Northeastern Section–41st Annual Meeting (20–22 March 2006)

Paper No. 7
Presentation Time: 8:00 AM-12:00 PM


UNDERWOOD, H. Ray, CRESPI, Jean and PARMELEE, David, Center for Integrative Geosciences, University of Connecticut, Storrs, CT 06269,

Detailed mapping of cleavage (S2), intersection lineations, fold axes, and late-stage brittle faults was conducted in a 140 sq. km area in the Giddings Brook thrust sheet of the Taconic Allochthon north and west of Middle Granville, NY. Strain fringes around pyrite framboids in oriented samples of slate were analyzed to determine sense of shear, state of strain, strain magnitude, and orientation of L2. In XZ sections, a top-to-the-NW sense of shear was observed for nearly all oriented samples. In XY sections, both flattening and plane strain were observed. On the basis of strain magnitude and L2 orientation two domains were recognized. In one, strains are relatively high (mean 1 + e1 = 2.03), and L2 is oriented approximately down the dip of S2 (mean L2 = 102, 40). In addition, mesoscale fold axes locally show unusually steep plunges and little variation in orientation despite a wide range of interlimb angles. In the other domain, strains are relatively low (mean 1 + e1 = 1.43), and L2 rakes moderately from the south on S2 (mean L2 = 131, 34). Strain fringe fibers of some low strain domain samples demonstrate evidence of triclinic strain symmetry, exhibiting as much as 35° curvature in the XY plane. Previously triclinic strain symmetry has only been recognized in high strain mylonites. Similar triclinic strain symmetry was not observed in samples from the high strain domain, even though the fibers are significantly longer.

The high strain domain is located near a recess in the Middle Granville Thrust (MGT) where the fault trace narrows substantially compared to low strain domain sites to the south where the trace broadens into a fault zone. Although the MGT is interpreted as a post-S2, out-of-sequence thrust, its trace may represent a zone of weakness influenced by the strain distribution during S2 formation and top-to-NW shearing. The high strain results near the narrow section of the inferred shear zone differ not only in intensity but also in kinematics with respect to the results in the low strain domain. The obliquity of L2 on S2 and evidence of triclinic symmetry suggest a lack of strain partitioning in the low strain domain and consequently a more complete manifestation of (albeit not parallel to) the far field orientation of the convergence vector.