THE IMPORTANCE OF SYN-CONVERGENCE FLEXURAL NORMAL FAULTS IN THE CHAMPLAIN THRUST SYSTEM

The Champlain Thrust System (CTS) of the Taconic thrust belt in west-central Vermont contains transverse structures and frontal ramps that were controlled by reactivated normal faults. These reactivated normal faults were inherited from a phase of syn-convergence lithospheric flexure during medial Ordovician Taconic collision between the Laurentian margin and island arc terranes. The normal faults cut the pre-transport stratigraphic section, affected syn-convergence sedimentation, bounded local flysch basins, and partitioned displacement along strike. Map-unit relationships where the amount of stratigraphic offset across thrusts and thrust-linking structures does not match the corresponding structural offset are the best evidence for these conclusions. The CTS has several major splays that have along-strike displacement gradients determined by normal faults reactivated as lateral thrust-structures. One lateral structure links the apparent "tip-line" of the Champlain thrust with a large-displacement thrust that continues into New York. In the autochthon, the flexural normal faults are in a dominantly paleo-trench parallel orientation with a subordinate paleo-trench normal set. In the transported portion of the foreland (the par-autochthon), the thrust structures mask the set of paleo-trench parallel normal faults. However, an out of sequence thrust that led to the emplacement of shelf rocks above the par-autochthonous shale nucleated on a paleo-trench parallel normal fault in the outer shelf. The flexural normal faults thus controlled the structural style and the distribution of sedimentary facies along the Laurentian margin and the thrust faults occupy the same structural level as normal faults in the present-day subsurface. Our findings are consistent with a generally foreland-propagating far-traveled Taconic thrust system with a decollement that traces under transported shelf-sequence rocks, and a detached Green Mountain crystalline core on an out-of-sequence thrust, with the decollement level perhaps partly determined by the pre-thrust flexural normal fault system. Other foreland thrust systems may prove to contain significant syn-convergence normal fault components, and their recognition may help in resolving current geodynamic problems of thick-skinned thrusting, displacement partitioning along thrusts, and reactivation/inversion.