EN ECHELON GEOMETRY AND 2-DIMENSIONAL MODEL OF THE TRIANGLE ZONE, GREASE CREEK SYNCLINE AREA, ALBERTA

The triangle zone structure examined in this study is a deformed wedge inserted along a delaminated horizon in the basal Upper Brazeau Forma tion (Upper Cretaceous elastic unit). The surface expression of the tr iangle zone is an antiform, with an east-dipping thrust cutting throug h the core of the antiform. Surface antiforms delineate three en echel on structures in the Grease Creek area, with a strike length of approx imately 20 km. A comparison of three balanced cross-sections suggests the en echelon surface geometry reflects the changing position of the lower bounding detachment of the intercutaneous wedge. From south to n orth the structures consistently step eastward. A ''relict'' triangle zone is interpreted beneath the west flank of the Grease Creek Synclin e based on interpreted seismic data. The surface culmination of the Wi ldcat Hills triangle zone, located at the deformation front south of t he study area, can be traced to the position of this relict triangle z one, demonstrating that there are two separate en echelon intercutaneo us wedges. An important implication of the en echelon geometry is that it produces closure of the structural traps along strike because indi vidual structures terminate along strike. A composite two-dimensional model of the triangle zone is presented from the published models and the Grease Creek study. Important features of this model include a ram p and flat geometry of the upper detachment and upper and lower detach ments of the intercutaneous wedge that do not merge. Instead, the toe of the wedge is a zone between the upper and lower detachments rather than a distinct tip line. The internal geometry of the triangle zone s tructure is hinterland-dipping in the study area.