SELKIRK FAN STRUCTURE, SOUTHEASTERN CANADIAN CORDILLERA - TECTONIC WEDGING AGAINST AN INHERITED BASEMENT RAMP

A revised cross section through the Selkirk fan structure provides the basis for a new model for the Middle Jurassic tectonic evolution of t he southern Omineca belt, Canadian Cordillera, Palinspastic restoratio n of this cross section shows that the southwest-verging structures al ong the west flank of the Selkirk fan structure formed as a result of tectonic wedging of distal North American strata (Clachnacudainn compl ex) beneath more proximal North American strata, and that the Selkirk fan structure developed outboard from a crustal ramp (Dogtooth high) i nherited from Late Proterozoic-early Paleozoic rifting along the weste rn margin of North America. The first episode of Mesozoic deformation in southeastern British Columbia occurred between 187 and 173 Ma and i nvolved the northeastward juxtaposition of the Intermontane superterra ne over the outer part of the North American continental terrace wedge . It resulted in deep burial (20-25 km) of the outer margin of North A merica. A crustal ramp, localized along the western edge of the Late P roterozoic-early Paleozoic Dogtooth high, impeded the northeastward pr opagation of the orogenic wedge comprising the Intermontane superterra ne and the imbricate, underlying northeast-verging thrust sheets of No rth American supracrustal rocks. Tectonic wedging, involving southwest -verging deformation, occurred within the orogenic wedge, and the resu lting crustal thickening established sufficient topography and gravita tional potential to drive the propagation of the deformation eastward into the Dogtooth Range and the Rocky Mountains. The southwest-verging structures along the west flank of the Selkirk fan developed between approximately 173 and 168 Ma concurrent with synorogenic extension and similar to 10 lan of exhumation. The initial subsidence of the forela nd basin during Kimmeridgian time (ca, 154 Ma) provides the first indi cation of tectonic loading and lithospheric flexure of the NorthAmeric an plate. It is interpreted to mark the time at which the orogenic wed ge overrode the crustal ramp of the Dogtooth high and advanced onto re latively thick and rigid continental lithosphere. The tectonic model p roposed for the Selkirk fan structure illustrates how the configuratio n of the rifted margin influenced the style of crustal thickening duri ng subsequent compressional deformation.