STRAIN GRADIENTS IN THE DAY MOUNTAIN THRUST SHEET, WESTERN MASSACHUSETTS

The Day Mountain thrust sheet is part of the Berkshire massif in western Massachusetts. Mesoproterozoic basement gneisses and unconformably overlying Neoproterozoic Dalton Formation and Cambrian Cheshire Quartzite were thrust westward over Cambrian to Ordovician marbles of the Stockbridge Formation. Thrusting was interpreted by Ratcliffe (1984) to be part of the Ordovician Taconic orogeny, although Karabinos et al. (2005) reported monazite ages suggesting that faulting was Silurian to Early Devonian. The basal unit of the Dalton Formation is a well-exposed quartz-pebble conglomerate. Deformed quartz pebbles serve as excellent strain markers, and we applied the R_f/φ method to 10 outcrops and 10 hand-samples to estimate strain gradients within the thrust sheet. Where possible, we measured 30 to 50 pebbles on each of three nearly orthogonal joint faces in outcrops or slabbed surfaces on oriented samples. Strain ellipsoids vary from nearly prolate (4:1:1) to nearly oblate (4:4:1); most ellipsoids are triaxial. Typically, one principal strain axis is approximately vertical and the other two plunge shallowly and trend roughly east-west and north-south. The long axis of the strain ellipsoid most commonly plunges gently to the east and the intermediate axis is either horizontal (north-south) or vertical. Locally, however, the long ellipsoid axis is oriented north-south, and the short axis plunges gently to the east. The observed strain gradients indicate that emplacement of the thrust sheet was not accomplished by simple plane-strain. Thrusting involved complex ductile deformation with regions of extending and constricting flow. Indentation of adjacent quartz pebbles indicates that volume loss was an important deformation mechanism not taken into account by our measurements. Furthermore, the mica-rich matrix of the conglomerate deformed more readily than the quartz pebbles. Thus, although the quartz pebbles record a significant shape change, they underestimate the true amount of strain in the conglomerate. Strong thrust-related deformation fabrics are restricted to the conglomerate; they did not develop in the underlying basement gneisses or in the overlying Cheshire Quartzite. Strain partitioning concentrated deformation in the intermediate Dalton Formation between the two other more competent units.