INTERMEDIATE-TERM HISTORY OF THRUST FAULTING IN THE TACONIC ACCRETIONARY WEDGE

Short-term history of thrust faults is reflected in cycles of seismic energy accumulation and release or stable sliding (102 - 103 years). Long term history of thrust faults is tied to cycles of plate boundary interactions (106-107 years). Intermediate term history (104-105 years)is more difficult to discern. In the Taconic slate belt of eastern NY and western Vt. we have identified very large pyrite framboid pressure shadows which record the strain history during the Taconic orogeny. The pressure shadows record three episodes of largely coaxial strain resulting in a cumulative strain of approximately 100%. At geologically reasonable strain rates it would take approximately 106 years to accumulate this strain. We have measured stable isotope compositions in quartz and calcite in these shadows and have combined these data with fluid inclusion data to yield a history of temperature and fluid pressure during the course of the strain accumulation. The framboids lie immediately below the Bird Mountain Thrust and allow the interpretation of fault history over a time period of a million years. We find regular variations in delta 18O of quartz, cycling from ~19.5 to 20 and back to 19.2. Coupled with calcite O isotopes, these data reveal a history of temperatures beginning at 250oC, falling to 200oC and increasing to 280oC. We interpret these results as indicating a period of cooling of approximately 300,000 years following a thrust motion episode, a period of heating during a second thrust motion episode of similar duration and a second cooling of equal duration. Periods of heating correlate with periods of high fluid pressure and both these correlate to changes in orientations of incremental strain. We believe that these record two episodes during which the Taconic accretionary prism was built up by thrust faulting to attain a critical taper angle and two episodes of horizontal extension during which the wedge was acting to decrease the taper angle.