CONSTRAINING THE 3D GEOMETRY OF CONJUGATE OROGENIC STRIKE-SLIP SHEAR ZONES IN THE NEW ENGLAND APPALACHIANS

The mid-lower crustal structure of conjugate orogenic strike-slip fault systems, such as the Karakorum-Altyn Tagh fault system in the Tibetan-Himalaya, remains a topic of widespread debate. End-member models of orogenic strike-slip fault systems propose either trans-lithospheric strike-slip, or alternatively, upper crustal strike-slip terminating in a shallowly dipping to subhorizontal mid-crustal shear zone or weak layer (Tapponnier et al., 2001 and Van Buer et al., 2015, respectively). The exhumed Acadian-Alleghenian orogen in the southern New England Appalachians presents an unparalleled opportunity to investigate extensive exposure of the orogenic mid-lower crust. The exhumed crust in this region underwent deformation and metamorphism during orogenic conjugate strike-slip shear along the Norumbega (dextral) and Western Bronson Hill (sinistral) shear zones during the Carboniferous Alleghenian orogeny (Massey and Moecher, 2013; McWilliams et al., 2013). The distribution of Alleghenian deformation and metamorphism in New England exhibits a distinct north-to-south variation in Neoacadian and Alleghenian metamorphic grade and deformation style. This variation allows for direct observations of an orogenic conjugate strike-slip shear system as a function of depth in the orogen. We present new regional macro-structural analysis, microstructural observations, Electron Backscatter Diffraction (EBSD) data, and in situ monazite and titanite U-Pb geochronology, suggesting the spatial distribution of Alleghenian deformation and metamorphism in the New England Appalachians is the result of rigid indentation of Gondwana to the south and highly oblique convergence to the north. This resulted in upper crustal strike-slip conjugate shear terminating on a low angle mid-lower crustal decollement and moderately west dipping coaxial shear zone. These observations of mid-lower crustal structure in association with an upper crustal conjugate shear system support an end-member model akin to that proposed by Van Buer et al. (2015), wherein upper crustal deformation is decoupled from the mid-lower crust along mid-lower crustal structures.