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.

    Thanks to my son, DJ, for the maps.

                                       
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INTRODUCTION AND REGIONAL GEOLOGY

Geology by Joseph P. Kopera and Charles E. Shaw, Jr.


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