Duplex Structures and Imbricate Thrust Systems: Geometry, Structural Position, and Hydrocarbon Potential

Abstract

Duplexes and imbricate thrust systems form some of the most complex hydrocarbon traps in overthrust belts. The geometry of a duplex is controlled by the ramp angle (^Thgr) and height (hr), the final spacing between adjacent thrusts (a^prime), and the relative displacements on them (d1-d2). For constant ^Thgr and hr, three different classes are recognized: (1) independent ramp anticlines and hinterland sloping duplexes, (2) true duplexes, and (3) overlapping ramp anticlines. Imbricate thrust systems consist of several thrust faults, each of which loses displacement upsection and eventually dies out by progressively transferring its displacement to a fold at its tip, or by distributing it among several splays. Hybrid duplexes are ormed from systems of fault propagation folds that are subsequently carried over ramps, so they have a more complex geometry and a greater structural relief. These different types of thrust systems occur as outcrops and prospect-scale structures, and they define the regional structural styles of most fold and thrust belts. Second- and third-order duplexes and imbricate thrust systems occur in specific structural positions, such as footwalls, hanging walls, and frontal zones of major thrust ramps, and in cores of major anticlines. Several types of duplexes and imbricate thrust systems form important hydrocarbon traps. Examples include the system of independent anticlines of the Turner Valley and Highwood oil and gas fields, the hinterland sloping duplex consisting of the Chestnut Ridge-Sandy Ridge system of the Ben Hur oil field, the partly overlapping anticlines of the Waterton and Savanna Creek gas fields, and the completely overlapping anticlines of the Rose Hill oil field.