The map below displays the lithology and map-scale structural features underlying the Milford quadrangle in Massachusetts. The bedrock of Milford quadrangle lies completely within the Southeastern New England Avalon terrane. (Figure 1), which is an exotic terrane that was accreted to the southeastern margin of Laurentia during the Paleozoic Acadian and/or Alleghenian orogeny. The Avalon terrane in the Milford quadrangle is comprised mostly of Neo-Proterozoic batholithic granites (Milford granite) and granitic gneisses presumably equivalent to the Ponaganset, Hope Valley, and Scituate granite gneisses of Connecticut (Zen et al., 1983; Hatch, et al., 1991). These rocks intrude the meta-sedimentary / metavolcanic quartzite-amphibolite assemblage of the Blackstone group (after Zen et al., 1983), which may represent sedimentation on the margins of a mature Proterozoic volcanic arc (Hatch et al., 1991, pg. E13). Several late mafic dikes, presumably Triassic in age, cross-cut all other rocks in the quadrangle. Structural Geology and Metamorphism The rocks of the Milford quadrangle are folded into broad north-to-northeast plunging antiforms and synforms. Shaw (1966) divided these into first and second order folds, with the second order folds being parasitic folds on the flanks of the larger first order folds. These second order folds locally refold the first order folds. Shaw (1966) attributed this to local strain variation arising from progressive regional deformation and competency contrasts between rock units. In the granitic rocks, these folds are defined by a foliation (Sheet 2 - Figure 3) which weakens from west to east across the quadrangle. The axes of these folds are generally parallel to a pervasive northeast trending lineation in all rocks (Photo 1; Sheet 2- Figure 4). Shallow to moderately north-dipping shear zones are developed parallel to this foliation with shear-related lineations parallel to the regional lineation. Contacts between units are also commonly sheared. Later, north- to northwest-trending, steeply dipping shear zones and brittle faults are also present, that presumably range from Permian to Mesozoic in age. Although the age of the deformation Milford quadrangle is uncertain, studies in the Blackstone Group to the south suggest that the bulk of ductile deformation and folding observed in the Milford quadrangle can be attributed to the Permian Alleghenian orogeny (Dreier, 1985). A component of the deformation in the Blackstone group may be related to the intrusion of the batholithic granites during the Neo-Proterozoic (Dreier, 1985; Hatch et al., 1991, pg. H23). Rocks in the Milford quadrangle have been metamorphosed to the middle-amphibolite facies. The age of metamorphism is uncertain, but most likely contains components of Neo-Proterozoic metamorphism that have been overprinted by Permian-age metamorphism associated with the Alleghenian orogeny (Shaw, 1966; Hatch et al., 1991).Office of the Massachusetts State Geologist.
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INTRODUCTION AND REGIONAL GEOLOGY Geology by Joseph P. Kopera and Charles E. Shaw, Jr. |
