Three-dimensional structure across the Tintina strike-slip fault, northern Canadian Cordillera, from seismic refraction and reflection tomography

Accepted 2006 June 5. Received 2006 June 3; in original form 2005 September 4 SUMMARY The development of the northern Canadian Cordillera involved major strike-slip displacement of accreted terranes relative to North America along faults such as the Tintina, which has experienced ∼425 km of dextral motion since the Palaeocene. The SNORE seismic refraction/wideangle reflection experiment was carried out in 1997 as one component of Lithoprobe’s SlaveNorthern Cordillera Lithospheric Evolution (SNORCLE) transect. In addition to four 2-D profiles, two sets of broadside recordings were acquired to image the 3-D structure across the Tintina fault (TF) in areas centred at about 59.5 ◦ N and 62 ◦ N. Simultaneous and independent refraction and reflection traveltime tomography were applied to the combined inline and broadside data set for each region to establish the range of lower crustal velocity, Moho depth and upper mantle velocity structure consistent with the data. Our preferred models are the average of the simultaneous and independent models since they represent the robust features required by the data. The preferred 3-D models are generally consistent with the 2-D models obtained from the inline data in previous independent studies. There are along-strike variations across the TF, perhaps due to the change in strike direction or the amount of motion along the fault in the north compared to the south. In the lower crust, the only correlation with the TF that is required by the data is a 0.1 km s −1 drop in velocity to the southwest of the fault in the northern study area. The absence of a strong correlation with the TF in the lower crust is consistent with the interpreted continuity of lower crustal units across the fault in the SNORCLE reflection data. The Moho is relatively flat throughout the study area, 34‐35 km depth, but there is broad crustal thickening of a few kilometres centred ∼50 km southwest of the TF in the northern and southern study areas. This thickening may be the result of a period when there was a component of compression along the TF. There is strong evidence for a 0.3‐0.4 km s −1 drop in upper mantle velocity to the west of the TF in the south, and weak evidence for ∼0.1 km s −1 drop to the southwest in the north. The upper mantle variations in the south indicate that the TF is the boundary between cooler and/or more refractory ancestral North American mantle to the east and warmer and/or more fertile mantle beneath accreted North America to the west. In the north, the mantle appears to be more intermediate in its properties on both sides of the TF.