THE NATURE OF THE BOUNDARY BETWEEN THE ST. CROIX AND MERRIMACK-HARPSWELL TERRANES, EASTERN MAINE

The contact between rocks of the Fredericton Trough (Merrimack-Harpswell terrane) and of the St. Croix terrane is poorly exposed in eastern Maine. Well preserved sedimentary structures (graded bedding, cross lamination) in outcrops in the vicinity of this contact indicates that Silurian turbidites (Digdeguash Formation) and calcareous flysch (Flume Ridge Formation) of the Fredericton Trough young towards early Ordovician metapelites and metasandstones (Calais and Woodland Formations) of the St. Croix terrane. Rocks of the St. Croix terrane are deformed and are intruded by quartz veins near the contact. These observations together with regional studies confirm that the contact is tectonic, probably a high-angle reverse fault, named here the Old Stream fault. This fault is presumably the continuation of the Sennebec Pond fault that outcrops to the west in the vicinity of Penobscot Bay. Along the Sennebec Pond Fault, rocks of the St. Croix terrane have been thrust over rocks of the Fredericton trough. Repetition of Ordovician strata of the St. Croix terrane across strike and parallel to the terrane boundary indicate that the Old Stream fault may actually be a complex zone with more than one fault splay.

Quartz veins in rocks near the Old Stream fault are boudinaged, and there is other evidence for strong shearing including development of kinematic indicators that record dextral motion along the fault. This shearing predates intrusion of the large Devonian (384 Ma) Deblois granite pluton, because there is no evidence for cataclasis or shearing where this pluton cross cuts the boundary between the Fredericton tough and the St. Croix terrane. In contrast, movement along the dextral, strike-slip Norumbega fault has sheared parts of the Deblois granite. Shearing along the Old Stream fault therefore does not reflect reactivation associated with movement along the Norumbega fault.

Published data indicate that the Merrimack-Harspwell and St. Croix terranes were juxtaposed in the late Ordovician or early Silurian. The data described here suggest that amalgamation of these terranes involved both thrusting and strike slip motion. The Sennebec Pond-Old Stream fault system thus provides evidence for transpression during terrane accretion in the Salinic orogeny.