ABSTRACT The Berkshire massif in western Massachusetts is one of several external basement massifs in the New England Appalachians. The Day Mountain thrust is a segment of the western frontal thrust of the Berkshire massif that carried Mesoproterozoic basement gneisses and unconformably overlying cover rocks of the Neoproterozoic (?) Dalton Formation and Cambrian Cheshire Quartzite over the Cambrian to Ordovician Stockbridge Formation. The basal unit of the Dalton Formation is a distinctive deformed quartz-pebble conglomerate. We made 27 strain estimates at 18 locations using the deformed conglomerate to investigate the strain field in the Day Mountain thrust sheet and test the plane-strain model of thrust emplacement. Although the strain ellipsoids vary from prolate to oblate shapes over distances as small as 200 m, and the orientations of the principal directions of strain range widely, a remarkably simple strain pattern, broadly consistent with simple shear, emerges when the strain data are plotted on contoured stereograms. The preferred orientation of the maximum elongation direction plunges gently and approximately coincides with the west-northwest transport direction of the thrust sheet, the preferred orientation of the intermediate principal strain axis is nearly horizontal and perpendicular to the transport direction, and the preferred orientation of the short axis plunges steeply. Most of the strain ellipsoids fall in the prolate field, which is indicative of constrictive flow, especially in the northern part of the thrust sheet. We suggest that the steep gradients in three-dimensional strain type were caused by flow of the more ductile conglomerate over an irregular surface of relatively rigid basement rocks, which were little affected by Paleozoic deformation. The constrictive flow conditions that dominate the strain field in the northern part of the thrust sheet may reflect the irregular paleotopography of the unconformity surface and/or a lateral ramp oriented at an oblique angle to the transport direction that impeded west-northwest–directed thrusting.

Detailed mapping in the Ashley Falls, East Lee, Stockbridge, Great Barrington, and Monterey quadragles of Massachusetts and Connecticut has established 12 lithostratigraphic paragneiss and metavolcanic units, approximately 1,830 m in total thickness, in the Precambrian gneiss of the Berkshire massif. Coarsely blastopor-phyritic, microcline, perthite, biotite, and ferrohastingsite granite gneiss (the Tyringham Gneiss of Emerson, 1898) intruded the foliated paragneiss sequence in the form of broadly concordant granite-granodiorite sills as much as 457 m thick and also in the form of other less well defined intrusions. Fine-grained granodiorite and foliated alaskite locally rich in magnetite form a narrow discontinuous border facies of the granite gneiss. The Tyringham and its country rocks were isoclinally folded along easterly trends, granulated, and foliated prior to deposition of the Dalton Formation of late Precambrian(?) and Early Cambrian age. U-Pb isotopic data for zircon from the Tyringham and a paragneiss unit, the Washington Gneiss on Beartown Mountain (Great Barrington and Stockbridge quadrangles), are only slightly discordant and give Pb 207 /Pb 206 ages of 1,040 to 1,080 m.y. for both units. The zircons from the Tyringham Gneiss have an elongate, euhedral habit compatible with an igneous origin, whereas zircons from the Washington Gneiss commonly are rounded, have overgrowths, and occur as detrital grains in a blue quartz metaconglomerate, which interfingers with metadacitic volcanic rocks. The close agreement in age between the intrusive rock and its country rock may mean either that (1) the age of the source material for the Washington Gneiss and the intrusion of the Tyringham Gneiss are little separated in time, (2) the Tyringham Gneiss was intruded during the same dynamothermal event that reset the Washington Gneiss zircon, or (3) the zircons in both the Tyringham and Washington Gneiss record a metamorphic age. Because of geologic evidence that the Tyringham was intruded syntectonically during a time of deep-seated plutonic dynamothermal metamorphism in Precambrian time, either (2) or (3) is the favored explanation. Some evidence of an inherited component in the zircon from the Washington Gneiss is given by its older (~1,200 m.y.) Th-Pb age. The Berkshire gneiss sequence resembles closely in total thickness, probable protoliths, character of Precambrian deformation and plutonism, and isotopically determined age the “Grenville” metasedimentary sequence of the Adirondack Mountains described by Buddington and Leonard (1962) and de Waard and Walton (1967). Upper Precambrian (?) and Lower Cambrian quartzofeldspathic metasedimentary rocks of the Dalton Formation and Cheshire Quartzite overlie with profound angular unconformity the gneissic rocks of the Berkshire massif. Comparison of the metamorphic mineralogy of the Paleozioc and Precambrian rocks indicates that the Precambrian gneiss was affected by dynamothermal metamorphism at least as high as muscovite plus sillimanite grade prior to deposition of the Dalton-Cheshire sequence. Multiple dynamothermal metamorphism and deformation during Ordovician and Devonian time, however, have greatly affected the configuration and mineralogy of the basement rocks. Large-scale synmetamorphic westward overthrusting of imbricate slices of basement rocks transported the gneiss of the massif at least 21 km across underlying and highly deformed miogeosynclinal and exogeosynclinal metasedimentary rocks of Cambrian to Middle Ordovician age. High-grade (staurolite-kyanite-sillimanite plus muscovite) Barrovian metamophism postdated thrusting during a probable Acadian dynamothermal event. The complex internal structures seen in the Precambrian gneiss of the Berkshire massif result from multiple deep-seated deformations during the “Grenville” orogeny (~1 b.y. ago), Taconic orogeny (~460 m.y. ago), and Acadian orogeny (350 m.y. ago).