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Master of Science (M.S.)
Year Degree Awarded
The Connecticut Valley border fault is a major, probably listric, west-dipping normal fault, that uplifted Precambrian to Devonian schists and gneisses of the Pelham Dome on the east, and formed a lowland to the west where Triassic and Jurassic sediments accumulated. It extends from northwestern New Hampshire south to Long Island Sound. Five kilometers of vertical displacement, and dips of 20 to 40 degrees have been estimated for central Massachusetts.
Mylonitic and silicified rocks were examined from the Village of Millers Falls, in north-central Massachusetts, south to Belchertown, in central Massachusetts. Silicified rocks occur on the footwall of the fault in seven locations, and mylonitic rocks were studied in one location.
Mylonitic rocks involve protomylonite with small areas of rock that have characteristics of orthomylonite and ultramylonite. This indicates that some rocks were affected ally by the faulting process at depth in a ductile environment with heterogeneous strain rates. S - C fabrics (schistosite-cisaillement) within the mylonites indicate a west-side-down motion direction, supporting the usual observation that the border fault is a normal fault. The mylonites were cut by a cataclastic intrusion breccia which was subsequently mylonitized, developing an S-C fabric Similar to that in the host mylonite rock. Mylonites are indicative of low strain rates in a ductile regime, while cataclasites indicate higher strain rates. This suggests that the fault surface was not a smooth plane, but had irregularities that temporarily changed the strain rates imposed on the rocks.
Conditions nearer to the surfaces involved a brittle extensional environment where volumes of rock borderin the fault in the footwall were brecciated. The mylonites also were uplifted into the brittle regime. Joints, veins and minor normal faults were developed within the mylonites, cutting previous ductile features. Silicification occurred in the brecciated rock masses where hydrothermal fluids circulated through the fractured volumes of rock replacing primary metamorphic and igneous minerals with new minerals, primarily quartz.
Several groups of joints and veins within the silicified rocks have north-south strikes similar to the trend of the border fault in many areas. One style of joint strikes northeast to northwest, dips west, has a platy character, and appears to mimic the local fault plane orientation. Fluids that initially silicified the breccia masses also produced the quartz, and later, hematite veins in at least two subsequent hydrothermal pulses. Combined joint data show no conclusive evidence as to whether joint development and orientation was controlled by the geometry of the border fault, or by regional stresses. Combined vein data, however, show strong evidence that vein orientation was controlled by a regional stress field and not directly by border fault geometry. A mean strike for the veins of N15E suggests an extensional stress of N75W-S75E for the region at the time of vein formation. Extensional stresses of N60W-S60E and N68W-S68E have been estimated in previous studies for the Northfield basin and Amherst areas during the early Mesozoic. Vein formation was later and suggests that the regional stress field rotated counterclockwise over time.
Donald U. Wise
Stearns A. Morse