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Seismic structure of the crust and upper mantle in the Peace River Arch region Zelt, Colin Andrew


The Peace River Arch (PRA) is a regional ~E-W trending geological structure within the Western Canada Sedimentary Basin whose Phanerozoic history of vertical movements is anomalous with respect to the basin as a whole. Four intersecting ~300-km-long reversed refraction lines within the PRA region in northwestern Alberta and northeastern British Columbia have been interpreted for crustal and upper mantle P-wave velocity structure. The data have been analyzed using a new two-dimensional ray-trace forward modelling algorithm to match travel times and amplitudes of first and coherent later arrivals. An inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the in-line profiles. 5-waves and the observed spectra of the refraction data have been analyzed to infer a regional Poisson's ratio and Q structure, respectively. The consistency of the seismic models with the observed Bouguer gravity data was studied. The new algorithm for tracing rays and calculating amplitudes in two-dimensional media is based on a simple, layered, large-block velocity model parameterization in which velocity is an analytic function of position. This allows for computationally efficient ray tracing. The user's ability to specify kinematically-similar ray families permits practical and rapid forward modelling of refraction data. In addition, the routine allows for 5-wave propagation, converted phases, multiple and surface reflections, approximate attenuation, head waves, a simulation of smooth layer boundaries, and a reverse ray-direction amplitude calculation. Amplitude calculations are based on zero- and first-order asymptotic ray theory. The main attributes of the routine are illustrated with several examples. The major features of the interpreted structural model of the PRA region are (1) weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low-velocity zones within the crust, (2) an average sub-basement RMS crustal velocity of 6.6 km/s, average upper mantle velocity of 8.25 km/s and average crustal thickness of 40 km, (3) a high-velocity (> 7.0 km/s) lower crust of 5 to 10 km thickness, (4) westward crustal thinning north of the arch, (5) regional variations in structure that appear related to the N-S Precambrian trends as revealed by aeromagnetic data, including crustal thickness, upper crustal and upper mantle velocities and P[sub m]P character, and (6) subtle variations in structure that may be associated with the E-W trending Devonian axis of the PRA, including a shallowing of high lower-crustal velocities, thickening of the crust, and an anisotropic P[sub m]P character beneath the arch and along-axis low-amplitude P[sub n] arrivals. A high-velocity lower crust and localized shallowing of high lower-crustal velocities are commonly observed in continental rift zones. These features and the ~E-W trend of the arch perpendicular to the ancient western margin suggest that the PRA originated as a Paleozoic failed-rift. The results of supplementary studies show (1) an average crustal Poisson's ratio of 0.25, (2) Q increases with depth from ~500 to ~1000 in the crust and is ~1000 in the upper mantle, and (3) a seismic-gravity relationship that suggests that localized velocity anomalies of the refraction models are not associated with density anomalies. Also, extended-listen-time processing of a 10-km-long industry Vibroseis reflection line coincident with one of the refraction lines shows prominent dipping events that correlate with the zero-offset two-way travel time of a strong intracrustal reflector and the crust-mantle boundary of the refraction model. A series of reflections over 1.5 s terminating at the refraction Moho indicates a complex, possibly layered crust-mantle transition zone of 5 km thickness.

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