POTENTIAL FIELD AND PETROPHYSICAL INVESTIGATIONS APPLIED TO TECTONIC ANALYSIS IN THE NORTHERN APPALACHIAN OROGEN

Several distinct lithostratigraphic subdivisions or terranes have been recognized in the northern Appalachian orogen. They occur for the most part in discrete belts that can be traced from Newfoundland through Cape Breton Island and New Brunswick and into New England. Based on interpreted "basement" and/or mid- to shallow-crustal character these areas generally have been grouped into three broad divisions: Humber, "Central", and Avalon. Qualitative interpretations of regional gravity and magnetic data appear to support this interpretation; however, systematic differences in potential field character also support geological evidence for lithostratigraphic subdivisions. For example, Avalon terrane sensu stricto differs in geophysical character from the adjacent Bras d'Or, Gander, and Exploits terranes.

In order to develop a quantitative interpretation of magnetic and gravity data at the terrane scale, physical properties were measured on more than 4000 samples representing map units in southern Cape Breton Island and southern New Brunswick. These data were used to constrain potential field map pattern interpretations and to construct 2.5-D potential field models across terrane boundaries. The results of these investigations show systematic differences in physical property character between Avalon terrane sensu stricto and the adjacent Bras d'Or-Brookville terrane. Furthermore, constraints on modeling results from geological mapping and the physical property database allowed the subdivision of the upper crust (<10 km depth but below exposed rock units)into separate blocks based on distinctive physical property combinations. The upper crustal blocks and their physical properties are consistent between southern Cape Breton Island and southern New Brunswick, and are also compatible with published data for eastern Newfoundland. The data imply that the terranes identified at surface extend to depths of at least 10 km and are bounded mainly by steep faults.