FIELD EVIDENCE FOR THE DUCTILE FAULT ORIGIN OF MAFIC AND GRANITIC SCHIST IN THE NORTHERN STAMFORD AND POUND RIDGE QUADRANGLES, SOUTHWESTERN CONNECTICUT

New bedrock mapping at a scale of 1:24000 in the northern half of the Stamford and adjacent Pound Ridge quadrangles in southwestern CT has led to re-interpreting the origin of rocks previously mapped as part of the regional tectono-stratigraphy, and re-interpreting the tectonic significance of deformed mafic to felsic orthogneiss. The map area forms part of the southwestern corner of the Hartland Belt of western CT and is bordered to the north and west by parautochthonous to allochthonous Neoproterozoic to Cambrian Laurentian-margin rift- to drift- sequence rocks west of Cameron’s Line. Previously, Frank (unpublished 1971) mapped metasedimentary Harrison and Hartland schist and granofels, Harrison bt-hbl schist and gneiss, and interlayered sheets of various granitic orthogneiss. Rocks in the Stamford and Pound Ridge quads are shown on the state bedrock map (Rodgers, 1985) as meta-sedimentary+volcanic Trap Falls Fm., and in large parts of the area as undifferentiated Trap Falls+granite where schist interlayered with granite occurs at outcrop scale. Several features of the rocks and the revised map pattern indicate that this is an area of pervasive ductile deformation of plutonic rocks and that north of the Merritt Parkway, meta-sedimentary units are absent. Weakly deformed bt+hbl quartz diorite with small, unaligned xenoliths can be traced into strongly foliated bt+hbl schist. Sheets up to 0.5 m across of fine-grained, well-foliated bt-ms-pl-qtz-kfs schist (w/o garnet or an aluminosilicate phase), some with S-C fabrics, occur within deformed bt+ms granite. Roches moutonnées of foliated bt-ms granite are wrapped by similar schist (which occupies most of the low-lying parts of the map area). Many mappable layers of granitic schist are migmatitic, characterized by anastomosing layers and lenses of bt-rich schist and granite, with the layers and lenses ranging in size from 5-15 mm to 10s of cm thick. Rather than meta-sedimentary units intruded by mafic to felsic plutons, we interpret the rocks in northern Stamford and Pound Ridge to be part of a large-scale ductile fault zone where high strain zones are marked by “tectonic” diorite schist and granitic schist. All of these tectonized rocks are deformed by polyphase folding. Exposed in our map area is a deeply deformed part of the accretionary margin of Laurentia.