A sub-critical seismic crustal reflection survey over the Aulneau batholith, Kenora region, Ontario

Abstract

A sub-critical seismic reflection survey has been conducted across the Aulneau granite batholith in northwest Ontario. Shot to receiver distances ranged from 2.23 to 71.9 km and the total amount of explosives used for each shot-receiver configuration ranged from 54 to 327 kg. Multiple shots with vertical stacking, frequency filtering and time varying velocity filtering were used to increase the signal to noise ratio for each trace. Reflections from shallow vertical faults and deep horizontal crustal discontinuities were observed on the processed records. From these data a model of the crustal structure in the region of the Aulneau batholith has been derived. The model has a number of near vertical faults and an essentially three layered crust in which the upper layer is 16 km thick with P-wave and S-wave velocities of 6.1 and 3.6 kmls, respectively, in the top part of the layer. The average P-wave velocity throughout the layer is 6.2 kmls. This layer probably represents the relatively homogeneous part of the batholith. The middle layer is 5 km thick beneath the shot point and thins to the southeast. It has a P-wave velocity of 6.9 kmls. The lower layer extends to the Mohorovicic discontinuity at 38 km depth and has an interval velocity of 7.2 kmls. The proposed crustal model is generally supported by other geophysical data collected in northwest Ontario.On a procede 2 un leve de sismique-reflexion sous le niveau critique a travers le batholite granitique d3Aulneau dans le nord-ouest de I'Ontario. On afait varier les distances entre les points d'impact et les recepteurs de 2.23 a 71.9 km et la quantite totale d'explosifs utilisee pour chaque configuration source-recepteur allait de 54 a 327 kg. On autilise des sources multiples configuration verticale, un filtrage de frequence et un filtrage des vitesses en variant le temps pour augmenter le rapport signallbruit de chaque trace. On a observe sur les donnees traitees des reflexions de failles verticales peu profondes et des discontinuites horizontales dans la croiite en profondeur. A partir de ces donnees, on a Clabore un modele pour la structure de la croDte dans la region du batholite d'Aulneau. Le modele possede un certain nombre de failles presque verticales et une croDte composee de trois couches dans laquelle la couche superieure a une epaisseur de 16 km avec des vitesses d'ondes P de 6.1 kmls et des vitesses d'ondes S de 3.6 kmls au somrnet de la couche. La vitesse moyenne des ondes P dans cette couche est de 6.2 kmls. La couche superieure represente probablement la portion relativement homogene du batholite. La couche mediane a une epaisseur de 5 km au point d'impact et s'amincit vers le sud-est. La vitesse des ondes P y est de 6.9 kmls. La couche inferieure vajusqu'a la discontinuite de MohoroviCic a une profondeur de 38 km et a une vitesse de 7.2 kmls. Ce modele de croiite est compatible avec d'autres donnees geophysiques recueillies dans le nord-ouest de I'Ontario. Can. J. Earth Sci., 15,301-315 (1978) [Traduit par le journal]

Crustal refraction surveys across the Trans‐Hudson Orogen/Williston Basin of south central Canada

“…In the region where the Williston Basin is deepest beneath our composite E-W profile (Figure 11) there appears to be a 3-to 4-km decrease in crustal thickness and a related west to east thinning of the lower crustal layer. Low-resolution seismic refraction surveys conducted during the 1960s [e.g., Hajnal, 1969, 1973;Mereu and Hunter, 1969;Gurbuz, 1970] and limited seismic reflection data collected during the 1970s [Green et al, 1978[Green et al, , 1979a have revealed a relatively thin (30-38 km) crust beneath the exposed Superior craton. Differences of 3-5 km between the older crustal thickness estimates and those reported here could result from the lower resolution of the older data and from the different modeling techniques used.…”