Topography of the crust–mantle interface under the Western Superior craton from gravity data

Abstract

The Moho undulations beneath the western part of the Archean Superior Province have been investigated with a three-dimensional gravity inversion algorithm for a single interface of constant density contrast. Inversion of the complete gravity data set produces unreal effects in the solution due to the ambiguity in the possible sources of some crustal gravity anomalies. To avoid these effects a censored gravity data set was used instead. The inversion results are consistent with reflection and refraction seismic data from the region and, therefore, provide a basis for the lateral correlation of the Moho topography between parallel seismic lines. The results indicate the existence of a major linear east-west-trending rise of the Moho below the metasedimentary English River subprovince, which is paralleled by crustal roots below the granite-greenstone Uchi and Wabigoon subprovinces. This correlation between the subprovincial structure at the surface and deep Moho undulations suggests that the topography of the crust-mantle boundary is related to the tectonic evolution of the Western Superior belts. Although certain features of the crust-mantle boundary are likely inherited from the accretionary and collisional stages of the Western Superior craton, gravity-driven processes triggered by subsequent magmatism and crustal softening may have played a role in both the preservation of those features, as well as in the development of new ones.Résumé : Les ondulations de la discontinuité Moho sous la partie ouest de la Province du Supérieur (Archéen) ont été étudiées au moyen d'un algorithme d'inversion gravimétrique 3-D pour une interface simple à contraste de densité constant. L'inversion de l'ensemble complet de données gravimétriques produit des effets irréels dans la solution étant donné l'ambiguïté des sources possibles de quelques anomalies gravimétriques de la croûte. Afin d'éviter ces effets, on a plutôt utilisé un ensemble de données gravimétriques épurées. Les résultats de l'inversion concordent avec les données de réflexion et de réfraction sismiques de la région et fournissent donc une base pour la corrélation latérale de la topographie de la discontinuité Moho entre les lignes sismiques parallèles. Les résultats indiquent l'existence d'une montée linéaire majeure à tendance est-ouest du Moho sous la sous-province métasédimentaire de English River; les racines de la croûte sous les sous-provinces Uchi et de Wabigoon, composées de granites et de roches vertes, sont parallèles à cette montée. Cette corrélation entre la structure de la sous-province à la surface et les ondulations du Moho en profondeur suggère un lien entre la topographie de la limite croûte-manteau et l'évolution tectonique des ceintures de la Province du Supérieur Ouest. Bien que certaines caractéristiques de la limite croûte-manteau aient probablement été héritées des phases d'accrétion et de collision du craton Supérieur Ouest, des processus gravitationnels déclenchés par du magmatisme subséquent et un ramollissement de la croûte peuvent avoir j...

FTIR spectroscopy of OH in olivine: A new tool in kimberlite exploration

“…8). The estimated pressure of crystallization based on water contents of A154-S olivines roughly coincides with the depth of the Moho, located at 35-45 km depth beneath the North American craton (Nitescu et al, 2003;Snyder et al, 2004;Németh et al, 2005). We speculate that kimberlitic magmas may sometimes become stagnant at depth of the Moho due to abrupt changes in rheology and density at the crust-mantle boundary.…”

Crustal structure and implications for the tectonic evolution of the Archean Western Superior craton from forward and inverse gravity modeling

“…From the calculation of the gravity effect of this low-velocity zone for a negative density contrast of 50-60 kg/m 3 we obtained that at the point where the Ochig Lake gravity anomaly is largest, its amplitude reaches 13-16 mGal, which implies a level of the regional field in the range À60 to À63 mGal at this point (Figure 4). This conclusion is confirmed by the study of Nitescu et al [2003], which, on the basis of the inversion of the long-wavelength components of the gravity field related to deep sources, suggests that the gravity low associated with the Ochig Lake pluton has an amplitude of $13 mGal [see Nitescu et al, 2003, Figure 10].…”

“…The presence of a crustal suture under the Uchi Subprovince was also suggested by Calvert et al [2004] on the basis of reflection seismic data acquired 200 km to the west, along line 2b (Figure 3c), where, similar to line 1, a crustal root characterized by north dipping reflectivity is observed. In addition to accretion‐ or shortening‐related reflection fabrics, the line 2b seismic section reveals several features (e.g., the upwarp of the crust‐mantle boundary underneath the English River Subprovince; listric south dipping reflectors that extend into the lower crust from close to the surface contact between the Uchi and Berens River belts) that were previously interpreted as extensional structures [ Nitescu et al , 2003; Calvert et al , 2004].…”

“…In order to identify the major gravity anomaly sources at depth and to assess the relative contributions of shallow and deep sources to the WS gravity field (Figure 2), two‐and‐a‐half‐dimensional forward gravity modeling was carried out for the gravity data collected along Lithoprobe seismic lines 1 and 2b (Figure 1), which cross perpendicularly or at high angle the regional belt pattern of the WS, as well as the major topographic features of the crust‐mantle boundary [see Nitescu et al , 2003, Figure 8]. The actual gravity profiles were obtained by projecting the positions of the gravity stations on the N‐S direction.…”

“…The topographic features of the Moho, as well as the deep intracrustal mass anomalies (i.e., depressions of the intracrustal boundaries and the region of anisotropic lower crust), were modeled assuming strike extents of 100 km on each side of the profiles. Although some of these features may have larger lateral extents [ Nitescu et al , 2003], the value we considered for their strike extent leads to realistic modeled gravity effects, since no significant difference in the calculated effects is obtained for larger values of strike extent.…”