Charles T. O'Reilly

The presence of gravitational lows common to the southern portion of Nova Scotia is attributed to a large granitic mass of Devonian age intruded into a sequence of Lower Paleozoic quartzites, slates, schists and greywackes. This study is an attempt to interpret and establish a possible model of this mass. It was undertaken in the light of data available from a recent petrographic study and the availability of modern 2-D and 3-D computer programs to assist in gravity modelling computations.

Previous attempts at interpretation (Garland, 1953) have assumed a more or less homogeneous intrusion having a constant density contrast with the country rock. Garland's model of the batholith composed of a cylindrical "pipe" some 29 km deep and 59 km in diameter with a sheet or "slab" of 5 km thickness extending seaward to undetermined limits.

The petrologic study by McKenzie shows the granite as consisting of four main adamellite (quartz monzonite) pods, set in a background of biotite granodiorite. Subsequent information on densities, density contrasts and distribution was obtained by direct measurements; and through the use of 2-D and 3-D gravity programs, a new model was established which agreed with observed gravity patterns. Although very simplistic, initial 3-D models reveal the possibility of a more massive somewhat elliptical sheet of granodiorite with depths of 15 to 20 km extending over large areas. The adamellite bodies were modelled as being contained within this sheet to equivalent depths of the granodiorite (a maximum of 20 km). This model maintains that variations in petrology, and not dimensions, accounts for the observed gravity patterns.

Misfits of model contours to those observed may indicate the possible presence of extensive "roots" or interconnecting channels at depth between the adamellites bodies. Such would likely exist if one regards the adamellites as representing a secondary intrusive event(s).

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Pages: 84
Supervisors: M. J. Keen