ALONG-STRIKE VARIATION IN PARTITIONING OF HORIZONTAL SHORTENING BETWEEN OUT-OF-SEQUENCE THRUST FAULTS AND SLATY CLEAVAGE IN THE GIDDINGS BROOK THRUST SHEET OF THE TACONIC ALLOCHTHON, VERMONT AND NEW YORK

The Giddings Brook thrust sheet in the northern Taconic allochthon of Vermont and New York is characterized by a zone of out-of-sequence thrust faults in the back part of the thrust sheet. The out-of-sequence thrust faults developed where the stratigraphy changes across strike such that formations dominated by black slate are present in the west but not the east. Our previous work has shown that the cleavage-related strain within the zone of out-of-sequence thrust faults in the Middle Granville–Poultney region varies along strike. In the north, the strain is described by thrusting simple shear + pure shear; the strain magnitude is relatively high and the stretching lineation is down-dip. In the south, the strain is described by thrusting simple shear + wrenching simple shear; the strain magnitude is relatively low and the stretching lineation has a moderate rake from the south. The difference in kinematics between the north and the south has resulted in a difference in the amount of horizontal shortening related to slaty cleavage development in the two regions. In the north, the average horizontal stretch related to slaty cleavage development is about 0.7 whereas, in the south, it is essentially zero. Here, we evaluate the extent to which the deficit in cleavage-related horizontal shortening in the south is balanced by increased displacement along the out-of-sequence thrust faults. The 1979 NEIGC field guide produced by Rowley and others contains cross sections that transect these two regions, and we use these to estimate the horizontal component of displacement across the Middle Granville thrust and an unnamed thrust between the South Poultney and Bird Mountain thrusts. Estimates for two other thrusts in the cross sections cannot be made because of complications produced by the across-strike change in stratigraphy. Our results support the possibility that the total horizontal shortening across the two regions is similar but partitioned in different ways between out-of-sequence thrust faults and slaty cleavage.