Crustal structure across the northern Cordillera, Yukon Territory, from seismic wide-angle studies: Omineca Belt to Intermontane Belt

Abstract

A seismic refraction – wide-angle reflection experiment shot in 1997 in the southern Yukon Territory crosses the Omineca Belt, which includes the strike-slip Tintina Fault, and terminates within the Intermontane Belt of the northern Canadian Cordillera. Lithospheric structure is interpreted from two-dimensional forward and inverse modelling of traveltimes, combined with forward-amplitude modelling, and from 2.5-dimensional modelling of gravity data. Beneath the Cassiar terrane and the North America miogeocline, average velocities in the upper 20 km of crust are < 6.1 km/s. In the west beneath the accreted Cache Creek, Slide Mountain, and Yukon–Tanana terranes, average velocities increase to ∼6.3 km/s. In the upper crust, the velocity model beneath these terranes thus correlates with more mafic accreted material and not with a subsurface extension of the Cassiar terrane. The Tintina Fault is a crustal-scale structure across which significant structural differences occur. A mid-crustal reflector terminates to the east of the Tintina Fault. The crust immediately west of the fault is thicker (∼37 km) than the crust to the east (∼34 km); the thick crust may suggest movement along the fault from a region of thicker crust to the south. Lower crustal velocities range from 6.4 to 6.7 km/s, with the lowest velocities located 25–50 km west of the Tintina Fault, coincident with the location of the thickest crust. A reflector at 28 km depth may correspond to the top of Proterozoic cratonic basement in the lowermost crust. Upper mantle velocities just below the Moho range from 7.8 to 7.9 km/s, consistent with the high heat flow in the region.