Teleseismic studies of the lithosphere below the Abitibi-Grenville Lithoprobe transect

Abstract

Abstract:In the past decade, the Abitibi-Grenville Lithoprobe transect has been the site of numerous geological and geophysical surveys oriented towards understanding the lithospheric evolution of the southeastern Superior and adjoining Grenville provinces. Among the different geophysical methods that have been employed, earthquake seismology provides the widest range of information on the deep structures of the upper mantle. This paper presents a review of studies, both complete and ongoing, involving teleseismic datasets that were collected in 1994 and 1996 along the transect. A complete shear-wave splitting analysis has been performed on the 1994 dataset as part of a comparative study on electrical and seismic anisotropies. Results suggest a correlation between the two anisotropies (supported by xenolith data) and favour a lithospheric origin for the seismic anisotropy. The two anisotropies are believed to represent the fossilized remnants of Archean strain fields in the lithospheric roots of the Canadian Shield. Preliminary splitting results for the 1996 experiment suggest that the S-wave azimuthal anisotropy may be depth dependent and laterally varying. Ongoing receiver function analysis and traveltime inversion studies provide velocity models of the crust and upper mantle beneath the study area. Preliminary receiver function results reveal the presence of an S-velocity increase at -90-100 km depth which appears to be laterally continuous over 200 km. Traveltime inversion models indicate the presence of an elongate, low-velocity anomaly beneath the southern portion of the 1996 array which strikes obliquely to major geological structures at the surface (e.g., Grenville Front). Preliminary interpretation relates this anomaly to the same process (e.g., fixed mantle plume, continental rifting) responsible for the emplacement of the Monteregian Hills igneous province.Résumé : Au cours de la dernière décennie, de nombreuses campagnes géologiques et géophysiques ont été effectuées le long de la traverse Abitibi-Grenville du projet Lithoprobe, dans le but de comprendre l'évolution lithosphérique de la partie sud-est de la Province du Supérieur et de la Province de Grenville. Parmi les différentes méthodes géophysiques employées, la sismologie passive est sans doute celle qui procure le plus d'informations sur les structures profondes du manteau supérieur. Cet article passe en revue les études, complétées et en cours, s'appuyant sur les données télésismi-ques enregistrées en 1994 et en 1996, le long de la traverse. Une analyse complète de biréfringence des ondes S a été effectuée sur les données de 1994, dans le cadre d'une étude comparative entre l'anisotropie électrique et sismique. Les résultats obtenus suggèrent une corrélation entre les deux anisotropies (observation supportée par l'analyse de xénoli-thes), et favorisent une anisotropie sismique d'origine lithosphérique. Les deux anisotropies semblent associées à des champs de déformation archéens fossilisés dans les racines lithosphériques du Bouclier canadien...

The Lithoprobe Abitibi-Grenville transect: two billion years of crust formation and recycling in the Precambrian Shield of Canada

“…Thus, if the offset of the Moho at the Abitibi-Grenville line 53 -line 54 boundary is due to Ottawan normal faulting, lithospheric delamination cannot have characterized the Grenvillian orogen this far west. This conclusion is also supported by the teleseismic evidence for a mantle lithospheric root in the western Grenville Province (Rondenay et al 2000).…”

“…As evident from teleseismic studies (Rondenay et al 2000), electromagnetic studies ) and shear-wave splitting studies on xenoliths (Ji et al 1997), this root continues below the Archean parautochthon in the Grenville Province and would have provided a stable, cold, and relatively light basement to the Grenvillian orogen, which may have been able to support the overthickened crust that is recorded in these reworked Archean rocks (Indares and Martignole 1990). …”

“…These were complemented by a seismic-refraction experiment to constrain the velocity structure of the crust and upper mantle (Winardhi and Mereu 1997;Mereu 2000), and a seismic tomography study extending from the central Abitibi region across the Grenville Province (Rondenay et al 2000). Locations of the seismic-reflection lines and the refraction experiments are indicated on Fig.…”

Grenville Province and Monteregian carbonatite and nepheline syenite distribution related to rifting, collision, and plume passage

“…130 Ma and ca. 120 Ma at the base of the lithosphere (at a depth of ~200 km; Rondenay et al, 2000;White et al, 2000). The most southeastern Monteregian Hills volcano is shown as actively erupting.…”

Lithospheric variations across the Superior Province, Ontario, Canada: Evidence from tomography and shear wave splitting

“…Such features would require a thermal or chemical contrast with surrounding material [ Sobolev et al , 1996], through the incorporation of more enriched or higher temperature mantle, or localized thinning of the continental lithosphere. Low‐velocity features beneath cratons have in several cases been attributed to hot spot activity heating, eroding, or modifying the lithosphere [see e.g., VanDecar et al , 1995; Rondenay et al , 2000a, 2000b]. …”