A DETAILED TRANSECT ACROSS THE NORUMBEGA FAULT ZONE IN THE LIBERTY AND MORRILL 7.5' QUADRANGLES, SOUTH-CENTRAL MAINE

Detailed mapping of the Norumbega fault zone in the Liberty and Morrill 7.5’ quadrangles of south-central Maine has revealed an approximately 14 km wide dextral shear zone which separates metasandstones of the Central Maine sequence on the northwest from rocks of the coastal lithotectonic belt on the southeast. Mylonites of various types are present across the shear zone, and the nature of these are dependent upon protolith. Mylonite with well defined Casco Bay Group protoliths are mostly ultramylonite or phyllonite. Additionally, two separate migmatized zones are recognized. In these areas granitoid dikes have been disassembled into sigmoidally shaped boudins, which in turn have been locally and progressively sheared into mylonite and striped gneiss. In the migmatized zones, the rocks have been metamorphosed to the sillimanite or sillimanite + K-feldspar zones.

The fault zone consists of several map scale components. From northwest to southeast these are: 1) A 2.5 to 2.7 km wide zone of gneisses assigned to the Falmouth–Brunswick sequence. 2) An approximate 800 m wide zone consisting of several thin slivers of phyllonite interlayered with thin sequences characteristic of both Falmouth–Brunswick and Casco Bay Group rock types. 3) A 700 to 1000 m wide zone of stratified rocks assigned to the Casco Bay Group (dominantly Cape Elizabeth Formation). 4) A 2 to 2.4 km wide zone of migmatized and strongly sheared rocks with locally developed mylonite zones. The protolith for this group of rocks is tentatively and partially considered to have been the Cape Elizabeth Formation. 5) A 4 to 4.5 km wide zone of sheared but moderately stratified rocks assigned to various formations of the Casco Bay Group. In this seqence local ultramylonites range in width from a few centimeters to approximately 40 m. 6) A second migmatized and strongly sheared gneissose zone is approximately 4.5 to 5 km wide. In this zone granitoid boudins of varying size commonly exhibit a sigmoidal shape. Local within this zone are mylonite and striped gneiss. This zone has incorporated within it a microcline–rich orthogneiss. 7) The most southeasterly zone ranges in width from approximately 160 to 240 meters, and is predominantly mylonite and striped gneiss. This zone marks the southeastern boundary of the Norumbega fault zone with the coastal lithotectonic belt.