CONTACT RELATIONS BETWEEN THE AMMONOOSUC VOLCANCS AND JEFFERSON DOME, NORTHERN NH

The study examines the contact relationships between the Ordovician Ammonoosuc Volcanics (Oam) and Oliverian Jefferson Dome, a biotite quartz monzonite (Obqm), along the Bronson Hill Anticlinorium, in northern NH.  The purpose of the study is to evaluate the pre-, syn- and post-doming kinematics and timing of the contact. The contact has been variably interpreted as intrusive and domed or a fault that juxtaposed geochemically contrasting units and was then domed. Mylonites with reverse slip in the Oam have been interpreted to be early, later domed, normal faults. Three detailed strip maps across the contact were made and oriented samples were collected for microstructural and geochronologic study. The detailed maps show that Obqm is intercalated with a heterogeneous assemblage of Oam lithologies with pervasive shear fabrics along the contact zone decreasing in intensity away from the boundary. A strong foliation (58, 44 SE) is accompanied by a variably strong lineation (110, 41) in the contact zone. Microstructural shear sense indicators from samples in their present domed orientation show normal slip in the Oam and reverse slip in the Obqm. These fabrics indicate a range of possible kinematic models for each unit and the contact itself: 1) diapiric doming causing normal slip with no earlier deformation; 2) earlier thrust faulting that became domed; and/or 3) earlier normal faulting with dip reversal upon doming. Monazite microprobe dating of samples of Oam and Obqm will constrain the timing of the shearing and distinguish between these models.