Wyoming thrust belt discoveries have demonstrated that lateral ramps and strike-extension can also produce paired longitudinal fields separated by high angle transverse faults.
Explorationists are becoming increasingly aware that tectonic provinces cannot be treated in isolation. By expanding our viewpoint to the regional and world scales, we find that many concepts applicable to one structural province also apply to adjacent provinces. For example, tectonic forces responsible for thrusting, when combined with the sinusoidal crustal loading of stacked sheets, can produce basement-involved foreland structures. The principles of geometry and trap formation in ancient thrust belts can also be applied to Cenozoic thrust belts and accretionary complexes at convergent plat margins.
The simplest of thrust belt structural traps occur within a single thrust sheet and are of three types: trailing edge, intraplate, and leading edge anticlines (Figure 1).