STRUCTURE AND DEFORMATION OF THE JEFFERSON OLIVERIAN DOME IN THE SOUTHERN HALF OF THE MT. CRESCENT 7.5 QUADRANGLE, NEW HAMPSHIRE

New detailed structural mapping and zircon geochronology in the southern half of the Mt. Crescent 7.5’ quadrangle in Randolph, NH was completed for the USGS/NHGS StateMap program in an effort to better understand the geometry and deformation history of the Jefferson Dome, a mantled gneiss dome that is part of the Bronson Hill Anticlinorium. Overall structures support a Devonian Acadian or Neoacadian timing of deformation for the dome with subsequent Alleghenian shearing, followed by later Permian or Triassic brittle faulting.

Structural data in the form of foliations in the Jefferson Dome were collected for this study. Three different types of foliations were found: 1) zones of no foliation; 2) zones of single foliations that are likely dome related and 3) mylonitic S-C foliations defining cross cutting shear zones. The zones of no foliation are somewhat randomly distributed throughout the Dome. Similarly the dips of the singly foliated rocks are variably distributed across the Dome. Equal area projections of the dome foliations indicate a classic anticlinal dome shape with a trend and plunge of 65.4°, 12.7°. However, the spatial distribution of the foliation data does not support the dome shape. Narrow zones with abundant mylonitic foliations that present S-C fabrics and sigma porphyroclasts striking NE-SW show primarily reverse dip slip shear and cut the single and no foliation zones. These represent a complex array of shear zones that agree with kinematics from the Moose River fault studied by Dupee (2002) and Foley (2009). Newly mapped silicified pods up to 10 m in width are indicative of late brittle faults striking NE-SW and NW-SE. Kinematic indicators for the faults are absent.

New detrital and crystallization zircon geochronology constrains the timing of deformational events in the area. A newly mapped foliated metasedimentary xenolith within an unfoliated granite yielded a maximum depositional age of 429.3 +/- 7.0 Ma constraining the dome deformation to post-Ordovician. A fine grained granite with no foliation except for zones of S-C fabrics yielded a crystallization age of 334.0 +/- 2.2 Ma constraining the dome deformation between the Silurian to Devonian. The timing of the shear zones and brittle faults is constrained to the Permian to Jurassic since they are not present in the 178.4 +/- 1.1 Ma granite porphyry unit.