Crustal structure of the southeastern Grenville Province, northern New York State and eastern Ontario

Abstract

The Grenville province exposes an oblique cross section through mid‐lower crustal lithologies that were pervasively deformed and subjected to regional thermal overprinting during the Grenvillian orogeny (1.1 Ga). The southeastern Grenville province is divided into two subterranes by the Carthage‐Colton mylonite zone, a 110‐km‐long lineament characterized by intense ductile shear and igneous intrusion, which separates the amphibolite facies metasediments of the Central Metasedimentary Belt to the west from the granulite facies metaplutonics of the Central Granulite Terrane to the east. The recognition of distinct lithotectonic domains separated by zones of intense ductile shear in the Grenville province raises questions concerning the deep structure of these subterranes and, in particular, the means by which the mid‐lower crustal rocks exposed in the Grenville province were emplaced. Seismic refraction/wide‐angle reflection data were acquired to investigate the deep structural interrelationships within the southeastern Grenville province. A travel time inversion for velocity and interface depth was applied to the seismic data, together with constraints from amplitude modeling to produce a seismic velocity model of the crust in the southeastern Grenville province. In the Central Metasedimentary Belt the upper crust is characterized by velocities in the range 6.3–6.4 km/s and a Poisson's ratio of 0.26 ± 0.01 which are attributed to quartzofeldspathic rocks. Farther east in the Central Granulite Terrane, upper crustal velocities of 6.55 km/s and a Poisson's ratio of 0.28 ± 0.01 are associated with the Marcy Anorthosite. The seismic homogeneity of the upper crust in the region of the Carthage‐Colton mylonite zone suggests that this boundary is a shallow feature, limited to the upper 2–3 km of the crust. The deep crustal structure of the southeastern Grenville province is characterized by two discrete and laterally discontinuous seismic interfaces. In the Central Metasedimentary Belt the top of the lower crust is delineated by an eastward dipping interface at 24–28 km depth. In the Central Granulite Terrane, prominent en echelon reflections, referred to as the Tahawus complex, form a gently arched dome at 17–22 km depth. Interpretation of the Tahawus complex as a zone of layered mafic cumulates is supported by its high velocity (7.1 km/s) and Poisson's ratio (0.27 ± 0.02). The lower crust is characterized by a velocity of 7.0–7.2 km/s and an anomalously high Poisson's ratio of 0.30 ± 0.02, which are representative of pyroxene‐garnet granulites. In contrast, velocities of 6.8–7.0 km/s are modeled beneath the Central Granulite Terrane and appear to signify a lateral change in composition. The Moho lies at 44–45 km depth and is characterized by pronounced en echelon reflection segments, suggesting compositional interlayering around the crust‐mantle boundary. The velocity of the upper mantle is 8.0–8.2 km/s. An anomalous upper mantle layer with a reversed velocity of 8.6 km/s dips eastward from 50 to 60 km depth beneath the southeastern Grenville province. Our results indicate that remnants of magmatic intrusions that mobilized and thickened the crust during the Grenvillian orogeny are preserved in the mid‐lower crust as a layered cumulate body (Tahawus complex) and in the upper mantle as an eclogitic lens, possibly delaminated from the overthickened crust during uplift of the southeastern Grenville province.